Preface

Working with both Object-Oriented software and Relational Databases can be cumbersome and time-consuming. Development costs are significantly higher due to a paradigm mismatch between how data is represented in objects versus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The term Object/Relational Mapping refers to the technique of mapping data from an object model representation to a relational data model representation (and vice versa).

Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities. It can significantly reduce development time otherwise spent with manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of common data persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQL and JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always.

Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and will help with the common task of result set translation from a tabular representation to a graph of objects.

Get Involved

  • Use Hibernate and report any bugs or issues you find. See Issue Tracker for details.

  • Try your hand at fixing some bugs or implementing enhancements. Again, see Issue Tracker.

  • Engage with the community using mailing lists, forums, IRC, or other ways listed in the Community section.

  • Help improve or translate this documentation. Contact us on the developer mailing list if you have interest.

  • Spread the word. Let the rest of your organization know about the benefits of Hibernate.

System Requirements

Hibernate 6.0 and later versions require at least Java 11 and JDBC 4.2.

Getting Started Guide

New users may want to first look through the Hibernate Getting Started Guide for basic information as well as tutorials. There is also a series of topical guides providing deep dives into various topics.

While having a strong background in SQL is not required to use Hibernate, it certainly helps a lot because it all boils down to SQL statements. Probably even more important is an understanding of data modeling principles. You might want to consider these resources as a good starting point:

Understanding the basics of transactions and design patterns such as Unit of Work (PoEAA) or Application Transaction are important as well. These topics will be discussed in the documentation, but a prior understanding will certainly help.

1. Architecture

1.1. Overview

Data Access Layers

Hibernate, as an ORM solution, effectively "sits between" the Java application data access layer and the Relational Database, as can be seen in the diagram above. The Java application makes use of the Hibernate APIs to load, store, query, etc. its domain data. Here we will introduce the essential Hibernate APIs. This will be a brief introduction; we will discuss these contracts in detail later.

As a Jakarta Persistence provider, Hibernate implements the Java Persistence API specifications and the association between Jakarta Persistence interfaces and Hibernate specific implementations can be visualized in the following diagram:

image

SessionFactory (org.hibernate.SessionFactory)

A thread-safe (and immutable) representation of the mapping of the application domain model to a database. Acts as a factory for org.hibernate.Session instances. The EntityManagerFactory is the Jakarta Persistence equivalent of a SessionFactory and basically, those two converge into the same SessionFactory implementation.

A SessionFactory is very expensive to create, so, for any given database, the application should have only one associated SessionFactory. The SessionFactory maintains services that Hibernate uses across all Session(s) such as second level caches, connection pools, transaction system integrations, etc.

Session (org.hibernate.Session)

A single-threaded, short-lived object conceptually modeling a "Unit of Work" (PoEAA). In Jakarta Persistence nomenclature, the Session is represented by an EntityManager.

Behind the scenes, the Hibernate Session wraps a JDBC java.sql.Connection and acts as a factory for org.hibernate.Transaction instances. It maintains a generally "repeatable read" persistence context (first level cache) of the application domain model.

Transaction (org.hibernate.Transaction)

A single-threaded, short-lived object used by the application to demarcate individual physical transaction boundaries. EntityTransaction is the Jakarta Persistence equivalent and both act as an abstraction API to isolate the application from the underlying transaction system in use (JDBC or JTA).

2. Domain Model

The term domain model comes from the realm of data modeling. It is the model that ultimately describes the problem domain you are working in. Sometimes you will also hear the term persistent classes.

Ultimately the application domain model is the central character in an ORM. They make up the classes you wish to map. Hibernate works best if these classes follow the Plain Old Java Object (POJO) / JavaBean programming model. However, none of these rules are hard requirements. Indeed, Hibernate assumes very little about the nature of your persistent objects. You can express a domain model in other ways (using trees of java.util.Map instances, for example).

Historically applications using Hibernate would have used its proprietary XML mapping file format for this purpose. With the coming of Jakarta Persistence, most of this information is now defined in a way that is portable across ORM/Jakarta Persistence providers using annotations (and/or standardized XML format). This chapter will focus on Jakarta Persistence mapping where possible. For Hibernate mapping features not supported by Jakarta Persistence we will prefer Hibernate extension annotations.

This chapter mostly uses "implicit naming" for table names, column names, etc. For details on adjusting these names see Naming strategies.

2.1. Mapping types

Hibernate understands both the Java and JDBC representations of application data. The ability to read/write this data from/to the database is the function of a Hibernate type. A type, in this usage, is an implementation of the org.hibernate.type.Type interface. This Hibernate type also describes various behavioral aspects of the Java type such as how to check for equality, how to clone values, etc.

Usage of the word type

The Hibernate type is neither a Java type nor a SQL data type. It provides information about mapping a Java type to an SQL type as well as how to persist and fetch a given Java type to and from a relational database.

When you encounter the term type in discussions of Hibernate, it may refer to the Java type, the JDBC type, or the Hibernate type, depending on the context.

To help understand the type categorizations, let’s look at a simple table and domain model that we wish to map.

Example 1. A simple table and domain model
create table Contact (
    id integer not null,
    first varchar(255),
    last varchar(255),
    middle varchar(255),
    notes varchar(255),
    starred boolean not null,
    website varchar(255),
    primary key (id)
)
@Entity(name = "Contact")
public static class Contact {

	@Id
	private Integer id;

	private Name name;

	private String notes;

	private URL website;

	private boolean starred;

	//Getters and setters are omitted for brevity
}

@Embeddable
public class Name {

	private String firstName;

	private String middleName;

	private String lastName;

	// getters and setters omitted
}

In the broadest sense, Hibernate categorizes types into two groups:

2.1.1. Value types

A value type is a piece of data that does not define its own lifecycle. It is, in effect, owned by an entity, which defines its lifecycle.

Looked at another way, all the state of an entity is made up entirely of value types. These state fields or JavaBean properties are termed persistent attributes. The persistent attributes of the Contact class are value types.

Value types are further classified into three sub-categories:

Basic types

in mapping the Contact table, all attributes except for name would be basic types. Basic types are discussed in detail in Basic types

Embeddable types

the name attribute is an example of an embeddable type, which is discussed in details in Embeddable types

Collection types

although not featured in the aforementioned example, collection types are also a distinct category among value types. Collection types are further discussed in Collections

2.1.2. Entity types

Entities, by nature of their unique identifier, exist independently of other objects whereas values do not. Entities are domain model classes which correlate to rows in a database table, using a unique identifier. Because of the requirement for a unique identifier, entities exist independently and define their own lifecycle. The Contact class itself would be an example of an entity.

Mapping entities is discussed in detail in Entity types.

2.2. Basic values

A basic type is a mapping between a Java type and a single database column.

Hibernate can map many standard Java types (Integer, String, etc.) as basic types. The mapping for many come from tables B-3 and B-4 in the JDBC specification[jdbc]. Others (URL as VARCHAR, e.g.) simply make sense.

Additionally, Hibernate provides multiple, flexible ways to indicate how the Java type should be mapped to the database.

The Jakarta Persistence specification strictly limits the Java types that can be marked as basic to the following:

Category Package Types

Java primitive types

boolean, int, double, etc.

Primitive wrappers

java.lang

Boolean, Integer, Double, etc.

Strings

java.lang

String

Arbitrary-precision numeric types

java.math

BigInteger and BigDecimal

Date/time types

java.time

LocalDate, LocalTime, LocalDateTime, OffsetTime, OffsetDateTime, Instant

Deprecated date/time types

java.util

Date and Calendar

Deprecated date/time types from

java.sql

Date, Time, Timestamp

Byte and character arrays

byte[] or Byte[], char[] or Character[]

Java enumerated types

Any enum

Serializable types

Any type that implements java.io.Serializable[1]

If provider portability is a concern, you should stick to just these basic types.

Java Persistence 2.1 introduced the jakarta.persistence.AttributeConverter providing support for handling types beyond those defined in the specification. See AttributeConverters for more on this topic.

2.2.1. @Basic

Strictly speaking, a basic type is denoted by the jakarta.persistence.Basic annotation.

Generally, the @Basic annotation can be ignored as it is assumed by default. Both of the following examples are ultimately the same.

Example 2. @Basic explicit
@Entity(name = "Product")
public class Product {

	@Id
	@Basic
	private Integer id;

	@Basic
	private String sku;

	@Basic
	private String name;

	@Basic
	private String description;
}
Example 3. @Basic implied
@Entity(name = "Product")
public class Product {

	@Id
	private Integer id;

	private String sku;

	private String name;

	private String description;
}

The @Basic annotation defines 2 attributes.

optional - boolean (defaults to true)

Defines whether this attribute allows nulls. Jakarta Persistence defines this as "a hint", which means the provider is free to ignore it. Jakarta Persistence also says that it will be ignored if the type is primitive. As long as the type is not primitive, Hibernate will honor this value. Works in conjunction with @Column#nullable - see @Column.

fetch - FetchType (defaults to EAGER)

Defines whether this attribute should be fetched eagerly or lazily. EAGER indicates that the value will be fetched as part of loading the owner. LAZY values are fetched only when the value is accessed. Jakarta Persistence requires providers to support EAGER, while support for LAZY is optional meaning that a provider is free to not support it. Hibernate supports lazy loading of basic values as long as you are using its bytecode enhancement support.

2.2.2. @Column

Jakarta Persistence defines rules for implicitly determining the name of tables and columns. For a detailed discussion of implicit naming see Naming strategies.

For basic type attributes, the implicit naming rule is that the column name is the same as the attribute name. If that implicit naming rule does not meet your requirements, you can explicitly tell Hibernate (and other providers) the column name to use.

Example 4. Explicit column naming
@Entity(name = "Product")
public class Product {

	@Id
	private Integer id;

	private String sku;

	private String name;

	@Column(name = "NOTES")
	private String description;
}

Here we use @Column to explicitly map the description attribute to the NOTES column, as opposed to the implicit column name description. See Naming strategies for additional details.

The @Column annotation defines other mapping information as well. See its Javadocs for details.

2.2.3. @Formula

@Formula allows mapping any database computed value as a virtual read-only column.

  • The @Formula annotation takes a native SQL clause which may affect database portability.

  • @Formula is a Hibernate-specific mapping construct and not covered by Jakarta Persistence. Applications interested in portability should avoid its use.

Example 5. @Formula mapping usage
@Entity(name = "Account")
public static class Account {

	@Id
	private Long id;

	private Double credit;

	private Double rate;

	@Formula(value = "credit * rate")
	private Double interest;

	//Getters and setters omitted for brevity

}

When loading the Account entity, Hibernate is going to calculate the interest property using the configured @Formula:

Example 6. Persisting an entity with a @Formula mapping
doInJPA(this::entityManagerFactory, entityManager -> {
	Account account = new Account();
	account.setId(1L);
	account.setCredit(5000d);
	account.setRate(1.25 / 100);
	entityManager.persist(account);
});

doInJPA(this::entityManagerFactory, entityManager -> {
	Account account = entityManager.find(Account.class, 1L);
	assertEquals(Double.valueOf(62.5d), account.getInterest());
});
INSERT INTO Account (credit, rate, id)
VALUES (5000.0, 0.0125, 1)

SELECT
    a.id as id1_0_0_,
    a.credit as credit2_0_0_,
    a.rate as rate3_0_0_,
    a.credit * a.rate as formula0_0_
FROM
    Account a
WHERE
    a.id = 1

The SQL fragment defined by the @Formula annotation can be as complex as you want, and it can even include sub-selects.

2.2.4. Mapping basic values

To deal with values of basic type, Hibernate needs to understand a few things about the mapping:

  • The capabilities of the Java type. For example:

    • How to compare values

    • How to calculate a hash-code

    • How to coerce values of this type to another type

  • The JDBC type it should use

    • How to bind values to JDBC statements

    • How to extract from JDBC results

  • Any conversion it should perform on the value to/from the database

  • The mutability of the value - whether the internal state can change like java.util.Date or is immutable like java.lang.String

This section covers how Hibernate determines these pieces and how to influence that determination process.

The following sections focus on approaches introduced in version 6 to influence how Hibernate will map basic value to the database.

This includes removal of the following deprecated legacy annotations:

  • @TypeDef

  • @TypeDefs

  • @CollectionId#type

  • @AnyMetaDef#metaType

  • @AnyMetaDef#idType

See the 6.0 migration guide for discussions about migrating uses of these annotations

The new annotations added as part of 6.0 support composing mappings in annotations through "meta-annotations".

Looking at this example, how does Hibernate know what mapping to use for these attributes? The annotations do not really provide much information.

This is an illustration of Hibernate’s implicit basic-type resolution, which is a series of checks to determine the appropriate mapping to use. Describing the complete process for implicit resolution is beyond the scope of this documentation[2].

This is primarily driven by the Java type defined for the basic type, which can generally be determined through reflection. Is the Java type an enum? Is it temporal? These answers can indicate certain mappings be used.

The fallback is to map the value to the "recommended" JDBC type.

Worst case, if the Java type is Serializable Hibernate will try to handle it via binary serialization.

For cases where the Java type is not a standard type or if some specialized handling is desired, Hibernate provides 2 main approaches to influence this mapping resolution:

  • A compositional approach using a combination of one-or-more annotations to describe specific aspects of the mapping. This approach is covered in Compositional basic mapping.

  • The UserType contract, which is covered in Custom type mapping

These 2 approaches should be considered mutually exclusive. A custom UserType will always take precedence over compositional annotations.

The next few sections look at common, standard Java types and discusses various ways to map them. See Case Study : BitSet for examples of mapping BitSet as a basic type using all of these approaches.

2.2.5. Enums

Hibernate supports the mapping of Java enums as basic value types in a number of different ways.

@Enumerated

The original Jakarta Persistence-compliant way to map enums was via the @Enumerated or @MapKeyEnumerated annotations, working on the principle that the enum values are stored according to one of 2 strategies indicated by jakarta.persistence.EnumType:

ORDINAL

stored according to the enum value’s ordinal position within the enum class, as indicated by java.lang.Enum#ordinal

STRING

stored according to the enum value’s name, as indicated by java.lang.Enum#name

Assuming the following enumeration:

Example 7. PhoneType enumeration
public enum PhoneType {
    LAND_LINE,
    MOBILE;
}

In the ORDINAL example, the phone_type column is defined as a (nullable) INTEGER type and would hold:

NULL

For null values

0

For the LAND_LINE enum

1

For the MOBILE enum

Example 8. @Enumerated(ORDINAL) example
@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	@Column(name = "phone_number")
	private String number;

	@Enumerated(EnumType.ORDINAL)
	@Column(name = "phone_type")
	private PhoneType type;

	//Getters and setters are omitted for brevity

}

When persisting this entity, Hibernate generates the following SQL statement:

Example 9. Persisting an entity with an @Enumerated(ORDINAL) mapping
Phone phone = new Phone();
phone.setId(1L);
phone.setNumber("123-456-78990");
phone.setType(PhoneType.MOBILE);
entityManager.persist(phone);
INSERT INTO Phone (phone_number, phone_type, id)
VALUES ('123-456-78990', 1, 1)

In the STRING example, the phone_type column is defined as a (nullable) VARCHAR type and would hold:

NULL

For null values

LAND_LINE

For the LAND_LINE enum

MOBILE

For the MOBILE enum

Example 10. @Enumerated(STRING) example
@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	@Column(name = "phone_number")
	private String number;

	@Enumerated(EnumType.STRING)
	@Column(name = "phone_type")
	private PhoneType type;

	//Getters and setters are omitted for brevity

}

Persisting the same entity as in the @Enumerated(ORDINAL) example, Hibernate generates the following SQL statement:

Example 11. Persisting an entity with an @Enumerated(STRING) mapping
INSERT INTO Phone (phone_number, phone_type, id)
VALUES ('123-456-78990', 'MOBILE', 1)
Using AttributeConverter

Let’s consider the following Gender enum which stores its values using the 'M' and 'F' codes.

Example 12. Enum with a custom constructor
public enum Gender {

    MALE('M'),
    FEMALE('F');

    private final char code;

    Gender(char code) {
        this.code = code;
    }

    public static Gender fromCode(char code) {
        if (code == 'M' || code == 'm') {
            return MALE;
        }
        if (code == 'F' || code == 'f') {
            return FEMALE;
        }
        throw new UnsupportedOperationException(
            "The code " + code + " is not supported!"
       );
    }

    public char getCode() {
        return code;
    }
}

You can map enums in a Jakarta Persistence compliant way using a Jakarta Persistence AttributeConverter.

Example 13. Enum mapping with AttributeConverter example
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	@Convert(converter = GenderConverter.class)
	public Gender gender;

	//Getters and setters are omitted for brevity

}

@Converter
public static class GenderConverter
		implements AttributeConverter<Gender, Character> {

	public Character convertToDatabaseColumn(Gender value) {
		if (value == null) {
			return null;
		}

		return value.getCode();
	}

	public Gender convertToEntityAttribute(Character value) {
		if (value == null) {
			return null;
		}

		return Gender.fromCode(value);
	}
}

Here, the gender column is defined as a CHAR type and would hold:

NULL

For null values

'M'

For the MALE enum

'F'

For the FEMALE enum

For additional details on using AttributeConverters, see AttributeConverters section.

Jakarta Persistence explicitly disallows the use of an AttributeConverter with an attribute marked as @Enumerated.

So, when using the AttributeConverter approach, be sure not to mark the attribute as @Enumerated.

Custom type

You can also map enums using a Hibernate custom type mapping. Let’s again revisit the Gender enum example, this time using a custom Type to store the more standardized 'M' and 'F' codes.

Example 14. Enum mapping with custom Type example
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	@Type(org.hibernate.userguide.mapping.basic.GenderType.class)
	public Gender gender;

	//Getters and setters are omitted for brevity

}

public class GenderType extends UserTypeSupport<Gender> {
    public GenderType() {
        super(Gender.class, Types.CHAR);
    }
}

public class GenderJavaType extends AbstractClassJavaType<Gender> {

    public static final GenderJavaType INSTANCE =
        new GenderJavaType();

    protected GenderJavaType() {
        super(Gender.class);
    }

    public String toString(Gender value) {
        return value == null ? null : value.name();
    }

    public Gender fromString(CharSequence string) {
        return string == null ? null : Gender.valueOf(string.toString());
    }

    public <X> X unwrap(Gender value, Class<X> type, WrapperOptions options) {
        return CharacterJavaType.INSTANCE.unwrap(
            value == null ? null : value.getCode(),
            type,
            options
       );
    }

    public <X> Gender wrap(X value, WrapperOptions options) {
        return Gender.fromCode(
				CharacterJavaType.INSTANCE.wrap( value, options)
       );
    }
}

Again, the gender column is defined as a CHAR type and would hold:

NULL

For null values

'M'

For the MALE enum

'F'

For the FEMALE enum

For additional details on using custom types, see Custom type mapping section.

2.2.6. Boolean

By default, Boolean attributes map to BOOLEAN columns, at least when the database has a dedicated BOOLEAN type. On databases which don’t, Hibernate uses whatever else is available: BIT, TINYINT, or SMALLINT.

Example 15. Implicit boolean mapping
// this will be mapped to BIT or BOOLEAN on the database
@Basic
boolean implicit;

However, it is quite common to find boolean values encoded as a character or as an integer. Such cases are exactly the intention of AttributeConverter. For convenience, Hibernate provides 3 built-in converters for the common boolean mapping cases:

  • YesNoConverter encodes a boolean value as 'Y' or 'N',

  • TrueFalseConverter encodes a boolean value as 'T' or 'F', and

  • NumericBooleanConverter encodes the value as an integer, 1 for true, and 0 for false.

Example 16. Using AttributeConverter
// this will get mapped to CHAR or NCHAR with a conversion
@Basic
@Convert(converter = org.hibernate.type.YesNoConverter.class)
boolean convertedYesNo;

// this will get mapped to CHAR or NCHAR with a conversion
@Basic
@Convert(converter = org.hibernate.type.TrueFalseConverter.class)
boolean convertedTrueFalse;

// this will get mapped to TINYINT with a conversion
@Basic
@Convert(converter = org.hibernate.type.NumericBooleanConverter.class)
boolean convertedNumeric;

If the boolean value is defined in the database as something other than BOOLEAN, character or integer, the value can also be mapped using a custom AttributeConverter - see AttributeConverters.

A UserType may also be used - see Custom type mapping

2.2.7. Byte

By default, Hibernate maps values of Byte / byte to the TINYINT JDBC type.

Example 17. Mapping Byte
// these will both be mapped using TINYINT
Byte wrapper;
byte primitive;

See Byte array for mapping arrays of bytes.

2.2.8. Short

By default, Hibernate maps values of Short / short to the SMALLINT JDBC type.

Example 18. Mapping Short
// these will both be mapped using SMALLINT
Short wrapper;
short primitive;

2.2.9. Integer

By default, Hibernate maps values of Integer / int to the INTEGER JDBC type.

Example 19. Mapping Integer
// these will both be mapped using INTEGER
Integer wrapper;
int primitive;

2.2.10. Long

By default, Hibernate maps values of Long / long to the BIGINT JDBC type.

Example 20. Mapping Long
// these will both be mapped using BIGINT
Long wrapper;
long primitive;

2.2.11. BigInteger

By default, Hibernate maps values of BigInteger to the NUMERIC JDBC type.

Example 21. Mapping BigInteger
// will be mapped using NUMERIC
BigInteger wrapper;

2.2.12. Double

By default, Hibernate maps values of Double to the DOUBLE, FLOAT, REAL or NUMERIC JDBC type depending on the capabilities of the database

Example 22. Mapping Double
// these will be mapped using DOUBLE, FLOAT, REAL or NUMERIC
// depending on the capabilities of the database
Double wrapper;
double primitive;

A specific type can be influenced using any of the JDBC type influencers covered in [basic-mapping-explicit] section.

If @JdbcTypeCode is used, the Dialect is still consulted to make sure the database supports the requested type. If not, an appropriate type is selected

2.2.13. Float

By default, Hibernate maps values of Float to the FLOAT, REAL or NUMERIC JDBC type depending on the capabilities of the database.

Example 23. Mapping Float
// these will be mapped using FLOAT, REAL or NUMERIC
// depending on the capabilities of the database
Float wrapper;
float primitive;

A specific type can be influenced using any of the JDBC type influencers covered in Mapping basic values section.

If @JdbcTypeCode is used, the Dialect is still consulted to make sure the database supports the requested type. If not, an appropriate type is selected

2.2.14. BigDecimal

By default, Hibernate maps values of BigDecimal to the NUMERIC JDBC type.

Example 24. Mapping BigDecimal
// will be mapped using NUMERIC
BigDecimal wrapper;

2.2.15. Character

By default, Hibernate maps Character to the CHAR JDBC type.

Example 25. Mapping Character
// these will be mapped using CHAR
Character wrapper;
char primitive;

2.2.16. String

By default, Hibernate maps String to the VARCHAR JDBC type.

Example 26. Mapping String
// will be mapped using VARCHAR
String string;

// will be mapped using CLOB
@Lob
String clobString;

Optionally, you may specify the maximum length of the string using @Column(length=…​), or using the @Size annotation from Hibernate Validator. For very large strings, you can use one of the constant values defined by the class org.hibernate.Length, for example:

@Column(length=Length.LONG)
private String text;

Alternatively, you may explicitly specify the JDBC type LONGVARCHAR, which is treated as a VARCHAR mapping with default length=Length.LONG when no length is explicitly specified:

@JdbcTypeCode(Types.LONGVARCHAR)
private String text;

If you use Hibernate for schema generation, Hibernate will generate DDL with a column type that is large enough to accommodate the maximum length you’ve specified.

If the maximum length you specify is too long to fit in the largest VARCHAR column supported by your database, Hibernate’s schema exporter will automatically upgrade the column type to TEXT, CLOB, or whatever is the equivalent type for your database. Please don’t (ab)use JPA’s @Lob annotation just because you want a TEXT column. The purpose of the @Lob annotation is not to control DDL generation!

See Handling LOB data for details on mapping to a database CLOB.

For databases which support nationalized character sets, you can also store strings as nationalized data.

Example 27. Mapping String as nationalized
// will be mapped using NVARCHAR
@Nationalized
String nstring;

// will be mapped using NCLOB
@Lob
@Nationalized
String nclobString;

See Handling nationalized character data for details on mapping strings using nationalized character sets.

2.2.17. Character arrays

By default, Hibernate maps Character[] and char[] to the VARCHAR JDBC type.

Example 28. Mapping Character
// mapped as VARCHAR
char[] primitive;
Character[] wrapper;

// mapped as CLOB
@Lob
char[] primitiveClob;
@Lob
Character[] wrapperClob;

See Handling LOB data for details on mapping as database LOB.

For databases which support nationalized character sets, you can also store character arrays as nationalized data.

Example 29. Mapping character arrays as nationalized
// mapped as NVARCHAR
@Nationalized
char[] primitiveNVarchar;
@Nationalized
Character[] wrapperNVarchar;

// mapped as NCLOB
@Lob
@Nationalized
char[] primitiveNClob;
@Lob
@Nationalized
Character[] wrapperNClob;

See Handling nationalized character data for details on mapping strings using nationalized character sets.

2.2.18. Clob / NClob

Be sure to check out Handling LOB data which covers basics of LOB handling and Handling nationalized character data which covers basics of nationalized data handling.

By default, Hibernate will map the java.sql.Clob Java type to CLOB and java.sql.NClob to NCLOB.

Considering we have the following database table:

Example 30. CLOB - SQL
CREATE TABLE Product (
  id INTEGER NOT NULL,
  name VARCHAR(255),
  warranty CLOB,
  PRIMARY KEY (id)
)

Let’s first map this using the @Lob Jakarta Persistence annotation and the java.sql.Clob type:

Example 31. CLOB mapped to java.sql.Clob
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    private Clob warranty;

    //Getters and setters are omitted for brevity

}

To persist such an entity, you have to create a Clob using the ClobProxy Hibernate utility:

Example 32. Persisting a java.sql.Clob entity
String warranty = "My product warranty";

final Product product = new Product();
product.setId(1);
product.setName("Mobile phone");

product.setWarranty(ClobProxy.generateProxy(warranty));

entityManager.persist(product);

To retrieve the Clob content, you need to transform the underlying java.io.Reader:

Example 33. Returning a java.sql.Clob entity
Product product = entityManager.find(Product.class, productId);

try (Reader reader = product.getWarranty().getCharacterStream()) {
    assertEquals("My product warranty", toString(reader));
}

We could also map the CLOB in a materialized form. This way, we can either use a String or a char[].

Example 34. CLOB mapped to String
@Entity(name = "Product")
public static class Product {

	@Id
	private Integer id;

	private String name;

	@Lob
	private String warranty;

	//Getters and setters are omitted for brevity

}

We might even want the materialized data as a char array.

Example 35. CLOB - materialized char[] mapping
@Entity(name = "Product")
public static class Product {

	@Id
	private Integer id;

	private String name;

	@Lob
	private char[] warranty;

	//Getters and setters are omitted for brevity

}

Just like with CLOB, Hibernate can also deal with NCLOB SQL data types:

Example 36. NCLOB - SQL
CREATE TABLE Product (
    id INTEGER NOT NULL ,
    name VARCHAR(255) ,
    warranty nclob ,
    PRIMARY KEY ( id )
)

Hibernate can map the NCLOB to a java.sql.NClob

Example 37. NCLOB mapped to java.sql.NClob
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    @Nationalized
    // Clob also works, because NClob extends Clob.
    // The database type is still NCLOB either way and handled as such.
    private NClob warranty;

    //Getters and setters are omitted for brevity

}

To persist such an entity, you have to create an NClob using the NClobProxy Hibernate utility:

Example 38. Persisting a java.sql.NClob entity
String warranty = "My product warranty";

final Product product = new Product();
product.setId(1);
product.setName("Mobile phone");

product.setWarranty(NClobProxy.generateProxy(warranty));

entityManager.persist(product);

To retrieve the NClob content, you need to transform the underlying java.io.Reader:

Example 39. Returning a java.sql.NClob entity
Product product = entityManager.find(Product.class, 1);

try (Reader reader = product.getWarranty().getCharacterStream()) {
    assertEquals("My product warranty", toString(reader));
}

We could also map the NCLOB in a materialized form. This way, we can either use a String or a char[].

Example 40. NCLOB mapped to String
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    @Nationalized
    private String warranty;

    //Getters and setters are omitted for brevity

}

We might even want the materialized data as a char array.

Example 41. NCLOB - materialized char[] mapping
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    @Nationalized
    private char[] warranty;

    //Getters and setters are omitted for brevity

}

2.2.19. Byte array

By default, Hibernate maps values of type byte[] and Byte[] to the JDBC type VARBINARY.

Example 42. Mapping arrays of bytes
// mapped as VARBINARY
private byte[] primitive;
private Byte[] wrapper;

// mapped as (materialized) BLOB
@Lob
private byte[] primitiveLob;
@Lob
private Byte[] wrapperLob;

Just like with strings, you may specify the maximum length using @Column(length=…​) or the @Size annotation from Hibernate Validator. For very large arrays, you can use the constants defined by org.hibernate.Length. Alternatively @JdbcTypeCode(Types.LONGVARBINARY) is treated as a VARBINARY mapping with default length=Length.LONG when no length is explicitly specified.

If you use Hibernate for schema generation, Hibernate will generate DDL with a column type that is large enough to accommodate the maximum length you’ve specified.

If the maximum length you specify is too long to fit in the largest VARBINARY column supported by your database, Hibernate’s schema exporter will automatically upgrade the column type to IMAGE, BLOB, or whatever is the equivalent type for your database. Please don’t (ab)use JPA’s @Lob annotation for DDL customization.

See Handling LOB data for details on mapping to a database BLOB.

2.2.20. Blob

Be sure to check out Handling LOB data which covers basics of LOB handling.

By default, Hibernate will map the java.sql.Blob Java type to BLOB.

Considering we have the following database table:

Example 43. BLOB - SQL
CREATE TABLE Product (
    id INTEGER NOT NULL ,
    image blob ,
    name VARCHAR(255) ,
    PRIMARY KEY ( id )
)

Let’s first map this using the JDBC java.sql.Blob type.

Example 44. BLOB mapped to java.sql.Blob
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    private Blob image;

    //Getters and setters are omitted for brevity

}

To persist such an entity, you have to create a Blob using the BlobProxy Hibernate utility:

Example 45. Persisting a java.sql.Blob entity
byte[] image = new byte[] {1, 2, 3};

final Product product = new Product();
product.setId(1);
product.setName("Mobile phone");

product.setImage(BlobProxy.generateProxy(image));

entityManager.persist(product);

To retrieve the Blob content, you need to transform the underlying java.io.InputStream:

Example 46. Returning a java.sql.Blob entity
Product product = entityManager.find(Product.class, productId);

try (InputStream inputStream = product.getImage().getBinaryStream()) {
    assertArrayEquals(new byte[] {1, 2, 3}, toBytes(inputStream));
}

We could also map the BLOB in a materialized form (e.g. byte[]).

Example 47. BLOB mapped to byte[]
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Lob
    private byte[] image;

    //Getters and setters are omitted for brevity

}

2.2.21. Duration

By default, Hibernate will map Duration to the INTERVAL_SECOND SQL type and fallback to NUMERIC if necessary.

Example 48. Mapping Duration
private Duration duration;

2.2.22. Instant

Instant is mapped to the TIMESTAMP_UTC SQL type.

Example 49. Mapping Instant
// mapped as TIMESTAMP
private Instant instant;

See Handling temporal data for basics of temporal mapping

2.2.23. LocalDate

LocalDate is mapped to the DATE JDBC type.

Example 50. Mapping LocalDate
// mapped as DATE
private LocalDate localDate;

See Handling temporal data for basics of temporal mapping

2.2.24. LocalDateTime

LocalDateTime is mapped to the TIMESTAMP JDBC type.

Example 51. Mapping LocalDateTime
// mapped as TIMESTAMP
private LocalDateTime localDateTime;

See Handling temporal data for basics of temporal mapping

2.2.25. LocalTime

LocalTime is mapped to the TIME JDBC type.

Example 52. Mapping LocalTime
// mapped as TIME
private LocalTime localTime;

See Handling temporal data for basics of temporal mapping

2.2.26. OffsetDateTime

OffsetDateTime is mapped to the TIMESTAMP or TIMESTAMP_WITH_TIMEZONE JDBC type depending on the database.

Example 53. Mapping OffsetDateTime
// mapped as TIMESTAMP or TIMESTAMP_WITH_TIMEZONE
private OffsetDateTime offsetDateTime;

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.27. OffsetTime

OffsetTime is mapped to the TIME or TIME_WITH_TIMEZONE JDBC type depending on the database.

Example 54. Mapping OffsetTime
// mapped as TIME or TIME_WITH_TIMEZONE
private OffsetTime offsetTime;

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.28. TimeZone

TimeZone is mapped to VARCHAR JDBC type.

Example 55. Mapping OffsetTime
// mapped as VARCHAR
private TimeZone timeZone;

2.2.29. ZonedDateTime

ZonedDateTime is mapped to the TIMESTAMP or TIMESTAMP_WITH_TIMEZONE JDBC type depending on the database.

Example 56. Mapping ZonedDateTime
// mapped as TIMESTAMP or TIMESTAMP_WITH_TIMEZONE
private ZonedDateTime zonedDateTime;

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.30. ZoneOffset

ZoneOffset is mapped to VARCHAR JDBC type.

Example 57. Mapping ZoneOffset
// mapped as VARCHAR
private ZoneOffset zoneOffset;

2.2.31. Calendar

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.32. Date

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.33. Time

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.34. Timestamp

See Handling temporal data for basics of temporal mapping See Using a specific time zone for basics of time-zone handling

2.2.35. Class

Hibernate maps Class references to VARCHAR JDBC type

Example 58. Mapping Class
// mapped as VARCHAR
private Class<?> clazz;

2.2.36. Currency

Hibernate maps Currency references to VARCHAR JDBC type

Example 59. Mapping Currency
// mapped as VARCHAR
private Currency currency;

2.2.37. Locale

Hibernate maps Locale references to VARCHAR JDBC type

Example 60. Mapping Locale
// mapped as VARCHAR
private Locale locale;

2.2.38. UUID

Hibernate allows mapping UUID values in a number of ways. By default, Hibernate will store UUID values in the native form by using the SQL type UUID or in binary form with the BINARY JDBC type if the database does not have a native UUID type.

The default uses the binary representation because it uses a more efficient column storage.

However, many applications prefer the readability of the character-based column storage.

To switch the default mapping, set the hibernate.type.preferred_uuid_jdbc_type configuration to CHAR.

UUID as binary

As mentioned, the default mapping for UUID attributes. Maps the UUID to a byte[] using java.util.UUID#getMostSignificantBits and java.util.UUID#getLeastSignificantBits and stores that as BINARY data.

Chosen as the default simply because it is generally more efficient from a storage perspective.

UUID as (var)char

Maps the UUID to a String using java.util.UUID#toString and java.util.UUID#fromString and stores that as CHAR or VARCHAR data.

UUID as identifier

Hibernate supports using UUID values as identifiers, and they can even be generated on the user’s behalf. For details, see the discussion of generators in Identifiers.

2.2.39. InetAddress

By default, Hibernate will map InetAddress to the INET SQL type and fallback to BINARY if necessary.

Example 61. Mapping InetAddress
private InetAddress address;

2.2.40. JSON mapping

Hibernate will only use the JSON type if explicitly configured through @JdbcTypeCode( SqlTypes.JSON ). The JSON library used for serialization/deserialization is detected automatically, but can be overridden by setting hibernate.type.json_format_mapper as can be read in the Configurations section.

Example 62. Mapping JSON
@JdbcTypeCode( SqlTypes.JSON )
private Map<String, String> payload;

2.2.41. XML mapping

Hibernate will only use the XML type if explicitly configured through @JdbcTypeCode( SqlTypes.SQLXML ). The XML library used for serialization/deserialization is detected automatically, but can be overridden by setting hibernate.type.xml_format_mapper as can be read in the Configurations section.

Example 63. Mapping XML
@JdbcTypeCode( SqlTypes.SQLXML )
private Map<String, StringNode> stringMap;

2.2.42. Basic array mapping

Basic arrays, other than byte[]/Byte[] and char[]/Character[], map to the type code SqlTypes.ARRAY by default, which maps to the SQL standard array type if possible, as determined via the new methods getArrayTypeName and supportsStandardArrays of org.hibernate.dialect.Dialect. If SQL standard array types are not available, data will be modeled as SqlTypes.JSON, SqlTypes.XML or SqlTypes.VARBINARY, depending on the database support as determined via the new method org.hibernate.dialect.Dialect.getPreferredSqlTypeCodeForArray.

Example 64. Mapping basic arrays
Short[] wrapper;
short[] primitive;

2.2.43. Basic collection mapping

Basic collections (only subtypes of Collection), which are not annotated with @ElementCollection, map to the type code SqlTypes.ARRAY by default, which maps to the SQL standard array type if possible, as determined via the new methods getArrayTypeName and supportsStandardArrays of org.hibernate.dialect.Dialect. If SQL standard array types are not available, data will be modeled as SqlTypes.JSON, SqlTypes.XML or SqlTypes.VARBINARY, depending on the database support as determined via the new method org.hibernate.dialect.Dialect.getPreferredSqlTypeCodeForArray.

Example 65. Mapping basic collections
List<Short> list;
SortedSet<Short> sortedSet;

2.2.44. Compositional basic mapping

The compositional approach allows defining how the mapping should work in terms of influencing individual parts that make up a basic-value mapping. This section will look at these individual parts and the specifics of influencing each.

JavaType

Hibernate needs to understand certain aspects of the Java type to handle values properly and efficiently. Hibernate understands these capabilities through its org.hibernate.type.descriptor.java.JavaType contract. Hibernate provides built-in support for many JDK types (Integer, String, e.g.), but also supports the ability for the application to change the handling for any of the standard JavaType registrations as well as add in handling for non-standard types. Hibernate provides multiple ways for the application to influence the JavaType descriptor to use.

The resolution can be influenced locally using the @JavaType annotation on a particular mapping. The indicated descriptor will be used just for that mapping. There are also forms of @JavaType for influencing the keys of a Map (@MapKeyJavaType), the index of a List or array (@ListIndexJavaType), the identifier of an ID-BAG mapping (@CollectionIdJavaType) as well as the discriminator (@AnyDiscriminator) and key (@AnyKeyJavaClass, @AnyKeyJavaType) of an ANY mapping.

The resolution can also be influenced globally by registering the appropriate JavaType descriptor with the JavaTypeRegistry. This approach is able to both "override" the handling for certain Java types or to register new types. See Registries for discussion of JavaTypeRegistry.

See Resolving the composition for a discussion of the process used to resolve the mapping composition.

JdbcType

Hibernate also needs to understand aspects of the JDBC type it should use (how it should bind values, how it should extract values, etc.) which is the role of its org.hibernate.type.descriptor.jdbc.JdbcType contract. Hibernate provides multiple ways for the application to influence the JdbcType descriptor to use.

Locally, the resolution can be influenced using either the @JdbcType or @JdbcTypeCode annotations. There are also annotations for influencing the JdbcType in relation to Map keys (@MapKeyJdbcType, @MapKeyJdbcTypeCode), the index of a List or array (@ListIndexJdbcType, @ListIndexJdbcTypeCode), the identifier of an ID-BAG mapping (@CollectionIdJdbcType, @CollectionIdJdbcTypeCode) as well as the key of an ANY mapping (@AnyKeyJdbcType, @AnyKeyJdbcTypeCode). The @JdbcType specifies a specific JdbcType implementation to use while @JdbcTypeCode specifies a "code" that is then resolved against the JdbcTypeRegistry.

The "type code" relative to a JdbcType generally maps to the corresponding value in java.sql.Types. registers entries in the JdbcTypeRegistry for all the standard java.sql.Types codes (aside from OTHER, which is special). See Registries for more discussion.

Customizing the JdbcTypeRegistry can be accomplished through @JdbcTypeRegistration and TypeContributor. See Registries for discussion of JavaTypeRegistry. See TypeContributor for discussion of TypeContributor.

See the @JdbcTypeCode Javadoc for details.

See Resolving the composition for a discussion of the process used to resolve the mapping composition.

MutabilityPlan

MutabilityPlan is the means by which Hibernate understands how to deal with the domain value in terms of its internal mutability as well as related concerns such as making copies. While it seems like a minor concern, it can have a major impact on performance. See AttributeConverter Mutability Plan for one case where this can manifest. See also Case Study : BitSet for another discussion.

The MutabilityPlan for a mapping can be influenced by any of the following annotations:

  • @Mutability

  • @Immutable

  • @MapKeyMutability

  • @CollectionIdMutability

Hibernate checks the following places for @Mutability and @Immutable, in order of precedence:

  1. Local to the mapping

  2. On the associated AttributeConverter implementation class (if one)

  3. On the value’s Java type

In most cases, the fallback defined by JavaType#getMutabilityPlan is the proper strategy.

Hibernate uses MutabilityPlan to:

  1. Check whether a value is considered dirty

  2. Make deep copies

  3. Marshal values to and from the second-level cache

Generally speaking, immutable values perform better in all of these cases

  1. To check for dirtiness, Hibernate just needs to check object identity (==) as opposed to equality (Object#equals).

  2. The same value instance can be used as the deep copy of itself.

  3. The same value can be used from the second-level cache as well as the value we put into the second-level cache.

If a particular Java type is considered mutable (a Date e.g.), @Immutable or a immutable-specific MutabilityPlan implementation can be specified to have Hibernate treat the value as immutable. This also acts as a contract from the application that the internal state of these objects is not changed by the application. Specifying that a mutable type is immutable and then changing the internal state will lead to problems; so only do this if the application unequivocally does not change the internal state.

See Resolving the composition for a discussion of the process used to resolve the mapping composition.

BasicValueConverter

BasicValueConverter is roughly analogous to AttributeConverter in that it describes a conversion to happen when reading or writing values of a basic-valued model part. In fact, internally Hibernate wraps an applied AttributeConverter in a BasicValueConverter. It also applies implicit BasicValueConverter converters in certain cases such as enum handling, etc.

Hibernate does not provide an explicit facility to influence these conversions beyond AttributeConverter. See AttributeConverters.

See Resolving the composition for a discussion of the process used to resolve the mapping composition.

Resolving the composition

Using this composition approach, Hibernate will need to resolve certain parts of this mapping. Often this involves "filling in the blanks" as it will be configured for just parts of the mapping. This section outlines how this resolution happens.

This is a complicated process and is only covered at a high level for the most common cases here.

For the full specifics, consult the source code for org.hibernate.mapping.BasicValue#buildResolution

First, we look for a custom type. If found, this takes predence. See Custom type mapping for details

If an AttributeConverter is applied, we use it as the basis for the resolution

  1. If @JavaType is also used, that specific JavaType is used for the converter’s "domain type". Otherwise, the Java type defined by the converter as its "domain type" is resolved against the JavaTypeRegistry

  2. If @JdbcType or @JdbcTypeCode is used, the indicated JdbcType is used and the converted "relational Java type" is determined by JdbcType#getJdbcRecommendedJavaTypeMapping. Otherwise, the Java type defined by the converter as its relational type is used and the JdbcType is determined by JdbcType#getRecommendedJdbcType

  3. The MutabilityPlan can be specified using @Mutability or @Immutable on the AttributeConverter implementation, the basic value mapping or the Java type used as the domain-type. Otherwise, JdbcType#getJdbcRecommendedJavaTypeMapping for the conversion’s domain-type is used to determine the mutability-plan.

Next we try to resolve the JavaType to use for the mapping. We check for an explicit @JavaType and use the specified JavaType if found. Next any "implicit" indication is checked; for example, the index for a List has the implicit Java type of Integer. Next, we use reflection if possible. If we are unable to determine the JavaType to use through the preceeding steps, we try to resolve an explicitly specified JdbcType to use and, if found, use its JdbcType#getJdbcRecommendedJavaTypeMapping as the mapping’s JavaType. If we are not able to determine the JavaType by this point, an error is thrown.

The JavaType resolved earlier is then inspected for a number of special cases.

  1. For enum values, we check for an explicit @Enumerated and create an enumeration mapping. Note that this resolution still uses any explicit JdbcType indicators

  2. For temporal values, we check for @Temporal and create an enumeration mapping. Note that this resolution still uses any explicit JdbcType indicators; this includes @JdbcType and @JdbcTypeCode, as well as @TimeZoneStorage and @TimeZoneColumn if appropriate.

The fallback at this point is to use the JavaType and JdbcType determined in earlier steps to create a JDBC-mapping (which encapsulates the JavaType and JdbcType) and combines it with the resolved MutabilityPlan

When using the compositional approach, there are other ways to influence the resolution as covered in Enums, Handling temporal data, Handling LOB data and Handling nationalized character data

See TypeContributor for an alternative to @JavaTypeRegistration and @JdbcTypeRegistration.

2.2.45. Custom type mapping

Another approach is to supply the implementation of the org.hibernate.usertype.UserType contract using @Type.

There are also corresponding, specialized forms of @Type for specific model parts:

  • When mapping a Map, @Type describes the Map value while @MapKeyType describe the Map key

  • When mapping an id-bag, @Type describes the elements while @CollectionIdType describes the collection-id

  • For other collection mappings, @Type describes the elements

  • For discriminated association mappings (@Any and @ManyToAny), @Type describes the discriminator value

@Type allows for more complex mapping concerns; but, AttributeConverter and Compositional basic mapping should generally be preferred as simpler solutions

2.2.46. Handling nationalized character data

How nationalized character data is handled and stored depends on the underlying database.

Most databases support storing nationalized character data through the standardized SQL NCHAR, NVARCHAR, LONGNVARCHAR and NCLOB variants.

Others support storing nationalized data as part of CHAR, VARCHAR, LONGVARCHAR and CLOB. Generally these databases do not support NCHAR, NVARCHAR, LONGNVARCHAR and NCLOB, even as aliased types.

Ultimately Hibernate understands this through Dialect#getNationalizationSupport()

To ensure nationalized character data gets stored and accessed correctly, @Nationalized can be used locally or hibernate.use_nationalized_character_data can be set globally.

@Nationalized and hibernate.use_nationalized_character_data can be used regardless of the specific database support for nationalized data and allows the application to work portably across databases with varying support.

For databases with no NCLOB data type, attributes of type java.sql.NClob are simply unsupported. Use java.sql.Clob (which NClob extends) or a materialized mapping like String or char[] instead.

See also Handling LOB data regarding similar limitation for databases which do not support explicit CLOB data-type.

Considering we have the following database table:

Example 66. NVARCHAR - SQL
CREATE TABLE Product (
    id INTEGER NOT NULL ,
    name VARCHAR(255) ,
    warranty NVARCHAR(255) ,
    PRIMARY KEY ( id )
)

To map a specific attribute to a nationalized variant data type, Hibernate defines the @Nationalized annotation.

Example 67. NVARCHAR mapping
@Entity(name = "Product")
public static class Product {

    @Id
    private Integer id;

    private String name;

    @Nationalized
    private String warranty;

    //Getters and setters are omitted for brevity

}

2.2.47. Handling LOB data

The @Lob annotation specifies that character or binary data should be written to the database using the special JDBC APIs for handling database LOB (Large OBject) types.

How JDBC deals with LOB data varies from driver to driver. Hibernate tries to take care of all these differences, and protect you as much as possible from inconsistent driver behavior. Sadly, Hibernate is only partially successful at achieving this goal.

Some database drivers (i.e. PostgreSQL) are especially problematic and in such cases you might have to do some extra work to get LOBs functioning. But that’s beyond the scope of this guide.

For databases with no CLOB type, attributes of type java.sql.Clob are simply unsupported. Use a materialized type like String or char[] instead.

There’s two ways a LOB may be represented in the Java domain model:

  • using a special JDBC-defined LOB locator type, or

  • using a regular "materialized" type like String, char[], or byte[].

LOB Locator

The JDBC LOB locator types are:

  • java.sql.Blob

  • java.sql.Clob

  • java.sql.NClob

These types represent references to off-table LOB data. In principle, they allow JDBC drivers to support more efficient access to the LOB data. Some drivers stream parts of the LOB data as needed, potentially consuming less memory.

However, java.sql.Blob and java.sql.Clob can be unnatural to deal with and suffer certain limitations. For example, it’s not portable to access a LOB locator after the end of the transaction in which it was obtained.

Materialized LOB

Alternatively, Hibernate lets you access LOB data via the familiar Java types String, char[], and byte[]. But of course this requires materializing the entire contents of the LOB in memory when the object is first retrieved. Whether this performance cost is acceptable depends on many factors, including the vagaries of the JDBC driver.

You don’t need to use a @Lob mapping for every database column of type BLOB or CLOB. The @Lob annotation is a special-purpose tool that should only be used when a default basic mapping to String would result in unacceptable performance characteristics.

2.2.48. Handling temporal data

Hibernate supports mapping temporal values in numerous ways, though ultimately these strategies boil down to the 3 main Date/Time types defined by the SQL specification:

DATE

Represents a calendar date by storing years, months and days.

TIME

Represents the time of a day by storing hours, minutes and seconds.

TIMESTAMP

Represents both a DATE and a TIME plus nanoseconds.

TIMESTAMP WITH TIME ZONE

Represents both a DATE and a TIME plus nanoseconds and zone id or offset.

The mapping of java.time temporal types to the specific SQL Date/Time types is implied as follows:

DATE

java.time.LocalDate

TIME

java.time.LocalTime, java.time.OffsetTime

TIMESTAMP

java.time.Instant, java.time.LocalDateTime, java.time.OffsetDateTime and java.time.ZonedDateTime

TIMESTAMP WITH TIME ZONE

java.time.OffsetDateTime, java.time.ZonedDateTime

Although Hibernate recommends the use of the java.time package for representing temporal values, it does support using java.sql.Date, java.sql.Time, java.sql.Timestamp, java.util.Date and java.util.Calendar.

The mappings for java.sql.Date, java.sql.Time, java.sql.Timestamp are implicit:

DATE

java.sql.Date

TIME

java.sql.Time

TIMESTAMP

java.sql.Timestamp

Applying @Temporal to java.sql.Date, java.sql.Time, java.sql.Timestamp or any of the java.time types is considered an exception

When using java.util.Date or java.util.Calendar, Hibernate assumes TIMESTAMP. To alter that, use @Temporal.

Example 68. Mapping java.util.Date
// mapped as TIMESTAMP by default
Date dateAsTimestamp;

// explicitly mapped as DATE
@Temporal(TemporalType.DATE)
Date dateAsDate;

// explicitly mapped as TIME
@Temporal(TemporalType.TIME)
Date dateAsTime;
Using a specific time zone

By default, Hibernate is going to use the PreparedStatement.setTimestamp(int parameterIndex, java.sql.Timestamp) or PreparedStatement.setTime(int parameterIndex, java.sql.Time x) when saving a java.sql.Timestamp or a java.sql.Time property.

When the time zone is not specified, the JDBC driver is going to use the underlying JVM default time zone, which might not be suitable if the application is used from all across the globe. For this reason, it is very common to use a single reference time zone (e.g. UTC) whenever saving/loading data from the database.

One alternative would be to configure all JVMs to use the reference time zone:

Declaratively
java -Duser.timezone=UTC ...
Programmatically
TimeZone.setDefault( TimeZone.getTimeZone( "UTC" ) );

However, as explained in this article, this is not always practical, especially for front-end nodes. For this reason, Hibernate offers the hibernate.jdbc.time_zone configuration property which can be configured:

Declaratively, at the SessionFactory level
settings.put(
    AvailableSettings.JDBC_TIME_ZONE,
    TimeZone.getTimeZone( "UTC" )
);
Programmatically, on a per Session basis
Session session = sessionFactory()
    .withOptions()
    .jdbcTimeZone( TimeZone.getTimeZone( "UTC" ) )
    .openSession();

With this configuration property in place, Hibernate is going to call the PreparedStatement.setTimestamp(int parameterIndex, java.sql.Timestamp, Calendar cal) or PreparedStatement.setTime(int parameterIndex, java.sql.Time x, Calendar cal), where the java.util.Calendar references the time zone provided via the hibernate.jdbc.time_zone property.

Handling time zoned temporal data

By default, Hibernate will convert and normalize OffsetDateTime and ZonedDateTime to java.sql.Timestamp in UTC. This behavior can be altered by configuring the hibernate.timezone.default_storage property

settings.put(
    AvailableSettings.TIMEZONE_DEFAULT_STORAGE,
    TimeZoneStorageType.AUTO
);

Other possible storage types are AUTO, COLUMN, NATIVE and NORMALIZE (the default). With COLUMN, Hibernate will save the time zone information into a dedicated column, whereas NATIVE will require the support of database for a TIMESTAMP WITH TIME ZONE data type that retains the time zone information. NORMALIZE doesn’t store time zone information and will simply convert the timestamp to UTC. Hibernate understands what a database/dialect supports through Dialect#getTimeZoneSupport and will abort with a boot error if the NATIVE is used in conjunction with a database that doesn’t support this. For AUTO, Hibernate tries to use NATIVE if possible and falls back to COLUMN otherwise.

2.2.49. @TimeZoneStorage

Hibernate supports defining the storage to use for time zone information for individual properties via the @TimeZoneStorage and @TimeZoneColumn annotations. The storage type can be specified via the @TimeZoneStorage by specifying a org.hibernate.annotations.TimeZoneStorageType. The default storage type is AUTO which will ensure that the time zone information is retained. The @TimeZoneColumn annotation can be used in conjunction with AUTO or COLUMN and allows to define the column details for the time zone information storage.

Storing the zone offset might be problematic for future timestamps as zone rules can change. Due to this, storing the offset is only safe for past timestamps, and we advise sticking to the NORMALIZE strategy by default.

Example 69. @TimeZoneColumn usage
@TimeZoneStorage(TimeZoneStorageType.COLUMN)
@TimeZoneColumn(name = "birthday_offset_offset")
@Column(name = "birthday_offset")
private OffsetDateTime offsetDateTimeColumn;

@TimeZoneStorage(TimeZoneStorageType.COLUMN)
@TimeZoneColumn(name = "birthday_zoned_offset")
@Column(name = "birthday_zoned")
private ZonedDateTime zonedDateTimeColumn;

2.2.50. AttributeConverters

With a custom AttributeConverter, the application developer can map a given JDBC type to an entity basic type.

In the following example, the java.time.Period is going to be mapped to a VARCHAR database column.

Example 70. java.time.Period custom AttributeConverter
@Converter
public class PeriodStringConverter
        implements AttributeConverter<Period, String> {

    @Override
    public String convertToDatabaseColumn(Period attribute) {
        return attribute.toString();
    }

    @Override
    public Period convertToEntityAttribute(String dbData) {
        return Period.parse(dbData);
    }
}

To make use of this custom converter, the @Convert annotation must decorate the entity attribute.

Example 71. Entity using the custom java.time.Period AttributeConverter mapping
@Entity(name = "Event")
public static class Event {

    @Id
    @GeneratedValue
    private Long id;

    @Convert(converter = PeriodStringConverter.class)
    @Column(columnDefinition = "")
    private Period span;

    //Getters and setters are omitted for brevity

}

When persisting such entity, Hibernate will do the type conversion based on the AttributeConverter logic:

Example 72. Persisting entity using the custom AttributeConverter
INSERT INTO Event ( span, id )
VALUES ( 'P1Y2M3D', 1 )

An AttributeConverter can be applied globally for (@Converter( autoApply=true )) or locally.

AttributeConverter Java and JDBC types

In cases when the Java type specified for the "database side" of the conversion (the second AttributeConverter bind parameter) is not known, Hibernate will fallback to a java.io.Serializable type.

If the Java type is not known to Hibernate, you will encounter the following message:

HHH000481: Encountered Java type for which we could not locate a JavaType and which does not appear to implement equals and/or hashCode. This can lead to significant performance problems when performing equality/dirty checking involving this Java type. Consider registering a custom JavaType or at least implementing equals/hashCode.

A Java type is "known" if it has an entry in the JavaTypeRegistry. While Hibernate does load many JDK types into the JavaTypeRegistry, an application can also expand the JavaTypeRegistry by adding new JavaType entries as discussed in Compositional basic mapping and TypeContributor.

Mapping an AttributeConverter using HBM mappings

When using HBM mappings, you can still make use of the Jakarta Persistence AttributeConverter because Hibernate supports such mapping via the type attribute as demonstrated by the following example.

Let’s consider we have an application-specific Money type:

Example 73. Application-specific Money type
public class Money {

    private long cents;

    public Money(long cents) {
        this.cents = cents;
    }

    public long getCents() {
        return cents;
    }

    public void setCents(long cents) {
        this.cents = cents;
    }
}

Now, we want to use the Money type when mapping the Account entity:

Example 74. Account entity using the Money type
public class Account {

    private Long id;

    private String owner;

    private Money balance;

    //Getters and setters are omitted for brevity
}

Since Hibernate has no knowledge how to persist the Money type, we could use a Jakarta Persistence AttributeConverter to transform the Money type as a Long. For this purpose, we are going to use the following MoneyConverter utility:

Example 75. MoneyConverter implementing the Jakarta Persistence AttributeConverter interface
public class MoneyConverter
        implements AttributeConverter<Money, Long> {

    @Override
    public Long convertToDatabaseColumn(Money attribute) {
        return attribute == null ? null : attribute.getCents();
    }

    @Override
    public Money convertToEntityAttribute(Long dbData) {
        return dbData == null ? null : new Money(dbData);
    }
}

To map the MoneyConverter using HBM configuration files you need to use the converted:: prefix in the type attribute of the property element.

Example 76. HBM mapping for AttributeConverter
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="org.hibernate.userguide.mapping.converter.hbm">
    <class name="Account" table="account" >
        <id name="id"/>

        <property name="owner"/>

        <property name="balance"
            type="converted::org.hibernate.userguide.mapping.converter.hbm.MoneyConverter"/>

    </class>
</hibernate-mapping>
AttributeConverter Mutability Plan

A basic type that’s converted by a Jakarta Persistence AttributeConverter is immutable if the underlying Java type is immutable and is mutable if the associated attribute type is mutable as well.

Therefore, mutability is given by the JavaType#getMutabilityPlan of the associated entity attribute type.

This can be adjusted by using @Immutable or @Mutability on any of:

  1. the basic value

  2. the AttributeConverter class

  3. the basic value type

See Mapping basic values for additional details.

Immutable types

If the entity attribute is a String, a primitive wrapper (e.g. Integer, Long), an Enum type, or any other immutable Object type, then you can only change the entity attribute value by reassigning it to a new value.

Considering we have the same Period entity attribute as illustrated in the AttributeConverters section:

@Entity(name = "Event")
public static class Event {

    @Id
    @GeneratedValue
    private Long id;

    @Convert(converter = PeriodStringConverter.class)
    @Column(columnDefinition = "")
    private Period span;

    //Getters and setters are omitted for brevity

}

The only way to change the span attribute is to reassign it to a different value:

 Event event = entityManager.createQuery("from Event", Event.class).getSingleResult();
 event.setSpan(Period
     .ofYears(3)
     .plusMonths(2)
     .plusDays(1)
);
Mutable types

On the other hand, consider the following example where the Money type is a mutable.

public static class Money {

	private long cents;

	//Getters and setters are omitted for brevity
}

@Entity(name = "Account")
public static class Account {

	@Id
	private Long id;

	private String owner;

	@Convert(converter = MoneyConverter.class)
	private Money balance;

	//Getters and setters are omitted for brevity
}

public static class MoneyConverter
		implements AttributeConverter<Money, Long> {

	@Override
	public Long convertToDatabaseColumn(Money attribute) {
		return attribute == null ? null : attribute.getCents();
	}

	@Override
	public Money convertToEntityAttribute(Long dbData) {
		return dbData == null ? null : new Money(dbData);
	}
}

A mutable Object allows you to modify its internal structure, and Hibernate’s dirty checking mechanism is going to propagate the change to the database:

Account account = entityManager.find(Account.class, 1L);
account.getBalance().setCents(150 * 100L);
entityManager.persist(account);

Although the AttributeConverter types can be mutable so that dirty checking, deep copying, and second-level caching work properly, treating these as immutable (when they really are) is more efficient.

For this reason, prefer immutable types over mutable ones whenever possible.

Using the AttributeConverter entity property as a query parameter

Assuming you have the following entity:

Example 77. Photo entity with AttributeConverter
@Entity(name = "Photo")
public static class Photo {

	@Id
	private Integer id;

	@Column(length = 256)
	private String name;

	@Column(length = 256)
	@Convert(converter = CaptionConverter.class)
	private Caption caption;

	//Getters and setters are omitted for brevity
}

And the Caption class looks as follows:

Example 78. Caption Java object
public static class Caption {

	private String text;

	public Caption(String text) {
		this.text = text;
	}

	public String getText() {
		return text;
	}

	public void setText(String text) {
		this.text = text;
	}

	@Override
	public boolean equals(Object o) {
		if ( this == o ) {
			return true;
		}
		if ( o == null || getClass() != o.getClass() ) {
			return false;
		}
		Caption caption = (Caption) o;
		return text != null ? text.equals( caption.text ) : caption.text == null;

	}

	@Override
	public int hashCode() {
		return text != null ? text.hashCode() : 0;
	}
}

And we have an AttributeConverter to handle the Caption Java object:

Example 79. Caption Java object AttributeConverter
public static class CaptionConverter
		implements AttributeConverter<Caption, String> {

	@Override
	public String convertToDatabaseColumn(Caption attribute) {
		return attribute.getText();
	}

	@Override
	public Caption convertToEntityAttribute(String dbData) {
		return new Caption( dbData );
	}
}

Traditionally, you could only use the DB data Caption representation, which in our case is a String, when referencing the caption entity property.

Example 80. Filtering by the Caption property using the DB data representation
Photo photo = entityManager.createQuery(
				"select p " +
						"from Photo p " +
						"where upper(caption) = upper(:caption) ", Photo.class )
		.setParameter( "caption", "Nicolae Grigorescu" )
		.getSingleResult();

In order to use the Java object Caption representation, you have to get the associated Hibernate Type.

Example 81. Filtering by the Caption property using the Java Object representation
SessionFactoryImplementor sessionFactory = entityManager.getEntityManagerFactory()
		.unwrap( SessionFactoryImplementor.class );
final MappingMetamodelImplementor mappingMetamodel = sessionFactory
		.getRuntimeMetamodels()
		.getMappingMetamodel();

Type captionType = mappingMetamodel
		.getEntityDescriptor( Photo.class )
		.getPropertyType( "caption" );

Photo photo = (Photo) entityManager.createQuery(
				"select p " +
						"from Photo p " +
						"where upper(caption) = upper(:caption) ", Photo.class )
		.unwrap( Query.class )
		.setParameter(
				"caption",
				new Caption( "Nicolae Grigorescu" ),
				(BindableType) captionType
		)
		.getSingleResult();

By passing the associated Hibernate Type, you can use the Caption object when binding the query parameter value.

2.2.51. Registries

We’ve covered JavaTypeRegistry and JdbcTypeRegistry a few times now, mainly in regards to mapping resolution as discussed in Resolving the composition. But they each also serve additional important roles.

The JavaTypeRegistry is a registry of JavaType references keyed by Java type. In addition to mapping resolution, this registry is used to handle Class references exposed in various APIs such as Query parameter types. JavaType references can be registered through @JavaTypeRegistration.

The JdbcTypeRegistry is a registry of JdbcType references keyed by an integer code. As discussed in JdbcType, these type-codes typically match with the corresponding code from java.sql.Types, but that is not a requirement - integers other than those defined by java.sql.Types can be used. This might be useful for mapping JDBC User Data Types (UDTs) or other specialized database-specific types (PostgreSQL’s UUID type, e.g.). In addition to its use in mapping resolution, this registry is also used as the primary source for resolving "discovered" values in a JDBC ResultSet. JdbcType references can be registered through @JdbcTypeRegistration.

See TypeContributor for an alternative to @JavaTypeRegistration and @JdbcTypeRegistration for registration.

2.2.52. TypeContributor

org.hibernate.boot.model.TypeContributor is a contract for overriding or extending parts of the Hibernate type system.

There are many ways to integrate a TypeContributor. The most common is to define the TypeContributor as a Java service (see java.util.ServiceLoader).

TypeContributor is passed a TypeContributions reference, which allows registration of custom JavaType, JdbcType and BasicType references.

While TypeContributor still exposes the ability to register BasicType references, this is considered deprecated. As of 6.0, these BasicType registrations are only used while interpreting hbm.xml mappings, which are themselves considered deprecated. Use Custom type mapping or Compositional basic mapping instead.

2.2.53. Case Study : BitSet

We’ve covered many ways to specify basic value mappings so far. This section will look at mapping the java.util.BitSet type by applying the different techniques covered so far.

Example 82. Implicit BitSet mapping
@Entity(name = "Product")
public static class Product {
	@Id
	private Integer id;

	private BitSet bitSet;

	//Getters and setters are omitted for brevity
}

As mentioned previously, the worst-case fallback for Hibernate mapping a basic type which implements Serializable is to simply serialize it to the database. BitSet does implement Serializable, so by default Hibernate would handle this mapping by serialization.

That is not an ideal mapping. In the following sections we will look at approaches to change various aspects of how the BitSet gets mapped to the database.

Using AttributeConverter

We’ve seen uses of AttributeConverter previously.

This works well in most cases and is portable across Jakarta Persistence providers.

Example 83. BitSet AttributeConverter
@Entity(name = "Product")
public static class Product {
	@Id
	private Integer id;

	@Convert(converter = BitSetConverter.class)
	private BitSet bitSet;

	//Getters and setters are omitted for brevity
}

@Converter(autoApply = true)
public static class BitSetConverter implements AttributeConverter<BitSet,String> {
	@Override
	public String convertToDatabaseColumn(BitSet attribute) {
		return BitSetHelper.bitSetToString(attribute);
	}

	@Override
	public BitSet convertToEntityAttribute(String dbData) {
		return BitSetHelper.stringToBitSet(dbData);
	}
}

The @Convert annotation was used for illustration. Generally such a converter would be auto-applied instead

See AttributeConverters for details.

This greatly improves the reading and writing performance of dealing with these BitSet values because the AttributeConverter does that more efficiently using a simple externalizable form of the BitSet rather than serializing and deserializing the values.

Using a custom JavaTypeDescriptor

As covered in [basic-mapping-explicit], we will define a JavaType for BitSet that maps values to VARCHAR for storage by default.

Example 84. BitSet JavaTypeDescriptor
public class BitSetJavaType extends AbstractClassJavaType<BitSet> {
    public static final BitSetJavaType INSTANCE = new BitSetJavaType();

    public BitSetJavaType() {
        super(BitSet.class);
    }

    @Override
    public MutabilityPlan<BitSet> getMutabilityPlan() {
        return BitSetMutabilityPlan.INSTANCE;
    }

    @Override
    public JdbcType getRecommendedJdbcType(JdbcTypeIndicators indicators) {
        return indicators.getTypeConfiguration()
                .getJdbcTypeRegistry()
                .getDescriptor(Types.VARCHAR);
    }

    @Override
    public String toString(BitSet value) {
        return BitSetHelper.bitSetToString(value);
    }

    @Override
    public BitSet fromString(CharSequence string) {
        return BitSetHelper.stringToBitSet(string.toString());
    }

    @SuppressWarnings("unchecked")
    public <X> X unwrap(BitSet value, Class<X> type, WrapperOptions options) {
        if (value == null) {
            return null;
        }
        if (BitSet.class.isAssignableFrom(type)) {
            return (X) value;
        }
        if (String.class.isAssignableFrom(type)) {
            return (X) toString(value);
        }
        if (type.isArray()) {
            if (type.getComponentType() == byte.class) {
                return (X) value.toByteArray();
            }
        }
        throw unknownUnwrap(type);
    }

    public <X> BitSet wrap(X value, WrapperOptions options) {
        if (value == null) {
            return null;
        }
        if (value instanceof CharSequence) {
            return fromString((CharSequence) value);
        }
        if (value instanceof BitSet) {
            return (BitSet) value;
        }
        throw unknownWrap(value.getClass());
    }

}

We can either apply that type locally using @JavaType

Example 85. @JavaType
@Entity(name = "Product")
public static class Product {
	@Id
	private Integer id;

	@JavaType(BitSetJavaType.class)
	private BitSet bitSet;

	//Constructors, getters, and setters are omitted for brevity
}

Or we can apply it globally using @JavaTypeRegistration. This allows the registered JavaType to be used as the default whenever we encounter the BitSet type

Example 86. @JavaTypeRegistration
@Entity(name = "Product")
@JavaTypeRegistration(javaType = BitSet.class, descriptorClass = BitSetJavaType.class)
public static class Product {
	@Id
	private Integer id;

	private BitSet bitSet;

	//Constructors, getters, and setters are omitted for brevity
}
Selecting different JdbcTypeDescriptor

Our custom BitSetJavaType maps BitSet values to VARCHAR by default. That was a better option than direct serialization. But as BitSet is ultimately binary data we would probably really want to map this to VARBINARY type instead. One way to do that would be to change BitSetJavaType#getRecommendedJdbcType to instead return VARBINARY descriptor. Another option would be to use a local @JdbcType or @JdbcTypeCode.

The following examples for specifying the JdbcType assume our BitSetJavaType is globally registered.

We will again store the values as VARBINARY in the database. The difference now however is that the coercion methods #wrap and #unwrap will be used to prepare the value rather than relying on serialization.

Example 87. @JdbcTypeCode
@Entity(name = "Product")
public static class Product {
	@Id
	private Integer id;

	@JdbcTypeCode(Types.VARBINARY)
	private BitSet bitSet;

	//Constructors, getters, and setters are omitted for brevity
}

In this example, @JdbcTypeCode has been used to indicate that the JdbcType registered for JDBC’s VARBINARY type should be used.

Example 88. @JdbcType
@Entity(name = "Product")
public static class Product {
	@Id
	private Integer id;

	@JdbcType(CustomBinaryJdbcType.class)
	private BitSet bitSet;

	//Constructors, getters, and setters are omitted for brevity
}

In this example, @JdbcType has been used to specify our custom BitSetJdbcType descriptor locally for this attribute.

We could instead replace how Hibernate deals with all VARBINARY handling with our custom impl using @JdbcTypeRegistration

Example 89. @JdbcType
@Entity(name = "Product")
@JdbcTypeRegistration(CustomBinaryJdbcType.class)
public static class Product {
	@Id
	private Integer id;

	private BitSet bitSet;

	//Constructors, getters, and setters are omitted for brevity
}

2.2.54. SQL quoted identifiers

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. While traditionally, Hibernate used backticks for escaping SQL reserved keywords, Jakarta Persistence uses double quotes instead.

Once the reserved keywords are escaped, Hibernate will use the correct quotation style for the SQL Dialect. This is usually double quotes, but SQL Server uses brackets and MySQL uses backticks.

Example 90. Hibernate quoting
@Entity(name = "Product")
public static class Product {

	@Id
	private Long id;

	@Column(name = "`name`")
	private String name;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}
Example 91. Jakarta Persistence quoting
@Entity(name = "Product")
public static class Product {

	@Id
	private Long id;

	@Column(name = "\"name\"")
	private String name;

	@Column(name = "\"number\"")
	private String number;

	//Getters and setters are omitted for brevity

}

Because name and number are reserved words, the Product entity mapping uses backticks to quote these column names.

When saving the following Product entity, Hibernate generates the following SQL insert statement:

Example 92. Persisting a quoted column name
Product product = new Product();
product.setId(1L);
product.setName("Mobile phone");
product.setNumber("123-456-7890");
entityManager.persist(product);
INSERT INTO Product ("name", "number", id)
VALUES ('Mobile phone', '123-456-7890', 1)
Global quoting

Hibernate can also quote all identifiers (e.g. table, columns) using the following configuration property:

<property
    name="hibernate.globally_quoted_identifiers"
    value="true"
/>

This way, we don’t need to manually quote any identifier:

Example 93. Jakarta Persistence quoting
@Entity(name = "Product")
public static class Product {

	@Id
	private Long id;

	private String name;

	private String number;

	//Getters and setters are omitted for brevity

}

When persisting a Product entity, Hibernate is going to quote all identifiers as in the following example:

INSERT INTO "Product" ("name", "number", "id")
VALUES ('Mobile phone', '123-456-7890', 1)

As you can see, both the table name and all the column have been quoted.

For more about quoting-related configuration properties, check out the Mapping configurations section as well.

2.2.55. Generated properties

NOTE

This section talks about generating values for non-identifier attributes. For discussion of generated identifier values, see Generated identifier values.

Generated attributes have their values generated as part of performing a SQL INSERT or UPDATE. Applications can generate these values in any number of ways (SQL DEFAULT value, trigger, etc). Typically, the application needs to refresh objects that contain any properties for which the database was generating values, which is a major drawback.

Applications can also delegate generation to Hibernate, in which case Hibernate will manage the value generation and (potential[3]) state refresh itself.

Only @Basic and @Version attributes can be marked as generated.

Generated attributes must additionally be non-insertable and non-updateable.

Hibernate supports both in-VM and in-DB generation. A generation that uses the current JVM timestamp as the generated value is an example of an in-VM strategy. A generation that uses the database’s current_timestamp function is an example of an in-DB strategy.

Hibernate supports the following timing (when) for generation:

NEVER (the default)

the given attribute value is not generated

INSERT

the attribute value is generated on insert but is not regenerated on subsequent updates

ALWAYS

the attribute value is generated both on insert and update.

Hibernate supports multiple ways to mark an attribute as generated:

@CurrentTimestamp

The @CurrentTimestamp annotation is an in-DB strategy that can be configured for either INSERT or ALWAYS timing. It uses the database’s current_timestamp function as the generated value

Example 94. @UpdateTimestamp mapping example
@CurrentTimestamp( timing = GenerationTiming.INSERT )
public Instant createdAt;

@CurrentTimestamp( timing = GenerationTiming.ALWAYS )
public Instant lastUpdatedAt;
@CreationTimestamp

The @CreationTimestamp annotation is an in-VM INSERT strategy. Hibernate will use the current timestamp of the JVM as the insert value for the attribute.

Supports most temporal types (java.time.Instant, java.util.Date, java.util.Calendar, etc)

Example 95. @CreationTimestamp mapping example
@Entity(name = "Event")
public static class Event {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`timestamp`")
	@CreationTimestamp
	private Date timestamp;

	//Constructors, getters, and setters are omitted for brevity
}

While inserting the Event, Hibernate will populate the underlying timestamp column with the current JVM timestamp value

@UpdateTimestamp annotation

The @UpdateTimestamp annotation is an in-VM INSERT strategy. Hibernate will use the current timestamp of the JVM as the insert and update value for the attribute.

Supports most temporal types (java.time.Instant, java.util.Date, java.util.Calendar, etc)

Example 96. @UpdateTimestamp mapping example
@Entity(name = "Bid")
public static class Bid {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "updated_on")
	@UpdateTimestamp
	private Date updatedOn;

	@Column(name = "updated_by")
	private String updatedBy;

	private Long cents;

	//Getters and setters are omitted for brevity

}
@Generated annotation

The @Generated annotation is an in-DB strategy that can be configured for either INSERT or ALWAYS timing

This is the legacy mapping for in-DB generated values.

Example 97. @Generated mapping example
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String firstName;

	private String lastName;

	private String middleName1;

	private String middleName2;

	private String middleName3;

	private String middleName4;

	private String middleName5;

	@Generated(value = GenerationTime.ALWAYS)
	@Column(columnDefinition =
		"AS CONCAT(" +
		"	COALESCE(firstName, ''), " +
		"	COALESCE(' ' + middleName1, ''), " +
		"	COALESCE(' ' + middleName2, ''), " +
		"	COALESCE(' ' + middleName3, ''), " +
		"	COALESCE(' ' + middleName4, ''), " +
		"	COALESCE(' ' + middleName5, ''), " +
		"	COALESCE(' ' + lastName, '') " +
		")")
	private String fullName;

}
Custom generation strategy

Hibernate also supports value generation via a pluggable API using @ValueGenerationType and AnnotationValueGeneration allowing users to define any generation strategy they wish.

Let’s look at an example of generating UUID values. First the attribute mapping

Example 98. Custom generation mapping example
@GeneratedUuidValue( timing = GenerationTiming.INSERT )
public UUID createdUuid;

@GeneratedUuidValue( timing = GenerationTiming.ALWAYS )
   public UUID updatedUuid;

This example makes use of an annotation named @GeneratedUuidValue - but where is that annotation defined? This is a custom annotations provided by the application.

Example 99. Custom generation mapping example
@ValueGenerationType( generatedBy = UuidValueGeneration.class )
@Retention(RetentionPolicy.RUNTIME)
@Target( { ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE } )
@Inherited
public @interface GeneratedUuidValue {
	GenerationTiming timing();
}

The @ValueGenerationType( generatedBy = UuidValueGeneration.class ) here is the important piece; it tells Hibernate how to generate values for the attribute - here it will use the specified UuidValueGeneration class

Example 100. Custom generation mapping example
public static class UuidValueGeneration implements AnnotationValueGeneration<GeneratedUuidValue>, ValueGenerator<UUID> {
	private GenerationTiming timing;

	@Override
	public void initialize(GeneratedUuidValue annotation, Class<?> propertyType) {
		timing = annotation.timing();
	}

	@Override
	public GenerationTiming getGenerationTiming() {
		return timing;
	}

	@Override
	public ValueGenerator<?> getValueGenerator() {
		return this;
	}

	@Override
	public boolean referenceColumnInSql() {
		return false;
	}

	@Override
	public String getDatabaseGeneratedReferencedColumnValue() {
		return null;
	}

	@Override
	public UUID generateValue(Session session, Object owner) {
		return UUID.randomUUID();
	}
}

See @ValueGenerationType and AnnotationValueGeneration for details of each contract

2.2.56. Column transformers: read and write expressions

Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped to @Basic types. For example, if your database provides a set of data encryption functions, you can invoke them for individual columns like in the following example.

Example 101. @ColumnTransformer example
	@Entity(name = "Employee")
	public static class Employee {

		@Id
		private Long id;

		@NaturalId
		private String username;

		@Column(name = "pswd")
		@ColumnTransformer(
			read = "decrypt('AES', '00', pswd )",
			write = "encrypt('AES', '00', ?)"
		)
// For H2 2.0.202+ one must use the varbinary DDL type
//		@Column(name = "pswd", columnDefinition = "varbinary")
//		@ColumnTransformer(
//			read = "trim(trailing u&'\\0000' from cast(decrypt('AES', '00', pswd ) as character varying))",
//			write = "encrypt('AES', '00', ?)"
//		)
		private String password;

		private int accessLevel;

		@ManyToOne(fetch = FetchType.LAZY)
		private Department department;

		@ManyToMany(mappedBy = "employees")
		private List<Project> projects = new ArrayList<>();

		//Getters and setters omitted for brevity
	}

If a property uses more than one column, you must use the forColumn attribute to specify which column the @ColumnTransformer read and write expressions are targeting.

Example 102. @ColumnTransformer forColumn attribute usage
@Entity(name = "Savings")
public static class Savings {

	@Id
	private Long id;

	@CompositeType(MonetaryAmountUserType.class)
	@AttributeOverrides({
		@AttributeOverride(name = "amount", column = @Column(name = "money")),
		@AttributeOverride(name = "currency", column = @Column(name = "currency"))
	})
	@ColumnTransformer(
			forColumn = "money",
			read = "money / 100",
			write = "? * 100"
	)
	private MonetaryAmount wallet;

	//Getters and setters omitted for brevity

}

Hibernate applies the custom expressions automatically whenever the property is referenced in a query. This functionality is similar to a derived-property [mapping-column-formula] with two differences:

  • The property is backed by one or more columns that are exported as part of automatic schema generation.

  • The property is read-write, not read-only.

The write expression, if specified, must contain exactly one '?' placeholder for the value.

Example 103. Persisting an entity with a @ColumnTransformer and a composite type
doInJPA(this::entityManagerFactory, entityManager -> {
	Savings savings = new Savings();
	savings.setId(1L);
	savings.setWallet(new MonetaryAmount(BigDecimal.TEN, Currency.getInstance(Locale.US)));
	entityManager.persist(savings);
});

doInJPA(this::entityManagerFactory, entityManager -> {
	Savings savings = entityManager.find(Savings.class, 1L);
	assertEquals(10, savings.getWallet().getAmount().intValue());
	assertEquals(Currency.getInstance(Locale.US), savings.getWallet().getCurrency());
});
INSERT INTO Savings (money, currency, id)
VALUES (10 * 100, 'USD', 1)

SELECT
    s.id as id1_0_0_,
    s.money / 100 as money2_0_0_,
    s.currency as currency3_0_0_
FROM
    Savings s
WHERE
    s.id = 1

2.3. Embeddable values

Historically Hibernate called these components. Jakarta Persistence calls them embeddables. Either way, the concept is the same: a composition of values.

For example, we might have a Publisher class that is a composition of name and country, or a Location class that is a composition of country and city.

Usage of the word embeddable

To avoid any confusion with the annotation that marks a given embeddable type, the annotation will be further referred to as @Embeddable.

Throughout this chapter and thereafter, for brevity sake, embeddable types may also be referred to as embeddable.

Example 104. Embeddable type example
@Embeddable
public static class Publisher {

	private String name;

	private Location location;

	public Publisher(String name, Location location) {
		this.name = name;
		this.location = location;
	}

	private Publisher() {}

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class Location {

	private String country;

	private String city;

	public Location(String country, String city) {
		this.country = country;
		this.city = city;
	}

	private Location() {}

	//Getters and setters are omitted for brevity
}

An embeddable type is another form of a value type, and its lifecycle is bound to a parent entity type, therefore inheriting the attribute access from its parent (for details on attribute access, see Access strategies).

Embeddable types can be made up of basic values as well as associations, with the caveat that, when used as collection elements, they cannot define collections themselves.

2.3.1. Component / Embedded

Most often, embeddable types are used to group multiple basic type mappings and reuse them across several entities.

Example 105. Simple Embeddable
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	private Publisher publisher;

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class Publisher {

	@Column(name = "publisher_name")
	private String name;

	@Column(name = "publisher_country")
	private String country;

	//Getters and setters, equals and hashCode methods omitted for brevity

}
create table Book (
    id bigint not null,
    author varchar(255),
    publisher_country varchar(255),
    publisher_name varchar(255),
    title varchar(255),
    primary key (id)
)

Jakarta Persistence defines two terms for working with an embeddable type: @Embeddable and @Embedded.

@Embeddable is used to describe the mapping type itself (e.g. Publisher).

@Embedded is for referencing a given embeddable type (e.g. book.publisher).

So, the embeddable type is represented by the Publisher class and the parent entity makes use of it through the book#publisher object composition.

The composed values are mapped to the same table as the parent table. Composition is part of good object-oriented data modeling (idiomatic Java). In fact, that table could also be mapped by the following entity type instead.

Example 106. Alternative to embeddable type composition
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	@Column(name = "publisher_name")
	private String publisherName;

	@Column(name = "publisher_country")
	private String publisherCountry;

	//Getters and setters are omitted for brevity
}

The composition form is certainly more object-oriented, and that becomes more evident as we work with multiple embeddable types.

2.3.2. Overriding Embeddable types

Although from an object-oriented perspective, it’s much more convenient to work with embeddable types, when we reuse the same embeddable multiple times on the same class, the Jakarta Persistence specification requires to set the associated column names explicitly.

This requirement is due to how object properties are mapped to database columns. By default, Jakarta Persistence expects a database column having the same name with its associated object property. When including multiple embeddables, the implicit name-based mapping rule doesn’t work anymore because multiple object properties could end-up being mapped to the same database column.

When an embeddable type is used multiple times, Jakarta Persistence defines the @AttributeOverride and @AssociationOverride annotations to handle this scenario to override the default column names defined by the Embeddable.

See Embeddables and ImplicitNamingStrategy for an alternative to using @AttributeOverride and @AssociationOverride

Considering you have the following Publisher embeddable type which defines a @ManyToOne association with the Country entity:

Example 107. Embeddable type with a @ManyToOne association
@Embeddable
public static class Publisher {

	private String name;

	@ManyToOne(fetch = FetchType.LAZY)
	private Country country;

	//Getters and setters, equals and hashCode methods omitted for brevity

}

@Entity(name = "Country")
public static class Country {

	@Id
	@GeneratedValue
	private Long id;

	@NaturalId
	private String name;

	//Getters and setters are omitted for brevity
}
create table Country (
    id bigint not null,
    name varchar(255),
    primary key (id)
)

alter table Country
    add constraint UK_p1n05aafu73sbm3ggsxqeditd
    unique (name)

Now, if you have a Book entity which declares two Publisher embeddable types for the ebook and paperback versions, you cannot use the default Publisher embeddable mapping since there will be a conflict between the two embeddable column mappings.

Therefore, the Book entity needs to override the embeddable type mappings for each Publisher attribute:

Example 108. Overriding embeddable type attributes
@Entity(name = "Book")
@AttributeOverrides({
		@AttributeOverride(
				name = "ebookPublisher.name",
				column = @Column(name = "ebook_pub_name")
		),
		@AttributeOverride(
				name = "paperBackPublisher.name",
				column = @Column(name = "paper_back_pub_name")
		)
})
@AssociationOverrides({
		@AssociationOverride(
				name = "ebookPublisher.country",
				joinColumns = @JoinColumn(name = "ebook_pub_country_id")
		),
		@AssociationOverride(
				name = "paperBackPublisher.country",
				joinColumns = @JoinColumn(name = "paper_back_pub_country_id")
		)
})
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	private Publisher ebookPublisher;

	private Publisher paperBackPublisher;

	//Getters and setters are omitted for brevity
}
create table Book (
    id bigint not null,
    author varchar(255),
    ebook_pub_name varchar(255),
    paper_back_pub_name varchar(255),
    title varchar(255),
    ebook_pub_country_id bigint,
    paper_back_pub_country_id bigint,
    primary key (id)
)

alter table Book
    add constraint FKm39ibh5jstybnslaoojkbac2g
    foreign key (ebook_pub_country_id)
    references Country

alter table Book
    add constraint FK7kqy9da323p7jw7wvqgs6aek7
    foreign key (paper_back_pub_country_id)
    references Country

2.3.3. Collections of embeddable types

Collections of embeddable types are specifically valued collections (as embeddable types are value types). Value collections are covered in detail in Collections of value types.

2.3.4. Embeddable type as a Map key

Embeddable types can also be used as Map keys. This topic is converted in detail in Map - key.

2.3.5. Embeddable type as identifier

Embeddable types can also be used as entity type identifiers. This usage is covered in detail in Composite identifiers.

Embeddable types that are used as collection entries, map keys or entity type identifiers cannot include their own collection mappings.

2.3.6. @Target mapping

The @Target annotation is used to specify the implementation class of a given association that is mapped via an interface. The @ManyToOne, @OneToOne, @OneToMany, and @ManyToMany feature a targetEntity attribute to specify the actual class of the entity association when an interface is used for the mapping.

The @ElementCollection association has a targetClass attribute for the same purpose.

However, for simple embeddable types, there is no such construct and so you need to use the Hibernate-specific @Target annotation instead.

Example 109. @Target mapping usage
public interface Coordinates {
	double x();
	double y();
}

@Embeddable
public static class GPS implements Coordinates {

	private double latitude;

	private double longitude;

	private GPS() {
	}

	public GPS(double latitude, double longitude) {
		this.latitude = latitude;
		this.longitude = longitude;
	}

	@Override
	public double x() {
		return latitude;
	}

	@Override
	public double y() {
		return longitude;
	}
}

@Entity(name = "City")
public static class City {

	@Id
	@GeneratedValue
	private Long id;

	private String name;

	@Embedded
	@Target(GPS.class)
	private Coordinates coordinates;

	//Getters and setters omitted for brevity

}

The coordinates embeddable type is mapped as the Coordinates interface. However, Hibernate needs to know the actual implementation tye, which is GPS in this case, hence the @Target annotation is used to provide this information.

Assuming we have persisted the following City entity:

Example 110. @Target persist example
doInJPA(this::entityManagerFactory, entityManager -> {

	City cluj = new City();
	cluj.setName("Cluj");
	cluj.setCoordinates(new GPS(46.77120, 23.62360));

	entityManager.persist(cluj);
});

When fetching the City entity, the coordinates property is mapped by the @Target expression:

Example 111. @Target fetching example
doInJPA(this::entityManagerFactory, entityManager -> {

	City cluj = entityManager.find(City.class, 1L);

	assertEquals(46.77120, cluj.getCoordinates().x(), 0.00001);
	assertEquals(23.62360, cluj.getCoordinates().y(), 0.00001);
});
SELECT
    c.id as id1_0_0_,
    c.latitude as latitude2_0_0_,
    c.longitude as longitud3_0_0_,
    c.name as name4_0_0_
FROM
    City c
WHERE
    c.id = ?

-- binding parameter [1] as [BIGINT] - [1]

-- extracted value ([latitude2_0_0_] : [DOUBLE])  - [46.7712]
-- extracted value ([longitud3_0_0_] : [DOUBLE])  - [23.6236]
-- extracted value ([name4_0_0_]     : [VARCHAR]) - [Cluj]

Therefore, the @Target annotation is used to define a custom join association between the parent-child association.

2.3.7. @Parent mapping

The Hibernate-specific @Parent annotation allows you to reference the owner entity from within an embeddable.

Example 112. @Parent mapping usage
@Embeddable
public static class GPS {

	private double latitude;

	private double longitude;

	@Parent
	private City city;

	//Getters and setters omitted for brevity

}

@Entity(name = "City")
public static class City {

	@Id
	@GeneratedValue
	private Long id;

	private String name;

	@Embedded
	@Target(GPS.class)
	private GPS coordinates;

	//Getters and setters omitted for brevity

}

Assuming we have persisted the following City entity:

Example 113. @Parent persist example
doInJPA(this::entityManagerFactory, entityManager -> {

	City cluj = new City();
	cluj.setName("Cluj");
	cluj.setCoordinates(new GPS(46.77120, 23.62360));

	entityManager.persist(cluj);
});

When fetching the City entity, the city property of the embeddable type acts as a back reference to the owning parent entity:

Example 114. @Parent fetching example
doInJPA(this::entityManagerFactory, entityManager -> {

	City cluj = entityManager.find(City.class, 1L);

	assertSame(cluj, cluj.getCoordinates().getCity());
});

Therefore, the @Parent annotation is used to define the association between an embeddable type and the owning entity.

2.3.8. Custom instantiation

Jakarta Persistence requires embeddable classes to follow Java Bean conventions. Part of this is the definition of a non-arg constructor. However, not all value compositions applications might map as embeddable values follow Java Bean conventions - e.g. a struct or Java 15 record.

Hibernate allows the use of a custom instantiator for creating the embeddable instances through the org.hibernate.metamodel.spi.EmbeddableInstantiator contract. For example, consider the following embeddable:

Example 115. EmbeddableInstantiator - Embeddable
@Embeddable
public class Name {
	@Column(name = "first_name")
	private final String first;
	@Column(name = "last_name")
	private final String last;

	private Name() {
		throw new UnsupportedOperationException();
	}

	public Name(String first, String last) {
		this.first = first;
		this.last = last;
	}

	public String getFirstName() {
		return first;
	}

	public String getLastName() {
		return last;
	}
}

Here, Name only allows use of the constructor accepting its state. Because this class does not follow Java Bean conventions, in terms of constructor, a custom strategy for instantiation is needed.

Example 116. EmbeddableInstantiator - Implementation
public class NameInstantiator implements EmbeddableInstantiator {
	@Override
	public Object instantiate(ValueAccess valueAccess, SessionFactoryImplementor sessionFactory) {
		// alphabetical
		final String first = valueAccess.getValue( 0, String.class );
		final String last = valueAccess.getValue( 1, String.class );
		return new Name( first, last );
	}

	// ...

}

There are a few ways to specify the custom instantiator. The @org.hibernate.annotations.EmbeddableInstantiator annotation can be used on the embedded attribute:

Example 117. @EmbeddableInstantiator on attribute
@Entity
public class Person {
	@Id
	public Integer id;
	@Embedded
	@EmbeddableInstantiator( NameInstantiator.class )
	public Name name;
	@ElementCollection
	@Embedded
	@EmbeddableInstantiator( NameInstantiator.class )
	public Set<Name> aliases;

}

@EmbeddableInstantiator may also be specified on the embeddable class:

Example 118. @EmbeddableInstantiator on class
@Embeddable
@EmbeddableInstantiator( NameInstantiator.class )
public class Name {
	@Column(name = "first_name")
	private final String first;
	@Column(name = "last_name")
	private final String last;

	private Name() {
		throw new UnsupportedOperationException();
	}

	public Name(String first, String last) {
		this.first = first;
		this.last = last;
	}

	public String getFirstName() {
		return first;
	}

	public String getLastName() {
		return last;
	}
}

@Entity
public class Person {
	@Id
	public Integer id;
	@Embedded
	public Name name;
	@ElementCollection
	@Embedded
	public Set<Name> aliases;
}

Lastly, @org.hibernate.annotations.EmbeddableInstantiatorRegistration may be used, which is useful when the application developer does not control the embeddable to be able to apply the instantiator on the embeddable.

Example 119. @EmbeddableInstantiatorRegistration
@Entity
@EmbeddableInstantiatorRegistration( embeddableClass = Name.class, instantiator = NameInstantiator.class )
public class Person {
	@Id
	public Integer id;
	@Embedded
	public Name name;
	@ElementCollection
	@Embedded
	public Set<Name> aliases;

}

2.3.9. Custom type mapping

Another approach is to supply the implementation of the org.hibernate.usertype.CompositeUserType contract using @CompositeType, which is an extension to the org.hibernate.metamodel.spi.EmbeddableInstantiator contract.

There are also corresponding, specialized forms of @CompositeType for specific model parts:

  • When mapping a Map, @CompositeType describes the Map value while @MapKeyCompositeType describes the Map key

  • For collection mappings, @CompositeType describes the elements

For example, consider the following custom type:

Example 120. CompositeUserType - Domain type
public class Name {
	private final String first;
	private final String last;

	public Name(String first, String last) {
		this.first = first;
		this.last = last;
	}

	public String firstName() {
		return first;
	}

	public String lastName() {
		return last;
	}
}

Here, Name only allows use of the constructor accepting its state. Because this class does not follow Java Bean conventions, a custom user type for instantiation and state access is needed.

Example 121. CompositeUserType - Implementation
public class NameCompositeUserType implements CompositeUserType<Name> {

	public static class NameMapper {
		String firstName;
		String lastName;
	}

	@Override
	public Class<?> embeddable() {
		return NameMapper.class;
	}

	@Override
	public Class<Name> returnedClass() {
		return Name.class;
	}

	@Override
	public Name instantiate(ValueAccess valueAccess, SessionFactoryImplementor sessionFactory) {
		// alphabetical
		final String first = valueAccess.getValue( 0, String.class );
		final String last = valueAccess.getValue( 1, String.class );
		return new Name( first, last );
	}

	@Override
	public Object getPropertyValue(Name component, int property) throws HibernateException {
		// alphabetical
		switch ( property ) {
			case 0:
				return component.firstName();
			case 1:
				return component.lastName();
		}
		return null;
	}

	@Override
	public boolean equals(Name x, Name y) {
		return x == y || x != null && Objects.equals( x.firstName(), y.firstName() )
				&& Objects.equals( x.lastName(), y.lastName() );
	}

	@Override
	public int hashCode(Name x) {
		return Objects.hash( x.firstName(), x.lastName() );
	}

	@Override
	public Name deepCopy(Name value) {
		return value; // immutable
	}

	@Override
	public boolean isMutable() {
		return false;
	}

	@Override
	public Serializable disassemble(Name value) {
		return new String[] { value.firstName(), value.lastName() };
	}

	@Override
	public Name assemble(Serializable cached, Object owner) {
		final String[] parts = (String[]) cached;
		return new Name( parts[0], parts[1] );
	}

	@Override
	public Name replace(Name detached, Name managed, Object owner) {
		return detached;
	}

}

A composite user type needs an embeddable mapper class, which represents the embeddable mapping structure of the type i.e. the way the type would look like if you had the option to write a custom @Embeddable class.

In addition to the instantiation logic, a composite user type also has to provide a way to decompose the returned type into the individual components/properties of the embeddable mapper class through getPropertyValue. The property index, just like in the instantiate method, is based on the alphabetical order of the attribute names of the embeddable mapper class.

The composite user type also needs to provide methods to handle the mutability, equals, hashCode and the cache serialization and deserialization of the returned type.

There are a few ways to specify the composite user type. The @org.hibernate.annotations.CompositeType annotation can be used on the embedded and element collection attributes:

Example 122. @CompositeType on attribute
@Entity
public class Person {
	@Id
	public Integer id;

	@Embedded
	@AttributeOverride(name = "firstName", column = @Column(name = "first_name"))
	@AttributeOverride(name = "lastName", column = @Column(name = "last_name"))
	@CompositeType( NameCompositeUserType.class )
	public Name name;

	@ElementCollection
	@AttributeOverride(name = "firstName", column = @Column(name = "first_name"))
	@AttributeOverride(name = "lastName", column = @Column(name = "last_name"))
	@CompositeType( NameCompositeUserType.class )
	public Set<Name> aliases;

}

Or @org.hibernate.annotations.CompositeTypeRegistration may be used, which is useful when the application developer wants to apply the composite user type for all domain type uses.

Example 123. @CompositeTypeRegistration
@Entity
@CompositeTypeRegistration( embeddableClass = Name.class, userType = NameCompositeUserType.class )
public class Person {
	@Id
	public Integer id;

	@Embedded
	@AttributeOverride(name = "firstName", column = @Column(name = "first_name"))
	@AttributeOverride(name = "lastName", column = @Column(name = "last_name"))
	public Name name;

	@ElementCollection
	@AttributeOverride(name = "firstName", column = @Column(name = "first_name"))
	@AttributeOverride(name = "lastName", column = @Column(name = "last_name"))
	public Set<Name> aliases;

}

2.3.10. Embeddables and ImplicitNamingStrategy

The ImplicitNamingStrategyComponentPathImpl is a Hibernate-specific feature. Users concerned with Jakarta Persistence provider portability should instead prefer explicit column naming with @AttributeOverride.

Hibernate naming strategies are covered in detail in Naming. However, for the purposes of this discussion, Hibernate has the capability to interpret implicit column names in a way that is safe for use with multiple embeddable types.

Example 124. Implicit multiple embeddable type mapping
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	private Publisher ebookPublisher;

	private Publisher paperBackPublisher;

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class Publisher {

	private String name;

	@ManyToOne(fetch = FetchType.LAZY)
	private Country country;

	//Getters and setters, equals and hashCode methods omitted for brevity
}

@Entity(name = "Country")
public static class Country {

	@Id
	@GeneratedValue
	private Long id;

	@NaturalId
	private String name;

	//Getters and setters are omitted for brevity
}

To make it work, you need to use the ImplicitNamingStrategyComponentPathImpl naming strategy.

Example 125. Enabling implicit embeddable type mapping using the component path naming strategy
metadataBuilder.applyImplicitNamingStrategy(
	ImplicitNamingStrategyComponentPathImpl.INSTANCE
);

Now the "path" to attributes are used in the implicit column naming:

create table Book (
    id bigint not null,
    author varchar(255),
    ebookPublisher_name varchar(255),
    paperBackPublisher_name varchar(255),
    title varchar(255),
    ebookPublisher_country_id bigint,
    paperBackPublisher_country_id bigint,
    primary key (id)
)

You could even develop your own naming strategy to do other types of implicit naming strategies.

2.4. Entity types

Usage of the word entity

The entity type describes the mapping between the actual persistable domain model object and a database table row. To avoid any confusion with the annotation that marks a given entity type, the annotation will be further referred to as @Entity.

Throughout this chapter and thereafter, entity types will be simply referred to as entity.

2.4.1. POJO Models

Section 2.1 The Entity Class of the Java Persistence 2.1 specification defines its requirements for an entity class. Applications that wish to remain portable across Jakarta Persistence providers should adhere to these requirements:

  • The entity class must be annotated with the jakarta.persistence.Entity annotation (or be denoted as such in XML mapping).

  • The entity class must have a public or protected no-argument constructor. It may define additional constructors as well.

  • The entity class must be a top-level class.

  • An enum or interface may not be designated as an entity.

  • The entity class must not be final. No methods or persistent instance variables of the entity class may be final.

  • If an entity instance is to be used remotely as a detached object, the entity class must implement the Serializable interface.

  • Both abstract and concrete classes can be entities. Entities may extend non-entity classes as well as entity classes, and non-entity classes may extend entity classes.

  • The persistent state of an entity is represented by instance variables, which may correspond to JavaBean-style properties. An instance variable must be directly accessed only from within the methods of the entity by the entity instance itself. The state of the entity is available to clients only through the entity’s accessor methods (getter/setter methods) or other business methods.

Hibernate, however, is not as strict in its requirements. The differences from the list above include:

  • The entity class must have a no-argument constructor, which may be public, protected or package visibility. It may define additional constructors as well.

  • The entity class need not be a top-level class.

  • Technically Hibernate can persist final classes or classes with final persistent state accessor (getter/setter) methods. However, it is generally not a good idea as doing so will stop Hibernate from being able to generate proxies for lazy-loading the entity.

  • Hibernate does not restrict the application developer from exposing instance variables and referencing them from outside the entity class itself. The validity of such a paradigm, however, is debatable at best.

Let’s look at each requirement in detail.

2.4.2. Prefer non-final classes

A central feature of Hibernate is the ability to load lazily certain entity instance variables (attributes) via runtime proxies. This feature depends upon the entity class being non-final or else implementing an interface that declares all the attribute getters/setters. You can still persist final classes that do not implement such an interface with Hibernate, but you will not be able to use proxies for fetching lazy associations, therefore limiting your options for performance tuning. For the very same reason, you should also avoid declaring persistent attribute getters and setters as final.

Starting with 5.0, Hibernate offers a more robust version of bytecode enhancement as another means for handling lazy loading. Hibernate had some bytecode re-writing capabilities prior to 5.0 but they were very rudimentary. See the Bytecode Enhancement for additional information on fetching and on bytecode enhancement.

2.4.3. Implement a no-argument constructor

The entity class should have a no-argument constructor. Both Hibernate and Jakarta Persistence require this.

Jakarta Persistence requires that this constructor be defined as public or protected. Hibernate, for the most part, does not care about the constructor visibility, as long as the system SecurityManager allows overriding the visibility setting. That said, the constructor should be defined with at least package visibility if you wish to leverage runtime proxy generation.

2.4.4. Declare getters and setters for persistent attributes

The Jakarta Persistence specification requires this, otherwise, the model would prevent accessing the entity persistent state fields directly from outside the entity itself.

Although Hibernate does not require it, it is recommended to follow the JavaBean conventions and define getters and setters for entity persistent attributes. Nevertheless, you can still tell Hibernate to directly access the entity fields.

Attributes (whether fields or getters/setters) need not be declared public. Hibernate can deal with attributes declared with the public, protected, package or private visibility. Again, if wanting to use runtime proxy generation for lazy loading, the getter/setter should grant access to at least package visibility.

2.4.5. Providing identifier attribute(s)

Historically, providing identifier attributes was considered optional.

However, not defining identifier attributes on the entity should be considered a deprecated feature that will be removed in an upcoming release.

The identifier attribute does not necessarily need to be mapped to the column(s) that physically define the primary key. However, it should map to column(s) that can uniquely identify each row.

We recommend that you declare consistently-named identifier attributes on persistent classes and that you use a wrapper (i.e., non-primitive) type (e.g. Long or Integer).

The placement of the @Id annotation marks the persistence state access strategy.

Example 126. Identifier mapping
@Id
private Long id;

Hibernate offers multiple identifier generation strategies, see the Identifier Generators chapter for more about this topic.

2.4.6. Mapping the entity

The main piece in mapping the entity is the jakarta.persistence.Entity annotation.

The @Entity annotation defines just the name attribute which is used to give a specific entity name for use in JPQL queries.

By default, if the name attribute of the @Entity annotation is missing, the unqualified name of the entity class itself will be used as the entity name.

Because the entity name is given by the unqualified name of the class, Hibernate does not allow registering multiple entities with the same name even if the entity classes reside in different packages.

Without imposing this restriction, Hibernate would not know which entity class is referenced in a JPQL query if the unqualified entity name is associated with more then one entity classes.

In the following example, the entity name (e.g. Book) is given by the unqualified name of the entity class name.

Example 127. @Entity mapping with an implicit name
@Entity
public class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

However, the entity name can also be set explicitly as illustrated by the following example.

Example 128. @Entity mapping with an explicit name
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

An entity models a database table. The identifier uniquely identifies each row in that table. By default, the name of the table is assumed to be the same as the name of the entity. To explicitly give the name of the table or to specify other information about the table, we would use the jakarta.persistence.Table annotation.

Example 129. Simple @Entity with @Table
 @Entity(name = "Book")
 @Table(
         catalog = "public",
         schema = "store",
         name = "book"
)
 public static class Book {

     @Id
     private Long id;

     private String title;

     private String author;

     //Getters and setters are omitted for brevity
 }
Mapping the catalog of the associated table

Without specifying the catalog of the associated database table a given entity is mapped to, Hibernate will use the default catalog associated with the current database connection.

However, if your database hosts multiple catalogs, you can specify the catalog where a given table is located using the catalog attribute of the Jakarta Persistence @Table annotation.

Let’s assume we are using MySQL and want to map a Book entity to the book table located in the public catalog which looks as follows.

Example 130. The book table located in the public catalog
create table public.book (
  id bigint not null,
  author varchar(255),
  title varchar(255),
  primary key (id)
) engine=InnoDB

Now, to map the Book entity to the book table in the public catalog we can use the catalog attribute of the @Table Jakarta Persistence annotation.

Example 131. Specifying the database catalog using the @Table annotation
@Entity(name = "Book")
@Table(
	catalog = "public",
	name = "book"
)
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}
Mapping the schema of the associated table

Without specifying the schema of the associated database table a given entity is mapped to, Hibernate will use the default schema associated with the current database connection.

However, if your database supports schemas, you can specify the schema where a given table is located using the schema attribute of the Jakarta Persistence @Table annotation.

Let’s assume we are using PostgreSQL and want to map a Book entity to the book table located in the library schema which looks as follows.

Example 132. The book table located in the library schema
create table library.book (
  id int8 not null,
  author varchar(255),
  title varchar(255),
  primary key (id)
)

Now, to map the Book entity to the book table in the library schema we can use the schema attribute of the @Table Jakarta Persistence annotation.

Example 133. Specifying the database schema using the @Table annotation
@Entity(name = "Book")
@Table(
	schema = "library",
	name = "book"
)
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

The schema attribute of the @Table annotation works only if the underlying database supports schemas (e.g. PostgreSQL).

Therefore, if you’re using MySQL or MariaDB, which do not support schemas natively (schemas being just an alias for catalog), you need to use the catalog attribute, and not the schema one.

2.4.7. Implementing equals() and hashCode()

Much of the discussion in this section deals with the relation of an entity to a Hibernate Session, whether the entity is managed, transient or detached. If you are unfamiliar with these topics, they are explained in the Persistence Context chapter.

Whether to implement equals() and hashCode() methods in your domain model, let alone how to implement them, is a surprisingly tricky discussion when it comes to ORM.

There is really just one absolute case: a class that acts as an identifier must implement equals/hashCode based on the id value(s). Generally, this is pertinent for user-defined classes used as composite identifiers. Beyond this one very specific use case and few others we will discuss below, you may want to consider not implementing equals/hashCode altogether.

So what’s all the fuss? Normally, most Java objects provide a built-in equals() and hashCode() based on the object’s identity, so each new object will be different from all others. This is generally what you want in ordinary Java programming. Conceptually, however, this starts to break down when you start to think about the possibility of multiple instances of a class representing the same data.

This is, in fact, exactly the case when dealing with data coming from a database. Every time we load a specific Person from the database we would naturally get a unique instance. Hibernate, however, works hard to make sure that does not happen within a given Session. In fact, Hibernate guarantees equivalence of persistent identity (database row) and Java identity inside a particular session scope. So if we ask a Hibernate Session to load that specific Person multiple times we will actually get back the same instance:

Example 134. Scope of identity
Book book1 = entityManager.find(Book.class, 1L);
Book book2 = entityManager.find(Book.class, 1L);

assertTrue(book1 == book2);

Consider we have a Library parent entity which contains a java.util.Set of Book entities:

Example 135. Library entity mapping
@Entity(name = "Library")
public static class Library {

	@Id
	private Long id;

	private String name;

	@OneToMany(cascade = CascadeType.ALL)
	@JoinColumn(name = "book_id")
	private Set<Book> books = new HashSet<>();

	//Getters and setters are omitted for brevity
}
Example 136. Set usage with Session-scoped identity
Library library = entityManager.find(Library.class, 1L);

Book book1 = entityManager.find(Book.class, 1L);
Book book2 = entityManager.find(Book.class, 1L);

library.getBooks().add(book1);
library.getBooks().add(book2);

assertEquals(1, library.getBooks().size());

However, the semantic changes when we mix instances loaded from different Sessions:

Example 137. Mixed Sessions
Book book1 = doInJPA(this::entityManagerFactory, entityManager -> {
	return entityManager.find(Book.class, 1L);
});

Book book2 = doInJPA(this::entityManagerFactory, entityManager -> {
	return entityManager.find(Book.class, 1L);
});

assertFalse(book1 == book2);
doInJPA(this::entityManagerFactory, entityManager -> {
	Set<Book> books = new HashSet<>();

	books.add(book1);
	books.add(book2);

	assertEquals(2, books.size());
});

Specifically, the outcome in this last example will depend on whether the Book class implemented equals/hashCode, and, if so, how.

If the Book class did not override the default equals/hashCode, then the two Book object references are not going to be equal since their references are different.

Consider yet another case:

Example 138. Sets with transient entities
Library library = entityManager.find(Library.class, 1L);

Book book1 = new Book();
book1.setId(100L);
book1.setTitle("High-Performance Java Persistence");

Book book2 = new Book();
book2.setId(101L);
book2.setTitle("Java Persistence with Hibernate");

library.getBooks().add(book1);
library.getBooks().add(book2);

assertEquals(2, library.getBooks().size());

In cases where you will be dealing with entities outside of a Session (whether they be transient or detached), especially in cases where you will be using them in Java collections, you should consider implementing equals/hashCode.

A common initial approach is to use the entity’s identifier attribute as the basis for equals/hashCode calculations:

Example 139. Naive equals/hashCode implementation
@Entity(name = "Library")
public static class Library {

	@Id
	private Long id;

	private String name;

	@OneToMany(cascade = CascadeType.ALL)
	@JoinColumn(name = "book_id")
	private Set<Book> books = new HashSet<>();

	//Getters and setters are omitted for brevity
}

@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Book book = (Book) o;
		return Objects.equals(id, book.id);
	}

	@Override
	public int hashCode() {
		return Objects.hash(id);
	}
}

It turns out that this still breaks when adding transient instance of Book to a set as we saw in the last example:

Example 140. Auto-generated identifiers with Sets and naive equals/hashCode
Book book1 = new Book();
book1.setTitle("High-Performance Java Persistence");

Book book2 = new Book();
book2.setTitle("Java Persistence with Hibernate");

Library library = doInJPA(this::entityManagerFactory, entityManager -> {
	Library _library = entityManager.find(Library.class, 1L);

	_library.getBooks().add(book1);
	_library.getBooks().add(book2);

	return _library;
});

assertFalse(library.getBooks().contains(book1));
assertFalse(library.getBooks().contains(book2));

The issue here is a conflict between the use of the generated identifier, the contract of Set, and the equals/hashCode implementations. Set says that the equals/hashCode value for an object should not change while the object is part of the Set. But that is exactly what happened here because the equals/hasCode are based on the (generated) id, which was not set until the Jakarta Persistence transaction is committed.

Note that this is just a concern when using generated identifiers. If you are using assigned identifiers this will not be a problem, assuming the identifier value is assigned prior to adding to the Set.

Another option is to force the identifier to be generated and set prior to adding to the Set:

Example 141. Forcing the flush before adding to the Set
Book book1 = new Book();
book1.setTitle("High-Performance Java Persistence");

Book book2 = new Book();
book2.setTitle("Java Persistence with Hibernate");

Library library = doInJPA(this::entityManagerFactory, entityManager -> {
	Library _library = entityManager.find(Library.class, 1L);

	entityManager.persist(book1);
	entityManager.persist(book2);
	entityManager.flush();

	_library.getBooks().add(book1);
	_library.getBooks().add(book2);

	return _library;
});

assertTrue(library.getBooks().contains(book1));
assertTrue(library.getBooks().contains(book2));

But this is often not feasible.

The final approach is to use a "better" equals/hashCode implementation, making use of a natural-id or business-key.

Example 142. Natural Id equals/hashCode
@Entity(name = "Library")
public static class Library {

	@Id
	private Long id;

	private String name;

	@OneToMany(cascade = CascadeType.ALL)
	@JoinColumn(name = "book_id")
	private Set<Book> books = new HashSet<>();

	//Getters and setters are omitted for brevity
}

@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	@NaturalId
	private String isbn;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Book book = (Book) o;
		return Objects.equals(isbn, book.isbn);
	}

	@Override
	public int hashCode() {
		return Objects.hash(isbn);
	}
}

This time, when adding a Book to the Library Set, you can retrieve the Book even after it’s being persisted:

Example 143. Natural Id equals/hashCode persist example
Book book1 = new Book();
book1.setTitle("High-Performance Java Persistence");
book1.setIsbn("978-9730228236");

Library library = doInJPA(this::entityManagerFactory, entityManager -> {
	Library _library = entityManager.find(Library.class, 1L);

	_library.getBooks().add(book1);

	return _library;
});

assertTrue(library.getBooks().contains(book1));

As you can see the question of equals/hashCode is not trivial, nor is there a one-size-fits-all solution.

Although using a natural-id is best for equals and hashCode, sometimes you only have the entity identifier that provides a unique constraint.

It’s possible to use the entity identifier for equality check, but it needs a workaround:

  • you need to provide a constant value for hashCode so that the hash code value does not change before and after the entity is flushed.

  • you need to compare the entity identifier equality only for non-transient entities.

For details on mapping the identifier, see the Identifiers chapter.

2.4.8. Mapping the entity to a SQL query

You can map an entity to a SQL query using the @Subselect annotation.

Example 144. @Subselect entity mapping
@Entity(name = "Client")
@Table(name = "client")
public static class Client {

	@Id
	private Long id;

	@Column(name = "first_name")
	private String firstName;

	@Column(name = "last_name")
	private String lastName;

	//Getters and setters omitted for brevity

}

@Entity(name = "Account")
@Table(name = "account")
public static class Account {

	@Id
	private Long id;

	@ManyToOne
	private Client client;

	private String description;

	//Getters and setters omitted for brevity

}

@Entity(name = "AccountTransaction")
@Table(name = "account_transaction")
public static class AccountTransaction {

	@Id
	@GeneratedValue
	private Long id;

	@ManyToOne
	private Account account;

	private Integer cents;

	private String description;

	//Getters and setters omitted for brevity

}

@Entity(name = "AccountSummary")
@Subselect(
	"select " +
	"	a.id as id, " +
	"	concat(concat(c.first_name, ' '), c.last_name) as clientName, " +
	"	sum(atr.cents) as balance " +
	"from account a " +
	"join client c on c.id = a.client_id " +
	"join account_transaction atr on a.id = atr.account_id " +
	"group by a.id, concat(concat(c.first_name, ' '), c.last_name)"
)
@Synchronize({"client", "account", "account_transaction"})
public static class AccountSummary {

	@Id
	private Long id;

	private String clientName;

	private int balance;

	//Getters and setters omitted for brevity

}

In the example above, the Account entity does not retain any balance since every account operation is registered as an AccountTransaction. To find the Account balance, we need to query the AccountSummary which shares the same identifier with the Account entity.

However, the AccountSummary is not mapped to a physical table, but to an SQL query.

So, if we have the following AccountTransaction record, the AccountSummary balance will match the proper amount of money in this Account.

Example 145. Finding a @Subselect entity
scope.inTransaction(
		(entityManager) -> {
			Client client = new Client();
			client.setId(1L);
			client.setFirstName("John");
			client.setLastName("Doe");
			entityManager.persist(client);

			Account account = new Account();
			account.setId(1L);
			account.setClient(client);
			account.setDescription("Checking account");
			entityManager.persist(account);

			AccountTransaction transaction = new AccountTransaction();
			transaction.setAccount(account);
			transaction.setDescription("Salary");
			transaction.setCents(100 * 7000);
			entityManager.persist(transaction);

			AccountSummary summary = entityManager.createQuery(
				"select s " +
				"from AccountSummary s " +
				"where s.id = :id", AccountSummary.class)
			.setParameter("id", account.getId())
			.getSingleResult();

			assertEquals("John Doe", summary.getClientName());
			assertEquals(100 * 7000, summary.getBalance());
		}
);

If we add a new AccountTransaction entity and refresh the AccountSummary entity, the balance is updated accordingly:

Example 146. Refreshing a @Subselect entity
scope.inTransaction(
		(entityManager) -> {
			AccountSummary summary = entityManager.find(AccountSummary.class, 1L);
			assertEquals("John Doe", summary.getClientName());
			assertEquals(100 * 7000, summary.getBalance());

			AccountTransaction transaction = new AccountTransaction();
			transaction.setAccount(entityManager.getReference(Account.class, 1L));
			transaction.setDescription("Shopping");
			transaction.setCents(-100 * 2200);
			entityManager.persist(transaction);
			entityManager.flush();

			entityManager.refresh(summary);
			assertEquals(100 * 4800, summary.getBalance());
		}
);

The goal of the @Synchronize annotation in the AccountSummary entity mapping is to instruct Hibernate which database tables are needed by the underlying @Subselect SQL query. This is because, unlike JPQL and HQL queries, Hibernate cannot parse the underlying native SQL query.

With the @Synchronize annotation in place, when executing an HQL or JPQL which selects from the AccountSummary entity, Hibernate will trigger a Persistence Context flush if there are pending Account, Client or AccountTransaction entity state transitions.

2.4.9. Define a custom entity proxy

By default, when it needs to use a proxy instead of the actual POJO, Hibernate is going to use a Bytecode manipulation library like Byte Buddy.

However, if the entity class is final, a proxy will not be created; you will get a POJO even when you only need a proxy reference. In this case, you could proxy an interface that this particular entity implements, as illustrated by the following example.

Example 147. Final entity class implementing the Identifiable interface
public interface Identifiable {

	Long getId();

	void setId(Long id);
}

@Entity(name = "Book")
@Proxy(proxyClass = Identifiable.class)
public static final class Book implements Identifiable {

	@Id
	private Long id;

	private String title;

	private String author;

	@Override
	public Long getId() {
		return id;
	}

	@Override
	public void setId(Long id) {
		this.id = id;
	}

	//Other getters and setters omitted for brevity
}

The @Proxy annotation is used to specify a custom proxy implementation for the current annotated entity.

When loading the Book entity proxy, Hibernate is going to proxy the Identifiable interface instead as illustrated by the following example:

Example 148. Proxying the final entity class implementing the Identifiable interface
doInHibernate(this::sessionFactory, session -> {
	Book book = new Book();
	book.setId(1L);
	book.setTitle("High-Performance Java Persistence");
	book.setAuthor("Vlad Mihalcea");

	session.persist(book);
});

doInHibernate(this::sessionFactory, session -> {
	Identifiable book = session.getReference(Book.class, 1L);

	assertTrue(
		"Loaded entity is not an instance of the proxy interface",
		book instanceof Identifiable
	);
	assertFalse(
		"Proxy class was not created",
		book instanceof Book
	);
});
insert
into
    Book
    (author, title, id)
values
    (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Vlad Mihalcea]
-- binding parameter [2] as [VARCHAR] - [High-Performance Java Persistence]
-- binding parameter [3] as [BIGINT]  - [1]

As you can see in the associated SQL snippet, Hibernate issues no SQL SELECT query since the proxy can be constructed without needing to fetch the actual entity POJO.

2.4.10. Define a custom entity persister

The @Persister annotation is used to specify a custom entity or collection persister.

For entities, the custom persister must implement the EntityPersister interface.

For collections, the custom persister must implement the CollectionPersister interface.

Example 149. Entity persister mapping
@Entity
@Persister(impl = EntityPersister.class)
public class Author {

    @Id
    public Integer id;

    @OneToMany(mappedBy = "author")
    @Persister(impl = CollectionPersister.class)
    public Set<Book> books = new HashSet<>();

    //Getters and setters omitted for brevity

    public void addBook(Book book) {
        this.books.add(book);
        book.setAuthor(this);
    }
}
@Entity
@Persister(impl = EntityPersister.class)
public class Book {

    @Id
    public Integer id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    public Author author;

    //Getters and setters omitted for brevity
}

By providing your own EntityPersister and CollectionPersister implementations, you can control how entities and collections are persisted into the database.

2.5. Naming strategies

Part of the mapping of an object model to the relational database is mapping names from the object model to the corresponding database names. Hibernate looks at this as 2-stage process:

  • The first stage is determining a proper logical name from the domain model mapping. A logical name can be either explicitly specified by the user (e.g., using @Column or @Table) or it can be implicitly determined by Hibernate through an ImplicitNamingStrategy contract.

  • Second is the resolving of this logical name to a physical name which is defined by the PhysicalNamingStrategy contract.

Historical NamingStrategy contract

Historically Hibernate defined just a single org.hibernate.cfg.NamingStrategy. That singular NamingStrategy contract actually combined the separate concerns that are now modeled individually as ImplicitNamingStrategy and PhysicalNamingStrategy.

Also, the NamingStrategy contract was often not flexible enough to properly apply a given naming "rule", either because the API lacked the information to decide or because the API was honestly not well defined as it grew.

Due to these limitations, org.hibernate.cfg.NamingStrategy has been deprecated in favor of ImplicitNamingStrategy and PhysicalNamingStrategy.

At the core, the idea behind each naming strategy is to minimize the amount of repetitive information a developer must provide for mapping a domain model.

Jakarta Persistence Compatibility

Jakarta Persistence defines inherent rules about implicit logical name determination. If Jakarta Persistence provider portability is a major concern, or if you really just like the Jakarta Persistence-defined implicit naming rules, be sure to stick with ImplicitNamingStrategyJpaCompliantImpl (the default).

Also, Jakarta Persistence defines no separation between logical and physical name. Following the Jakarta Persistence specification, the logical name is the physical name. If Jakarta Persistence provider portability is important, applications should prefer not to specify a PhysicalNamingStrategy.

2.5.1. ImplicitNamingStrategy

When an entity does not explicitly name the database table that it maps to, we need to implicitly determine that table name. Or when a particular attribute does not explicitly name the database column that it maps to, we need to implicitly determine that column name. There are examples of the role of the org.hibernate.boot.model.naming.ImplicitNamingStrategy contract to determine a logical name when the mapping did not provide an explicit name.

Implicit Naming Strategy Diagram

Hibernate defines multiple ImplicitNamingStrategy implementations out-of-the-box. Applications are also free to plug in custom implementations.

There are multiple ways to specify the ImplicitNamingStrategy to use. First, applications can specify the implementation using the hibernate.implicit_naming_strategy configuration setting which accepts:

  • pre-defined "short names" for the out-of-the-box implementations

    default

    for org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl - an alias for jpa

    jpa

    for org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl - the Jakarta Persistence compliant naming strategy

    legacy-hbm

    for org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyHbmImpl - compliant with the original Hibernate NamingStrategy

    legacy-jpa

    for org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyJpaImpl - compliant with the legacy NamingStrategy developed for Java Persistence 1.0, which was unfortunately unclear in many respects regarding implicit naming rules

    component-path

    for org.hibernate.boot.model.naming.ImplicitNamingStrategyComponentPathImpl - mostly follows ImplicitNamingStrategyJpaCompliantImpl rules, except that it uses the full composite paths, as opposed to just the ending property part

  • reference to a Class that implements the org.hibernate.boot.model.naming.ImplicitNamingStrategy contract

  • FQN of a class that implements the org.hibernate.boot.model.naming.ImplicitNamingStrategy contract

Secondly, applications and integrations can leverage org.hibernate.boot.MetadataBuilder#applyImplicitNamingStrategy to specify the ImplicitNamingStrategy to use. See Bootstrap for additional details on bootstrapping.

2.5.2. PhysicalNamingStrategy

Many organizations define rules around the naming of database objects (tables, columns, foreign keys, etc). The idea of a PhysicalNamingStrategy is to help implement such naming rules without having to hard-code them into the mapping via explicit names.

While the purpose of an ImplicitNamingStrategy is to determine that an attribute named accountNumber maps to a logical column name of accountNumber when not explicitly specified, the purpose of a PhysicalNamingStrategy would be, for example, to say that the physical column name should instead be abbreviated to acct_num.

It is true that the resolution to acct_num could have been handled using an ImplicitNamingStrategy in this case.

But the point here is the separation of concerns. The PhysicalNamingStrategy will be applied regardless of whether the attribute explicitly specified the column name or whether we determined that implicitly. The ImplicitNamingStrategy would only be applied if an explicit name was not given. So, it all depends on needs and intent.

The default implementation is to simply use the logical name as the physical name. However applications and integrations can define custom implementations of this PhysicalNamingStrategy contract. Here is an example PhysicalNamingStrategy for a fictitious company named Acme Corp whose naming standards are to:

  • prefer underscore-delimited words rather than camel casing

  • replace certain words with standard abbreviations

Example 150. Example PhysicalNamingStrategy implementation
/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
 * See the lgpl.txt file in the root directory or <http://www.gnu.org/licenses/lgpl-2.1.html>.
 */
package org.hibernate.userguide.naming;

import java.util.Arrays;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;

import org.hibernate.boot.model.naming.Identifier;
import org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl;
import org.hibernate.engine.jdbc.env.spi.JdbcEnvironment;

import org.apache.commons.lang3.StringUtils;

/**
 * An example PhysicalNamingStrategy that implements database object naming standards
 * for our fictitious company Acme Corp.
 * <p/>
 * In general Acme Corp prefers underscore-delimited words rather than camel casing.
 * <p/>
 * Additionally standards call for the replacement of certain words with abbreviations.
 *
 * @author Steve Ebersole
 * @author Nathan Xu
 */
public class AcmeCorpPhysicalNamingStrategy extends PhysicalNamingStrategyStandardImpl {
	private static final Map<String, String> ABBREVIATIONS;

	static {
		ABBREVIATIONS = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
		ABBREVIATIONS.put("account", "acct");
		ABBREVIATIONS.put("number", "num");
	}

	@Override
	public Identifier toPhysicalTableName(Identifier logicalName, JdbcEnvironment jdbcEnvironment) {
		final List<String> parts = splitAndReplace( logicalName.getText());
		return jdbcEnvironment.getIdentifierHelper().toIdentifier(
				StringUtils.join(parts, '_'),
				logicalName.isQuoted()
		);
	}

	@Override
	public Identifier toPhysicalSequenceName(Identifier logicalName, JdbcEnvironment jdbcEnvironment) {
		final List<String> parts = splitAndReplace( logicalName.getText());
		// Acme Corp says all sequences should end with _seq
		if (!"seq".equals(parts.get(parts.size() - 1))) {
			parts.add("seq");
		}
		return jdbcEnvironment.getIdentifierHelper().toIdentifier(
				StringUtils.join(parts, '_'),
				logicalName.isQuoted()
		);
	}

	@Override
	public Identifier toPhysicalColumnName(Identifier logicalName, JdbcEnvironment jdbcEnvironment) {
		final List<String> parts = splitAndReplace( logicalName.getText());
		return jdbcEnvironment.getIdentifierHelper().toIdentifier(
				StringUtils.join(parts, '_'),
				logicalName.isQuoted()
		);
	}

	private List<String> splitAndReplace(String name) {
		return Arrays.stream(StringUtils.splitByCharacterTypeCamelCase(name))
				.filter(StringUtils::isNotBlank)
				.map(p -> ABBREVIATIONS.getOrDefault(p, p).toLowerCase(Locale.ROOT))
				.collect(Collectors.toList());
	}
}

There are multiple ways to specify the PhysicalNamingStrategy to use. First, applications can specify the implementation using the hibernate.physical_naming_strategy configuration setting which accepts:

  • reference to a Class that implements the org.hibernate.boot.model.naming.PhysicalNamingStrategy contract

  • FQN of a class that implements the org.hibernate.boot.model.naming.PhysicalNamingStrategy contract

Secondly, applications and integrations can leverage org.hibernate.boot.MetadataBuilder#applyPhysicalNamingStrategy. See Bootstrap for additional details on bootstrapping.

2.6. Access strategies

As a Jakarta Persistence provider, Hibernate can introspect both the entity attributes (instance fields) or the accessors (instance properties). By default, the placement of the @Id annotation gives the default access strategy. When placed on a field, Hibernate will assume field-based access. When placed on the identifier getter, Hibernate will use property-based access.

To avoid issues such as HCANN-63 - Property name beginning with at least two uppercase characters has odd functionality in HQL, you should pay attention to Java Bean specification in regard to naming properties.

Embeddable types inherit the access strategy from their parent entities.

2.6.1. Field-based access

Example 151. Field-based access
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

When using field-based access, adding other entity-level methods is much more flexible because Hibernate won’t consider those part of the persistence state. To exclude a field from being part of the entity persistent state, the field must be marked with the @Transient annotation.

Another advantage of using field-based access is that some entity attributes can be hidden from outside the entity.

An example of such attribute is the entity @Version field, which, usually, does not need to be manipulated by the data access layer.

With field-based access, we can simply omit the getter and the setter for this version field, and Hibernate can still leverage the optimistic concurrency control mechanism.

2.6.2. Property-based access

Example 152. Property-based access
@Entity(name = "Book")
public static class Book {

	private Long id;

	private String title;

	private String author;

	@Id
	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}
}

When using property-based access, Hibernate uses the accessors for both reading and writing the entity state. Every other method that will be added to the entity (e.g. helper methods for synchronizing both ends of a bidirectional one-to-many association) will have to be marked with the @Transient annotation.

2.6.3. Overriding the default access strategy

The default access strategy mechanism can be overridden with the Jakarta Persistence @Access annotation. In the following example, the @Version attribute is accessed by its field and not by its getter, like the rest of entity attributes.

Example 153. Overriding access strategy
@Entity(name = "Book")
public static class Book {

	private Long id;

	private String title;

	private String author;

	@Access(AccessType.FIELD)
	@Version
	private int version;

	@Id
	public Long getId() {
		return id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}
}

2.6.4. Embeddable types and access strategy

Because embeddables are managed by their owning entities, the access strategy is therefore inherited from the entity too. This applies to both simple embeddable types as well as for collection of embeddables.

The embeddable types can overrule the default implicit access strategy (inherited from the owning entity). In the following example, the embeddable uses property-based access, no matter what access strategy the owning entity is choosing:

Example 154. Embeddable with exclusive access strategy
@Embeddable
@Access(AccessType.PROPERTY)
public static class Author {

	private String firstName;

	private String lastName;

	public Author() {
	}

	public Author(String firstName, String lastName) {
		this.firstName = firstName;
		this.lastName = lastName;
	}

	public String getFirstName() {
		return firstName;
	}

	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}

	public String getLastName() {
		return lastName;
	}

	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
}

The owning entity can use field-based access while the embeddable uses property-based access as it has chosen explicitly:

Example 155. Entity including a single embeddable type
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	@Embedded
	private Author author;

	//Getters and setters are omitted for brevity
}

This works also for collection of embeddable types:

Example 156. Entity including a collection of embeddable types
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	@ElementCollection
	@CollectionTable(
		name = "book_author",
		joinColumns = @JoinColumn(name = "book_id")
	)
	private List<Author> authors = new ArrayList<>();

	//Getters and setters are omitted for brevity
}

2.7. Identifiers

Identifiers model the primary key of an entity. They are used to uniquely identify each specific entity.

Hibernate and Jakarta Persistence both make the following assumptions about the corresponding database column(s):

UNIQUE

The values must uniquely identify each row.

NOT NULL

The values cannot be null. For composite ids, no part can be null.

IMMUTABLE

The values, once inserted, can never be changed. In cases where the values for the PK you have chosen will be updated, Hibernate recommends mapping the mutable value as a natural id, and use a surrogate id for the PK. See Natural Ids.

Technically the identifier does not have to map to the column(s) physically defined as the table primary key. They just need to map to column(s) that uniquely identify each row. However, this documentation will continue to use the terms identifier and primary key interchangeably.

Every entity must define an identifier. For entity inheritance hierarchies, the identifier must be defined just on the entity that is the root of the hierarchy.

An identifier may be simple or composite.

2.7.1. Simple identifiers

Simple identifiers map to a single basic attribute, and are denoted using the jakarta.persistence.Id annotation.

According to Jakarta Persistence, only the following types are portably supported for use as identifier attribute types:

  • any Java primitive type

  • any primitive wrapper type

  • java.lang.String

  • java.util.Date (TemporalType#DATE)

  • java.sql.Date

  • java.math.BigDecimal

  • java.math.BigInteger

Hibernate, however, supports a more broad set of types to be used for identifiers (UUID, e.g.).

Assigned identifiers

Values for simple identifiers can be assigned, which simply means that the application itself will assign the value to the identifier attribute prior to persisting the entity.

Example 157. Simple assigned entity identifier
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}
Generated identifiers

Values for simple identifiers can be generated. To denote that an identifier attribute is generated, it is annotated with jakarta.persistence.GeneratedValue

Example 158. Simple generated identifier
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

When an entity with an identifier defined as generated is persisted, Hibernate will generate the value based on an associated generation strategy. Identifier value generations strategies are discussed in detail in the Generated identifier values section.

While Hibernate supports almost any valid basic type be used for generated identifier values, Jakarta Persistence restricts the allowable types to just integer types.

2.7.2. Composite identifiers

Composite identifiers correspond to one or more persistent attributes. Here are the rules governing composite identifiers, as defined by the Jakarta Persistence specification:

  • The composite identifier must be represented by a "primary key class". The primary key class may be defined using the jakarta.persistence.EmbeddedId annotation (see Composite identifiers with @EmbeddedId), or defined using the jakarta.persistence.IdClass annotation (see Composite identifiers with @IdClass).

  • The primary key class must be public and must have a public no-arg constructor.

  • The primary key class must be serializable.

  • The primary key class must define equals and hashCode methods, consistent with equality for the underlying database types to which the primary key is mapped.

The restriction that a composite identifier has to be represented by a "primary key class" (e.g. @EmbeddedId or @IdClass) is only Jakarta Persistence-specific.

Hibernate does allow composite identifiers to be defined without a "primary key class" via multiple @Id attributes, although that is generally considered poor design.

The attributes making up the composition can be either basic, composite or @ManyToOne. Note especially that collection and one-to-one are never appropriate.

2.7.3. Composite identifiers with @EmbeddedId

Modeling a composite identifier using an EmbeddedId simply means defining an embeddable to be a composition for the attributes making up the identifier, and then exposing an attribute of that embeddable type on the entity.

Example 159. Basic @EmbeddedId
@Entity(name = "SystemUser")
public static class SystemUser {

	@EmbeddedId
	private PK pk;

	private String name;

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class PK implements Serializable {

	private String subsystem;

	private String username;

	public PK(String subsystem, String username) {
		this.subsystem = subsystem;
		this.username = username;
	}

	private PK() {
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		PK pk = (PK) o;
		return Objects.equals(subsystem, pk.subsystem) &&
				Objects.equals(username, pk.username);
	}

	@Override
	public int hashCode() {
		return Objects.hash(subsystem, username);
	}
}

As mentioned before, EmbeddedIds can even contain @ManyToOne attributes:

Example 160. @EmbeddedId with @ManyToOne
@Entity(name = "SystemUser")
public static class SystemUser {

	@EmbeddedId
	private PK pk;

	private String name;

	//Getters and setters are omitted for brevity
}

@Entity(name = "Subsystem")
public static class Subsystem {

	@Id
	private String id;

	private String description;

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class PK implements Serializable {

	@ManyToOne(fetch = FetchType.LAZY)
	private Subsystem subsystem;

	private String username;

	public PK(Subsystem subsystem, String username) {
		this.subsystem = subsystem;
		this.username = username;
	}

	private PK() {
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		PK pk = (PK) o;
		return Objects.equals(subsystem, pk.subsystem) &&
				Objects.equals(username, pk.username);
	}

	@Override
	public int hashCode() {
		return Objects.hash(subsystem, username);
	}
}

Hibernate supports directly modeling @ManyToOne associations in the Primary Key class, whether @EmbeddedId or @IdClass.

However, that is not portably supported by the Jakarta Persistence specification. In Jakarta Persistence terms, one would use "derived identifiers". For more details, see Derived Identifiers.

2.7.4. Composite identifiers with @IdClass

Modeling a composite identifier using an IdClass differs from using an EmbeddedId in that the entity defines each individual attribute making up the composition. The IdClass is used as the representation of the identifier for load-by-id operations.

Example 161. Basic @IdClass
@Entity(name = "SystemUser")
@IdClass(PK.class)
public static class SystemUser {

	@Id
	private String subsystem;

	@Id
	private String username;

	private String name;

	public PK getId() {
		return new PK(
			subsystem,
			username
		);
	}

	public void setId(PK id) {
		this.subsystem = id.getSubsystem();
		this.username = id.getUsername();
	}

	//Getters and setters are omitted for brevity
}

public static class PK implements Serializable {

	private String subsystem;

	private String username;

	public PK(String subsystem, String username) {
		this.subsystem = subsystem;
		this.username = username;
	}

	private PK() {
	}

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		PK pk = (PK) o;
		return Objects.equals(subsystem, pk.subsystem) &&
				Objects.equals(username, pk.username);
	}

	@Override
	public int hashCode() {
		return Objects.hash(subsystem, username);
	}
}

Non-aggregated composite identifiers can also contain ManyToOne attributes as we saw with aggregated mappings, though still non-portably.

Example 162. IdClass with @ManyToOne
@Entity(name = "SystemUser")
@IdClass(PK.class)
public static class SystemUser {

	@Id
	@ManyToOne(fetch = FetchType.LAZY)
	private Subsystem subsystem;

	@Id
	private String username;

	private String name;

	//Getters and setters are omitted for brevity
}

@Entity(name = "Subsystem")
public static class Subsystem {

	@Id
	private String id;

	private String description;

	//Getters and setters are omitted for brevity
}

public static class PK implements Serializable {

	private Subsystem subsystem;

	private String username;

	public PK(Subsystem subsystem, String username) {
		this.subsystem = subsystem;
		this.username = username;
	}

	private PK() {
	}

	//Getters and setters are omitted for brevity
}

With non-aggregated composite identifiers, Hibernate also supports "partial" generation of the composite values.

Example 163. @IdClass with partial identifier generation using @GeneratedValue
@Entity(name = "SystemUser")
@IdClass(PK.class)
public static class SystemUser {

	@Id
	private String subsystem;

	@Id
	private String username;

	@Id
	@GeneratedValue
	private Integer registrationId;

	private String name;

	public PK getId() {
		return new PK(
			subsystem,
			username,
			registrationId
		);
	}

	public void setId(PK id) {
		this.subsystem = id.getSubsystem();
		this.username = id.getUsername();
		this.registrationId = id.getRegistrationId();
	}

	//Getters and setters are omitted for brevity
}

public static class PK implements Serializable {

	private String subsystem;

	private String username;

	private Integer registrationId;

	public PK(String subsystem, String username) {
		this.subsystem = subsystem;
		this.username = username;
	}

	public PK(String subsystem, String username, Integer registrationId) {
		this.subsystem = subsystem;
		this.username = username;
		this.registrationId = registrationId;
	}

	private PK() {
	}

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		PK pk = (PK) o;
		return Objects.equals(subsystem, pk.subsystem) &&
				Objects.equals(username, pk.username) &&
				Objects.equals(registrationId, pk.registrationId);
	}

	@Override
	public int hashCode() {
		return Objects.hash(subsystem, username, registrationId);
	}
}

This feature which allows auto-generated values in composite identifiers exists because of a highly questionable interpretation of the Jakarta Persistence specification made by the SpecJ committee.

Hibernate does not feel that Jakarta Persistence defines support for this, but added the feature simply to be usable in SpecJ benchmarks. Use of this feature may or may not be portable from a Jakarta Persistence perspective.

2.7.5. Composite identifiers with associations

Hibernate allows defining a composite identifier out of entity associations. In the following example, the Book entity identifier is formed of two @ManyToOne associations.

Example 164. Composite identifiers with associations
@Entity(name = "Book")
public static class Book implements Serializable {

	@Id
	@ManyToOne(fetch = FetchType.LAZY)
	private Author author;

	@Id
	@ManyToOne(fetch = FetchType.LAZY)
	private Publisher publisher;

	@Id
	private String title;

	public Book(Author author, Publisher publisher, String title) {
		this.author = author;
		this.publisher = publisher;
		this.title = title;
	}

	private Book() {
	}

	//Getters and setters are omitted for brevity


	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Book book = (Book) o;
		return Objects.equals(author, book.author) &&
				Objects.equals(publisher, book.publisher) &&
				Objects.equals(title, book.title);
	}

	@Override
	public int hashCode() {
		return Objects.hash(author, publisher, title);
	}
}

@Entity(name = "Author")
public static class Author implements Serializable {

	@Id
	private String name;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Author author = (Author) o;
		return Objects.equals(name, author.name);
	}

	@Override
	public int hashCode() {
		return Objects.hash(name);
	}
}

@Entity(name = "Publisher")
public static class Publisher implements Serializable {

	@Id
	private String name;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Publisher publisher = (Publisher) o;
		return Objects.equals(name, publisher.name);
	}

	@Override
	public int hashCode() {
		return Objects.hash(name);
	}
}

Although the mapping is much simpler than using an @EmbeddedId or an @IdClass, there’s no separation between the entity instance and the actual identifier. To query this entity, an instance of the entity itself must be supplied to the persistence context.

Example 165. Fetching with composite identifiers
Book book = entityManager.find(Book.class, new Book(
	author,
	publisher,
	"High-Performance Java Persistence"
));

assertEquals("Vlad Mihalcea", book.getAuthor().getName());

2.7.6. Composite identifiers with generated properties

When using composite identifiers, the underlying identifier properties must be manually assigned by the user.

Automatically generated properties are not supported to be used to generate the value of an underlying property that makes the composite identifier.

Therefore, you cannot use any of the automatic property generator described by the generated properties section like @Generated, @CreationTimestamp or @ValueGenerationType or database-generated values.

Nevertheless, you can still generate the identifier properties prior to constructing the composite identifier, as illustrated by the following examples.

Assuming we have the following EventId composite identifier and an Event entity which uses the aforementioned composite identifier.

Example 166. The Event entity and EventId composite identifier
@Entity
class Event {

    @Id
    private EventId id;

    @Column(name = "event_key")
    private String key;

    @Column(name = "event_value")
    private String value;

    //Getters and setters are omitted for brevity
}
@Embeddable
class EventId implements Serializable {

	private Integer category;

	private Timestamp createdOn;

	//Getters and setters are omitted for brevity
	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		EventId that = (EventId) o;
		return Objects.equals(category, that.category) &&
				Objects.equals(createdOn, that.createdOn);
	}

	@Override
	public int hashCode() {
		return Objects.hash(category, createdOn);
	}
}
In-memory generated composite identifier properties

If you want to generate the composite identifier properties in-memory, you need to do that as follows:

Example 167. In-memory generated composite identifier properties example
EventId id = new EventId();
id.setCategory(1);
id.setCreatedOn(new Timestamp(System.currentTimeMillis()));

Event event = new Event();
event.setId(id);
event.setKey("Temperature");
event.setValue("9");

entityManager.persist(event);

Notice that the createdOn property of the EventId composite identifier was generated by the data access code and assigned to the identifier prior to persisting the Event entity.

Database generated composite identifier properties

If you want to generate the composite identifier properties using a database function or stored procedure, you could to do it as illustrated by the following example.

Example 168. Database generated composite identifier properties example
OffsetDateTime currentTimestamp = (OffsetDateTime) entityManager
.createNativeQuery(
	"SELECT CURRENT_TIMESTAMP")
.getSingleResult();

EventId id = new EventId();
id.setCategory(1);
id.setCreatedOn(Timestamp.from(currentTimestamp.toInstant()));

Event event = new Event();
event.setId(id);
event.setKey("Temperature");
event.setValue("9");

entityManager.persist(event);

Notice that the createdOn property of the EventId composite identifier was generated by calling the CURRENT_TIMESTAMP database function, and we assigned it to the composite identifier prior to persisting the Event entity.

2.7.7. Generated identifier values

Hibernate supports identifier value generation across a number of different types. Remember that Jakarta Persistence portably defines identifier value generation just for integer types.

You can also auto-generate values for non-identifier attributes. For more details, see the Generated properties section.

Identifier value generation is indicated using the jakarta.persistence.GeneratedValue annotation. The most important piece of information here is the specified jakarta.persistence.GenerationType which indicates how values will be generated.

AUTO (the default)

Indicates that the persistence provider (Hibernate) should choose an appropriate generation strategy. See Interpreting AUTO.

IDENTITY

Indicates that database IDENTITY columns will be used for primary key value generation. See Using IDENTITY columns.

SEQUENCE

Indicates that database sequence should be used for obtaining primary key values. See Using sequences.

TABLE

Indicates that a database table should be used for obtaining primary key values. See Using the table identifier generator.

2.7.8. Interpreting AUTO

How a persistence provider interprets the AUTO generation type is left up to the provider.

The default behavior is to look at the Java type of the identifier attribute, plus what the underlying database supports.

If the identifier type is UUID, Hibernate is going to use a UUID identifier.

If the identifier type is numeric (e.g. Long, Integer), then Hibernate will use its SequenceStyleGenerator which resolves to a SEQUENCE generation if the underlying database supports sequences and a table-based generation otherwise.

2.7.9. Using sequences

For implementing database sequence-based identifier value generation Hibernate makes use of its org.hibernate.id.enhanced.SequenceStyleGenerator id generator. It is important to note that SequenceStyleGenerator is capable of working against databases that do not support sequences by transparently switching to a table as the underlying backing, which gives Hibernate a huge degree of portability across databases while still maintaining consistent id generation behavior (versus say choosing between SEQUENCE and IDENTITY).

Example 169. Implicit sequence
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue( strategy = SEQUENCE )
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}

Notice that the mapping does not specify the name of the sequence to use. In such cases, Hibernate will assume a sequence name based on the name of the table to which the entity is mapped. Here, since the entity is mapped to a table named product, Hibernate will use a sequence named product_seq.

When using @Subselect mappings, using the "table name" is not valid so Hibernate falls back to using the entity name as the base along with the _seq suffix.

To specify the sequence name explicitly, the simplest form is to specify @GeneratedValue#generator.

Example 170. Named sequence
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = SEQUENCE,
		generator = "explicit_product_sequence"
	)
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}

For this mapping, Hibernate will use explicit_product_sequence as the name of the sequence.

For more advanced configuration, Jakarta Persistence defines the @SequenceGenerator annotation.

Example 171. Simple @SequenceGenerator
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = SEQUENCE,
		generator = "sequence-generator"
	)
	@SequenceGenerator(
		name = "sequence-generator",
		sequenceName = "explicit_product_sequence"
	)
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}

This is simply a more verbose form of the mapping in Named sequence. However, the jakarta.persistence.SequenceGenerator annotation allows you to specify additional configurations as well.

Example 172. Sequence configuration
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = GenerationType.SEQUENCE,
		generator = "sequence-generator"
	)
	@SequenceGenerator(
		name = "sequence-generator",
		sequenceName = "explicit_product_sequence",
		allocationSize = 5
	)
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}

Again the mapping specifies explicit_product_sequence as the physical sequence name, but it also specifies an explicit allocation-size ("increment by").

2.7.10. Using IDENTITY columns

For implementing identifier value generation based on IDENTITY columns, Hibernate makes use of its org.hibernate.id.IdentityGenerator id generator which expects the identifier to be generated by INSERT into the table. IdentityGenerator understands 3 different ways that the INSERT-generated value might be retrieved:

  • If Hibernate believes the JDBC environment supports java.sql.Statement#getGeneratedKeys, then that approach will be used for extracting the IDENTITY generated keys.

  • Otherwise, if Dialect#supportsInsertSelectIdentity reports true, Hibernate will use the Dialect specific INSERT+SELECT statement syntax.

  • Otherwise, Hibernate will expect that the database supports some form of asking for the most recently inserted IDENTITY value via a separate SQL command as indicated by Dialect#getIdentitySelectString.

It is important to realize that using IDENTITY columns imposes a runtime behavior where the entity row must be physically inserted prior to the identifier value being known.

This can mess up extended persistence contexts (long conversations). Because of the runtime imposition/inconsistency, Hibernate suggests other forms of identifier value generation be used (e.g. SEQUENCE) with extended contexts.

In Hibernate 5.3, Hibernate attempts to delay the insert of entities if the flush-mode does not equal AUTO. This was slightly problematic for entities that used IDENTITY or SEQUENCE generated identifiers that were also involved in some form of association with another entity in the same transaction.

In Hibernate 5.4, Hibernate attempts to remedy the problem using an algorithm to decide if the insert should be delayed or if it requires immediate insertion. We wanted to restore the behavior prior to 5.3 only for very specific use cases where it made sense.

Entity mappings can sometimes be complex and it is possible a corner case was overlooked. Hibernate offers a way to completely disable the 5.3 behavior in the event problems occur with DelayedPostInsertIdentifier. To enable the legacy behavior, set hibernate.id.disable_delayed_identity_inserts=true.

This configuration option is meant to act as a temporary fix and bridge the gap between the changes in this behavior across Hibernate 5.x releases. If this configuration setting is necessary for a mapping, please open a JIRA and report the mapping so that the algorithm can be reviewed.

There is yet another important runtime impact of choosing IDENTITY generation: Hibernate will not be able to batch INSERT statements for the entities using the IDENTITY generation.

The importance of this depends on the application-specific use cases. If the application is not usually creating many new instances of a given entity type using the IDENTITY generator, then this limitation will be less important since batching would not have been very helpful anyway.

2.7.11. Using the table identifier generator

Hibernate achieves table-based identifier generation based on its org.hibernate.id.enhanced.TableGenerator which defines a table capable of holding multiple named value segments for any number of entities.

The basic idea is that a given table-generator table (hibernate_sequences for example) can hold multiple segments of identifier generation values.

Example 173. Unnamed table generator
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = GenerationType.TABLE
	)
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}
create table hibernate_sequences (
    sequence_name varchar2(255 char) not null,
    next_val number(19,0),
    primary key (sequence_name)
)

If no table name is given Hibernate assumes an implicit name of hibernate_sequences.

Additionally, because no jakarta.persistence.TableGenerator#pkColumnValue is specified, Hibernate will use the default segment (sequence_name='default') from the hibernate_sequences table.

However, you can configure the table identifier generator using the @TableGenerator annotation.

Example 174. Configured table generator
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = GenerationType.TABLE,
		generator = "table-generator"
	)
	@TableGenerator(
		name =  "table-generator",
		table = "table_identifier",
		pkColumnName = "table_name",
		valueColumnName = "product_id",
		allocationSize = 5
	)
	private Long id;

	@Column(name = "product_name")
	private String name;

	//Getters and setters are omitted for brevity

}
create table table_identifier (
    table_name varchar2(255 char) not null,
    product_id number(19,0),
    primary key (table_name)
)

Now, when inserting 3 Product entities, Hibernate generates the following statements:

Example 175. Configured table generator persist example
for (long i = 1; i <= 3; i++) {
	Product product = new Product();
	product.setName(String.format("Product %d", i));
	entityManager.persist(product);
}
select
    tbl.product_id
from
    table_identifier tbl
where
    tbl.table_name = ?
for update

-- binding parameter [1] - [Product]

insert
into
    table_identifier
    (table_name, product_id)
values
    (?, ?)

-- binding parameter [1] - [Product]
-- binding parameter [2] - [1]

update
    table_identifier
set
    product_id= ?
where
    product_id= ?
    and table_name= ?

-- binding parameter [1] - [6]
-- binding parameter [2] - [1]

select
    tbl.product_id
from
    table_identifier tbl
where
    tbl.table_name= ? for update

update
    table_identifier
set
    product_id= ?
where
    product_id= ?
    and table_name= ?

-- binding parameter [1] - [11]
-- binding parameter [2] - [6]

insert
into
    Product
    (product_name, id)
values
    (?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 1]
-- binding parameter [2] as [BIGINT]  - [1]

insert
into
    Product
    (product_name, id)
values
    (?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 2]
-- binding parameter [2] as [BIGINT]  - [2]

insert
into
    Product
    (product_name, id)
values
    (?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 3]
-- binding parameter [2] as [BIGINT]  - [3]

2.7.12. Using UUID generation

As mentioned above, Hibernate supports UUID identifier value generation. This is supported through its org.hibernate.id.UUIDGenerator id generator.

NOTE

org.hibernate.id.UUIDGenerator is an example of @IdGeneratorType discussed in Using @IdGeneratorType

UUIDGenerator supports pluggable strategies for exactly how the UUID is generated. These strategies are defined by the org.hibernate.id.UUIDGenerationStrategy contract. The default strategy is a version 4 (random) strategy according to IETF RFC 4122. Hibernate does ship with an alternative strategy which is a RFC 4122 version 1 (time-based) strategy (using IP address rather than mac address).

Example 176. Implicitly using the random UUID strategy
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue
	private UUID id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

To specify an alternative generation strategy, we’d have to define some configuration via @GenericGenerator. Here we choose the RFC 4122 version 1 compliant strategy named org.hibernate.id.uuid.CustomVersionOneStrategy.

Example 177. Implicitly using the random UUID strategy
@Entity(name = "Book")
public static class Book {

	@Id
	@GeneratedValue(generator = "custom-uuid")
	@GenericGenerator(
		name = "custom-uuid",
		strategy = "org.hibernate.id.UUIDGenerator",
		parameters = {
			@Parameter(
				name = "uuid_gen_strategy_class",
				value = "org.hibernate.id.uuid.CustomVersionOneStrategy"
			)
		}
	)
	private UUID id;

	private String title;

	private String author;

	//Getters and setters are omitted for brevity
}

2.7.13. Optimizers

Most of the Hibernate generators that separately obtain identifier values from database structures support the use of pluggable optimizers. Optimizers help manage the number of times Hibernate has to talk to the database in order to generate identifier values. For example, with no optimizer applied to a sequence-generator, every time the application asked Hibernate to generate an identifier it would need to grab the next sequence value from the database. But if we can minimize the number of times we need to communicate with the database here, the application will be able to perform better, which is, in fact, the role of these optimizers.

none

No optimization is performed. We communicate with the database each and every time an identifier value is needed from the generator.

pooled-lo

The pooled-lo optimizer works on the principle that the increment-value is encoded into the database table/sequence structure. In sequence-terms, this means that the sequence is defined with a greater-than-1 increment size.

For example, consider a brand new sequence defined as create sequence m_sequence start with 1 increment by 20. This sequence essentially defines a "pool" of 20 usable id values each and every time we ask it for its next-value. The pooled-lo optimizer interprets the next-value as the low end of that pool.

So when we first ask it for next-value, we’d get 1. We then assume that the valid pool would be the values from 1-20 inclusive.

The next call to the sequence would result in 21, which would define 21-40 as the valid range. And so on. The "lo" part of the name indicates that the value from the database table/sequence is interpreted as the pool lo(w) end.

pooled

Just like pooled-lo, except that here the value from the table/sequence is interpreted as the high end of the value pool.

hilo; legacy-hilo

Define a custom algorithm for generating pools of values based on a single value from a table or sequence.

These optimizers are not recommended for use. They are maintained (and mentioned) here simply for use by legacy applications that used these strategies previously.

Applications can also implement and use their own optimizer strategies, as defined by the org.hibernate.id.enhanced.Optimizer contract.

2.7.14. Using @IdGeneratorType

@IdGeneratorType is a meta-annotation that allows the creation of custom annotations that support simple, concise and type-safe definition and configuration of custom org.hibernate.id.IdentifierGenerator implementations.

Example 178. @IdGeneratorType
public class CustomSequenceGenerator implements IdentifierGenerator {

	public CustomSequenceGenerator(
			Sequence config,
			Member annotatedMember,
			CustomIdGeneratorCreationContext context) {
		//...
	}

	@Override
	public Object generate(
			SharedSessionContractImplementor session,
			Object object) {
		//...
}

@IdGeneratorType( CustomSequenceGenerator.class )
@Target({METHOD, FIELD})
@Retention(RUNTIME)
public @interface Sequence {
	String name();
	int startWith() default 1;
	int incrementBy() default 50;
	Class<? extends Optimizer> optimizer() default Optimizer.class;
}

The example illustrates using @IdGeneratorType to define a custom sequence-based annotation @Sequence to apply and configure a custom IdentifierGenerator implementation CustomSequenceGenerator.

Notice the CustomSequenceGenerator constructor. Custom generator defined through @IdGeneratorType receive the following arguments:

  1. The configuration annotation - here, @Sequence. This is the type-safety aspect, rather than relying on untyped configuration properties in a Map, etc.

  2. The Member to which annotation was applied. This allows access to the Java type of the identifier attribute, etc.

  3. CustomIdGeneratorCreationContext is a "parameter object" providing access to things often useful for identifier generators.

2.7.15. Using @GenericGenerator

@GenericGenerator is generally considered deprecated in favor of @IdGeneratorType

@GenericGenerator allows integration of any Hibernate org.hibernate.id.IdentifierGenerator implementation, including any of the specific ones discussed here and any custom ones.

Example 179. Pooled-lo optimizer mapping using @GenericGenerator mapping
@Entity(name = "Product")
public static class Product {

	@Id
	@GeneratedValue(
		strategy = GenerationType.SEQUENCE,
		generator = "product_generator"
	)
	@GenericGenerator(
		name = "product_generator",
		strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator",
		parameters = {
			@Parameter(name = "sequence_name", value = "product_sequence"),
			@Parameter(name = "initial_value", value = "1"),
			@Parameter(name = "increment_size", value = "3"),
			@Parameter(name = "optimizer", value = "pooled-lo")
		}
	)
	private Long id;

	@Column(name = "p_name")
	private String name;

	@Column(name = "p_number")
	private String number;

	//Getters and setters are omitted for brevity

}

Now, when saving 5 Person entities and flushing the Persistence Context after every 3 entities:

Example 180. Pooled-lo optimizer mapping using @GenericGenerator mapping
for (long i = 1; i <= 5; i++) {
	if(i % 3 == 0) {
		entityManager.flush();
	}
	Product product = new Product();
	product.setName(String.format("Product %d", i));
	product.setNumber(String.format("P_100_%d", i));
	entityManager.persist(product);
}
CALL NEXT VALUE FOR product_sequence

INSERT INTO Product (p_name, p_number, id)
VALUES (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 1]
-- binding parameter [2] as [VARCHAR] - [P_100_1]
-- binding parameter [3] as [BIGINT]  - [1]

INSERT INTO Product (p_name, p_number, id)
VALUES (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 2]
-- binding parameter [2] as [VARCHAR] - [P_100_2]
-- binding parameter [3] as [BIGINT]  - [2]

CALL NEXT VALUE FOR product_sequence

INSERT INTO Product (p_name, p_number, id)
VALUES (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 3]
-- binding parameter [2] as [VARCHAR] - [P_100_3]
-- binding parameter [3] as [BIGINT]  - [3]

INSERT INTO Product (p_name, p_number, id)
VALUES (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 4]
-- binding parameter [2] as [VARCHAR] - [P_100_4]
-- binding parameter [3] as [BIGINT]  - [4]

INSERT INTO Product (p_name, p_number, id)
VALUES (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Product 5]
-- binding parameter [2] as [VARCHAR] - [P_100_5]
-- binding parameter [3] as [BIGINT]  - [5]

As you can see from the list of generated SQL statements, you can insert 3 entities with just one database sequence call. This way, the pooled and the pooled-lo optimizers allow you to reduce the number of database roundtrips, therefore reducing the overall transaction response time.

2.7.16. Derived Identifiers

Java Persistence 2.0 added support for derived identifiers which allow an entity to borrow the identifier from a many-to-one or one-to-one association.

Example 181. Derived identifier with @MapsId
@Entity(name = "Person")
public static class Person  {

	@Id
	private Long id;

	@NaturalId
	private String registrationNumber;

	public Person() {}

	public Person(String registrationNumber) {
		this.registrationNumber = registrationNumber;
	}

	//Getters and setters are omitted for brevity
}

@Entity(name = "PersonDetails")
public static class PersonDetails  {

	@Id
	private Long id;

	private String nickName;

	@OneToOne
	@MapsId
	private Person person;

	//Getters and setters are omitted for brevity
}

In the example above, the PersonDetails entity uses the id column for both the entity identifier and for the one-to-one association to the Person entity. The value of the PersonDetails entity identifier is "derived" from the identifier of its parent Person entity.

Example 182. Derived identifier with @MapsId persist example
doInJPA(this::entityManagerFactory, entityManager -> {
	Person person = new Person("ABC-123");
	person.setId(1L);
	entityManager.persist(person);

	PersonDetails personDetails = new PersonDetails();
	personDetails.setNickName("John Doe");
	personDetails.setPerson(person);

	entityManager.persist(personDetails);
});

doInJPA(this::entityManagerFactory, entityManager -> {
	PersonDetails personDetails = entityManager.find(PersonDetails.class, 1L);

	assertEquals("John Doe", personDetails.getNickName());
});

The @MapsId annotation can also reference columns from an @EmbeddedId identifier as well.

The previous example can also be mapped using @PrimaryKeyJoinColumn.

Example 183. Derived identifier @PrimaryKeyJoinColumn
@Entity(name = "Person")
public static class Person  {

	@Id
	private Long id;

	@NaturalId
	private String registrationNumber;

	public Person() {}

	public Person(String registrationNumber) {
		this.registrationNumber = registrationNumber;
	}

	//Getters and setters are omitted for brevity
}

@Entity(name = "PersonDetails")
public static class PersonDetails  {

	@Id
	private Long id;

	private String nickName;

	@OneToOne
	@PrimaryKeyJoinColumn
	private Person person;

	public void setPerson(Person person) {
		this.person = person;
		this.id = person.getId();
	}

	//Other getters and setters are omitted for brevity
}

Unlike @MapsId, the application developer is responsible for ensuring that the entity identifier and the many-to-one (or one-to-one) association are in sync, as you can see in the PersonDetails#setPerson method.

2.7.17. @RowId

If you annotate a given entity with the @RowId annotation and the underlying database supports fetching a record by ROWID (e.g. Oracle), then Hibernate can use the ROWID pseudo-column for CRUD operations.

Example 184. @RowId entity mapping
@Entity(name = "Product")
@RowId("ROWID")
public static class Product {

	@Id
	private Long id;

	@Column(name = "`name`")
	private String name;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}

Now, when fetching an entity and modifying it, Hibernate uses the ROWID pseudo-column for the UPDATE SQL statement.

Example 185. @RowId example
Product product = entityManager.find(Product.class, 1L);

product.setName("Smart phone");
SELECT
    p.id as id1_0_0_,
    p."name" as name2_0_0_,
    p."number" as number3_0_0_,
    p.ROWID as rowid_0_
FROM
    Product p
WHERE
    p.id = ?

-- binding parameter [1] as [BIGINT] - [1]

-- extracted value ([name2_0_0_] : [VARCHAR]) - [Mobile phone]
-- extracted value ([number3_0_0_] : [VARCHAR]) - [123-456-7890]
-- extracted ROWID value: AAAwkBAAEAAACP3AAA

UPDATE
    Product
SET
    "name" = ?,
    "number" = ?
WHERE
    ROWID = ?

-- binding parameter [1] as [VARCHAR] - [Smart phone]
-- binding parameter [2] as [VARCHAR] - [123-456-7890]
-- binding parameter [3] as ROWID     - [AAAwkBAAEAAACP3AAA]

2.8. Associations

Associations describe how two or more entities form a relationship based on a database joining semantics.

2.8.1. @ManyToOne

@ManyToOne is the most common association, having a direct equivalent in the relational database as well (e.g. foreign key), and so it establishes a relationship between a child entity and a parent.

Example 186. @ManyToOne association
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	//Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@ManyToOne
	@JoinColumn(name = "person_id",
			foreignKey = @ForeignKey(name = "PERSON_ID_FK")
	)
	private Person person;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    person_id BIGINT ,
    PRIMARY KEY ( id )
 )

ALTER TABLE Phone
ADD CONSTRAINT PERSON_ID_FK
FOREIGN KEY (person_id) REFERENCES Person

Each entity has a lifecycle of its own. Once the @ManyToOne association is set, Hibernate will set the associated database foreign key column.

Example 187. @ManyToOne association lifecycle
Person person = new Person();
entityManager.persist(person);

Phone phone = new Phone("123-456-7890");
phone.setPerson(person);
entityManager.persist(phone);

entityManager.flush();
phone.setPerson(null);
INSERT INTO Person ( id )
VALUES ( 1 )

INSERT INTO Phone ( number, person_id, id )
VALUES ( '123-456-7890', 1, 2 )

UPDATE Phone
SET    number = '123-456-7890',
       person_id = NULL
WHERE  id = 2

2.8.2. @OneToMany

The @OneToMany association links a parent entity with one or more child entities. If the @OneToMany doesn’t have a mirroring @ManyToOne association on the child side, the @OneToMany association is unidirectional. If there is a @ManyToOne association on the child side, the @OneToMany association is bidirectional and the application developer can navigate this relationship from both ends.

Unidirectional @OneToMany

When using a unidirectional @OneToMany association, Hibernate resorts to using a link table between the two joining entities.

Example 188. Unidirectional @OneToMany association
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person_Phone (
    Person_id BIGINT NOT NULL ,
    phones_id BIGINT NOT NULL
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    PRIMARY KEY ( id )
)

ALTER TABLE Person_Phone
ADD CONSTRAINT UK_9uhc5itwc9h5gcng944pcaslf
UNIQUE (phones_id)

ALTER TABLE Person_Phone
ADD CONSTRAINT FKr38us2n8g5p9rj0b494sd3391
FOREIGN KEY (phones_id) REFERENCES Phone

ALTER TABLE Person_Phone
ADD CONSTRAINT FK2ex4e4p7w1cj310kg2woisjl2
FOREIGN KEY (Person_id) REFERENCES Person

The @OneToMany association is by definition a parent association, regardless of whether it’s a unidirectional or a bidirectional one. Only the parent side of an association makes sense to cascade its entity state transitions to children.

Example 189. Cascading @OneToMany association
Person person = new Person();
Phone phone1 = new Phone("123-456-7890");
Phone phone2 = new Phone("321-654-0987");

person.getPhones().add(phone1);
person.getPhones().add(phone2);
entityManager.persist(person);
entityManager.flush();

person.getPhones().remove(phone1);
INSERT INTO Person
       ( id )
VALUES ( 1 )

INSERT INTO Phone
       ( number, id )
VALUES ( '123-456-7890', 2 )

INSERT INTO Phone
       ( number, id )
VALUES ( '321-654-0987', 3 )

INSERT INTO Person_Phone
       ( Person_id, phones_id )
VALUES ( 1, 2 )

INSERT INTO Person_Phone
       ( Person_id, phones_id )
VALUES ( 1, 3 )

DELETE FROM Person_Phone
WHERE  Person_id = 1

INSERT INTO Person_Phone
       ( Person_id, phones_id )
VALUES ( 1, 3 )

DELETE FROM Phone
WHERE  id = 2

When persisting the Person entity, the cascade will propagate the persist operation to the underlying Phone children as well. Upon removing a Phone from the phones collection, the association row is deleted from the link table, and the orphanRemoval attribute will trigger a Phone removal as well.

The unidirectional associations are not very efficient when it comes to removing child entities. In the example above, upon flushing the persistence context, Hibernate deletes all database rows from the link table (e.g. Person_Phone) that are associated with the parent Person entity and reinserts the ones that are still found in the @OneToMany collection.

On the other hand, a bidirectional @OneToMany association is much more efficient because the child entity controls the association.

Bidirectional @OneToMany

The bidirectional @OneToMany association also requires a @ManyToOne association on the child side. Although the Domain Model exposes two sides to navigate this association, behind the scenes, the relational database has only one foreign key for this relationship.

Every bidirectional association must have one owning side only (the child side), the other one being referred to as the inverse (or the mappedBy) side.

Example 190. @OneToMany association mappedBy the @ManyToOne side
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

	public void addPhone(Phone phone) {
		phones.add(phone);
		phone.setPerson(this);
	}

	public void removePhone(Phone phone) {
		phones.remove(phone);
		phone.setPerson(null);
	}
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@NaturalId
	@Column(name = "`number`", unique = true)
	private String number;

	@ManyToOne
	private Person person;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    person_id BIGINT ,
    PRIMARY KEY ( id )
)

ALTER TABLE Phone
ADD CONSTRAINT UK_l329ab0g4c1t78onljnxmbnp6
UNIQUE (number)

ALTER TABLE Phone
ADD CONSTRAINT FKmw13yfsjypiiq0i1osdkaeqpg
FOREIGN KEY (person_id) REFERENCES Person

Whenever a bidirectional association is formed, the application developer must make sure both sides are in-sync at all times.

The addPhone() and removePhone() are utility methods that synchronize both ends whenever a child element is added or removed.

Because the Phone class has a @NaturalId column (the phone number being unique), the equals() and the hashCode() can make use of this property, and so the removePhone() logic is reduced to the remove() Java Collection method.

Example 191. Bidirectional @OneToMany with an owner @ManyToOne side lifecycle
Person person = new Person();
Phone phone1 = new Phone("123-456-7890");
Phone phone2 = new Phone("321-654-0987");

person.addPhone(phone1);
person.addPhone(phone2);
entityManager.persist(person);
entityManager.flush();

person.removePhone(phone1);
INSERT INTO Person
       ( id )
VALUES ( 1 )

INSERT INTO Phone
       ( "number", person_id, id )
VALUES ( '123-456-7890', 1, 2 )

INSERT INTO Phone
       ( "number", person_id, id )
VALUES ( '321-654-0987', 1, 3 )

DELETE FROM Phone
WHERE  id = 2

Unlike the unidirectional @OneToMany, the bidirectional association is much more efficient when managing the collection persistence state. Every element removal only requires a single update (in which the foreign key column is set to NULL), and, if the child entity lifecycle is bound to its owning parent so that the child cannot exist without its parent, then we can annotate the association with the orphanRemoval attribute and dissociating the child will trigger a delete statement on the actual child table row as well.

2.8.3. @OneToOne

The @OneToOne association can either be unidirectional or bidirectional. A unidirectional association follows the relational database foreign key semantics, the client-side owning the relationship. A bidirectional association features a mappedBy @OneToOne parent side too.

Unidirectional @OneToOne
Example 192. Unidirectional @OneToOne
@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OneToOne
	@JoinColumn(name = "details_id")
	private PhoneDetails details;

	//Getters and setters are omitted for brevity

}

@Entity(name = "PhoneDetails")
public static class PhoneDetails {

	@Id
	@GeneratedValue
	private Long id;

	private String provider;

	private String technology;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    details_id BIGINT ,
    PRIMARY KEY ( id )
)

CREATE TABLE PhoneDetails (
    id BIGINT NOT NULL ,
    provider VARCHAR(255) ,
    technology VARCHAR(255) ,
    PRIMARY KEY ( id )
)

ALTER TABLE Phone
ADD CONSTRAINT FKnoj7cj83ppfqbnvqqa5kolub7
FOREIGN KEY (details_id) REFERENCES PhoneDetails

From a relational database point of view, the underlying schema is identical to the unidirectional @ManyToOne association, as the client-side controls the relationship based on the foreign key column.

But then, it’s unusual to consider the Phone as a client-side and the PhoneDetails as the parent-side because the details cannot exist without an actual phone. A much more natural mapping would be the Phone were the parent-side, therefore pushing the foreign key into the PhoneDetails table. This mapping requires a bidirectional @OneToOne association as you can see in the following example:

Bidirectional @OneToOne
Example 193. Bidirectional @OneToOne
@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OneToOne(
		mappedBy = "phone",
		cascade = CascadeType.ALL,
		orphanRemoval = true,
		fetch = FetchType.LAZY
	)
	private PhoneDetails details;

	//Getters and setters are omitted for brevity

	public void addDetails(PhoneDetails details) {
		details.setPhone(this);
		this.details = details;
	}

	public void removeDetails() {
		if (details != null) {
			details.setPhone(null);
			this.details = null;
		}
	}
}

@Entity(name = "PhoneDetails")
public static class PhoneDetails {

	@Id
	@GeneratedValue
	private Long id;

	private String provider;

	private String technology;

	@OneToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "phone_id")
	private Phone phone;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE PhoneDetails (
    id BIGINT NOT NULL ,
    provider VARCHAR(255) ,
    technology VARCHAR(255) ,
    phone_id BIGINT ,
    PRIMARY KEY ( id )
)

ALTER TABLE PhoneDetails
ADD CONSTRAINT FKeotuev8ja8v0sdh29dynqj05p
FOREIGN KEY (phone_id) REFERENCES Phone

This time, the PhoneDetails owns the association, and, like any bidirectional association, the parent-side can propagate its lifecycle to the child-side through cascading.

Example 194. Bidirectional @OneToOne lifecycle
Phone phone = new Phone("123-456-7890");
PhoneDetails details = new PhoneDetails("T-Mobile", "GSM");

phone.addDetails(details);
entityManager.persist(phone);
INSERT INTO Phone ( number, id )
VALUES ( '123-456-7890', 1 )

INSERT INTO PhoneDetails ( phone_id, provider, technology, id )
VALUES ( 1, 'T-Mobile', 'GSM', 2 )

When using a bidirectional @OneToOne association, Hibernate enforces the unique constraint upon fetching the child-side. If there are more than one children associated with the same parent, Hibernate will throw a org.hibernate.exception.ConstraintViolationException. Continuing the previous example, when adding another PhoneDetails, Hibernate validates the uniqueness constraint when reloading the Phone object.

Example 195. Bidirectional @OneToOne unique constraint
PhoneDetails otherDetails = new PhoneDetails("T-Mobile", "CDMA");
otherDetails.setPhone(phone);
entityManager.persist(otherDetails);
entityManager.flush();
entityManager.clear();

//throws jakarta.persistence.PersistenceException: org.hibernate.HibernateException: More than one row with the given identifier was found: 1
phone = entityManager.find(Phone.class, phone.getId());
Bidirectional @OneToOne lazy association

Although you might annotate the parent-side association to be fetched lazily, Hibernate cannot honor this request since it cannot know whether the association is null or not.

The only way to figure out whether there is an associated record on the child side is to fetch the child association using a secondary query. Because this can lead to N+1 query issues, it’s much more efficient to use unidirectional @OneToOne associations with the @MapsId annotation in place.

However, if you really need to use a bidirectional association and want to make sure that this is always going to be fetched lazily, then you need to enable lazy state initialization bytecode enhancement and use the @LazyToOne annotation as well.

Example 196. Bidirectional @OneToOne lazy parent-side association
@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OneToOne(
		mappedBy = "phone",
		cascade = CascadeType.ALL,
		orphanRemoval = true,
		fetch = FetchType.LAZY
	)
	@LazyToOne(LazyToOneOption.NO_PROXY)
	private PhoneDetails details;

	//Getters and setters are omitted for brevity

	public void addDetails(PhoneDetails details) {
		details.setPhone(this);
		this.details = details;
	}

	public void removeDetails() {
		if (details != null) {
			details.setPhone(null);
			this.details = null;
		}
	}
}

@Entity(name = "PhoneDetails")
public static class PhoneDetails {

	@Id
	@GeneratedValue
	private Long id;

	private String provider;

	private String technology;

	@OneToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "phone_id")
	private Phone phone;

	//Getters and setters are omitted for brevity

}

For more about how to enable Bytecode enhancement, see the Bytecode Enhancement chapter.

2.8.4. @ManyToMany

The @ManyToMany association requires a link table that joins two entities. Like the @OneToMany association, @ManyToMany can be either unidirectional or bidirectional.

Unidirectional @ManyToMany
Example 197. Unidirectional @ManyToMany
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
	private List<Address> addresses = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Address")
public static class Address {

	@Id
	@GeneratedValue
	private Long id;

	private String street;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Address (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    street VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person_Address (
    Person_id BIGINT NOT NULL ,
    addresses_id BIGINT NOT NULL
)

ALTER TABLE Person_Address
ADD CONSTRAINT FKm7j0bnabh2yr0pe99il1d066u
FOREIGN KEY (addresses_id) REFERENCES Address

ALTER TABLE Person_Address
ADD CONSTRAINT FKba7rc9qe2vh44u93u0p2auwti
FOREIGN KEY (Person_id) REFERENCES Person

Just like with unidirectional @OneToMany associations, the link table is controlled by the owning side.

When an entity is removed from the @ManyToMany collection, Hibernate simply deletes the joining record in the link table. Unfortunately, this operation requires removing all entries associated with a given parent and recreating the ones that are listed in the current running persistent context.

Example 198. Unidirectional @ManyToMany lifecycle
Person person1 = new Person();
Person person2 = new Person();

Address address1 = new Address("12th Avenue", "12A");
Address address2 = new Address("18th Avenue", "18B");

person1.getAddresses().add(address1);
person1.getAddresses().add(address2);

person2.getAddresses().add(address1);

entityManager.persist(person1);
entityManager.persist(person2);

entityManager.flush();

person1.getAddresses().remove(address1);
INSERT INTO Person ( id )
VALUES ( 1 )

INSERT INTO Address ( number, street, id )
VALUES ( '12A', '12th Avenue', 2 )

INSERT INTO Address ( number, street, id )
VALUES ( '18B', '18th Avenue', 3 )

INSERT INTO Person ( id )
VALUES ( 4 )

INSERT INTO Person_Address ( Person_id, addresses_id )
VALUES ( 1, 2 )
INSERT INTO Person_Address ( Person_id, addresses_id )
VALUES ( 1, 3 )
INSERT INTO Person_Address ( Person_id, addresses_id )
VALUES ( 4, 2 )

DELETE FROM Person_Address
WHERE  Person_id = 1

INSERT INTO Person_Address ( Person_id, addresses_id )
VALUES ( 1, 3 )

For @ManyToMany associations, the REMOVE entity state transition doesn’t make sense to be cascaded because it will propagate beyond the link table. Since the other side might be referenced by other entities on the parent-side, the automatic removal might end up in a ConstraintViolationException.

For example, if @ManyToMany(cascade = CascadeType.ALL) was defined and the first person would be deleted, Hibernate would throw an exception because another person is still associated with the address that’s being deleted.

Person person1 = entityManager.find(Person.class, personId);
entityManager.remove(person1);

Caused by: jakarta.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
Caused by: java.sql.SQLIntegrityConstraintViolationException: integrity constraint violation: foreign key no action; FKM7J0BNABH2YR0PE99IL1D066U table: PERSON_ADDRESS

By simply removing the parent-side, Hibernate can safely remove the associated link records as you can see in the following example:

Example 199. Unidirectional @ManyToMany entity removal
Person person1 = entityManager.find(Person.class, personId);
entityManager.remove(person1);
DELETE FROM Person_Address
WHERE  Person_id = 1

DELETE FROM Person
WHERE  id = 1
Bidirectional @ManyToMany

A bidirectional @ManyToMany association has an owning and a mappedBy side. To preserve synchronicity between both sides, it’s good practice to provide helper methods for adding or removing child entities.

Example 200. Bidirectional @ManyToMany
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@NaturalId
	private String registrationNumber;

	@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
	private List<Address> addresses = new ArrayList<>();

	//Getters and setters are omitted for brevity

	public void addAddress(Address address) {
		addresses.add(address);
		address.getOwners().add(this);
	}

	public void removeAddress(Address address) {
		addresses.remove(address);
		address.getOwners().remove(this);
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Person person = (Person) o;
		return Objects.equals(registrationNumber, person.registrationNumber);
	}

	@Override
	public int hashCode() {
		return Objects.hash(registrationNumber);
	}
}

@Entity(name = "Address")
public static class Address {

	@Id
	@GeneratedValue
	private Long id;

	private String street;

	@Column(name = "`number`")
	private String number;

	private String postalCode;

	@ManyToMany(mappedBy = "addresses")
	private List<Person> owners = new ArrayList<>();

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Address address = (Address) o;
		return Objects.equals(street, address.street) &&
				Objects.equals(number, address.number) &&
				Objects.equals(postalCode, address.postalCode);
	}

	@Override
	public int hashCode() {
		return Objects.hash(street, number, postalCode);
	}
}
CREATE TABLE Address (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    postalCode VARCHAR(255) ,
    street VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person (
    id BIGINT NOT NULL ,
    registrationNumber VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person_Address (
    owners_id BIGINT NOT NULL ,
    addresses_id BIGINT NOT NULL
)

ALTER TABLE Person
ADD CONSTRAINT UK_23enodonj49jm8uwec4i7y37f
UNIQUE (registrationNumber)

ALTER TABLE Person_Address
ADD CONSTRAINT FKm7j0bnabh2yr0pe99il1d066u
FOREIGN KEY (addresses_id) REFERENCES Address

ALTER TABLE Person_Address
ADD CONSTRAINT FKbn86l24gmxdv2vmekayqcsgup
FOREIGN KEY (owners_id) REFERENCES Person

With the helper methods in place, the synchronicity management can be simplified, as you can see in the following example:

Example 201. Bidirectional @ManyToMany lifecycle
Person person1 = new Person("ABC-123");
Person person2 = new Person("DEF-456");

Address address1 = new Address("12th Avenue", "12A", "4005A");
Address address2 = new Address("18th Avenue", "18B", "4007B");

person1.addAddress(address1);
person1.addAddress(address2);

person2.addAddress(address1);

entityManager.persist(person1);
entityManager.persist(person2);

entityManager.flush();

person1.removeAddress(address1);
INSERT INTO Person ( registrationNumber, id )
VALUES ( 'ABC-123', 1 )

INSERT INTO Address ( number, postalCode, street, id )
VALUES ( '12A', '4005A', '12th Avenue', 2 )

INSERT INTO Address ( number, postalCode, street, id )
VALUES ( '18B', '4007B', '18th Avenue', 3 )

INSERT INTO Person ( registrationNumber, id )
VALUES ( 'DEF-456', 4 )

INSERT INTO Person_Address ( owners_id, addresses_id )
VALUES ( 1, 2 )

INSERT INTO Person_Address ( owners_id, addresses_id )
VALUES ( 1, 3 )

INSERT INTO Person_Address ( owners_id, addresses_id )
VALUES ( 4, 2 )

DELETE FROM Person_Address
WHERE  owners_id = 1

INSERT INTO Person_Address ( owners_id, addresses_id )
VALUES ( 1, 3 )

If a bidirectional @OneToMany association performs better when removing or changing the order of child elements, the @ManyToMany relationship cannot benefit from such an optimization because the foreign key side is not in control. To overcome this limitation, the link table must be directly exposed and the @ManyToMany association split into two bidirectional @OneToMany relationships.

To most natural @ManyToMany association follows the same logic employed by the database schema, and the link table has an associated entity which controls the relationship for both sides that need to be joined.

Both the Person and the Address have a mappedBy @OneToMany side, while the PersonAddress owns the person and the address @ManyToOne associations. Because this mapping is formed out of two bidirectional associations, the helper methods are even more relevant.

The aforementioned example uses a Hibernate-specific mapping for the link entity since Jakarta Persistence doesn’t allow building a composite identifier out of multiple @ManyToOne associations.

For more details, see the composite identifiers with associations section.

The entity state transitions are better managed than in the previous bidirectional @ManyToMany case.

There is only one delete statement executed because, this time, the association is controlled by the @ManyToOne side which only has to monitor the state of the underlying foreign key relationship to trigger the right DML statement.

2.8.5. @NotFound

When dealing with associations which are not enforced by a physical foreign-key, it is possible for a non-null foreign-key value to point to a non-existent value on the associated entity’s table.

Not enforcing physical foreign-keys at the database level is highly discouraged.

Hibernate provides support for such models using the @NotFound annotation, which accepts a NotFoundAction value which indicates how Hibernate should behave when such broken foreign-keys are encountered -

EXCEPTION

(default) Hibernate will throw an exception (FetchNotFoundException)

IGNORE

the association will be treated as null

Both @NotFound(IGNORE) and @NotFound(EXCEPTION) cause Hibernate to assume that there is no physical foreign-key.

@ManyToOne and @OneToOne associations annotated with @NotFound are always fetched eagerly even if the fetch strategy is set to FetchType.LAZY.

If the application itself manages the referential integrity and can guarantee that there are no broken foreign-keys, jakarta.persistence.ForeignKey(NO_CONSTRAINT) can be used instead. This will force Hibernate to not export physical foreign-keys, but still behave as if there is in terms of avoiding the downsides to @NotFound.

Considering the following City and Person entity mappings:

Example 204. @NotFound mapping example
@Entity(name = "Person")
@Table(name = "Person")
public static class Person {

	@Id
	private Integer id;
	private String name;

	@ManyToOne
	@NotFound(action = NotFoundAction.IGNORE)
	@JoinColumn(name = "city_fk", referencedColumnName = "id")
	private City city;

	//Getters and setters are omitted for brevity

}

@Entity(name = "City")
@Table(name = "City")
public static class City implements Serializable {

	@Id
	private Integer id;

	private String name;

	//Getters and setters are omitted for brevity

}

If we have the following entities in our database:

Example 205. @NotFound persist example
City newYork = new City( 1, "New York" );
entityManager.persist( newYork );

Person person = new Person( 1, "John Doe", newYork );
entityManager.persist( person );

When loading the Person entity, Hibernate is able to locate the associated City parent entity:

Example 206. @NotFound - find existing entity example
Person person = entityManager.find(Person.class, 1);
assertEquals("New York", person.getCity().getName());

However, if we break the foreign-key:

Example 207. Break the foreign-key
// the database allows this because there is no physical foreign-key
em.createMutationQuery( "delete City" ).executeUpdate();

Hibernate is not going to throw any exception, and it will assign a value of null for the non-existing City entity reference:

Example 208. @NotFound - find non-existing City example
Person person = entityManager.find(Person.class, 1);

assertNull(null, person.getCity());

@NotFound also affects how the association is treated as "implicit joins" in HQL and Criteria. When there is a physical foreign-key, Hibernate can safely assume that the value in the foreign-key’s key-column(s) will match the value in the target-column(s) because the database makes sure that is the case. However, @NotFound forces Hibernate to perform a physical join for implicit joins when it might not be needed otherwise.

Using the Person / City model, consider the query from Person p where p.city.id is null.

Normally Hibernate would not need the join between the Person table and the City table because a physical foreign-key would ensure that any non-null value in the Person.cityName column has a matching non-null value in the City.name column.

However, with @NotFound mappings it is possible to have a broken association because there is no physical foreign-key enforcing the relation. As seen in Break the foreign-key, the Person.cityName column for John Doe has been changed from "New York" to "Atlantis" even though there is no City in the database named "Atlantis". Hibernate is not able to trust the referring foreign-key value ("Atlantis") has a matching target value, so it must join to the City table to resolve the city.id value.

Example 209. Implicit join example
final List<Person> nullResults = entityManager
		.createSelectionQuery( "from Person p where p.city.id is null", Person.class )
		.list();
assertThat( nullResults ).isEmpty();

final List<Person> nonNullResults = entityManager
		.createSelectionQuery( "from Person p where p.city.id is not null", Person.class )
		.list();
assertThat( nonNullResults ).isEmpty();

Neither result includes a match for "John Doe" because the inner-join filters out that row.

Hibernate does support a means to refer specifically to the key column (Person.cityName) in a query using the special fk(..) function. E.g.

Example 210. Implicit join example
final List<String> nullResults = entityManager
		.createSelectionQuery( "select p.name from Person p where fk( p.city ) is null", String.class )
		.list();

assertThat( nullResults ).isEmpty();

final List<String> nonNullResults = entityManager
		.createSelectionQuery( "select p.name from Person p where fk( p.city ) is not null", String.class )
		.list();
assertThat( nonNullResults ).hasSize( 1 );
assertThat( nonNullResults.get( 0 ) ).isEqualTo( "John Doe" );

2.8.6. @Any mapping

The @Any mapping is useful to emulate a unidirectional @ManyToOne association when there can be multiple target entities.

Because the @Any mapping defines a polymorphic association to classes from multiple tables, this association type requires the FK column which provides the associated parent identifier and a metadata information for the associated entity type.

This is not the usual way of mapping polymorphic associations and you should use this only in special cases (e.g. audit logs, user session data, etc).

To map such an association, Hibernate needs to understand 3 things:

  1. The column and mapping for the discriminator

  2. The column and mapping for the key

  3. The mapping between discriminator values and entity classes

The discriminator

The discriminator of an any-style association holds the value that indicates which entity is referred to by a row.

Its "column" can be specified with either @Column or @Formula. The mapping type can be influenced by any of:

  1. @AnyDiscriminator allows re-using the DiscriminatorType simplified mappings from Jakarta Persistence for the common cases

  2. @JavaType

  3. @JdbcType

  4. @JdbcTypeCode

The key

The key of an any-style association holds the matching key for the row

Its "column" can be specified with either @JoinColumn (@JoinFormula not supported). The mapping type can be influenced by any of:

  1. @AnyKeyJavaClass

  2. @AnyKeyJavaType

  3. @AnyKeyJdbcType

  4. @AnyKeyJdbcTypeCode

The discriminator value mappings

@AnyDiscriminatorValue is used to map the discriminator values to the corresponding entity classes

2.8.7. Example using @Any mapping

For this example, consider the following Property class hierarchy:

Example 211. Property class hierarchy
public interface Property<T> {

    String getName();

    T getValue();
}


@Entity
@Table(name="integer_property")
public class IntegerProperty implements Property<Integer> {

    @Id
    private Long id;

    @Column(name = "`name`")
    private String name;

    @Column(name = "`value`")
    private Integer value;

    @Override
    public String getName() {
        return name;
    }

    @Override
    public Integer getValue() {
        return value;
    }

    //Getters and setters omitted for brevity
}


@Entity
@Table(name="string_property")
public class StringProperty implements Property<String> {

    @Id
    private Long id;

    @Column(name = "`name`")
    private String name;

    @Column(name = "`value`")
    private String value;

    @Override
    public String getName() {
        return name;
    }

    @Override
    public String getValue() {
        return value;
    }

    //Getters and setters omitted for brevity
}

A PropertyHolder entity defines an attribute of type Property:

Example 212. @Any mapping usage
@Entity
@Table(name = "property_holder")
public class PropertyHolder {

    @Id
    private Long id;

    @Any
    @AnyDiscriminator(DiscriminatorType.STRING)
    @AnyDiscriminatorValue(discriminator = "S", entity = StringProperty.class)
    @AnyDiscriminatorValue(discriminator = "I", entity = IntegerProperty.class)
    @AnyKeyJavaClass(Long.class)
    @Column(name = "property_type")
    @JoinColumn(name = "property_id")
    private Property property;

    //Getters and setters are omitted for brevity

}
CREATE TABLE property_holder (
    id BIGINT NOT NULL,
    property_type VARCHAR(255),
    property_id BIGINT,
    PRIMARY KEY ( id )
)

PropertyHolder#property can refer to either StringProperty or IntegerProperty references, as indicated by the associated discriminator according to the @DiscriminatorValue annotations.

As you can see, there are two columns used to reference a Property instance: property_id and property_type. The property_id is used to match the id column of either the string_property or integer_property tables, while the property_type is used to match the string_property or the integer_property table.

To see the @Any annotation in action, consider the next examples.

If we persist an IntegerProperty as well as a StringProperty entity, and associate the StringProperty entity with a PropertyHolder, Hibernate will generate the following SQL queries:

Example 213. @Any mapping persist example
IntegerProperty ageProperty = new IntegerProperty();
ageProperty.setId(1L);
ageProperty.setName("age");
ageProperty.setValue(23);

session.persist(ageProperty);

StringProperty nameProperty = new StringProperty();
nameProperty.setId(1L);
nameProperty.setName("name");
nameProperty.setValue("John Doe");

session.persist(nameProperty);

PropertyHolder namePropertyHolder = new PropertyHolder();
namePropertyHolder.setId(1L);
namePropertyHolder.setProperty(nameProperty);

session.persist(namePropertyHolder);
INSERT INTO integer_property
       ( "name", "value", id )
VALUES ( 'age', 23, 1 )

INSERT INTO string_property
       ( "name", "value", id )
VALUES ( 'name', 'John Doe', 1 )

INSERT INTO property_holder
       ( property_type, property_id, id )
VALUES ( 'S', 1, 1 )

When fetching the PropertyHolder entity and navigating its property association, Hibernate will fetch the associated StringProperty entity like this:

Example 214. @Any mapping query example
PropertyHolder propertyHolder = session.get(PropertyHolder.class, 1L);

assertEquals("name", propertyHolder.getProperty().getName());
assertEquals("John Doe", propertyHolder.getProperty().getValue());
SELECT ph.id AS id1_1_0_,
       ph.property_type AS property2_1_0_,
       ph.property_id AS property3_1_0_
FROM   property_holder ph
WHERE  ph.id = 1


SELECT sp.id AS id1_2_0_,
       sp."name" AS name2_2_0_,
       sp."value" AS value3_2_0_
FROM   string_property sp
WHERE  sp.id = 1
Using meta-annotations

As mentioned in Mapping basic values, Hibernate’s ANY-related annotations can be composed using meta-annotations to re-use ANY mapping details.

Looking back at @Any mapping usage, we can see how cumbersome it would be to duplicate that information every time Property is mapped in the domain model. This description can also be moved into a single annotation that we can apply in each usage.

Example 215. @Any mapping usage
@Entity
@Table(name = "property_holder")
public class PropertyHolder2 {

    @Id
    private Long id;

    @Any
    @PropertyDiscriminationDef
    @Column(name = "property_type")
    @JoinColumn(name = "property_id")
    private Property property;

    //Getters and setters are omitted for brevity

}

Though the mapping has been "simplified", the mapping works exactly as shown in @Any mapping usage.

@ManyToAny mapping

While the @Any mapping is useful to emulate a @ManyToOne association when there can be multiple target entities, to emulate a @OneToMany association, the @ManyToAny annotation must be used.

The mapping details are the same between @Any and @ManyToAny except for:

  1. The use of @ManyToAny instead of @Any

  2. The use of @JoinTable, @JoinTable#joinColumns and @JoinTable#inverseJoinColumns instead of just @JoinColumn

In the following example, the PropertyRepository entity has a collection of Property entities.

The repository_properties link table holds the associations between PropertyRepository and Property entities.

Example 216. @ManyToAny mapping usage
@Entity
@Table(name = "property_repository")
public class PropertyRepository {

    @Id
    private Long id;

    @ManyToAny
    @AnyDiscriminator(DiscriminatorType.STRING)
    @Column(name = "property_type")
    @AnyKeyJavaClass(Long.class)
    @AnyDiscriminatorValue(discriminator = "S", entity = StringProperty.class)
    @AnyDiscriminatorValue(discriminator = "I", entity = IntegerProperty.class)
    @Cascade({ org.hibernate.annotations.CascadeType.ALL })
    @JoinTable(name = "repository_properties",
            joinColumns = @JoinColumn(name = "repository_id"),
            inverseJoinColumns = @JoinColumn(name = "property_id")
   )
    private List<Property<?>> properties = new ArrayList<>();

    //Getters and setters are omitted for brevity

}
CREATE TABLE property_repository (
    id BIGINT NOT NULL,
    PRIMARY KEY ( id )
)

CREATE TABLE repository_properties (
    repository_id BIGINT NOT NULL,
    property_type VARCHAR(255),
    property_id BIGINT NOT NULL
)

To see the @ManyToAny annotation in action, consider the next examples.

If we persist an IntegerProperty as well as a StringProperty entity, and associate both of them with a PropertyRepository parent entity, Hibernate will generate the following SQL queries:

Example 217. @ManyToAny mapping persist example
IntegerProperty ageProperty = new IntegerProperty();
ageProperty.setId(1L);
ageProperty.setName("age");
ageProperty.setValue(23);

session.persist(ageProperty);

StringProperty nameProperty = new StringProperty();
nameProperty.setId(1L);
nameProperty.setName("name");
nameProperty.setValue("John Doe");

session.persist(nameProperty);

PropertyRepository propertyRepository = new PropertyRepository();
propertyRepository.setId(1L);

propertyRepository.getProperties().add(ageProperty);
propertyRepository.getProperties().add(nameProperty);

session.persist(propertyRepository);
INSERT INTO integer_property
       ( "name", "value", id )
VALUES ( 'age', 23, 1 )

INSERT INTO string_property
       ( "name", "value", id )
VALUES ( 'name', 'John Doe', 1 )

INSERT INTO property_repository ( id )
VALUES ( 1 )

INSERT INTO repository_properties
    ( repository_id , property_type , property_id )
VALUES
    ( 1 , 'I' , 1 )

When fetching the PropertyRepository entity and navigating its properties association, Hibernate will fetch the associated IntegerProperty and StringProperty entities like this:

Example 218. @ManyToAny mapping query example
PropertyRepository propertyRepository = session.get(PropertyRepository.class, 1L);

assertEquals(2, propertyRepository.getProperties().size());

for(Property property : propertyRepository.getProperties()) {
    assertNotNull(property.getValue());
}
SELECT pr.id AS id1_1_0_
FROM   property_repository pr
WHERE  pr.id = 1

SELECT ip.id AS id1_0_0_ ,
       ip."name" AS name2_0_0_ ,
       ip."value" AS value3_0_0_
FROM   integer_property ip
WHERE  ip.id = 1

SELECT sp.id AS id1_3_0_ ,
       sp."name" AS name2_3_0_ ,
       sp."value" AS value3_3_0_
FROM   string_property sp
WHERE  sp.id = 1

2.8.8. @JoinFormula mapping

The @JoinFormula annotation is used to customize the join between a child Foreign Key and a parent row Primary Key.

Example 219. @JoinFormula mapping usage
@Entity(name = "User")
@Table(name = "users")
public static class User {

	@Id
	private Long id;

	private String firstName;

	private String lastName;

	private String phoneNumber;

	@ManyToOne
	@JoinFormula("REGEXP_REPLACE(phoneNumber, '\\+(\\d+)-.*', '\\1')::int")
	private Country country;

	//Getters and setters omitted for brevity

}

@Entity(name = "Country")
@Table(name = "countries")
public static class Country {

	@Id
	private Integer id;

	private String name;

	//Getters and setters, equals and hashCode methods omitted for brevity

}
CREATE TABLE countries (
    id int4 NOT NULL,
    name VARCHAR(255),
    PRIMARY KEY ( id )
)

CREATE TABLE users (
    id int8 NOT NULL,
    firstName VARCHAR(255),
    lastName VARCHAR(255),
    phoneNumber VARCHAR(255),
    PRIMARY KEY ( id )
)

The country association in the User entity is mapped by the country identifier provided by the phoneNumber property.

Considering we have the following entities:

Example 220. @JoinFormula mapping usage
Country US = new Country();
US.setId(1);
US.setName("United States");

Country Romania = new Country();
Romania.setId(40);
Romania.setName("Romania");

doInJPA(this::entityManagerFactory, entityManager -> {
	entityManager.persist(US);
	entityManager.persist(Romania);
});

doInJPA(this::entityManagerFactory, entityManager -> {
	User user1 = new User();
	user1.setId(1L);
	user1.setFirstName("John");
	user1.setLastName("Doe");
	user1.setPhoneNumber("+1-234-5678");
	entityManager.persist(user1);

	User user2 = new User();
	user2.setId(2L);
	user2.setFirstName("Vlad");
	user2.setLastName("Mihalcea");
	user2.setPhoneNumber("+40-123-4567");
	entityManager.persist(user2);
});

When fetching the User entities, the country property is mapped by the @JoinFormula expression:

Example 221. @JoinFormula mapping usage
doInJPA(this::entityManagerFactory, entityManager -> {
	log.info("Fetch User entities");

	User john = entityManager.find(User.class, 1L);
	assertEquals(US, john.getCountry());

	User vlad = entityManager.find(User.class, 2L);
	assertEquals(Romania, vlad.getCountry());
});
-- Fetch User entities

SELECT
    u.id as id1_1_0_,
    u.firstName as firstNam2_1_0_,
    u.lastName as lastName3_1_0_,
    u.phoneNumber as phoneNum4_1_0_,
    REGEXP_REPLACE(u.phoneNumber, '\+(\d+)-.*', '\1')::int as formula1_0_,
    c.id as id1_0_1_,
    c.name as name2_0_1_
FROM
    users u
LEFT OUTER JOIN
    countries c
        ON REGEXP_REPLACE(u.phoneNumber, '\+(\d+)-.*', '\1')::int = c.id
WHERE
    u.id=?

-- binding parameter [1] as [BIGINT] - [1]

SELECT
    u.id as id1_1_0_,
    u.firstName as firstNam2_1_0_,
    u.lastName as lastName3_1_0_,
    u.phoneNumber as phoneNum4_1_0_,
    REGEXP_REPLACE(u.phoneNumber, '\+(\d+)-.*', '\1')::int as formula1_0_,
    c.id as id1_0_1_,
    c.name as name2_0_1_
FROM
    users u
LEFT OUTER JOIN
    countries c
        ON REGEXP_REPLACE(u.phoneNumber, '\+(\d+)-.*', '\1')::int = c.id
WHERE
    u.id=?

-- binding parameter [1] as [BIGINT] - [2]

Therefore, the @JoinFormula annotation is used to define a custom join association between the parent-child association.

2.8.9. @JoinColumnOrFormula mapping

The @JoinColumnOrFormula annotation is used to customize the join between a child Foreign Key and a parent row Primary Key when we need to take into consideration a column value as well as a @JoinFormula.

Example 222. @JoinColumnOrFormula mapping usage
@Entity(name = "User")
@Table(name = "users")
public static class User {

	@Id
	private Long id;

	private String firstName;

	private String lastName;

	private String language;

	@ManyToOne
	@JoinColumnOrFormula(column =
		@JoinColumn(
			name = "language",
			referencedColumnName = "primaryLanguage",
			insertable = false,
			updatable = false
		)
	)
	@JoinColumnOrFormula(formula =
		@JoinFormula(
			value = "true",
			referencedColumnName = "is_default"
		)
	)
	private Country country;

	//Getters and setters omitted for brevity

}

@Entity(name = "Country")
@Table(name = "countries")
public static class Country implements Serializable {

	@Id
	private Integer id;

	private String name;

	private String primaryLanguage;

	@Column(name = "is_default")
	private boolean _default;

	//Getters and setters, equals and hashCode methods omitted for brevity

}
CREATE TABLE countries (
    id INTEGER NOT NULL,
    is_default boolean,
    name VARCHAR(255),
    primaryLanguage VARCHAR(255),
    PRIMARY KEY ( id )
)

CREATE TABLE users (
    id BIGINT NOT NULL,
    firstName VARCHAR(255),
    language VARCHAR(255),
    lastName VARCHAR(255),
    PRIMARY KEY ( id )
)

The country association in the User entity is mapped by the language property value and the associated Country is_default column value.

Considering we have the following entities:

Example 223. @JoinColumnOrFormula persist example
Country US = new Country();
US.setId(1);
US.setDefault(true);
US.setPrimaryLanguage("English");
US.setName("United States");

Country Romania = new Country();
Romania.setId(40);
Romania.setDefault(true);
Romania.setName("Romania");
Romania.setPrimaryLanguage("Romanian");

doInJPA(this::entityManagerFactory, entityManager -> {
	entityManager.persist(US);
	entityManager.persist(Romania);
});

doInJPA(this::entityManagerFactory, entityManager -> {
	User user1 = new User();
	user1.setId(1L);
	user1.setFirstName("John");
	user1.setLastName("Doe");
	user1.setLanguage("English");
	entityManager.persist(user1);

	User user2 = new User();
	user2.setId(2L);
	user2.setFirstName("Vlad");
	user2.setLastName("Mihalcea");
	user2.setLanguage("Romanian");
	entityManager.persist(user2);

});

When fetching the User entities, the country property is mapped by the @JoinColumnOrFormula expression:

Example 224. @JoinColumnOrFormula fetching example
doInJPA(this::entityManagerFactory, entityManager -> {
	log.info("Fetch User entities");

	User john = entityManager.find(User.class, 1L);
	assertEquals(US, john.getCountry());

	User vlad = entityManager.find(User.class, 2L);
	assertEquals(Romania, vlad.getCountry());
});
SELECT
    u.id as id1_1_0_,
    u.language as language3_1_0_,
    u.firstName as firstNam2_1_0_,
    u.lastName as lastName4_1_0_,
    1 as formula1_0_,
    c.id as id1_0_1_,
    c.is_default as is_defau2_0_1_,
    c.name as name3_0_1_,
    c.primaryLanguage as primaryL4_0_1_
FROM
    users u
LEFT OUTER JOIN
    countries c
        ON u.language = c.primaryLanguage
        AND 1 = c.is_default
WHERE
    u.id = ?

-- binding parameter [1] as [BIGINT] - [1]

SELECT
    u.id as id1_1_0_,
    u.language as language3_1_0_,
    u.firstName as firstNam2_1_0_,
    u.lastName as lastName4_1_0_,
    1 as formula1_0_,
    c.id as id1_0_1_,
    c.is_default as is_defau2_0_1_,
    c.name as name3_0_1_,
    c.primaryLanguage as primaryL4_0_1_
FROM
    users u
LEFT OUTER JOIN
    countries c
        ON u.language = c.primaryLanguage
        AND 1 = c.is_default
WHERE
    u.id = ?

-- binding parameter [1] as [BIGINT] - [2]

Therefore, the @JoinColumnOrFormula annotation is used to define a custom join association between the parent-child association.

2.9. Collections

Hibernate supports mapping collections (java.util.Collection and java.util.Map subtypes) in a variety of ways.

Hibernate even allows mapping a collection as @Basic, but that should generally be avoided. See Collections as basic value type for details of such a mapping.

This section is limited to discussing @ElementCollection, @OneToMany and @ManyToMany.

Two entities cannot share a reference to the same collection instance.

Collection-valued properties do not support null value semantics.

Collections cannot be nested, meaning Hibernate does not support mapping List<List<?>>, for example.

Embeddables which are used as a collection element, Map value or Map key may not themselves define collections

2.9.1. Collection Semantics

The semantics of a collection describes how to handle the collection, including

  • the collection subtype to use - java.util.List, java.util.Set, java.util.SortedSet, etc.

  • how to access elements of the collection

  • how to create instances of the collection - both "raw" and "wrapper" forms. See Wrappers

Hibernate supports the following semantics:

ARRAY

Object and primitive arrays. See Mapping Arrays.

BAG

A collection that may contain duplicate entries and has no defined ordering. See Mapping Collections.

ID_BAG

A bag that defines a per-element identifier to uniquely identify elements in the collection. See Mapping Collections.

LIST

Follows the semantics defined by java.util.List. See Ordered Lists.

SET

Follows the semantics defined by java.util.Set. See Mapping Sets.

ORDERED_SET

A set that is ordered by a SQL fragment defined on its mapping. See Mapping Sets.

SORTED_SET

A set that is sorted according to a Comparator defined on its mapping. See Mapping Sets.

MAP

Follows the semantics defined by java.util.Map. See Mapping Maps.

ORDERED_MAP

A map that is ordered by keys according to a SQL fragment defined on its mapping. See Mapping Maps.

SORTED_MAP

A map that is sorted by keys according to a Comparator defined on its mapping. See Mapping Maps.

By default, Hibernate interprets the defined type of the plural attribute and makes an interpretation as to which classification it fits in to, using the following checks:

  1. if an array → ARRAY

  2. if a List → LIST

  3. if a SortedSet → SORTED_SET

  4. if a Set → SET

  5. if a SortedMap → SORTED_MAP

  6. if a Map → MAP

  7. else Collection → BAG

2.9.2. Mapping Lists

java.util.List defines a collection of ordered, non-unique elements.

Example 225. Basic List Mapping
@Entity
public class EntityWithList {
	// ...
	@ElementCollection
	private List<Name> names;
}

Contrary to natural expectations, the ordering of a list is by default not maintained. To maintain the order, it is necessary to explicitly use the jakarta.persistence.OrderColumn annotation.

Starting in 6.0, Hibernate allows to configure the default semantics of List without @OrderColumn via the hibernate.mapping.default_list_semantics setting. To switch to the more natural LIST semantics with an implicit order-column, set the setting to LIST. Beware that default LIST semantics only affects owned collection mappings. Unowned mappings like @ManyToMany(mappedBy = "…​") and @OneToMany(mappedBy = "…​") do not retain the element order by default, and explicitly annotating @OrderColumn for @ManyToMany(mappedBy = "…​") mappings is illegal.

To retain the order of elements of a @OneToMany(mappedBy = "…​") the @OrderColumn annotation must be applied explicitly. In addition to that, it is important that both sides of the relationship, the @OneToMany(mappedBy = "…​") and the @ManyToOne, must be kept in sync. Otherwise, the element position will not be updated accordingly.

The default column name that stores the index is derived from the attribute name, by suffixing _ORDER.

Example 226. @OrderColumn
@Entity
public class EntityWithOrderColumnList {
	// ...
	@ElementCollection
	@OrderColumn( name = "name_index" )
	private List<Name> names;
}

Now, a column named name_index will be used.

Hibernate stores index values into the order-column based on the element’s position in the list with no adjustment. The element at names[0] is stored with name_index=0 and so on. That is to say that the list index is considered 0-based just as list indexes themselves are 0-based. Some legacy schemas might map the position as 1-based, or any base really. Hibernate also defines support for such cases using its @ListIndexBase annotation.

Example 227. @ListIndexBase
@Entity
public class EntityWithIndexBasedList {
	// ...
	@ElementCollection
	@OrderColumn(name = "name_index")
	@ListIndexBase(1)
	private List<Name> names;
}

2.9.3. Mapping Sets

java.util.Set defines a collection of unique, though unordered elements. Hibernate supports mapping sets according to the requirements of the java.util.Set.

Example 228. Basic Set Mapping
@Entity
public class EntityWithSet {
	// ...
	@ElementCollection
	private Set<Name> names;
}

Hibernate also has the ability to map sorted and ordered sets. A sorted set orders its elements in memory via an associated Comparator; an ordered set is ordered via SQL when the set is loaded.

TIP

An ordered set does not perform any sorting in-memory. If an element is added after the collection is loaded, the collection would need to be refreshed to re-order the elements. For this reason, ordered sets are not recommended - if the application needs ordering of the set elements, a sorted set should be preferred. For this reason, it is not covered in the User Guide. See the javadocs for jakarta.persistence.OrderBy or org.hibernate.annotations.OrderBy for details.

There are 2 options for sorting a set - naturally or using an explicit comparator.

A set is naturally sorted using the natural sort comparator for its elements. Generally this implies that the element type is Comparable. E.g.

Example 229. @SortNatural
@Embeddable
@Access( AccessType.FIELD )
public class Name implements Comparable<Name> {
	private String first;
	private String last;

	// ...
}

@Entity
public class EntityWithNaturallySortedSet {
	// ...
	@ElementCollection
	@SortNatural
	private SortedSet<Name> names;
}

Because Name is defined as Comparable, its #compare method will be used to sort the elements in this set.

But Hibernate also allows sorting based on a specific Comparator implementation. Here, e.g., we map the Names as sorted by a NameComparator:

Example 230. @SortComparator
public class NameComparator implements Comparator<Name> {
	static final Comparator<Name> comparator = Comparator.comparing( Name::getLast ).thenComparing( Name::getFirst );

	@Override
	public int compare(Name o1, Name o2) {
		return comparator.compare( o1, o2 );
	}
}

@Entity
public class EntityWithSortedSet {
	// ...
	@ElementCollection
	@SortComparator( NameComparator.class )
	private SortedSet<Name> names;
}

Here, instead of Name#compare being use for the sorting, the explicit NameComparator will be used instead.

2.9.4. Mapping Maps

2.9.5. Mapping Collections

Without any other mapping influencers, java.util.Collection is interpreted using BAG semantics which means a collection that may contain duplicate entries and has no defined ordering.

Jakarta Persistence does not define support for BAG (nor ID_BAG) classification per-se. The specification does allow mapping of java.util.Collection attributes, but how such attributes are handled is largely undefined.

Example 231. Simple BAG mapping
@Entity
public class EntityWithBagAsCollection {
	// ..
	@ElementCollection
	private Collection<Name> names;
}

Some apps map BAG collections using java.util.List instead. Hibernate provides 2 ways to handle lists as bags. First an explicit annotation

Example 232. @Bag
@Entity
public class EntityWithBagAsList {
	// ..
	@ElementCollection
	@Bag
	private List<Name> names;
}

Specifically, the usage of @Bag forces the classification as BAG. Even though the names attribute is defined as List, Hibernate will treat it using the BAG semantics.

Additionally, as discussed in Mapping Lists, the hibernate.mapping.default_list_semantics setting is available to have Hibernate interpret a List with no @OrderColumn and no @ListIndexBase as a BAG.

An ID_BAG is similar to a BAG, except that it maps a generated, per-row identifier into the collection table. @CollectionId is the annotation to configure this identifier

2.9.6. Mapping Arrays

Hibernate is able to map Object and primitive arrays as collections. Mapping an array is essentially the same as mapping a list.

There is a major limitation of mapping arrays to be aware of - the array cannot be lazy using wrappers. It can, however, be lazy via bytecode enhancement of its owner.

Note that Jakarta Persistence does not define support for arrays as plural attributes; according to the specification, these would be mapped as binary data.

2.9.7. @ElementCollection

Element collections may contain values of either basic or embeddable types. They have a similar lifecycle to basic/embedded attributes in that their persistence is completely managed as part of the owner - they are created when referenced from an owner and automatically deleted when unreferenced. The specifics of how this lifecycle manifests in terms of database calls depends on the semantics of the mapping.

This section will discuss these lifecycle aspects using the example of mapping a collection of phone numbers. The examples use embeddable values, but the same aspects apply to collections of basic values as well.

The embeddable used in the examples is a PhoneNumber -

Example 233. PhoneNumber
@Embeddable
public class Phone {

	private String type;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}

First, a BAG mapping -

Example 234. Elemental BAG mapping
@Entity(name = "Person")
public static class Person {

	@Id
	private Integer id;

	@ElementCollection
	private Collection<String> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

}
Example 235. Elemental BAG lifecycle
// Clear element collection and add element
person.getPhones().clear();
person.getPhones().add( "123-456-7890" );
person.getPhones().add( "456-000-1234" );
delete from Person_phones where Person_id=1

INSERT INTO Person_phones ( Person_id, phones )
VALUES ( 1, '123-456-7890' )

INSERT INTO Person_phones  (Person_id, phones)
VALUES  ( 1, '456-000-1234' )
Collections of entities

If value type collections can only form a one-to-many association between an owner entity and multiple basic or embeddable types, entity collections can represent both @OneToMany and @ManyToMany associations.

From a relational database perspective, associations are defined by the foreign key side (the child-side). With value type collections, only the entity can control the association (the parent-side), but for a collection of entities, both sides of the association are managed by the persistence context.

For this reason, entity collections can be devised into two main categories: unidirectional and bidirectional associations. Unidirectional associations are very similar to value type collections since only the parent side controls this relationship. Bidirectional associations are more tricky since, even if sides need to be in-sync at all times, only one side is responsible for managing the association. A bidirectional association has an owning side and an inverse (mappedBy) side.

2.9.8. @CollectionType

The @CollectionType annotation provides the ability to use a custom UserCollectionType implementation to influence how the collection for a plural attribute behaves.

As an example, consider a requirement for a collection with the semantics of a "unique list" - a cross between the ordered-ness of a List and the uniqueness of a Set. First the entity:

Example 236. @CollectionType
@Entity
public class TheEntityWithUniqueList {
	@ElementCollection
	@CollectionType(type = UniqueListType.class)
	private List<String> strings;

	// ...
}

The mapping says to use the UniqueListType class for the mapping of the plural attribute.

Example 237. UniqueListType
public class UniqueListType implements UserCollectionType {
	@Override
	public CollectionClassification getClassification() {
		return CollectionClassification.LIST;
	}

	@Override
	public Class<?> getCollectionClass() {
		return List.class;
	}

	@Override
	public PersistentCollection instantiate(
			SharedSessionContractImplementor session,
			CollectionPersister persister) {
		return new UniqueListWrapper( session );
	}

	@Override
	public PersistentCollection wrap(
			SharedSessionContractImplementor session,
			Object collection) {
		return new UniqueListWrapper( session, (List) collection );
	}

	@Override
	public Iterator getElementsIterator(Object collection) {
		return ( (List) collection ).iterator();
	}

	@Override
	public boolean contains(Object collection, Object entity) {
		return ( (List) collection ).contains( entity );
	}

	@Override
	public Object indexOf(Object collection, Object entity) {
		return ( (List) collection ).indexOf( entity );
	}

	@Override
	public Object replaceElements(
			Object original,
			Object target,
			CollectionPersister persister,
			Object owner,
			Map copyCache,
			SharedSessionContractImplementor session) {
		List result = (List) target;
		result.clear();
		result.addAll( (List) original );
		return result;
	}

	@Override
	public Object instantiate(int anticipatedSize) {
		return new ArrayList<>();
	}
}

Most custom UserCollectionType implementations will want their own PersistentCollection implementation.

Example 238. UniqueListWrapper
public class UniqueListWrapper<E> extends PersistentList<E> {
	public UniqueListWrapper(SharedSessionContractImplementor session) {
		super( session );
	}

	public UniqueListWrapper(SharedSessionContractImplementor session, List<E> list) {
		super( session, list );
	}

	// ...
}

UniqueListWrapper is the PersistentCollection implementation for the "unique list" semantic. See Wrappers for more details.

2.9.9. @CollectionTypeRegistration

For cases where an application wants to apply the same custom type to all plural attributes of a given classification, Hibernate also provides the @CollectionTypeRegistration:

Example 239. UniqueListType Registration
@Entity
@CollectionTypeRegistration( type = UniqueListType.class, classification = CollectionClassification.LIST )
public class TheEntityWithUniqueListRegistration {
	@ElementCollection
	private List<String> strings;

	// ...
}

This example behaves exactly as in @CollectionType.

2.9.10. Wrappers

As mentioned in Collection Semantics, Hibernate provides its own implementations of the Java collection types. These are called wrappers as they wrap an underlying collection and provide support for things like lazy loading, queueing add/remove operations while detached, etc. Hibernate defines the following PersistentCollection implementations for each of its collection classifications -

  • PersistentArrayHolder

  • PersistentBag

  • PersistentIdentifierBag

  • PersistentList

  • PersistentMap

  • PersistentSet

  • PersistentSortedMap

  • PersistentSortedSet

ORDERED_SET uses PersistentSet for its wrapper and ORDERED_MAP uses PersistentMap.

The collections they wrap are called "raw" collections, which are generally the standard Java implementations (java.util.ArrayList, etc)

Original content below

2.9.11. Bags

Bags are unordered lists, and we can have unidirectional bags or bidirectional ones.

Unidirectional bags

The unidirectional bag is mapped using a single @OneToMany annotation on the parent side of the association. Behind the scenes, Hibernate requires an association table to manage the parent-child relationship, as we can see in the following example:

Example 240. Unidirectional bag
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL)
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	private String type;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Person_Phone (
    Person_id BIGINT NOT NULL ,
    phones_id BIGINT NOT NULL
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    type VARCHAR(255) ,
    PRIMARY KEY ( id )
)

ALTER TABLE Person_Phone
ADD CONSTRAINT UK_9uhc5itwc9h5gcng944pcaslf
UNIQUE (phones_id)

ALTER TABLE Person_Phone
ADD CONSTRAINT FKr38us2n8g5p9rj0b494sd3391
FOREIGN KEY (phones_id) REFERENCES Phone

ALTER TABLE Person_Phone
ADD CONSTRAINT FK2ex4e4p7w1cj310kg2woisjl2
FOREIGN KEY (Person_id) REFERENCES Person

Because both the parent and the child sides are entities, the persistence context manages each entity separately.

The cascading mechanism allows you to propagate an entity state transition from a parent entity to its children.

By marking the parent side with the CascadeType.ALL attribute, the unidirectional association lifecycle becomes very similar to that of a value type collection.

Example 241. Unidirectional bag lifecycle
Person person = new Person(1L);
person.getPhones().add(new Phone(1L, "landline", "028-234-9876"));
person.getPhones().add(new Phone(2L, "mobile", "072-122-9876"));
entityManager.persist(person);
INSERT INTO Person ( id )
VALUES ( 1 )

INSERT INTO Phone ( number, type, id )
VALUES ( '028-234-9876', 'landline', 1 )

INSERT INTO Phone ( number, type, id )
VALUES ( '072-122-9876', 'mobile', 2 )

INSERT INTO Person_Phone ( Person_id, phones_id )
VALUES ( 1, 1 )

INSERT INTO Person_Phone ( Person_id, phones_id )
VALUES ( 1, 2 )

In the example above, once the parent entity is persisted, the child entities are going to be persisted as well.

Just like value type collections, unidirectional bags are not as efficient when it comes to modifying the collection structure (removing or reshuffling elements).

Because the parent-side cannot uniquely identify each individual child, Hibernate deletes all link table rows associated with the parent entity and re-adds the remaining ones that are found in the current collection state.

Bidirectional bags

The bidirectional bag is the most common type of entity collection. The @ManyToOne side is the owning side of the bidirectional bag association, while the @OneToMany is the inverse side, being marked with the mappedBy attribute.

Example 242. Bidirectional bag
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

	public void addPhone(Phone phone) {
		phones.add(phone);
		phone.setPerson(this);
	}

	public void removePhone(Phone phone) {
		phones.remove(phone);
		phone.setPerson(null);
	}
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	private String type;

	@Column(name = "`number`", unique = true)
	@NaturalId
	private String number;

	@ManyToOne
	private Person person;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}
CREATE TABLE Person (
    id BIGINT NOT NULL, PRIMARY KEY (id)
)

CREATE TABLE Phone (
    id BIGINT NOT NULL,
    number VARCHAR(255),
    type VARCHAR(255),
    person_id BIGINT,
    PRIMARY KEY (id)
)

ALTER TABLE Phone
ADD CONSTRAINT UK_l329ab0g4c1t78onljnxmbnp6
UNIQUE (number)

ALTER TABLE Phone
ADD CONSTRAINT FKmw13yfsjypiiq0i1osdkaeqpg
FOREIGN KEy (person_id) REFERENCES Person
Example 243. Bidirectional bag lifecycle
person.addPhone(new Phone(1L, "landline", "028-234-9876"));
person.addPhone(new Phone(2L, "mobile", "072-122-9876"));
entityManager.flush();
person.removePhone(person.getPhones().get(0));
INSERT INTO Phone (number, person_id, type, id)
VALUES ( '028-234-9876', 1, 'landline', 1 )

INSERT INTO Phone (number, person_id, type, id)
VALUES ( '072-122-9876', 1, 'mobile', 2 )

UPDATE Phone
SET person_id = NULL, type = 'landline' where id = 1
Example 244. Bidirectional bag with orphan removal
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)
private List<Phone> phones = new ArrayList<>();
DELETE FROM Phone WHERE id = 1

When rerunning the previous example, the child will get removed because the parent-side propagates the removal upon dissociating the child entity reference.

2.9.12. Ordered Lists

Although they use the List interface on the Java side, bags don’t retain element order. To preserve the collection element order, there are two possibilities:

@OrderBy

the collection is ordered upon retrieval using a child entity property

@OrderColumn

the collection uses a dedicated order column in the collection link table

Unidirectional ordered lists

When using the @OrderBy annotation, the mapping looks as follows:

Example 245. Unidirectional @OrderBy list
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL)
	@OrderBy("number")
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	private String type;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}

The database mapping is the same as with the Unidirectional bags example, so it won’t be repeated. Upon fetching the collection, Hibernate generates the following select statement:

Example 246. Unidirectional @OrderBy list select statement
SELECT
   phones0_.Person_id AS Person_i1_1_0_,
   phones0_.phones_id AS phones_i2_1_0_,
   unidirecti1_.id AS id1_2_1_,
   unidirecti1_."number" AS number2_2_1_,
   unidirecti1_.type AS type3_2_1_
FROM
   Person_Phone phones0_
INNER JOIN
   Phone unidirecti1_ ON phones0_.phones_id=unidirecti1_.id
WHERE
   phones0_.Person_id = 1
ORDER BY
   unidirecti1_."number"

The child table column is used to order the list elements.

The @OrderBy annotation can take multiple entity properties, and each property can take an ordering direction too (e.g. @OrderBy("name ASC, type DESC")).

If no property is specified (e.g. @OrderBy), the primary key of the child entity table is used for ordering.

Another ordering option is to use the @OrderColumn annotation:

Example 247. Unidirectional @OrderColumn list
@OneToMany(cascade = CascadeType.ALL)
@OrderColumn(name = "order_id")
private List<Phone> phones = new ArrayList<>();
CREATE TABLE Person_Phone (
    Person_id BIGINT NOT NULL ,
    phones_id BIGINT NOT NULL ,
    order_id INTEGER NOT NULL ,
    PRIMARY KEY ( Person_id, order_id )
)

This time, the link table takes the order_id column and uses it to materialize the collection element order. When fetching the list, the following select query is executed:

Example 248. Unidirectional @OrderColumn list select statement
select
   phones0_.Person_id as Person_i1_1_0_,
   phones0_.phones_id as phones_i2_1_0_,
   phones0_.order_id as order_id3_0_,
   unidirecti1_.id as id1_2_1_,
   unidirecti1_.number as number2_2_1_,
   unidirecti1_.type as type3_2_1_
from
   Person_Phone phones0_
inner join
   Phone unidirecti1_
      on phones0_.phones_id=unidirecti1_.id
where
   phones0_.Person_id = 1

With the order_id column in place, Hibernate can order the list in-memory after it’s being fetched from the database.

Bidirectional ordered lists

The mapping is similar with the Bidirectional bags example, just that the parent side is going to be annotated with either @OrderBy or @OrderColumn.

Example 249. Bidirectional @OrderBy list
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
@OrderBy("number")
private List<Phone> phones = new ArrayList<>();

Just like with the unidirectional @OrderBy list, the number column is used to order the statement on the SQL level.

When using the @OrderColumn annotation, the order_id column is going to be embedded in the child table:

Example 250. Bidirectional @OrderColumn list
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
@OrderColumn(name = "order_id")
private List<Phone> phones = new ArrayList<>();
CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    type VARCHAR(255) ,
    person_id BIGINT ,
    order_id INTEGER ,
    PRIMARY KEY ( id )
)

When fetching the collection, Hibernate will use the fetched ordered columns to sort the elements according to the @OrderColumn mapping.

Customizing ordered list ordinal

You can customize the ordinal of the underlying ordered list by using the @ListIndexBase annotation.

Example 251. @ListIndexBase mapping example
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
@OrderColumn(name = "order_id")
@ListIndexBase(100)
private List<Phone> phones = new ArrayList<>();

When inserting two Phone records, Hibernate is going to start the List index from 100 this time.

Example 252. @ListIndexBase persist example
Person person = new Person(1L);
entityManager.persist(person);
person.addPhone(new Phone(1L, "landline", "028-234-9876"));
person.addPhone(new Phone(2L, "mobile", "072-122-9876"));
INSERT INTO Phone("number", person_id, type, id)
VALUES ('028-234-9876', 1, 'landline', 1)

INSERT INTO Phone("number", person_id, type, id)
VALUES ('072-122-9876', 1, 'mobile', 2)

UPDATE Phone
SET order_id = 100
WHERE id = 1

UPDATE Phone
SET order_id = 101
WHERE id = 2
Customizing ORDER BY SQL clause

While the Jakarta Persistence @OrderBy annotation allows you to specify the entity attributes used for sorting when fetching the current annotated collection, the Hibernate specific @OrderBy annotation is used to specify a SQL clause instead.

In the following example, the @OrderBy annotation uses the CHAR_LENGTH SQL function to order the Article entities by the number of characters of the name attribute.

Example 253. @OrderBy mapping example
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	@OneToMany(
		mappedBy = "person",
		cascade = CascadeType.ALL
	)
	@org.hibernate.annotations.OrderBy(
		clause = "CHAR_LENGTH(name) DESC"
	)
	private List<Article> articles = new ArrayList<>();

	//Getters and setters are omitted for brevity
}

@Entity(name = "Article")
public static class Article {

	@Id
	@GeneratedValue
	private Long id;

	private String name;

	private String content;

	@ManyToOne(fetch = FetchType.LAZY)
	private Person person;

	//Getters and setters are omitted for brevity
}

When fetching the articles collection, Hibernate uses the ORDER BY SQL clause provided by the mapping:

Example 254. @OrderBy fetching example
Person person = entityManager.find(Person.class, 1L);
assertEquals(
	"High-Performance Hibernate",
	person.getArticles().get(0).getName()
);
select
    a.person_id as person_i4_0_0_,
    a.id as id1_0_0_,
    a.content as content2_0_1_,
    a.name as name3_0_1_,
    a.person_id as person_i4_0_1_
from
    Article a
where
    a.person_id = ?
order by
    CHAR_LENGTH(a.name) desc

2.9.13. Sets

Sets are collections that don’t allow duplicate entries and Hibernate supports both the unordered Set and the natural-ordering SortedSet.

Unidirectional sets

The unidirectional set uses a link table to hold the parent-child associations and the entity mapping looks as follows:

Example 255. Unidirectional set
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL)
	private Set<Phone> phones = new HashSet<>();

	//Getters and setters are omitted for brevity
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	private String type;

	@NaturalId
	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}

The unidirectional set lifecycle is similar to that of the Unidirectional bags, so it can be omitted. The only difference is that Set doesn’t allow duplicates, but this constraint is enforced by the Java object contract rather than the database mapping.

When using Sets, it’s very important to supply proper equals/hashCode implementations for child entities.

In the absence of a custom equals/hashCode implementation logic, Hibernate will use the default Java reference-based object equality which might render unexpected results when mixing detached and managed object instances.

Bidirectional sets

Just like bidirectional bags, the bidirectional set doesn’t use a link table, and the child table has a foreign key referencing the parent table primary key. The lifecycle is just like with bidirectional bags except for the duplicates which are filtered out.

Example 256. Bidirectional set
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
	private Set<Phone> phones = new HashSet<>();

	//Getters and setters are omitted for brevity

	public void addPhone(Phone phone) {
		phones.add(phone);
		phone.setPerson(this);
	}

	public void removePhone(Phone phone) {
		phones.remove(phone);
		phone.setPerson(null);
	}
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	private String type;

	@Column(name = "`number`", unique = true)
	@NaturalId
	private String number;

	@ManyToOne
	private Person person;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}

2.9.14. Sorted sets

For sorted sets, the entity mapping must use the SortedSet interface instead. According to the SortedSet contract, all elements must implement the Comparable interface and therefore provide the sorting logic.

Unidirectional sorted sets

A SortedSet that relies on the natural sorting order given by the child element Comparable implementation logic might be annotated with the @SortNatural Hibernate annotation.

Example 257. Unidirectional natural sorted set
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL)
	@SortNatural
	private SortedSet<Phone> phones = new TreeSet<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Phone")
public static class Phone implements Comparable<Phone> {

	@Id
	private Long id;

	private String type;

	@NaturalId
	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

	@Override
	public int compareTo(Phone o) {
		return number.compareTo(o.getNumber());
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}

The lifecycle and the database mapping are identical to the Unidirectional bags, so they are intentionally omitted.

To provide a custom sorting logic, Hibernate also provides a @SortComparator annotation:

Example 258. Unidirectional custom comparator sorted set
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL)
	@SortComparator(ReverseComparator.class)
	private SortedSet<Phone> phones = new TreeSet<>();

	//Getters and setters are omitted for brevity

}

public static class ReverseComparator implements Comparator<Phone> {

	@Override
	public int compare(Phone o1, Phone o2) {
		return o2.compareTo(o1);
	}
}

@Entity(name = "Phone")
public static class Phone implements Comparable<Phone> {

	@Id
	private Long id;

	private String type;

	@NaturalId
	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

	@Override
	public int compareTo(Phone o) {
		return number.compareTo(o.getNumber());
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Phone phone = (Phone) o;
		return Objects.equals(number, phone.number);
	}

	@Override
	public int hashCode() {
		return Objects.hash(number);
	}
}
Bidirectional sorted sets

The @SortNatural and @SortComparator work the same for bidirectional sorted sets too:

Example 259. Bidirectional natural sorted set
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
@SortNatural
private SortedSet<Phone> phones = new TreeSet<>();


//Getters and setters are omitted for brevity

Before v6, @SortNatural must be used if collection element’s natural ordering is relied upon for sorting. Starting from v6, we can omit @SortNatural as it will take effect by default.

2.9.15. Maps

A java.util.Map is a ternary association because it requires a parent entity, a map key, and a value. An entity can either be a map key or a map value, depending on the mapping. Hibernate allows using the following map keys:

MapKeyColumn

for value type maps, the map key is a column in the link table that defines the grouping logic

MapKey

the map key is either the primary key or another property of the entity stored as a map entry value

MapKeyEnumerated

the map key is an Enum of the target child entity

MapKeyTemporal

the map key is a Date or a Calendar of the target child entity

MapKeyJoinColumn

the map key is an entity mapped as an association in the child entity that’s stored as a map entry key

Value type maps

A map of value type must use the @ElementCollection annotation, just like value type lists, bags or sets.

Example 260. Value type map with an entity as a map key
public enum PhoneType {
	LAND_LINE,
	MOBILE
}

@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@Temporal(TemporalType.TIMESTAMP)
	@ElementCollection
	@CollectionTable(name = "phone_register")
	@Column(name = "since")
	private Map<Phone, Date> phoneRegister = new HashMap<>();

	//Getters and setters are omitted for brevity

}

@Embeddable
public static class Phone {

	private PhoneType type;

	@Column(name = "`number`")
	private String number;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE phone_register (
    Person_id BIGINT NOT NULL ,
    since TIMESTAMP ,
    number VARCHAR(255) NOT NULL ,
    type INTEGER NOT NULL ,
    PRIMARY KEY ( Person_id, number, type )
)

ALTER TABLE phone_register
ADD CONSTRAINT FKrmcsa34hr68of2rq8qf526mlk
FOREIGN KEY (Person_id) REFERENCES Person

Adding entries to the map generates the following SQL statements:

Example 261. Adding value type map entries
person.getPhoneRegister().put(
	new Phone(PhoneType.LAND_LINE, "028-234-9876"), new Date()
);
person.getPhoneRegister().put(
	new Phone(PhoneType.MOBILE, "072-122-9876"), new Date()
);
INSERT INTO phone_register (Person_id, number, type, since)
VALUES (1, '072-122-9876', 1, '2015-12-15 17:16:45.311')

INSERT INTO phone_register (Person_id, number, type, since)
VALUES (1, '028-234-9876', 0, '2015-12-15 17:16:45.311')
Maps with a custom key type

Hibernate defines the @MapKeyType annotation which you can use to customize the Map key type.

Considering you have the following tables in your database:

create table person (
    id int8 not null,
    primary key (id)
)

create table call_register (
    person_id int8 not null,
    phone_number int4,
    call_timestamp_epoch int8 not null,
    primary key (person_id, call_timestamp_epoch)
)

alter table if exists call_register
    add constraint FKsn58spsregnjyn8xt61qkxsub
    foreign key (person_id)
    references person

The call_register records the call history for every person. The call_timestamp_epoch column stores the phone call timestamp as a Unix timestamp since the Unix epoch.

The @MapKeyColumn annotation is used to define the table column holding the key while the @Column mapping gives the value of the java.util.Map in question.

Since we want to map all the calls by their associated java.util.Date, not by their timestamp since epoch which is a number, the entity mapping looks as follows:

Example 262. @MapKeyType mapping example
	@Entity
	@Table(name = "person")
	public static class Person {

		@Id
		private Long id;

		@ElementCollection
		@CollectionTable(
			name = "call_register",
			joinColumns = @JoinColumn(name = "person_id")
		)
		@MapKeyJdbcTypeCode(Types.BIGINT)
// todo (6.0) : figure out why `@MapKeyTemporal` did not work.  imo it should
//		@MapKeyTemporal(TemporalType.TIMESTAMP)
		@MapKeyJavaType(JdbcTimestampJavaType.class)
		@MapKeyColumn(name = "call_timestamp_epoch")
		@Column(name = "phone_number")
		private Map<Date, Integer> callRegister = new HashMap<>();

		//Getters and setters are omitted for brevity

	}

The associated TimestampEpochType looks as follows:

Unresolved directive in chapters/domain/collections.adoc - include::../../../../../test/java/org/hibernate/userguide/collections/type/TimestampEpochType.java[tags=collections-map-custom-key-type-mapping-example,indent=0]

The TimestampEpochType allows us to map a Unix timestamp since epoch to a java.util.Date. But, without the @MapKeyType Hibernate annotation, it would not be possible to customize the Map key type.

Maps having an interface type as the key

Considering you have the following PhoneNumber interface with an implementation given by the MobilePhone class type:

Example 263. PhoneNumber interface and the MobilePhone class type
public interface PhoneNumber {

	String get();
}

@Embeddable
public static class MobilePhone
		implements PhoneNumber {

	static PhoneNumber fromString(String phoneNumber) {
		String[] tokens = phoneNumber.split("-");
		if (tokens.length != 3) {
			throw new IllegalArgumentException("invalid phone number: " + phoneNumber);
		}
		int i = 0;
		return new MobilePhone(
			tokens[i++],
			tokens[i++],
			tokens[i]
		);
	}

	private MobilePhone() {
	}

	public MobilePhone(
			String countryCode,
			String operatorCode,
			String subscriberCode) {
		this.countryCode = countryCode;
		this.operatorCode = operatorCode;
		this.subscriberCode = subscriberCode;
	}

	@Column(name = "country_code")
	private String countryCode;

	@Column(name = "operator_code")
	private String operatorCode;

	@Column(name = "subscriber_code")
	private String subscriberCode;

	@Override
	public String get() {
		return String.format(
			"%s-%s-%s",
			countryCode,
			operatorCode,
			subscriberCode
		);
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		MobilePhone that = (MobilePhone) o;
		return Objects.equals(countryCode, that.countryCode) &&
				Objects.equals(operatorCode, that.operatorCode) &&
				Objects.equals(subscriberCode, that.subscriberCode);
	}

	@Override
	public int hashCode() {
		return Objects.hash(countryCode, operatorCode, subscriberCode);
	}
}

If you want to use the PhoneNumber interface as a java.util.Map key, then you need to supply the @MapKeyClass annotation as well.

Example 264. @MapKeyClass mapping example
@Entity
@Table(name = "person")
public static class Person {

	@Id
	private Long id;

	@ElementCollection
	@CollectionTable(
		name = "call_register",
		joinColumns = @JoinColumn(name = "person_id")
	)
	@MapKeyColumn(name = "call_timestamp_epoch")
	@MapKeyClass(MobilePhone.class)
	@Column(name = "call_register")
	private Map<PhoneNumber, Integer> callRegister = new HashMap<>();

	//Getters and setters are omitted for brevity
}
create table person (
    id bigint not null,
    primary key (id)
)

create table call_register (
    person_id bigint not null,
    call_register integer,
    country_code varchar(255) not null,
    operator_code varchar(255) not null,
    subscriber_code varchar(255) not null,
    primary key (person_id, country_code, operator_code, subscriber_code)
)

alter table call_register
    add constraint FKqyj2at6ik010jqckeaw23jtv2
    foreign key (person_id)
    references person

When inserting a Person with a callRegister containing 2 MobilePhone references, Hibernate generates the following SQL statements:

Example 265. @MapKeyClass persist example
Person person = new Person();
person.setId(1L);
person.getCallRegister().put(new MobilePhone("01", "234", "567"), 101);
person.getCallRegister().put(new MobilePhone("01", "234", "789"), 102);

entityManager.persist(person);
insert into person (id) values (?)

-- binding parameter [1] as [BIGINT] - [1]

insert into call_register(
    person_id,
    country_code,
    operator_code,
    subscriber_code,
    call_register
)
values
    (?, ?, ?, ?, ?)

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [VARCHAR] - [01]
-- binding parameter [3] as [VARCHAR] - [234]
-- binding parameter [4] as [VARCHAR] - [789]
-- binding parameter [5] as [INTEGER] - [102]

insert into call_register(
    person_id,
    country_code,
    operator_code,
    subscriber_code,
    call_register
)
values
    (?, ?, ?, ?, ?)

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [VARCHAR] - [01]
-- binding parameter [3] as [VARCHAR] - [234]
-- binding parameter [4] as [VARCHAR] - [567]
-- binding parameter [5] as [INTEGER] - [101]

When fetching a Person and accessing the callRegister Map, Hibernate generates the following SQL statements:

Example 266. @MapKeyClass fetch example
Person person = entityManager.find(Person.class, 1L);
assertEquals(2, person.getCallRegister().size());

assertEquals(
	Integer.valueOf(101),
	person.getCallRegister().get(MobilePhone.fromString("01-234-567"))
);

assertEquals(
	Integer.valueOf(102),
	person.getCallRegister().get(MobilePhone.fromString("01-234-789"))
);
select
    cr.person_id as person_i1_0_0_,
    cr.call_register as call_reg2_0_0_,
    cr.country_code as country_3_0_,
    cr.operator_code as operator4_0_,
    cr.subscriber_code as subscrib5_0_
from
    call_register cr
where
    cr.person_id = ?

-- binding parameter [1] as [BIGINT] - [1]

-- extracted value ([person_i1_0_0_] : [BIGINT])  - [1]
-- extracted value ([call_reg2_0_0_] : [INTEGER]) - [101]
-- extracted value ([country_3_0_]   : [VARCHAR]) - [01]
-- extracted value ([operator4_0_]   : [VARCHAR]) - [234]
-- extracted value ([subscrib5_0_]   : [VARCHAR]) - [567]

-- extracted value ([person_i1_0_0_] : [BIGINT])  - [1]
-- extracted value ([call_reg2_0_0_] : [INTEGER]) - [102]
-- extracted value ([country_3_0_]   : [VARCHAR]) - [01]
-- extracted value ([operator4_0_]   : [VARCHAR]) - [234]
-- extracted value ([subscrib5_0_]   : [VARCHAR]) - [789]
Unidirectional maps

A unidirectional map exposes a parent-child association from the parent-side only.

The following example shows a unidirectional map which also uses a @MapKeyTemporal annotation. The map key is a timestamp, and it’s taken from the child entity table.

The @MapKey annotation is used to define the entity attribute used as a key of the java.util.Map in question.

Example 267. Unidirectional Map
public enum PhoneType {
	LAND_LINE,
	MOBILE
}

@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
	@JoinTable(
		name = "phone_register",
		joinColumns = @JoinColumn(name = "phone_id"),
		inverseJoinColumns = @JoinColumn(name = "person_id"))
	@MapKey(name = "since")
	@MapKeyTemporal(TemporalType.TIMESTAMP)
	private Map<Date, Phone> phoneRegister = new HashMap<>();

	//Getters and setters are omitted for brevity

	public void addPhone(Phone phone) {
		phoneRegister.put(phone.getSince(), phone);
	}
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	private PhoneType type;

	@Column(name = "`number`")
	private String number;

	private Date since;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    since TIMESTAMP ,
    type INTEGER ,
    PRIMARY KEY ( id )
)

CREATE TABLE phone_register (
    phone_id BIGINT NOT NULL ,
    person_id BIGINT NOT NULL ,
    PRIMARY KEY ( phone_id, person_id )
)

ALTER TABLE phone_register
ADD CONSTRAINT FKc3jajlx41lw6clbygbw8wm65w
FOREIGN KEY (person_id) REFERENCES Phone

ALTER TABLE phone_register
ADD CONSTRAINT FK6npoomh1rp660o1b55py9ndw4
FOREIGN KEY (phone_id) REFERENCES Person
Bidirectional maps

Like most bidirectional associations, this relationship is owned by the child-side while the parent is the inverse side and can propagate its own state transitions to the child entities.

In the following example, you can see that @MapKeyEnumerated was used so that the Phone enumeration becomes the map key.

Example 268. Bidirectional Map
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	@OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)
	@MapKey(name = "type")
	@MapKeyEnumerated
	private Map<PhoneType, Phone> phoneRegister = new HashMap<>();

	//Getters and setters are omitted for brevity

	public void addPhone(Phone phone) {
		phone.setPerson(this);
		phoneRegister.put(phone.getType(), phone);
	}
}

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	private PhoneType type;

	@Column(name = "`number`")
	private String number;

	private Date since;

	@ManyToOne
	private Person person;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    since TIMESTAMP ,
    type INTEGER ,
    person_id BIGINT ,
    PRIMARY KEY ( id )
)

ALTER TABLE Phone
ADD CONSTRAINT FKmw13yfsjypiiq0i1osdkaeqpg
FOREIGN KEY (person_id) REFERENCES Person

2.9.16. Arrays

When discussing arrays, it is important to understand the distinction between SQL array types and Java arrays that are mapped as part of the application’s domain model.

Not all databases implement the SQL-99 ARRAY type and, for this reason, Hibernate doesn’t support native database array types.

Hibernate does support the mapping of arrays in the Java domain model - conceptually the same as mapping a List. However, it is important to realize that it is impossible for Hibernate to offer lazy-loading for arrays of entities and, for this reason, it is strongly recommended to map a "collection" of entities using a List rather than an array.

2.9.17. Arrays as binary

By default, Hibernate will choose a BINARY type, as supported by the current Dialect.

Example 269. Arrays stored as binary
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String[] phones;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Person (
    id BIGINT NOT NULL ,
    phones VARBINARY(255) ,
    PRIMARY KEY ( id )
)

If you want to map arrays such as String[] or int[] to database-specific array types like PostgreSQL integer[] or text[], you need to write a custom Hibernate Type.

Check out this article for an example of how to write such a custom Hibernate Type.

2.9.18. Collections as basic value type

Notice how all the previous examples explicitly mark the collection attribute as either @ElementCollection, @OneToMany or @ManyToMany.

Attributes of collection or array type without any of those annotations are considered basic types and by default mapped to the database as VARBINARY.

Such mappings are not normal and beyond the scope of this documentation. The best way to map such a situation is using an AttributeConverter.

Example 270. Comma delimited collection
public class CommaDelimitedStringsConverter implements AttributeConverter<List<String>,String> {
	@Override
	public String convertToDatabaseColumn(List<String> attributeValue) {
		if ( attributeValue == null ) {
			return null;
		}
		return join( ",", attributeValue );
	}

	@Override
	public List<String> convertToEntityAttribute(String dbData) {
		if ( dbData == null ) {
			return null;
		}
		return Arrays.asList( dbData.split( "," ) );
	}
}

@Entity( name = "Person" )
public static class Person {
    @Id
    private Integer id;
    @Basic
	private String name;
	@Basic
	@Convert( converter = CommaDelimitedStringsConverter.class )
	private List<String> nickNames;

	// ...

}

2.10. Natural Ids

Natural ids represent domain model unique identifiers that have a meaning in the real world too. Even if a natural id does not make a good primary key (surrogate keys being usually preferred), it’s still useful to tell Hibernate about it. As we will see later, Hibernate provides a dedicated, efficient API for loading an entity by its natural id much like it offers for loading by its identifier (PK).

2.10.1. Natural Id Mapping

Natural ids are defined in terms of one or more persistent attributes.

Example 271. Natural id using single basic attribute
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	@NaturalId
	private String isbn;

	//Getters and setters are omitted for brevity
}
Example 272. Natural id using single embedded attribute
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	@NaturalId
	@Embedded
	private Isbn isbn;

	//Getters and setters are omitted for brevity
}

@Embeddable
public static class Isbn implements Serializable {

	private String isbn10;

	private String isbn13;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Isbn isbn = (Isbn) o;
		return Objects.equals(isbn10, isbn.isbn10) &&
				Objects.equals(isbn13, isbn.isbn13);
	}

	@Override
	public int hashCode() {
		return Objects.hash(isbn10, isbn13);
	}
}
Example 273. Natural id using multiple persistent attributes
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	@NaturalId
	private String productNumber;

	@NaturalId
	@ManyToOne(fetch = FetchType.LAZY)
	private Publisher publisher;

	//Getters and setters are omitted for brevity
}

@Entity(name = "Publisher")
public static class Publisher implements Serializable {

	@Id
	private Long id;

	private String name;

	//Getters and setters are omitted for brevity

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}
		Publisher publisher = (Publisher) o;
		return Objects.equals(id, publisher.id) &&
				Objects.equals(name, publisher.name);
	}

	@Override
	public int hashCode() {
		return Objects.hash(id, name);
	}
}

2.10.2. Natural Id API

As stated before, Hibernate provides an API for loading entities by their associated natural id. This is represented by the org.hibernate.NaturalIdLoadAccess contract obtained via Session#byNaturalId.

If the entity does not define a natural id, trying to load an entity by its natural id will throw an exception.

Example 274. Using NaturalIdLoadAccess
Book book = entityManager
	.unwrap(Session.class)
	.byNaturalId(Book.class)
	.using("isbn", "978-9730228236")
	.load();
Book book = entityManager
	.unwrap(Session.class)
	.byNaturalId(Book.class)
	.using(
		"isbn",
		new Isbn(
			"973022823X",
			"978-9730228236"
		))
	.load();
Book book = entityManager
	.unwrap(Session.class)
	.byNaturalId(Book.class)
	.using("productNumber", "973022823X")
	.using("publisher", publisher)
	.load();

NaturalIdLoadAccess offers 2 distinct methods for obtaining the entity:

load()

obtains a reference to the entity, making sure that the entity state is initialized.

getReference()

obtains a reference to the entity. The state may or may not be initialized. If the entity is already associated with the current running Session, that reference (loaded or not) is returned. If the entity is not loaded in the current Session and the entity supports proxy generation, an uninitialized proxy is generated and returned, otherwise the entity is loaded from the database and returned.

NaturalIdLoadAccess allows loading an entity by natural id and at the same time applies a pessimistic lock. For additional details on locking, see the Locking chapter.

We will discuss the last method available on NaturalIdLoadAccess ( setSynchronizationEnabled() ) in Natural Id - Mutability and Caching.

Because the Book entities in the first two examples define "simple" natural ids, we can load them as follows:

Example 275. Loading by simple natural id
Book book = entityManager
	.unwrap(Session.class)
	.bySimpleNaturalId(Book.class)
	.load("978-9730228236");
Book book = entityManager
	.unwrap(Session.class)
	.bySimpleNaturalId(Book.class)
	.load(
		new Isbn(
			"973022823X",
			"978-9730228236"
		)
	);

Here we see the use of the org.hibernate.SimpleNaturalIdLoadAccess contract, obtained via Session#bySimpleNaturalId().

SimpleNaturalIdLoadAccess is similar to NaturalIdLoadAccess except that it does not define the using method. Instead, because these simple natural ids are defined based on just one attribute we can directly pass the corresponding natural id attribute value directly to the load() and getReference() methods.

If the entity does not define a natural id, or if the natural id is not of a "simple" type, an exception will be thrown there.

2.10.3. Natural Id - Mutability and Caching

A natural id may be mutable or immutable. By default the @NaturalId annotation marks an immutable natural id attribute. An immutable natural id is expected to never change its value.

If the value(s) of the natural id attribute(s) change, @NaturalId(mutable = true) should be used instead.

Example 276. Mutable natural id mapping
@Entity(name = "Author")
public static class Author {

	@Id
	private Long id;

	private String name;

	@NaturalId(mutable = true)
	private String email;

	//Getters and setters are omitted for brevity
}

Within the Session, Hibernate maintains a mapping from natural id values to entity identifiers (PK) values. If natural ids values changed, it is possible for this mapping to become out of date until a flush occurs.

To work around this condition, Hibernate will attempt to discover any such pending changes and adjust them when the load() or getReference() methods are executed. To be clear: this is only pertinent for mutable natural ids.

This discovery and adjustment have a performance impact. If you are certain that none of the mutable natural ids already associated with the current Session have changed, you can disable this checking by calling setSynchronizationEnabled(false) (the default is true). This will force Hibernate to circumvent the checking of mutable natural ids.

Example 277. Mutable natural id synchronization use-case
Author author = entityManager
	.unwrap(Session.class)
	.bySimpleNaturalId(Author.class)
	.load("john@acme.com");

author.setEmail("john.doe@acme.com");

assertNull(
	entityManager
		.unwrap(Session.class)
		.bySimpleNaturalId(Author.class)
		.setSynchronizationEnabled(false)
		.load("john.doe@acme.com")
);

assertSame(author,
	entityManager
		.unwrap(Session.class)
		.bySimpleNaturalId(Author.class)
		.setSynchronizationEnabled(true)
		.load("john.doe@acme.com")
);

Not only can this NaturalId-to-PK resolution be cached in the Session, but we can also have it cached in the second-level cache if second level caching is enabled.

Example 278. Natural id caching
@Entity(name = "Book")
@NaturalIdCache
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	@NaturalId
	private String isbn;

	//Getters and setters are omitted for brevity
}

2.11. Dynamic Model

Jakarta Persistence only acknowledges the POJO entity model mapping so, if you are concerned about Jakarta Persistence provider portability, it’s best to stick to the strict POJO model. On the other hand, Hibernate can work with both POJO entities and dynamic entity models.

2.11.1. Dynamic mapping models

Persistent entities do not necessarily have to be represented as POJO/JavaBean classes. Hibernate also supports dynamic models (using Map of Maps at runtime). With this approach, you do not write persistent classes, only mapping files.

A given entity has just one entity mode within a given SessionFactory. This is a change from previous versions which allowed to define multiple entity modes for an entity and to select which to load. Entity modes can now be mixed within a domain model; a dynamic entity might reference a POJO entity and vice versa.

Example 279. Dynamic domain model Hibernate mapping
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class entity-name="Book">
        <id name="isbn" column="isbn" length="32" type="string"/>

        <property name="title" not-null="true" length="50" type="string"/>

        <property name="author" not-null="true" length="50" type="string"/>

    </class>
</hibernate-mapping>

After you defined your entity mapping, you need to instruct Hibernate to use the dynamic mapping mode:

Example 280. Dynamic domain model Hibernate mapping
settings.put("hibernate.default_entity_mode", "dynamic-map");

When you are going to save the following Book dynamic entity, Hibernate is going to generate the following SQL statement:

Example 281. Persist dynamic entity
Map<String, String> book = new HashMap<>();
book.put("isbn", "978-9730228236");
book.put("title", "High-Performance Java Persistence");
book.put("author", "Vlad Mihalcea");

entityManager
	.unwrap(Session.class)
	.save("Book", book);
insert
into
    Book
    (title, author, isbn)
values
    (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [High-Performance Java Persistence]
-- binding parameter [2] as [VARCHAR] - [Vlad Mihalcea]
-- binding parameter [3] as [VARCHAR] - [978-9730228236]

The main advantage of dynamic models is the quick turnaround time for prototyping without the need for entity class implementation. The main downfall is that you lose compile-time type checking and will likely deal with many exceptions at runtime. However, as a result of the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a proper domain model implementation on top later on.

It is also interesting to note that dynamic models are great for certain integration use cases as well. Envers, for example, makes extensive use of dynamic models to represent the historical data.

2.12. Inheritance

Although relational database systems don’t provide support for inheritance, Hibernate provides several strategies to leverage this object-oriented trait onto domain model entities:

MappedSuperclass

Inheritance is implemented in the domain model only without reflecting it in the database schema. See MappedSuperclass.

Single table

The domain model class hierarchy is materialized into a single table which contains entities belonging to different class types. See Single table.

Joined table

The base class and all the subclasses have their own database tables and fetching a subclass entity requires a join with the parent table as well. See Joined table.

Table per class

Each subclass has its own table containing both the subclass and the base class properties. See Table per class.

2.12.1. MappedSuperclass

In the following domain model class hierarchy, a DebitAccount and a CreditAccount share the same Account base class.

Inheritance class diagram

When using MappedSuperclass, the inheritance is visible in the domain model only, and each database table contains both the base class and the subclass properties.

Example 282. @MappedSuperclass inheritance
@MappedSuperclass
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE DebitAccount (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    overdraftFee NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

CREATE TABLE CreditAccount (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    creditLimit NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

Because the @MappedSuperclass inheritance model is not mirrored at the database level, it’s not possible to use polymorphic queries referencing the @MappedSuperclass when fetching persistent objects by their base class.

2.12.2. Single table

The single table inheritance strategy maps all subclasses to only one database table. Each subclass declares its own persistent properties. Version and id properties are assumed to be inherited from the root class.

When omitting an explicit inheritance strategy (e.g. @Inheritance), Jakarta Persistence will choose the SINGLE_TABLE strategy by default.

Example 283. Single Table inheritance
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Account (
    DTYPE VARCHAR(31) NOT NULL ,
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    overdraftFee NUMERIC(19, 2) ,
    creditLimit NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

Each subclass in a hierarchy must define a unique discriminator value, which is used to differentiate between rows belonging to separate subclass types. If this is not specified, the DTYPE column is used as a discriminator, storing the associated subclass name.

Example 284. Single Table inheritance discriminator column
DebitAccount debitAccount = new DebitAccount();
debitAccount.setId(1L);
debitAccount.setOwner("John Doe");
debitAccount.setBalance(BigDecimal.valueOf(100));
debitAccount.setInterestRate(BigDecimal.valueOf(1.5d));
debitAccount.setOverdraftFee(BigDecimal.valueOf(25));

CreditAccount creditAccount = new CreditAccount();
creditAccount.setId(2L);
creditAccount.setOwner("John Doe");
creditAccount.setBalance(BigDecimal.valueOf(1000));
creditAccount.setInterestRate(BigDecimal.valueOf(1.9d));
creditAccount.setCreditLimit(BigDecimal.valueOf(5000));

entityManager.persist(debitAccount);
entityManager.persist(creditAccount);
INSERT INTO Account (balance, interestRate, owner, overdraftFee, DTYPE, id)
VALUES (100, 1.5, 'John Doe', 25, 'DebitAccount', 1)

INSERT INTO Account (balance, interestRate, owner, creditLimit, DTYPE, id)
VALUES (1000, 1.9, 'John Doe', 5000, 'CreditAccount', 2)

When using polymorphic queries, only a single table is required to be scanned to fetch all associated subclass instances.

Example 285. Single Table polymorphic query
List<Account> accounts = entityManager
	.createQuery("select a from Account a")
	.getResultList();
SELECT  singletabl0_.id AS id2_0_ ,
        singletabl0_.balance AS balance3_0_ ,
        singletabl0_.interestRate AS interest4_0_ ,
        singletabl0_.owner AS owner5_0_ ,
        singletabl0_.overdraftFee AS overdraf6_0_ ,
        singletabl0_.creditLimit AS creditLi7_0_ ,
        singletabl0_.DTYPE AS DTYPE1_0_
FROM    Account singletabl0_

Among all other inheritance alternatives, the single table strategy performs the best since it requires access to one table only. Because all subclass columns are stored in a single table, it’s not possible to use NOT NULL constraints anymore, so integrity checks must be moved either into the data access layer or enforced through CHECK or TRIGGER constraints.

Discriminator

The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. Hibernate Core supports the following restricted set of types as discriminator column: String, char, int, byte, short, boolean(including yes_no, true_false).

Use the @DiscriminatorColumn to define the discriminator column as well as the discriminator type.

The enum DiscriminatorType used in jakarta.persistence.DiscriminatorColumn only contains the values STRING, CHAR and INTEGER which means that not all Hibernate supported types are available via the @DiscriminatorColumn annotation. You can also use @DiscriminatorFormula to express in SQL a virtual discriminator column. This is particularly useful when the discriminator value can be extracted from one or more columns of the table. Both @DiscriminatorColumn and @DiscriminatorFormula are to be set on the root entity (once per persisted hierarchy).

@org.hibernate.annotations.DiscriminatorOptions allows to optionally specify Hibernate-specific discriminator options which are not standardized in Jakarta Persistence. The available options are force and insert.

The force attribute is useful if the table contains rows with extra discriminator values that are not mapped to a persistent class. This could, for example, occur when working with a legacy database. If force is set to true, Hibernate will specify the allowed discriminator values in the SELECT query even when retrieving all instances of the root class.

The second option, insert, tells Hibernate whether or not to include the discriminator column in SQL INSERTs. Usually, the column should be part of the INSERT statement, but if your discriminator column is also part of a mapped composite identifier you have to set this option to false.

There used to be a @org.hibernate.annotations.ForceDiscriminator annotation which was deprecated in version 3.6 and later removed. Use @DiscriminatorOptions instead.

Discriminator formula

Assuming a legacy database schema where the discriminator is based on inspecting a certain column, we can take advantage of the Hibernate specific @DiscriminatorFormula annotation and map the inheritance model as follows:

Example 286. Single Table discriminator formula
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorFormula(
	"case when debitKey is not null " +
	"then 'Debit' " +
	"else (" +
	"   case when creditKey is not null " +
	"   then 'Credit' " +
	"   else 'Unknown' " +
	"   end) " +
	"end "
)
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
@DiscriminatorValue(value = "Debit")
public static class DebitAccount extends Account {

	private String debitKey;

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
@DiscriminatorValue(value = "Credit")
public static class CreditAccount extends Account {

	private String creditKey;

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Account (
    id int8 NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    debitKey VARCHAR(255) ,
    overdraftFee NUMERIC(19, 2) ,
    creditKey VARCHAR(255) ,
    creditLimit NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

The @DiscriminatorFormula defines a custom SQL clause that can be used to identify a certain subclass type. The @DiscriminatorValue defines the mapping between the result of the @DiscriminatorFormula and the inheritance subclass type.

Implicit discriminator values

Aside from the usual discriminator values assigned to each individual subclass type, the @DiscriminatorValue can take two additional values:

null

If the underlying discriminator column is null, the null discriminator mapping is going to be used.

not null

If the underlying discriminator column has a not-null value that is not explicitly mapped to any entity, the not-null discriminator mapping used.

To understand how these two values work, consider the following entity mapping:

Example 287. @DiscriminatorValue null and not-null entity mapping
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorValue("null")
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
@DiscriminatorValue("Debit")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
@DiscriminatorValue("Credit")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}

@Entity(name = "OtherAccount")
@DiscriminatorValue("not null")
public static class OtherAccount extends Account {

	private boolean active;

	//Getters and setters are omitted for brevity

}

The Account class has a @DiscriminatorValue( "null" ) mapping, meaning that any account row which does not contain any discriminator value will be mapped to an Account base class entity. The DebitAccount and CreditAccount entities use explicit discriminator values. The OtherAccount entity is used as a generic account type because it maps any database row whose discriminator column is not explicitly assigned to any other entity in the current inheritance tree.

To visualize how it works, consider the following example:

Example 288. @DiscriminatorValue null and not-null entity persistence
DebitAccount debitAccount = new DebitAccount();
debitAccount.setId(1L);
debitAccount.setOwner("John Doe");
debitAccount.setBalance(BigDecimal.valueOf(100));
debitAccount.setInterestRate(BigDecimal.valueOf(1.5d));
debitAccount.setOverdraftFee(BigDecimal.valueOf(25));

CreditAccount creditAccount = new CreditAccount();
creditAccount.setId(2L);
creditAccount.setOwner("John Doe");
creditAccount.setBalance(BigDecimal.valueOf(1000));
creditAccount.setInterestRate(BigDecimal.valueOf(1.9d));
creditAccount.setCreditLimit(BigDecimal.valueOf(5000));

Account account = new Account();
account.setId(3L);
account.setOwner("John Doe");
account.setBalance(BigDecimal.valueOf(1000));
account.setInterestRate(BigDecimal.valueOf(1.9d));

entityManager.persist(debitAccount);
entityManager.persist(creditAccount);
entityManager.persist(account);

entityManager.unwrap(Session.class).doWork(connection -> {
	try(Statement statement = connection.createStatement()) {
		statement.executeUpdate(
			"insert into Account (DTYPE, active, balance, interestRate, owner, id) " +
			"values ('Other', true, 25, 0.5, 'Vlad', 4)"
		);
	}
});

Map<Long, Account> accounts = entityManager.createQuery(
	"select a from Account a", Account.class)
.getResultList()
.stream()
.collect(Collectors.toMap(Account::getId, Function.identity()));

assertEquals(4, accounts.size());
assertEquals(DebitAccount.class, accounts.get(1L).getClass());
assertEquals(CreditAccount.class, accounts.get(2L).getClass());
assertEquals(Account.class, accounts.get(3L).getClass());
assertEquals(OtherAccount.class, accounts.get(4L).getClass());
INSERT INTO Account (balance, interestRate, owner, overdraftFee, DTYPE, id)
VALUES (100, 1.5, 'John Doe', 25, 'Debit', 1)

INSERT INTO Account (balance, interestRate, owner, overdraftFee, DTYPE, id)
VALUES (1000, 1.9, 'John Doe', 5000, 'Credit', 2)

INSERT INTO Account (balance, interestRate, owner, id)
VALUES (1000, 1.9, 'John Doe', 3)

INSERT INTO Account (DTYPE, active, balance, interestRate, owner, id)
VALUES ('Other', true, 25, 0.5, 'Vlad', 4)

SELECT a.id as id2_0_,
       a.balance as balance3_0_,
       a.interestRate as interest4_0_,
       a.owner as owner5_0_,
       a.overdraftFee as overdraf6_0_,
       a.creditLimit as creditLi7_0_,
       a.active as active8_0_,
       a.DTYPE as DTYPE1_0_
FROM   Account a

As you can see, the Account entity row has a value of NULL in the DTYPE discriminator column, while the OtherAccount entity was saved with a DTYPE column value of other which has not explicit mapping.

2.12.3. Joined table

Each subclass can also be mapped to its own table. This is also called table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass.

A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier.

Example 289. Join Table
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.JOINED)
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Account (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE CreditAccount (
    creditLimit NUMERIC(19, 2) ,
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE DebitAccount (
    overdraftFee NUMERIC(19, 2) ,
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

ALTER TABLE CreditAccount
ADD CONSTRAINT FKihw8h3j1k0w31cnyu7jcl7n7n
FOREIGN KEY (id) REFERENCES Account

ALTER TABLE DebitAccount
ADD CONSTRAINT FKia914478noepymc468kiaivqm
FOREIGN KEY (id) REFERENCES Account

The primary keys of the CreditAccount and DebitAccount tables are also foreign keys to the superclass table primary key and described by the @PrimaryKeyJoinColumns.

The table name still defaults to the non-qualified class name. Also, if @PrimaryKeyJoinColumn is not set, the primary key / foreign key columns are assumed to have the same names as the primary key columns of the primary table of the superclass.

Example 290. Join Table with @PrimaryKeyJoinColumn
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.JOINED)
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
@PrimaryKeyJoinColumn(name = "account_id")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
@PrimaryKeyJoinColumn(name = "account_id")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE CreditAccount (
    creditLimit NUMERIC(19, 2) ,
    account_id BIGINT NOT NULL ,
    PRIMARY KEY ( account_id )
)

CREATE TABLE DebitAccount (
    overdraftFee NUMERIC(19, 2) ,
    account_id BIGINT NOT NULL ,
    PRIMARY KEY ( account_id )
)

ALTER TABLE CreditAccount
ADD CONSTRAINT FK8ulmk1wgs5x7igo370jt0q005
FOREIGN KEY (account_id) REFERENCES Account

ALTER TABLE DebitAccount
ADD CONSTRAINT FK7wjufa570onoidv4omkkru06j
FOREIGN KEY (account_id) REFERENCES Account

When using polymorphic queries, the base class table must be joined with all subclass tables to fetch every associated subclass instance.

Example 291. Join Table polymorphic query
List<Account> accounts = entityManager
	.createQuery("select a from Account a")
	.getResultList();
SELECT jointablet0_.id AS id1_0_ ,
       jointablet0_.balance AS balance2_0_ ,
       jointablet0_.interestRate AS interest3_0_ ,
       jointablet0_.owner AS owner4_0_ ,
       jointablet0_1_.overdraftFee AS overdraf1_2_ ,
       jointablet0_2_.creditLimit AS creditLi1_1_ ,
       CASE WHEN jointablet0_1_.id IS NOT NULL THEN 1
            WHEN jointablet0_2_.id IS NOT NULL THEN 2
            WHEN jointablet0_.id IS NOT NULL THEN 0
       END AS clazz_
FROM   Account jointablet0_
       LEFT OUTER JOIN DebitAccount jointablet0_1_ ON jointablet0_.id = jointablet0_1_.id
       LEFT OUTER JOIN CreditAccount jointablet0_2_ ON jointablet0_.id = jointablet0_2_.id

The joined table inheritance polymorphic queries can use several JOINS which might affect performance when fetching a large number of entities.

2.12.4. Table per class

A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state.

In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class as a separate entity root. However, if you wish to use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the union subclass mapping.

Example 292. Table per class
@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public static class Account {

	@Id
	private Long id;

	private String owner;

	private BigDecimal balance;

	private BigDecimal interestRate;

	//Getters and setters are omitted for brevity

}

@Entity(name = "DebitAccount")
public static class DebitAccount extends Account {

	private BigDecimal overdraftFee;

	//Getters and setters are omitted for brevity

}

@Entity(name = "CreditAccount")
public static class CreditAccount extends Account {

	private BigDecimal creditLimit;

	//Getters and setters are omitted for brevity

}
CREATE TABLE Account (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE CreditAccount (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    creditLimit NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

CREATE TABLE DebitAccount (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    overdraftFee NUMERIC(19, 2) ,
    PRIMARY KEY ( id )
)

When using polymorphic queries, a UNION is required to fetch the base class table along with all subclass tables as well.

Example 293. Table per class polymorphic query
List<Account> accounts = entityManager
	.createQuery("select a from Account a")
	.getResultList();
SELECT tablepercl0_.id AS id1_0_ ,
       tablepercl0_.balance AS balance2_0_ ,
       tablepercl0_.interestRate AS interest3_0_ ,
       tablepercl0_.owner AS owner4_0_ ,
       tablepercl0_.overdraftFee AS overdraf1_2_ ,
       tablepercl0_.creditLimit AS creditLi1_1_ ,
       tablepercl0_.clazz_ AS clazz_
FROM (
    SELECT    id ,
             balance ,
             interestRate ,
             owner ,
             CAST(NULL AS INT) AS overdraftFee ,
             CAST(NULL AS INT) AS creditLimit ,
             0 AS clazz_
    FROM     Account
    UNION ALL
    SELECT   id ,
             balance ,
             interestRate ,
             owner ,
             overdraftFee ,
             CAST(NULL AS INT) AS creditLimit ,
             1 AS clazz_
    FROM     DebitAccount
    UNION ALL
    SELECT   id ,
             balance ,
             interestRate ,
             owner ,
             CAST(NULL AS INT) AS overdraftFee ,
             creditLimit ,
             2 AS clazz_
    FROM     CreditAccount
) tablepercl0_

Polymorphic queries require multiple UNION queries, so be aware of the performance implications of a large class hierarchy.

Unfortunately, not all database systems support UNION ALL, in which case, UNION is going to be used instead of UNION ALL.

The following Hibernate dialects support UNION ALL:

  • AbstractHANADialect

  • AbstractTransactSQLDialect

  • CUBRIDDialect

  • DB2Dialect

  • H2Dialect

  • HSQLDialect

  • Ingres9Dialect

  • MySQL5Dialect

  • Oracle8iDialect

  • Oracle9Dialect

  • PostgreSQL81Dialect

  • RDMSOS2200Dialect

2.12.5. Implicit and explicit polymorphism

By default, when you query a base class entity, the polymorphic query will fetch all subclasses belonging to the base type.

However, you can even query interfaces or base classes that don’t belong to the Jakarta Persistence entity inheritance model.

For instance, considering the following DomainModelEntity interface:

Example 294. DomainModelEntity interface
public interface DomainModelEntity<ID> {

    ID getId();

    Integer getVersion();
}

If we have two entity mappings, a Book and a Blog, and the Blog entity is mapped with the @Polymorphism annotation and taking the PolymorphismType.EXPLICIT setting:

Example 295. @Polymorphism entity mapping
@Entity(name = "Event")
public static class Book implements DomainModelEntity<Long> {

	@Id
	private Long id;

	@Version
	private Integer version;

	private String title;

	private String author;

	//Getter and setters omitted for brevity
}

@Entity(name = "Blog")
@Polymorphism(type = PolymorphismType.EXPLICIT)
public static class Blog implements DomainModelEntity<Long> {

	@Id
	private Long id;

	@Version
	private Integer version;

	private String site;

	//Getter and setters omitted for brevity
}

If we have the following entity objects in our system:

Example 296. Domain Model entity objects
Book book = new Book();
book.setId(1L);
book.setAuthor("Vlad Mihalcea");
book.setTitle("High-Performance Java Persistence");
entityManager.persist(book);

Blog blog = new Blog();
blog.setId(1L);
blog.setSite("vladmihalcea.com");
entityManager.persist(blog);

We can now query against the DomainModelEntity interface, and Hibernate is going to fetch only the entities that are either mapped with @Polymorphism(type = PolymorphismType.IMPLICIT) or they are not annotated at all with the @Polymorphism annotation (implying the IMPLICIT behavior):

Example 297. Fetching Domain Model entities using non-mapped base class polymorphism
List<DomainModelEntity> accounts = entityManager
.createQuery(
	"select e " +
	"from org.hibernate.userguide.inheritance.polymorphism.DomainModelEntity e")
.getResultList();

assertEquals(1, accounts.size());
assertTrue(accounts.get(0) instanceof Book);

Therefore, only the Book was fetched since the Blog entity was marked with the @Polymorphism(type = PolymorphismType.EXPLICIT) annotation, which instructs Hibernate to skip it when executing a polymorphic query against a non-mapped base class.

2.13. Immutability

Immutability can be specified for both entities and collections.

2.13.1. Entity immutability

If a specific entity is immutable, it is good practice to mark it with the @Immutable annotation.

Example 298. Immutable entity
@Entity(name = "Event")
@Immutable
public static class Event {

	@Id
	private Long id;

	private Date createdOn;

	private String message;

	//Getters and setters are omitted for brevity

}

Internally, Hibernate is going to perform several optimizations, such as:

  • reducing memory footprint since there is no need to retain the dehydrated state for the dirty checking mechanism

  • speeding-up the Persistence Context flushing phase since immutable entities can skip the dirty checking process

Considering the following entity is persisted in the database:

Example 299. Persisting an immutable entity
doInJPA(this::entityManagerFactory, entityManager -> {
	Event event = new Event();
	event.setId(1L);
	event.setCreatedOn(new Date());
	event.setMessage("Hibernate User Guide rocks!");

	entityManager.persist(event);
});

When loading the entity and trying to change its state, Hibernate will skip any modification, therefore no SQL UPDATE statement is executed.

Example 300. The immutable entity ignores any update
doInJPA(this::entityManagerFactory, entityManager -> {
	Event event = entityManager.find(Event.class, 1L);
	log.info("Change event message");
	event.setMessage("Hibernate User Guide");
});
doInJPA(this::entityManagerFactory, entityManager -> {
	Event event = entityManager.find(Event.class, 1L);
	assertEquals("Hibernate User Guide rocks!", event.getMessage());
});
SELECT e.id AS id1_0_0_,
       e.createdOn AS createdO2_0_0_,
       e.message AS message3_0_0_
FROM   event e
WHERE  e.id = 1

-- Change event message

SELECT e.id AS id1_0_0_,
       e.createdOn AS createdO2_0_0_,
       e.message AS message3_0_0_
FROM   event e
WHERE  e.id = 1

2.13.2. Collection immutability

Just like entities, collections can also be marked with the @Immutable annotation.

Considering the following entity mappings:

Example 301. Immutable collection
@Entity(name = "Batch")
public static class Batch {

	@Id
	private Long id;

	private String name;

	@OneToMany(cascade = CascadeType.ALL)
	@Immutable
	private List<Event> events = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Event")
@Immutable
public static class Event {

	@Id
	private Long id;

	private Date createdOn;

	private String message;

	//Getters and setters are omitted for brevity

}

This time, not only the Event entity is immutable, but the Event collection stored by the Batch parent entity. Once the immutable collection is created, it can never be modified.

Example 302. Persisting an immutable collection
doInJPA(this::entityManagerFactory, entityManager -> {
	Batch batch = new Batch();
	batch.setId(1L);
	batch.setName("Change request");

	Event event1 = new Event();
	event1.setId(1L);
	event1.setCreatedOn(new Date());
	event1.setMessage("Update Hibernate User Guide");

	Event event2 = new Event();
	event2.setId(2L);
	event2.setCreatedOn(new Date());
	event2.setMessage("Update Hibernate Getting Started Guide");

	batch.getEvents().add(event1);
	batch.getEvents().add(event2);

	entityManager.persist(batch);
});

The Batch entity is mutable. Only the events collection is immutable.

For instance, we can still modify the entity name:

Example 303. Changing the mutable entity
doInJPA(this::entityManagerFactory, entityManager -> {
	Batch batch = entityManager.find(Batch.class, 1L);
	log.info("Change batch name");
	batch.setName("Proposed change request");
});
SELECT b.id AS id1_0_0_,
       b.name AS name2_0_0_
FROM   Batch b
WHERE  b.id = 1

-- Change batch name

UPDATE batch
SET    name = 'Proposed change request'
WHERE  id = 1

However, when trying to modify the events collection:

Example 304. Immutable collections cannot be modified
try {
	doInJPA(this::entityManagerFactory, entityManager -> {
		Batch batch = entityManager.find(Batch.class, 1L);
		batch.getEvents().clear();
	});
}
catch (Exception e) {
	log.error("Immutable collections cannot be modified");
}
jakarta.persistence.RollbackException: Error while committing the transaction

Caused by: jakarta.persistence.PersistenceException: org.hibernate.HibernateException:

Caused by: org.hibernate.HibernateException: changed an immutable collection instance: [
    org.hibernate.userguide.immutability.CollectionImmutabilityTest$Batch.events#1
]

While immutable entity changes are simply discarded, modifying an immutable collection will end up in a HibernateException being thrown.

2.14. Customizing the domain model

For cases where Hibernate does not provide a built-in way to configure the domain model mapping based on requirements, it provides a very broad and flexible way to adjust the mapping model through its "boot-time model" (defined in the org.hibernate.mapping package) using its @AttributeBinderType meta annotation and corresponding AttributeBinder contract.

An example:

Example 305. AttributeBinder example
/**
 * Custom annotation applying 'Y'/'N' storage semantics to a boolean.
 *
 * The important piece here is `@AttributeBinderType`
 */
@Target({METHOD,FIELD})
@Retention(RUNTIME)
@AttributeBinderType( binder = YesNoBinder.class )
public @interface YesNo {
}

/**
 * The actual binder responsible for configuring the model objects
 */
public class YesNoBinder implements AttributeBinder<YesNo> {
	@Override
	public void bind(
			YesNo annotation,
			MetadataBuildingContext buildingContext,
			PersistentClass persistentClass,
			Property property) {
		final BasicValue booleanValueMapping = (BasicValue) property.getValue();

		final BasicJavaType<?> javaType = (BasicJavaType<?>) buildingContext.getBootstrapContext()
				.getTypeConfiguration()
				.getJavaTypeRegistry()
				.getDescriptor( Boolean.class );

		final JdbcType jdbcType = buildingContext.getBootstrapContext()
				.getTypeConfiguration()
				.getJdbcTypeRegistry()
				.getDescriptor( Types.CHAR );

		final InstanceBasedConverterDescriptor converter = new InstanceBasedConverterDescriptor(
				YesNoConverter.INSTANCE,
				buildingContext.getBootstrapContext().getClassmateContext()
		);

		booleanValueMapping.setExplicitJavaTypeAccess( (typeConfiguration) -> javaType );
		booleanValueMapping.setExplicitJdbcTypeAccess( (typeConfiguration) -> jdbcType );
		booleanValueMapping.setJpaAttributeConverterDescriptor( converter );
	}
}

The important thing to take away here is that both @YesNo and YesNoBinder are custom, user-written code. Hibernate has no inherent understanding of what a @YesNo does or is. It only understands that it has the @AttributeBinderType meta-annotation and knows how to apply that through the corresponding YesNoBinder.

Notice also that @AttributeBinderType provides a type-safe way to perform configuration because the AttributeBinder (YesNoBinder) is handed the custom annotation (@YesNo) to grab its configured attributes. @YesNo does not provide any attributes, but it easily could. Whatever YesNoBinder supports.

3. Bootstrap

The term bootstrapping refers to initializing and starting a software component. In Hibernate, we are specifically talking about the process of building a fully functional SessionFactory instance or EntityManagerFactory instance, for Jakarta Persistence. The process is very different for each.

During the bootstrap process, you might want to customize Hibernate behavior so make sure you check the Configurations section as well.

3.1. Native Bootstrapping

This section discusses the process of bootstrapping a Hibernate SessionFactory. Specifically, it addresses the bootstrapping APIs. For a discussion of the legacy bootstrapping API, see Legacy Bootstrapping.

3.1.1. Building the ServiceRegistry

The first step in native bootstrapping is the building of a ServiceRegistry holding the services Hibernate will need during bootstrapping and at run time.

Actually, we are concerned with building 2 different ServiceRegistries. First is the org.hibernate.boot.registry.BootstrapServiceRegistry. The BootstrapServiceRegistry is intended to hold services that Hibernate needs at both bootstrap and run time. This boils down to 3 services:

org.hibernate.boot.registry.classloading.spi.ClassLoaderService

which controls how Hibernate interacts with ClassLoaders.

org.hibernate.integrator.spi.IntegratorService

which controls the management and discovery of org.hibernate.integrator.spi.Integrator instances.

org.hibernate.boot.registry.selector.spi.StrategySelector

which controls how Hibernate resolves implementations of various strategy contracts. This is a very powerful service, but a full discussion of it is beyond the scope of this guide.

If you are ok with the default behavior of Hibernate in regards to these BootstrapServiceRegistry services (which is quite often the case, especially in stand-alone environments), then you don’t need to explicitly build the BootstrapServiceRegistry.

If you wish to alter how the BootstrapServiceRegistry is built, that is controlled through the org.hibernate.boot.registry.BootstrapServiceRegistryBuilder:

Example 306. Controlling BootstrapServiceRegistry building
BootstrapServiceRegistryBuilder bootstrapRegistryBuilder =
    new BootstrapServiceRegistryBuilder();
// add a custom ClassLoader
bootstrapRegistryBuilder.applyClassLoader(customClassLoader);
// manually add an Integrator
bootstrapRegistryBuilder.applyIntegrator(customIntegrator);

BootstrapServiceRegistry bootstrapRegistry = bootstrapRegistryBuilder.build();

The services of the BootstrapServiceRegistry cannot be extended (added to) nor overridden (replaced).

The second ServiceRegistry is the org.hibernate.boot.registry.StandardServiceRegistry. You will almost always need to configure the StandardServiceRegistry, which is done through org.hibernate.boot.registry.StandardServiceRegistryBuilder:

Example 307. Building a BootstrapServiceRegistryBuilder
// An example using an implicitly built BootstrapServiceRegistry
StandardServiceRegistryBuilder standardRegistryBuilder =
    new StandardServiceRegistryBuilder();

// An example using an explicitly built BootstrapServiceRegistry
BootstrapServiceRegistry bootstrapRegistry =
    new BootstrapServiceRegistryBuilder().build();

StandardServiceRegistryBuilder standardRegistryBuilder =
    new StandardServiceRegistryBuilder(bootstrapRegistry);

A StandardServiceRegistry is also highly configurable via the StandardServiceRegistryBuilder API. See the StandardServiceRegistryBuilder Javadocs for more details.

Some specific methods of interest:

Example 308. Configuring a MetadataSources
ServiceRegistry standardRegistry =
        new StandardServiceRegistryBuilder().build();

MetadataSources sources = new MetadataSources(standardRegistry);

// alternatively, we can build the MetadataSources without passing
// a service registry, in which case it will build a default
// BootstrapServiceRegistry to use.  But the approach shown
// above is preferred
// MetadataSources sources = new MetadataSources();

// add a class using JPA/Hibernate annotations for mapping
sources.addAnnotatedClass(MyEntity.class);

// add the name of a class using JPA/Hibernate annotations for mapping.
// differs from above in that accessing the Class is deferred which is
// important if using runtime bytecode-enhancement
sources.addAnnotatedClassName("org.hibernate.example.Customer");

// Read package-level metadata.
sources.addPackage("hibernate.example");

// Read package-level metadata.
sources.addPackage(MyEntity.class.getPackage());

// Adds the named hbm.xml resource as a source: which performs the
// classpath lookup and parses the XML
sources.addResource("org/hibernate/example/Order.hbm.xml");

// Adds the named JPA orm.xml resource as a source: which performs the
// classpath lookup and parses the XML
sources.addResource("org/hibernate/example/Product.orm.xml");

// Read all mapping documents from a directory tree.
// Assumes that any file named *.hbm.xml is a mapping document.
sources.addDirectory(new File("."));

// Read mappings from a particular XML file
sources.addFile(new File("./mapping.xml"));

// Read all mappings from a jar file.
// Assumes that any file named *.hbm.xml is a mapping document.
sources.addJar(new File("./entities.jar"));

3.1.2. Event Listener registration

The main use cases for an org.hibernate.integrator.spi.Integrator right now are registering event listeners.

Example 309. Configuring an event listener
public class MyIntegrator implements Integrator {

    @Override
    public void integrate(
            Metadata metadata,
            SessionFactoryImplementor sessionFactory,
            SessionFactoryServiceRegistry serviceRegistry) {

        // As you might expect, an EventListenerRegistry is the thing with which event
        // listeners are registered
        // It is a service so we look it up using the service registry
        final EventListenerRegistry eventListenerRegistry =
            serviceRegistry.getService(EventListenerRegistry.class);

        // If you wish to have custom determination and handling of "duplicate" listeners,
        // you would have to add an implementation of the
        // org.hibernate.event.service.spi.DuplicationStrategy contract like this
        eventListenerRegistry.addDuplicationStrategy(new CustomDuplicationStrategy());

        // EventListenerRegistry defines 3 ways to register listeners:

        // 1) This form overrides any existing registrations with
        eventListenerRegistry.setListeners(EventType.AUTO_FLUSH,
                                            DefaultAutoFlushEventListener.class);

        // 2) This form adds the specified listener(s) to the beginning of the listener chain
        eventListenerRegistry.prependListeners(EventType.PERSIST,
                                                DefaultPersistEventListener.class);

        // 3) This form adds the specified listener(s) to the end of the listener chain
        eventListenerRegistry.appendListeners(EventType.MERGE,
                                               DefaultMergeEventListener.class);
    }

    @Override
    public void disintegrate(
            SessionFactoryImplementor sessionFactory,
            SessionFactoryServiceRegistry serviceRegistry) {

    }
}

3.1.3. Building the Metadata

The second step in native bootstrapping is the building of an org.hibernate.boot.Metadata object containing the parsed representations of an application domain model and its mapping to a database. The first thing we obviously need to build a parsed representation is the source information to be parsed (annotated classes, hbm.xml files, orm.xml files). This is the purpose of org.hibernate.boot.MetadataSources.

MetadataSources has many other methods as well. Explore its API and Javadocs for more information. Also, all methods on MetadataSources offer fluent-style call chaining::

Example 310. Configuring a MetadataSources with method chaining
ServiceRegistry standardRegistry =
        new StandardServiceRegistryBuilder().build();

MetadataSources sources = new MetadataSources(standardRegistry)
    .addAnnotatedClass(MyEntity.class)
    .addAnnotatedClassName("org.hibernate.example.Customer")
    .addResource("org/hibernate/example/Order.hbm.xml")
    .addResource("org/hibernate/example/Product.orm.xml");

Once we have the sources of mapping information defined, we need to build the Metadata object. If you are ok with the default behavior in building the Metadata then you can simply call the buildMetadata method of the MetadataSources.

Notice that a ServiceRegistry can be passed at a number of points in this bootstrapping process. The suggested approach is to build a StandardServiceRegistry yourself and pass that along to the MetadataSources constructor. From there, MetadataBuilder, Metadata, SessionFactoryBuilder, and SessionFactory will all pick up that same StandardServiceRegistry.

However, if you wish to adjust the process of building Metadata from MetadataSources, you will need to use the MetadataBuilder as obtained via MetadataSources#getMetadataBuilder. MetadataBuilder allows a lot of control over the Metadata building process. See its Javadocs for full details.

Example 311. Building Metadata via MetadataBuilder
ServiceRegistry standardRegistry =
    new StandardServiceRegistryBuilder().build();

MetadataSources sources = new MetadataSources(standardRegistry);

MetadataBuilder metadataBuilder = sources.getMetadataBuilder();

// Use the JPA-compliant implicit naming strategy
metadataBuilder.applyImplicitNamingStrategy(
    ImplicitNamingStrategyJpaCompliantImpl.INSTANCE);

// specify the schema name to use for tables, etc when none is explicitly specified
metadataBuilder.applyImplicitSchemaName("my_default_schema");

// specify a custom Attribute Converter
metadataBuilder.applyAttributeConverter(myAttributeConverter);

Metadata metadata = metadataBuilder.build();

3.1.4. Building the SessionFactory

The final step in native bootstrapping is to build the SessionFactory itself. Much like discussed above, if you are ok with the default behavior of building a SessionFactory from a Metadata reference, you can simply call the buildSessionFactory method on the Metadata object.

However, if you would like to adjust that building process, you will need to use SessionFactoryBuilder as obtained via Metadata#getSessionFactoryBuilder. Again, see its Javadocs for more details.

Example 312. Native Bootstrapping - Putting it all together
StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
    .configure("org/hibernate/example/hibernate.cfg.xml")
    .build();

Metadata metadata = new MetadataSources(standardRegistry)
    .addAnnotatedClass(MyEntity.class)
    .addAnnotatedClassName("org.hibernate.example.Customer")
    .addResource("org/hibernate/example/Order.hbm.xml")
    .addResource("org/hibernate/example/Product.orm.xml")
    .getMetadataBuilder()
    .applyImplicitNamingStrategy(ImplicitNamingStrategyJpaCompliantImpl.INSTANCE)
    .build();

SessionFactory sessionFactory = metadata.getSessionFactoryBuilder()
    .applyBeanManager(getBeanManager())
    .build();

The bootstrapping API is quite flexible, but in most cases it makes the most sense to think of it as a 3 step process:

  1. Build the StandardServiceRegistry

  2. Build the Metadata

  3. Use those 2 to build the SessionFactory

Example 313. Building SessionFactory via SessionFactoryBuilder
StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
        .configure("org/hibernate/example/hibernate.cfg.xml")
        .build();

Metadata metadata = new MetadataSources(standardRegistry)
    .addAnnotatedClass(MyEntity.class)
    .addAnnotatedClassName("org.hibernate.example.Customer")
    .addResource("org/hibernate/example/Order.hbm.xml")
    .addResource("org/hibernate/example/Product.orm.xml")
    .getMetadataBuilder()
    .applyImplicitNamingStrategy(ImplicitNamingStrategyJpaCompliantImpl.INSTANCE)
    .build();

SessionFactoryBuilder sessionFactoryBuilder = metadata.getSessionFactoryBuilder();

// Supply a SessionFactory-level Interceptor
sessionFactoryBuilder.applyInterceptor(new CustomSessionFactoryInterceptor());

// Add a custom observer
sessionFactoryBuilder.addSessionFactoryObservers(new CustomSessionFactoryObserver());

// Apply a CDI BeanManager (for JPA event listeners)
sessionFactoryBuilder.applyBeanManager(getBeanManager());

SessionFactory sessionFactory = sessionFactoryBuilder.build();

3.2. Jakarta Persistence Bootstrapping

Bootstrapping Hibernate as a Jakarta Persistence provider can be done in a Jakarta Persistence-spec compliant manner or using a proprietary bootstrapping approach. The standardized approach has some limitations in certain environments, but aside from those, it is highly recommended that you use Jakarta Persistence-standardized bootstrapping.

3.2.1. Jakarta Persistence-compliant bootstrapping

In Jakarta Persistence, we are ultimately interested in bootstrapping a jakarta.persistence.EntityManagerFactory instance. The Jakarta Persistence specification defines two primary standardized bootstrap approaches depending on how the application intends to access the jakarta.persistence.EntityManager instances from an EntityManagerFactory.

It uses the terms EE and SE for these two approaches, but those terms are very misleading in this context. What the Jakarta Persistence spec calls EE bootstrapping implies the existence of a container (EE, OSGi, etc), who’ll manage and inject the persistence context on behalf of the application. What it calls SE bootstrapping is everything else. We will use the terms container-bootstrapping and application-bootstrapping in this guide.

For compliant container-bootstrapping, the container will build an EntityManagerFactory for each persistent-unit defined in the META-INF/persistence.xml configuration file and make that available to the application for injection via the jakarta.persistence.PersistenceUnit annotation or via JNDI lookup.

Example 314. Injecting the default EntityManagerFactory
@PersistenceUnit
private EntityManagerFactory emf;

Or, in case you have multiple Persistence Units (e.g. multiple persistence.xml configuration files), you can inject a specific EntityManagerFactory by Unit name:

Example 315. Injecting a specific EntityManagerFactory
 @PersistenceUnit(
     unitName = "CRM"
)
 private EntityManagerFactory entityManagerFactory;

The META-INF/persistence.xml file looks as follows:

Example 316. META-INF/persistence.xml configuration file
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
             http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
             version="2.1">

    <persistence-unit name="CRM">
        <description>
            Persistence unit for Hibernate User Guide
        </description>

        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

        <class>org.hibernate.documentation.userguide.Document</class>

        <properties>
            <property name="jakarta.persistence.jdbc.driver"
                      value="org.h2.Driver" />

            <property name="jakarta.persistence.jdbc.url"
                      value="jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1" />

            <property name="jakarta.persistence.jdbc.user"
                      value="sa" />

            <property name="jakarta.persistence.jdbc.password"
                      value="" />

            <property name="hibernate.show_sql"
                      value="true" />

            <property name="hibernate.hbm2ddl.auto"
                      value="update" />
        </properties>

    </persistence-unit>

</persistence>

For compliant application-bootstrapping, rather than the container building the EntityManagerFactory for the application, the application builds the EntityManagerFactory itself using the jakarta.persistence.Persistence bootstrap class. The application creates an EntityManagerFactory by calling the createEntityManagerFactory method:

Example 317. Application bootstrapped EntityManagerFactory
// Create an EMF for our CRM persistence-unit.
EntityManagerFactory emf = Persistence.createEntityManagerFactory("CRM");

If you don’t want to provide a persistence.xml configuration file, Jakarta Persistence allows you to provide all the configuration options in a PersistenceUnitInfo implementation and call HibernatePersistenceProvider.html#createContainerEntityManagerFactory.

To inject the default Persistence Context, you can use the @PersistenceContext annotation.

Example 318. Inject the default EntityManager
@PersistenceContext
private EntityManager em;

To inject a specific Persistence Context, you can use the @PersistenceContext annotation, and you can even pass EntityManager-specific properties using the @PersistenceProperty annotation.

Example 319. Inject a configurable EntityManager
 @PersistenceContext(
     unitName = "CRM",
     properties = {
         @PersistenceProperty(
             name="org.hibernate.flushMode",
             value= "MANUAL"
        )
     }
)
 private EntityManager entityManager;

If you would like additional details on accessing and using EntityManager instances, sections 7.6 and 7.7 of the Jakarta Persistence specification cover container-managed and application-managed EntityManagers, respectively.

3.2.2. Externalizing XML mapping files

Jakarta Persistence offers two mapping options:

  • annotations

  • XML mappings

Although annotations are much more common, there are projects where XML mappings are preferred. You can even mix annotations and XML mappings so that you can override annotation mappings with XML configurations that can be easily changed without recompiling the project source code. This is possible because if there are two conflicting mappings, the XML mappings take precedence over its annotation counterpart.

The Jakarta Persistence specification requires the XML mappings to be located on the classpath:

An object/relational mapping XML file named orm.xml may be specified in the META-INF directory in the root of the persistence unit or in the META-INF directory of any jar file referenced by the persistence.xml.

Alternatively, or in addition, one or more mapping files may be referenced by the mapping-file elements of the persistence-unit element. These mapping files may be present anywhere on the classpath.

— Section 8.2.1.6.2 of the Jakarta Persistence

Therefore, the mapping files can reside in the application jar artifacts, or they can be stored in an external folder location with the cogitation that that location be included in the classpath.

Hibernate is more lenient in this regard so you can use any external location even outside of the application configured classpath.

Example 320. META-INF/persistence.xml configuration file for external XML mappings
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
             http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
             version="2.1">

    <persistence-unit name="CRM">
        <description>
            Persistence unit for Hibernate User Guide
        </description>

        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

        <mapping-file>file:///etc/opt/app/mappings/orm.xml</mapping-file>

        <properties>
            <property name="jakarta.persistence.jdbc.driver"
                      value="org.h2.Driver" />

            <property name="jakarta.persistence.jdbc.url"
                      value="jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1" />

            <property name="jakarta.persistence.jdbc.user"
                      value="sa" />

            <property name="jakarta.persistence.jdbc.password"
                      value="" />

            <property name="hibernate.show_sql"
                      value="true" />

            <property name="hibernate.hbm2ddl.auto"
                      value="update" />
        </properties>

    </persistence-unit>

</persistence>

In the persistence.xml configuration file above, the orm.xml XML file containing all Jakarta Persistence entity mappings is located in the /etc/opt/app/mappings/ folder.

3.2.3. Configuring the SessionFactory Metadata via the Jakarta Persistence bootstrap

As previously seen, the Hibernate native bootstrap mechanism allows you to customize a great variety of configurations which are passed via the Metadata object.

When using Hibernate as a Jakarta Persistence provider, the EntityManagerFactory is backed by a SessionFactory. For this reason, you might still want to use the Metadata object to pass various settings which cannot be supplied via the standard Hibernate configuration settings.

For this reason, you can use the MetadataBuilderContributor class as you can see in the following examples.

Example 321. Implementing a MetadataBuilderContributor
public class SqlFunctionMetadataBuilderContributor
        implements MetadataBuilderContributor {

    @Override
    public void contribute(MetadataBuilder metadataBuilder) {
        metadataBuilder.applySqlFunction(
            "instr", new StandardSQLFunction( "instr", StandardBasicTypes.INTEGER )
        );
    }
}

org.hibernate.orm.test.bootstrap.spi.metadatabuildercontributor

The above MetadataBuilderContributor is used to register a SqlFuction which is not defined by the currently running Hibernate Dialect, but which we need to reference in our JPQL queries.

By having access to the MetadataBuilder class that’s used by the underlying SessionFactory, the Jakarta Persistence bootstrap becomes just as flexible as the Hibernate native bootstrap mechanism.

You can then pass the custom MetadataBuilderContributor via the hibernate.metadata_builder_contributor configuration property as explained in the Configuration chapter.

4. Schema generation

Hibernate allows you to generate the database from the entity mappings.

Although the automatic schema generation is very useful for testing and prototyping purposes, in a production environment, it’s much more flexible to manage the schema using incremental migration scripts.

Traditionally, the process of generating schema from entity mapping has been called HBM2DDL. To get a list of Hibernate-native and Jakarta Persistence-specific configuration properties consider reading the Configurations section.

Considering the following Domain Model:

Example 322. Schema generation Domain Model
Unresolved directive in chapters/schema/Schema.adoc - include::../../../../../test/java/org/hibernate/userguide/schema/SchemaGenerationTest.java[tags=schema-generation-domain-model-example]

If the hibernate.hbm2ddl.auto configuration is set to create, Hibernate is going to generate the following database schema:

Example 323. Auto-generated database schema
create table Customer (
    id integer not null,
    accountsPayableXrefId binary,
    image blob,
    name varchar(255),
    primary key (id)
)

create table Book (
    id bigint not null,
    isbn varchar(255),
    title varchar(255),
    author_id bigint,
    primary key (id)
)

create table Person (
    id bigint not null,
    name varchar(255),
    primary key (id)
)

alter table Book
    add constraint UK_u31e1frmjp9mxf8k8tmp990i unique (isbn)

alter table Book
    add constraint FKrxrgiajod1le3gii8whx2doie
    foreign key (author_id)
    references Person

4.1. Importing script files

To customize the schema generation process, the hibernate.hbm2ddl.import_files configuration property must be used to provide other scripts files that Hibernate can use when the SessionFactory is started.

For instance, considering the following schema-generation.sql import file:

Example 324. Schema generation import file
create sequence book_sequence start with 1 increment by 1

If we configure Hibernate to import the script above:

Example 325. Enabling schema generation import file
<property
    name="hibernate.hbm2ddl.import_files"
    value="schema-generation.sql" />

Hibernate is going to execute the script file after the schema is automatically generated.

4.2. Database objects

Hibernate allows you to customize the schema generation process via the HBM database-object element.

Considering the following HBM mapping:

Example 326. Schema generation HBM database-object
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd" >

<hibernate-mapping>
    <database-object>
        <create>
            CREATE OR REPLACE FUNCTION sp_count_books(
                IN authorId bigint,
                OUT bookCount bigint)
                RETURNS bigint AS
            $BODY$
                BEGIN
                    SELECT COUNT(*) INTO bookCount
                    FROM book
                    WHERE author_id = authorId;
                END;
            $BODY$
            LANGUAGE plpgsql;
        </create>
        <drop></drop>
        <dialect-scope name="org.hibernate.dialect.PostgreSQL95Dialect" />
    </database-object>
</hibernate-mapping>

When the SessionFactory is bootstrapped, Hibernate is going to execute the database-object, therefore creating the sp_count_books function.

4.3. Database-level checks

Hibernate offers the @Check annotation so that you can specify an arbitrary SQL CHECK constraint which can be defined as follows:

Example 327. Database check entity mapping example
@Entity(name = "Book")
@Check(constraints = "CASE WHEN isbn IS NOT NULL THEN LENGTH(isbn) = 13 ELSE true END")
public static class Book {

	@Id
	private Long id;

	private String title;

	@NaturalId
	private String isbn;

	private Double price;

	//Getters and setters omitted for brevity

}

Now, if you try to add a Book entity with an isbn attribute whose length is not 13 characters, a ConstraintViolationException is going to be thrown.

Example 328. Database check failure example
Book book = new Book();
book.setId(1L);
book.setPrice(49.99d);
book.setTitle("High-Performance Java Persistence");
book.setIsbn("11-11-2016");

entityManager.persist(book);
INSERT  INTO Book (isbn, price, title, id)
VALUES  ('11-11-2016', 49.99, 'High-Performance Java Persistence', 1)

-- WARN SqlExceptionHelper:129 - SQL Error: 0, SQLState: 23514
-- ERROR SqlExceptionHelper:131 - ERROR: new row for relation "book" violates check constraint "book_isbn_check"

4.4. Default value for a database column

With Hibernate, you can specify a default value for a given database column using the @ColumnDefault annotation.

Example 329. @ColumnDefault mapping example
@Entity(name = "Person")
@DynamicInsert
public static class Person {

    @Id
    private Long id;

    @ColumnDefault("'N/A'")
    private String name;

    @ColumnDefault("-1")
    private Long clientId;

    //Getter and setters omitted for brevity

}
CREATE TABLE Person (
  id BIGINT NOT NULL,
  clientId BIGINT DEFAULT -1,
  name VARCHAR(255) DEFAULT 'N/A',
  PRIMARY KEY (id)
)

In the mapping above, both the name and clientId table columns have a DEFAULT value.

The Person entity above is annotated with the @DynamicInsert annotation so that the INSERT statement does not include any entity attribute which is null.

This way, when the name and or clientId attribute is null, the database will set them according to their declared default values.

Example 330. @ColumnDefault mapping example
doInJPA(this::entityManagerFactory, entityManager -> {
    Person person = new Person();
    person.setId(1L);
    entityManager.persist(person);
});
doInJPA(this::entityManagerFactory, entityManager -> {
    Person person = entityManager.find(Person.class, 1L);
    assertEquals("N/A", person.getName());
    assertEquals(Long.valueOf(-1L), person.getClientId());
});
INSERT INTO Person (id) VALUES (?)

If the column value should be generated not only when a row is inserted, but also when it’s updated, the @GeneratedColumn annotation should be used.

4.5. Columns unique constraint

The @UniqueConstraint annotation is used to specify a unique constraint to be included by the automated schema generator for the primary or secondary table associated with the current annotated entity.

Considering the following entity mapping, Hibernate generates the unique constraint DDL when creating the database schema:

Example 331. @UniqueConstraint mapping example
 @Entity
 @Table(
     name = "book",
     uniqueConstraints =  @UniqueConstraint(
         name = "uk_book_title_author",
         columnNames = {
             "title",
             "author_id"
         }
    )
)
 public static class Book {

     @Id
     @GeneratedValue
     private Long id;

     private String title;

     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(
         name = "author_id",
         foreignKey = @ForeignKey(name = "fk_book_author_id")
    )
     private Author author;

     //Getter and setters omitted for brevity
 }

 @Entity
 @Table(name = "author")
 public static class Author {

     @Id
     @GeneratedValue
     private Long id;

     @Column(name = "first_name")
     private String firstName;

     @Column(name = "last_name")
     private String lastName;

     //Getter and setters omitted for brevity
 }
create table author (
    id bigint not null,
    first_name varchar(255),
    last_name varchar(255),
    primary key (id)
)

create table book (
    id bigint not null,
    title varchar(255),
    author_id bigint,
    primary key (id)
)

alter table book
   add constraint uk_book_title_author
   unique (title, author_id)

alter table book
   add constraint fk_book_author_id
   foreign key (author_id)
   references author

With the uk_book_title_author unique constraint in place, it’s no longer possible to add two books with the same title and for the same author.

Example 332. @UniqueConstraintTest persist example
     Author _author = doInJPA(this::entityManagerFactory, entityManager -> {
         Author author = new Author();
         author.setFirstName("Vlad");
         author.setLastName("Mihalcea");
         entityManager.persist(author);

         Book book = new Book();
         book.setTitle("High-Performance Java Persistence");
         book.setAuthor(author);
         entityManager.persist(book);

         return author;
     });

     try {
         doInJPA(this::entityManagerFactory, entityManager -> {
	Book book = new Book();
	book.setTitle("High-Performance Java Persistence");
	book.setAuthor(_author);
	entityManager.persist(book);
});
     }
     catch (Exception expected) {
         assertNotNull(ExceptionUtil.findCause(expected, ConstraintViolationException.class));
     }
insert
into
    author
    (first_name, last_name, id)
values
    (?, ?, ?)

-- binding parameter [1] as [VARCHAR] - [Vlad]
-- binding parameter [2] as [VARCHAR] - [Mihalcea]
-- binding parameter [3] as [BIGINT]  - [1]

insert
into
    book
    (author_id, title, id)
values
    (?, ?, ?)

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [VARCHAR] - [High-Performance Java Persistence]
-- binding parameter [3] as [BIGINT]  - [2]

insert
into
    book
    (author_id, title, id)
values
    (?, ?, ?)

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [VARCHAR] - [High-Performance Java Persistence]
-- binding parameter [3] as [BIGINT]  - [3]

-- SQL Error: 23505, SQLState: 23505
-- Unique index or primary key violation: "UK_BOOK_TITLE_AUTHOR_INDEX_1 ON PUBLIC.BOOK(TITLE, AUTHOR_ID) VALUES ( /* key:1 */ 3, 'High-Performance Java Persistence', 1)";

The second INSERT statement fails because of the unique constraint violation.

4.6. Columns index

The @Index annotation is used by the automated schema generation tool to create a database index.

Considering the following entity mapping. Hibernate generates the index when creating the database schema:

Example 333. @Index mapping example
 @Entity
 @Table(
     name = "author",
     indexes =  @Index(
         name = "idx_author_first_last_name",
         columnList = "first_name, last_name",
         unique = false
    )
)
 public static class Author {

     @Id
     @GeneratedValue
     private Long id;

     @Column(name = "first_name")
     private String firstName;

     @Column(name = "last_name")
     private String lastName;

     //Getter and setters omitted for brevity
 }
create table author (
    id bigint not null,
    first_name varchar(255),
    last_name varchar(255),
    primary key (id)
)

create index idx_author_first_last_name
    on author (first_name, last_name)

5. Persistence Context

Both the org.hibernate.Session API and jakarta.persistence.EntityManager API represent a context for dealing with persistent data. This concept is called a persistence context. Persistent data has a state in relation to both a persistence context and the underlying database.

transient

the entity has just been instantiated and is not associated with a persistence context. It has no persistent representation in the database and typically no identifier value has been assigned (unless the assigned generator was used).

managed or persistent

the entity has an associated identifier and is associated with a persistence context. It may or may not physically exist in the database yet.

detached

the entity has an associated identifier but is no longer associated with a persistence context (usually because the persistence context was closed or the instance was evicted from the context)

removed

the entity has an associated identifier and is associated with a persistence context, however, it is scheduled for removal from the database.

Much of the org.hibernate.Session and jakarta.persistence.EntityManager methods deal with moving entities among these states.

5.1. Accessing Hibernate APIs from Jakarta Persistence

Jakarta Persistence defines an incredibly useful method to allow applications access to the APIs of the underlying provider.

Example 334. Accessing Hibernate APIs from Jakarta Persistence
Session session = entityManager.unwrap(Session.class);
SessionImplementor sessionImplementor = entityManager.unwrap(SessionImplementor.class);

SessionFactory sessionFactory = entityManager.getEntityManagerFactory().unwrap(SessionFactory.class);

5.2. Bytecode Enhancement

Hibernate "grew up" not supporting bytecode enhancement at all. At that time, Hibernate only supported proxy-based alternative for lazy loading and always used diff-based dirty calculation. Hibernate 3.x saw the first attempts at bytecode enhancement support in Hibernate. We consider those initial attempts (up until 5.0) completely as an incubation. The support for bytecode enhancement in 5.0 onward is what we are discussing here.

See Bytecode Enhancement for discussion of performing enhancement.

5.2.1. Lazy attribute loading

Think of this as partial loading support. Essentially, you can tell Hibernate that only part(s) of an entity should be loaded upon fetching from the database and when the other part(s) should be loaded as well. Note that this is very much different from the proxy-based idea of lazy loading which is entity-centric where the entity’s state is loaded at once as needed. With bytecode enhancement, individual attributes or groups of attributes are loaded as needed.

Lazy attributes can be designated to be loaded together, and this is called a "lazy group". By default, all singular attributes are part of a single group, meaning that when one lazy singular attribute is accessed all lazy singular attributes are loaded. Lazy plural attributes, by default, are each a lazy group by themselves. This behavior is explicitly controllable through the @org.hibernate.annotations.LazyGroup annotation.

Example 335. @LazyGroup example
@Entity
public class Customer {

	@Id
	private Integer id;

	private String name;

	@Basic(fetch = FetchType.LAZY)
	private UUID accountsPayableXrefId;

	@Lob
	@Basic(fetch = FetchType.LAZY)
	@LazyGroup("lobs")
	private Blob image;

	//Getters and setters are omitted for brevity

}

In the above example, we have 2 lazy attributes: accountsPayableXrefId and image. Each is part of a different fetch group (accountsPayableXrefId is part of the default fetch group), which means that accessing accountsPayableXrefId will not force the loading of the image attribute, and vice-versa.

As a hopefully temporary legacy hold-over, it is currently required that all lazy singular associations (many-to-one and one-to-one) also include @LazyToOne(LazyToOneOption.NO_PROXY). The plan is to relax that requirement later.

5.2.2. In-line dirty tracking

Historically Hibernate only supported diff-based dirty calculation for determining which entities in a persistence context have changed. This essentially means that Hibernate would keep track of the last known state of an entity in regards to the database (typically the last read or write). Then, as part of flushing the persistence context, Hibernate would walk every entity associated with the persistence context and check its current state against that "last known database state". This is by far the most thorough approach to dirty checking because it accounts for data-types that can change their internal state (java.util.Date is the prime example of this). However, in a persistence context with a large number of associated entities, it can also be a performance-inhibiting approach.

If your application does not need to care about "internal state changing data-type" use cases, bytecode-enhanced dirty tracking might be a worthwhile alternative to consider, especially in terms of performance. In this approach Hibernate will manipulate the bytecode of your classes to add "dirty tracking" directly to the entity, allowing the entity itself to keep track of which of its attributes have changed. During the flush time, Hibernate asks your entity what has changed rather than having to perform the state-diff calculations.

5.2.3. Bidirectional association management

Hibernate strives to keep your application as close to "normal Java usage" (idiomatic Java) as possible. Consider a domain model with a normal Person/Book bidirectional association:

Example 336. Bidirectional association
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	@OneToMany(mappedBy = "author")
	private List<Book> books = new ArrayList<>();

	//Getters and setters are omitted for brevity

}

@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	@NaturalId
	private String isbn;

	@ManyToOne
	private Person author;

	//Getters and setters are omitted for brevity

}
Example 337. Incorrect normal Java usage
Person person = new Person();
person.setName("John Doe");

Book book = new Book();
person.getBooks().add(book);
try {
	book.getAuthor().getName();
}
catch (NullPointerException expected) {
	// This blows up (NPE) in normal Java usage
}

This blows up in normal Java usage. The correct normal Java usage is:

Example 338. Correct normal Java usage
Person person = new Person();
person.setName("John Doe");

Book book = new Book();
person.getBooks().add(book);
book.setAuthor(person);

book.getAuthor().getName();

Bytecode-enhanced bi-directional association management makes that first example work by managing the "other side" of a bi-directional association whenever one side is manipulated.

5.2.4. Internal performance optimizations

Additionally, we use the enhancement process to add some additional code that allows us to optimize certain performance characteristics of the persistence context. These are hard to discuss without diving into a discussion of Hibernate internals.

5.3. Making entities persistent

Once you’ve created a new entity instance (using the standard new operator) it is in new state. You can make it persistent by associating it to either an org.hibernate.Session or a jakarta.persistence.EntityManager.

Example 339. Making an entity persistent with Jakarta Persistence
Person person = new Person();
person.setId(1L);
person.setName("John Doe");

entityManager.persist(person);
Example 340. Making an entity persistent with Hibernate API
Person person = new Person();
person.setId(1L);
person.setName("John Doe");

session.save(person);

org.hibernate.Session also has a method named persist which follows the exact semantics defined in the Jakarta Persistence specification for the persist method. It is this org.hibernate.Session method to which the Hibernate jakarta.persistence.EntityManager implementation delegates.

If the DomesticCat entity type has a generated identifier, the value is associated with the instance when the save or persist is called. If the identifier is not automatically generated, the manually assigned (usually natural) key value has to be set on the instance before the save or persist methods are called.

5.4. Deleting (removing) entities

Entities can also be deleted.

Example 341. Deleting an entity with Jakarta Persistence
entityManager.remove(person);
Example 342. Deleting an entity with the Hibernate API
session.delete(person);

Hibernate itself can handle deleting entities in detached state. Jakarta Persistence, however, disallows this behavior.

The implication here is that the entity instance passed to the org.hibernate.Session delete method can be either in managed or detached state, while the entity instance passed to remove on jakarta.persistence.EntityManager must be in the managed state.

5.5. Obtain an entity reference without initializing its data

Sometimes referred to as lazy loading, the ability to obtain a reference to an entity without having to load its data is hugely important. The most common case being the need to create an association between an entity and another existing entity.

Example 343. Obtaining an entity reference without initializing its data with Jakarta Persistence
Book book = new Book();
book.setAuthor(entityManager.getReference(Person.class, personId));
Example 344. Obtaining an entity reference without initializing its data with Hibernate API
Book book = new Book();
book.setId(1L);
book.setIsbn("123-456-7890");
entityManager.persist(book);
book.setAuthor(session.load(Person.class, personId));

The above works on the assumption that the entity is defined to allow lazy loading, generally through use of runtime proxies. In both cases an exception will be thrown later if the given entity does not refer to actual database state when the application attempts to use the returned proxy in any way that requires access to its data.

Unless the entity class is declared final, the proxy extends the entity class. If the entity class is final, the proxy will implement an interface instead. See the @Proxy mapping section for more info.

5.6. Obtain an entity with its data initialized

It is also quite common to want to obtain an entity along with its data (e.g. like when we need to display it in the UI).

Example 345. Obtaining an entity reference with its data initialized with Jakarta Persistence
Person person = entityManager.find(Person.class, personId);
Example 346. Obtaining an entity reference with its data initialized with Hibernate API
Person person = session.get(Person.class, personId);
Example 347. Obtaining an entity reference with its data initialized using the byId() Hibernate API
Person person = session.byId(Person.class).load(personId);

In both cases null is returned if no matching database row was found.

It’s possible to return a Java 8 Optional as well:

Example 348. Obtaining an Optional entity reference with its data initialized using the byId() Hibernate API
Optional<Person> optionalPerson = session.byId(Person.class).loadOptional(personId);

5.7. Obtain multiple entities by their identifiers

If you want to load multiple entities by providing their identifiers, calling the EntityManager#find method multiple times is not only inconvenient, but also inefficient.

While the Jakarta Persistence standard does not support retrieving multiple entities at once, other than running a JPQL or Criteria API query, Hibernate offers this functionality via the byMultipleIds method of the Hibernate Session.

The byMultipleIds method returns a MultiIdentifierLoadAccess which you can use to customize the multi-load request.

The MultiIdentifierLoadAccess interface provides several methods which you can use to change the behavior of the multi-load call:

enableOrderedReturn(boolean enabled)

This setting controls whether the returned List is ordered and positional in relation to the incoming ids. If enabled (the default), the return List is ordered and positional relative to the incoming ids. In other words, a request to multiLoad([2,1,3]) will return [Entity#2, Entity#1, Entity#3].

An important distinction is made here in regards to the handling of unknown entities depending on this "ordered return" setting. If enabled, a null is inserted into the List at the proper position(s). If disabled, the nulls are not put into the return List.

In other words, consumers of the returned ordered List would need to be able to handle null elements.

enableSessionCheck(boolean enabled)

This setting, which is disabled by default, tells Hibernate to check the first-level cache (a.k.a Session or Persistence Context) first and, if the entity is found and already managed by the Hibernate Session, the cached entity will be added to the returned List, therefore skipping it from being fetched via the multi-load query.

enableReturnOfDeletedEntities(boolean enabled)

This setting instructs Hibernate if the multi-load operation is allowed to return entities that were deleted by the current Persistence Context. A deleted entity is one which has been passed to this Session.delete or Session.remove method, but the Session was not flushed yet, meaning that the associated row was not deleted in the database table.

The default behavior is to handle them as null in the return (see enableOrderedReturn). When enabled, the result set will contain deleted entities. When disabled (which is the default behavior), deleted entities are not included in the returning List.

with(LockOptions lockOptions)

This setting allows you to pass a given LockOptions mode to the multi-load query.

with(CacheMode cacheMode)

This setting allows you to pass a given CacheMode strategy so that we can load entities from the second-level cache, therefore skipping the cached entities from being fetched via the multi-load query.

withBatchSize(int batchSize)

This setting allows you to specify a batch size for loading the entities (e.g. how many at a time).

The default is to use a batch sizing strategy defined by the Dialect.getDefaultBatchLoadSizingStrategy() method.

Any greater-than-one value here will override that default behavior.

with(RootGraph<T> graph)

The RootGraph is a Hibernate extension to the Jakarta Persistence EntityGraph contract, and this method allows you to pass a specific RootGraph to the multi-load query so that it can fetch additional relationships of the current loading entity.

Now, assuming we have 3 Person entities in the database, we can load all of them with a single call as illustrated by the following example:

Example 349. Loading multiple entities using the byMultipleIds() Hibernate API
Session session = entityManager.unwrap(Session.class);

List<Person> persons = session
		.byMultipleIds(Person.class)
		.multiLoad(1L, 2L, 3L);

assertEquals(3, persons.size());

List<Person> samePersons = session
		.byMultipleIds(Person.class)
		.enableSessionCheck(true)
		.multiLoad(1L, 2L, 3L);

assertEquals(persons, samePersons);
SELECT p.id AS id1_0_0_,
       p.name AS name2_0_0_
FROM   Person p
WHERE  p.id IN ( 1, 2, 3 )

Notice that only one SQL SELECT statement was executed since the second call uses the enableSessionCheck method of the MultiIdentifierLoadAccess to instruct Hibernate to skip entities that are already loaded in the current Persistence Context.

If the entities are not available in the current Persistence Context but they could be loaded from the second-level cache, you can use the with(CacheMode) method of the MultiIdentifierLoadAccess object.

Example 350. Loading multiple entities from the second-level cache
SessionFactory sessionFactory = entityManagerFactory().unwrap(SessionFactory.class);
Statistics statistics = sessionFactory.getStatistics();

sessionFactory.getCache().evictAll();
statistics.clear();
sqlStatementInterceptor.clear();

assertEquals(0, statistics.getQueryExecutionCount());

doInJPA(this::entityManagerFactory, entityManager -> {
	Session session = entityManager.unwrap(Session.class);

	List<Person> persons = session
		.byMultipleIds(Person.class)
		.multiLoad(1L, 2L, 3L);

	assertEquals(3, persons.size());
});

assertEquals(0, statistics.getSecondLevelCacheHitCount());
assertEquals(3, statistics.getSecondLevelCachePutCount());
assertEquals(1, sqlStatementInterceptor.getSqlQueries().size());

doInJPA(this::entityManagerFactory, entityManager -> {
	Session session = entityManager.unwrap(Session.class);
	sqlStatementInterceptor.clear();

	List<Person> persons = session.byMultipleIds(Person.class)
		.with(CacheMode.NORMAL)
		.multiLoad(1L, 2L, 3L);

	assertEquals(3, persons.size());

});

assertEquals(3, statistics.getSecondLevelCacheHitCount());
assertEquals(0, sqlStatementInterceptor.getSqlQueries().size());

In the example above, we first make sure that we clear the second-level cache to demonstrate that the multi-load query will put the returning entities into the second-level cache.

After executing the first byMultipleIds call, Hibernate is going to fetch the requested entities, and as illustrated by the getSecondLevelCachePutCount method call, 3 entities were indeed added to the shared cache.

Afterward, when executing the second byMultipleIds call for the same entities in a new Hibernate Session, we set the CacheMode.NORMAL second-level cache mode so that entities are going to be returned from the second-level cache.

The getSecondLevelCacheHitCount statistics method returns 3 this time, since the 3 entities were loaded from the second-level cache, and, as illustrated by sqlStatementInterceptor.getSqlQueries(), no multi-load SELECT statement was executed this time.

5.8. Obtain an entity by natural-id

In addition to allowing to load the entity by its identifier, Hibernate allows applications to load entities by the declared natural identifier.

Example 351. Natural-id mapping
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	@NaturalId
	private String isbn;

	@ManyToOne
	private Person author;

	//Getters and setters are omitted for brevity

}

We can also opt to fetch the entity or just retrieve a reference to it when using the natural identifier loading methods.

Example 352. Get entity reference by simple natural-id
Book book = session.bySimpleNaturalId(Book.class).getReference(isbn);
Example 353. Load entity by natural-id
Book book = session
	.byNaturalId(Book.class)
	.using("isbn", isbn)
	.load();

We can also use a Java 8 Optional to load an entity by its natural id:

Example 354. Load an Optional entity by natural-id
Optional<Book> optionalBook = session
	.byNaturalId(Book.class)
	.using("isbn", isbn)
	.loadOptional();

Hibernate offers a consistent API for accessing persistent data by identifier or by the natural-id. Each of these defines the same two data access methods:

getReference

Should be used in cases where the identifier is assumed to exist, where non-existence would be an actual error. Should never be used to test existence. That is because this method will prefer to create and return a proxy if the data is not already associated with the Session rather than hit the database. The quintessential use-case for using this method is to create foreign key based associations.

load

Will return the persistent data associated with the given identifier value or null if that identifier does not exist.

Each of these two methods defines an overloading variant accepting a org.hibernate.LockOptions argument. Locking is discussed in a separate chapter.

5.9. Filtering entities and associations

Hibernate offers two options if you want to filter entities or entity associations:

static (e.g. @Where and @WhereJoinTable)

which are defined at mapping time and cannot change at runtime.

dynamic (e.g. @Filter and @FilterJoinTable)

which are applied and configured at runtime.

5.9.1. @Where

Sometimes, you want to filter out entities or collections using custom SQL criteria. This can be achieved using the @Where annotation, which can be applied to entities and collections.

Example 355. @Where mapping usage
public enum AccountType {
	DEBIT,
	CREDIT
}

@Entity(name = "Client")
public static class Client {

	@Id
	private Long id;

	private String name;

	@Where(clause = "account_type = 'DEBIT'")
	@OneToMany(mappedBy = "client")
	private List<Account> debitAccounts = new ArrayList<>();

	@Where(clause = "account_type = 'CREDIT'")
	@OneToMany(mappedBy = "client")
	private List<Account> creditAccounts = new ArrayList<>();

	//Getters and setters omitted for brevity

}

@Entity(name = "Account")
@Where(clause = "active = true")
public static class Account {

	@Id
	private Long id;

	@ManyToOne
	private Client client;

	@Column(name = "account_type")
	@Enumerated(EnumType.STRING)
	private AccountType type;

	private Double amount;

	private Double rate;

	private boolean active;

	//Getters and setters omitted for brevity

}

If the database contains the following entities:

Example 356. Persisting and fetching entities with a @Where mapping
doInJPA(this::entityManagerFactory, entityManager -> {

	Client client = new Client();
	client.setId(1L);
	client.setName("John Doe");
	entityManager.persist(client);

	Account account1 = new Account();
	account1.setId(1L);
	account1.setType(AccountType.CREDIT);
	account1.setAmount(5000d);
	account1.setRate(1.25 / 100);
	account1.setActive(true);
	account1.setClient(client);
	client.getCreditAccounts().add(account1);
	entityManager.persist(account1);

	Account account2 = new Account();
	account2.setId(2L);
	account2.setType(AccountType.DEBIT);
	account2.setAmount(0d);
	account2.setRate(1.05 / 100);
	account2.setActive(false);
	account2.setClient(client);
	client.getDebitAccounts().add(account2);
	entityManager.persist(account2);

	Account account3 = new Account();
	account3.setType(AccountType.DEBIT);
	account3.setId(3L);
	account3.setAmount(250d);
	account3.setRate(1.05 / 100);
	account3.setActive(true);
	account3.setClient(client);
	client.getDebitAccounts().add(account3);
	entityManager.persist(account3);
});
INSERT INTO Client (name, id)
VALUES ('John Doe', 1)

INSERT INTO Account (active, amount, client_id, rate, account_type, id)
VALUES (true, 5000.0, 1, 0.0125, 'CREDIT', 1)

INSERT INTO Account (active, amount, client_id, rate, account_type, id)
VALUES (false, 0.0, 1, 0.0105, 'DEBIT', 2)

INSERT INTO Account (active, amount, client_id, rate, account_type, id)
VALUES (true, 250.0, 1, 0.0105, 'DEBIT', 3)

When executing an Account entity query, Hibernate is going to filter out all records that are not active.

Example 357. Query entities mapped with @Where
doInJPA(this::entityManagerFactory, entityManager -> {
	List<Account> accounts = entityManager.createQuery(
		"select a from Account a", Account.class)
	.getResultList();
	assertEquals(2, accounts.size());
});
SELECT
    a.id as id1_0_,
    a.active as active2_0_,
    a.amount as amount3_0_,
    a.client_id as client_i6_0_,
    a.rate as rate4_0_,
    a.account_type as account_5_0_
FROM
    Account a
WHERE ( a.active = true )

When fetching the debitAccounts or the creditAccounts collections, Hibernate is going to apply the @Where clause filtering criteria to the associated child entities.

Example 358. Traversing collections mapped with @Where
doInJPA(this::entityManagerFactory, entityManager -> {
	Client client = entityManager.find(Client.class, 1L);
	assertEquals(1, client.getCreditAccounts().size());
	assertEquals(1, client.getDebitAccounts().size());
});
SELECT
    c.client_id as client_i6_0_0_,
    c.id as id1_0_0_,
    c.id as id1_0_1_,
    c.active as active2_0_1_,
    c.amount as amount3_0_1_,
    c.client_id as client_i6_0_1_,
    c.rate as rate4_0_1_,
    c.account_type as account_5_0_1_
FROM
    Account c
WHERE ( c.active = true and c.account_type = 'CREDIT' ) AND c.client_id = 1

SELECT
    d.client_id as client_i6_0_0_,
    d.id as id1_0_0_,
    d.id as id1_0_1_,
    d.active as active2_0_1_,
    d.amount as amount3_0_1_,
    d.client_id as client_i6_0_1_,
    d.rate as rate4_0_1_,
    d.account_type as account_5_0_1_
FROM
    Account d
WHERE ( d.active = true and d.account_type = 'DEBIT' ) AND d.client_id = 1

5.9.2. @WhereJoinTable

Just like @Where annotation, @WhereJoinTable is used to filter out collections using a joined table (e.g. @ManyToMany association).

Example 359. @WhereJoinTable mapping example
@Entity(name = "Book")
public static class Book {

	@Id
	private Long id;

	private String title;

	private String author;

	@ManyToMany
	@JoinTable(
		name = "Book_Reader",
		joinColumns = @JoinColumn(name = "book_id"),
		inverseJoinColumns = @JoinColumn(name = "reader_id")
	)
	@WhereJoinTable(clause = "created_on > DATEADD('DAY', -7, CURRENT_TIMESTAMP())")
	private List<Reader> currentWeekReaders = new ArrayList<>();

	//Getters and setters omitted for brevity

}

@Entity(name = "Reader")
public static class Reader {

	@Id
	private Long id;

	private String name;

	//Getters and setters omitted for brevity

}
create table Book (
    id bigint not null,
    author varchar(255),
    title varchar(255),
    primary key (id)
)

create table Book_Reader (
    book_id bigint not null,
    reader_id bigint not null
)

create table Reader (
    id bigint not null,
    name varchar(255),
    primary key (id)
)

alter table Book_Reader
    add constraint FKsscixgaa5f8lphs9bjdtpf9g
    foreign key (reader_id)
    references Reader

alter table Book_Reader
    add constraint FKoyrwu9tnwlukd1616qhck21ra
    foreign key (book_id)
    references Book

alter table Book_Reader
    add created_on timestamp
    default current_timestamp

In the example above, the current week Reader entities are included in the currentWeekReaders collection which uses the @WhereJoinTable annotation to filter the joined table rows according to the provided SQL clause.

Considering that the following two Book_Reader entries are added into our system:

Example 360. @WhereJoinTable test data
Book book = new Book();
book.setId(1L);
book.setTitle("High-Performance Java Persistence");
book.setAuthor("Vad Mihalcea");
entityManager.persist(book);

Reader reader1 = new Reader();
reader1.setId(1L);
reader1.setName("John Doe");
entityManager.persist(reader1);

Reader reader2 = new Reader();
reader2.setId(2L);
reader2.setName("John Doe Jr.");
entityManager.persist(reader2);

statement.executeUpdate(
	"INSERT INTO Book_Reader " +
	"	(book_id, reader_id) " +
	"VALUES " +
	"	(1, 1) "
);
statement.executeUpdate(
	"INSERT INTO Book_Reader " +
	"	(book_id, reader_id, created_on) " +
	"VALUES " +
	"	(1, 2, DATEADD('DAY', -10, CURRENT_TIMESTAMP())) "
);

When fetching the currentWeekReaders collection, Hibernate is going to find only one entry:

Example 361. @WhereJoinTable fetch example
Book book = entityManager.find(Book.class, 1L);
assertEquals(1, book.getCurrentWeekReaders().size());

5.9.3. @Filter

The @Filter annotation is another way to filter out entities or collections using custom SQL criteria. Unlike the @Where annotation, @Filter allows you to parameterize the filter clause at runtime.

Now, considering we have the following Account entity:

Example 362. @Filter mapping entity-level usage
 @Entity(name = "Account")
 @Table(name = "account")
 @FilterDef(
     name="activeAccount",
     parameters = @ParamDef(
         name="active",
         type=Boolean.class
    )
)
 @Filter(
     name="activeAccount",
     condition="active_status = :active"
)
 public static class Account {

     @Id
     private Long id;

     @ManyToOne(fetch = FetchType.LAZY)
     private Client client;

     @Column(name = "account_type")
     @Enumerated(EnumType.STRING)
     private AccountType type;

     private Double amount;

     private Double rate;

     @Column(name = "active_status")
     private boolean active;

     //Getters and setters omitted for brevity
 }

Notice that the active property is mapped to the active_status column.

This mapping was done to show you that the @Filter condition uses a SQL condition and not a JPQL filtering predicate.

As already explained, we can also apply the @Filter annotation for collections as illustrated by the Client entity:

Example 363. @Filter mapping collection-level usage
@Entity(name = "Client")
@Table(name = "client")
public static class Client {

    @Id
    private Long id;

    private String name;

    private AccountType type;

    @OneToMany(
        mappedBy = "client",
        cascade = CascadeType.ALL
   )
    @Filter(
        name="activeAccount",
        condition="active_status = :active"
   )
    private List<Account> accounts = new ArrayList<>();

    //Getters and setters omitted for brevity

    public void addAccount(Account account) {
        account.setClient(this);
        this.accounts.add(account);
    }
}

If we persist a Client with three associated Account entities, Hibernate will execute the following SQL statements:

Example 364. Persisting and fetching entities with a @Filter mapping
 Client client = new Client()
 .setId(1L)
 .setName("John Doe")
 .setType(AccountType.DEBIT);

 client.addAccount(
     new Account()
     .setId(1L)
     .setType(AccountType.CREDIT)
     .setAmount(5000d)
     .setRate(1.25 / 100)
     .setActive(true)
);

 client.addAccount(
     new Account()
     .setId(2L)
     .setType(AccountType.DEBIT)
     .setAmount(0d)
     .setRate(1.05 / 100)
     .setActive(false)
);

 client.addAccount(
     new Account()
     .setType(AccountType.DEBIT)
     .setId(3L)
     .setAmount(250d)
     .setRate(1.05 / 100)
     .setActive(true)
);

 entityManager.persist(client);
INSERT INTO Client (name, id)
VALUES ('John Doe', 1)

INSERT INTO Account (active_status, amount, client_id, rate, account_type, id)
VALUES (true, 5000.0, 1, 0.0125, 'CREDIT', 1)

INSERT INTO Account (active_status, amount, client_id, rate, account_type, id)
VALUES (false, 0.0, 1, 0.0105, 'DEBIT', 2)

INSERT INTO Account (active_status, amount, client_id, rate, account_type, id)
VALUES (true, 250.0, 1, 0.0105, 'DEBIT', 3)

By default, without explicitly enabling the filter, Hibernate is going to fetch all Account entities.

Example 365. Query entities mapped without activating the @Filter
List<Account> accounts = entityManager.createQuery(
    "select a from Account a", Account.class)
.getResultList();

assertEquals(3, accounts.size());
SELECT
    a.id as id1_0_,
    a.active_status as active2_0_,
    a.amount as amount3_0_,
    a.client_id as client_i6_0_,
    a.rate as rate4_0_,
    a.account_type as account_5_0_
FROM
    Account a

If the filter is enabled and the filter parameter value is provided, then Hibernate is going to apply the filtering criteria to the associated Account entities.

Example 366. Query entities mapped with @Filter
entityManager
    .unwrap(Session.class)
    .enableFilter("activeAccount")
    .setParameter("active", true);

List<Account> accounts = entityManager.createQuery(
    "select a from Account a", Account.class)
.getResultList();

assertEquals(2, accounts.size());
SELECT
    a.id as id1_0_,
    a.active_status as active2_0_,
    a.amount as amount3_0_,
    a.client_id as client_i6_0_,
    a.rate as rate4_0_,
    a.account_type as account_5_0_
FROM
    Account a
WHERE
    a.active_status = true

Filters apply to entity queries and to direct fetching.

Therefore, even in the following example, the filter is taken into consideration when fetching an entity from the Persistence Context.

Fetching entities mapped with @Filter
entityManager
    .unwrap(Session.class)
    .enableFilter("activeAccount")
    .setParameter("active", true);

Account account = entityManager.find(Account.class, 2L);

assertNull(account);
SELECT
    a.id as id1_0_0_,
    a.active_status as active2_0_0_,
    a.amount as amount3_0_0_,
    a.client_id as client_i6_0_0_,
    a.rate as rate4_0_0_,
    a.account_type as account_5_0_0_,
    c.id as id1_1_1_,
    c.name as name2_1_1_
FROM
    Account a
WHERE
    a.id = 2

As you can see from the example above, contrary to an entity query, the filter does not prevent the entity from being loaded.

Just like with entity queries, collections can be filtered as well, but only if the filter is explicitly enabled on the currently running Hibernate Session.

Example 367. Traversing collections without activating the @Filter
Client client = entityManager.find(Client.class, 1L);

assertEquals(3, client.getAccounts().size());
SELECT
    c.id as id1_1_0_,
    c.name as name2_1_0_
FROM
    Client c
WHERE
    c.id = 1

SELECT
    a.id as id1_0_,
    a.active_status as active2_0_,
    a.amount as amount3_0_,
    a.client_id as client_i6_0_,
    a.rate as rate4_0_,
    a.account_type as account_5_0_
FROM
    Account a
WHERE
    a.client_id = 1

When activating the @Filter and fetching the accounts collections, Hibernate is going to apply the filter condition to the associated collection entries.

Example 368. Traversing collections mapped with @Filter
entityManager
    .unwrap(Session.class)
    .enableFilter("activeAccount")
    .setParameter("active", true);

Client client = entityManager.find(Client.class, 1L);

assertEquals(2, client.getAccounts().size());
SELECT
    c.id as id1_1_0_,
    c.name as name2_1_0_
FROM
    Client c
WHERE
    c.id = 1

SELECT
    a.id as id1_0_,
    a.active_status as active2_0_,
    a.amount as amount3_0_,
    a.client_id as client_i6_0_,
    a.rate as rate4_0_,
    a.account_type as account_5_0_
FROM
    Account a
WHERE
    accounts0_.active_status = true
    and a.client_id = 1

The main advantage of @Filter over the @Where clause is that the filtering criteria can be customized at runtime.

It’s not possible to combine the @Filter and @Cache collection annotations. This limitation is due to ensuring consistency and because the filtering information is not stored in the second-level cache.

If caching were allowed for a currently filtered collection, then the second-level cache would store only a subset of the whole collection. Afterward, every other Session will get the filtered collection from the cache, even if the Session-level filters have not been explicitly activated.

For this reason, the second-level collection cache is limited to storing whole collections, and not subsets.

5.9.4. @Filter with @SqlFragmentAlias

When using the @Filter annotation and working with entities that are mapped onto multiple database tables, you will need to use the @SqlFragmentAlias annotation if the @Filter defines a condition that uses predicates across multiple tables.

Example 369. @SqlFragmentAlias mapping usage
@Entity(name = "Account")
@Table(name = "account")
@SecondaryTable(
	name = "account_details"
)
@SQLDelete(
	sql = "UPDATE account_details SET deleted = true WHERE id = ? "
)
@FilterDef(
	name="activeAccount",
	parameters = @ParamDef(
		name="active",
		type=Boolean.class
	)
)
@Filter(
	name="activeAccount",
	condition="{a}.active = :active and {ad}.deleted = false",
	aliases = {
		@SqlFragmentAlias(alias = "a", table= "account"),
		@SqlFragmentAlias(alias = "ad", table= "account_details"),
	}
)
public static class Account {

	@Id
	private Long id;

	private Double amount;

	private Double rate;

	private boolean active;

	@Column(table = "account_details")
	private boolean deleted;

	//Getters and setters omitted for brevity

}

Now, when fetching the Account entities and activating the filter, Hibernate is going to apply the right table aliases to the filter predicates:

Example 370. Fetching a collection filtered with @SqlFragmentAlias
entityManager
	.unwrap(Session.class)
	.enableFilter("activeAccount")
	.setParameter("active", true);

List<Account> accounts = entityManager.createQuery(
	"select a from Account a", Account.class)
.getResultList();
select
    filtersqlf0_.id as id1_0_,
    filtersqlf0_.active as active2_0_,
    filtersqlf0_.amount as amount3_0_,
    filtersqlf0_.rate as rate4_0_,
    filtersqlf0_1_.deleted as deleted1_1_
from
    account filtersqlf0_
left outer join
    account_details filtersqlf0_1_
        on filtersqlf0_.id=filtersqlf0_1_.id
where
    filtersqlf0_.active = ?
    and filtersqlf0_1_.deleted = false

-- binding parameter [1] as [BOOLEAN] - [true]

5.9.5. @FilterJoinTable

When using the @Filter annotation with collections, the filtering is done against the child entries (entities or embeddables). However, if you have a link table between the parent entity and the child table, then you need to use the @FilterJoinTable to filter child entries according to some column contained in the join table.

The @FilterJoinTable annotation can be, therefore, applied to a unidirectional @OneToMany collection as illustrated in the following mapping:

Example 371. @FilterJoinTable mapping usage
 @Entity(name = "Client")
 @FilterDef(
     name="firstAccounts",
     parameters=@ParamDef(
         name="maxOrderId",
         type=int.class
    )
)
 public static class Client {

     @Id
     private Long id;

     private String name;

     @OneToMany(cascade = CascadeType.ALL)
     @OrderColumn(name = "order_id")
     @FilterJoinTable(
         name="firstAccounts",
         condition="order_id <= :maxOrderId"
    )
     private List<Account> accounts = new ArrayList<>();

     //Getters and setters omitted for brevity

     public void addAccount(Account account) {
         this.accounts.add(account);
     }
 }

 @Entity(name = "Account")
 public static class Account {

     @Id
     private Long id;

     @Column(name = "account_type")
     @Enumerated(EnumType.STRING)
     private AccountType type;

     private Double amount;

     private Double rate;

     //Getters and setters omitted for brevity
 }

The firstAccounts filter will allow us to get only the Account entities that have the order_id (which tells the position of every entry inside the accounts collection) less than a given number (e.g. maxOrderId).

Let’s assume our database contains the following entities:

Example 372. Persisting and fetching entities with a @FilterJoinTable mapping
 Client client = new Client()
 .setId(1L)
 .setName("John Doe");

 client.addAccount(
     new Account()
     .setId(1L)
     .setType(AccountType.CREDIT)
     .setAmount(5000d)
     .setRate(1.25 / 100)
);

 client.addAccount(
     new Account()
     .setId(2L)
     .setType(AccountType.DEBIT)
     .setAmount(0d)
     .setRate(1.05 / 100)
);

 client.addAccount(
     new Account()
     .setType(AccountType.DEBIT)
     .setId(3L)
     .setAmount(250d)
     .setRate(1.05 / 100)
);

 entityManager.persist(client);
INSERT INTO Client (name, id)
VALUES ('John Doe', 1)

INSERT INTO Account (amount, client_id, rate, account_type, id)
VALUES (5000.0, 1, 0.0125, 'CREDIT', 1)

INSERT INTO Account (amount, client_id, rate, account_type, id)
VALUES (0.0, 1, 0.0105, 'DEBIT', 2)

INSERT INTO Account (amount, client_id, rate, account_type, id)
VALUES (250.0, 1, 0.0105, 'DEBIT', 3)

INSERT INTO Client_Account (Client_id, order_id, accounts_id)
VALUES (1, 0, 1)

INSERT INTO Client_Account (Client_id, order_id, accounts_id)
VALUES (1, 0, 1)

INSERT INTO Client_Account (Client_id, order_id, accounts_id)
VALUES (1, 1, 2)

INSERT INTO Client_Account (Client_id, order_id, accounts_id)
VALUES (1, 2, 3)

The collections can be filtered only if the associated filter is enabled on the currently running Hibernate Session.

Example 373. Traversing collections mapped with @FilterJoinTable without enabling the filter
Client client = entityManager.find(Client.class, 1L);

assertEquals(3, client.getAccounts().size());
SELECT
    ca.Client_id as Client_i1_2_0_,
    ca.accounts_id as accounts2_2_0_,
    ca.order_id as order_id3_0_,
    a.id as id1_0_1_,
    a.amount as amount3_0_1_,
    a.rate as rate4_0_1_,
    a.account_type as account_5_0_1_
FROM
    Client_Account ca
INNER JOIN
    Account a
ON  ca.accounts_id=a.id
WHERE
    ca.Client_id = ?

-- binding parameter [1] as [BIGINT] - [1]

If we enable the filter and set the maxOrderId to 1 when fetching the accounts collections, Hibernate is going to apply the @FilterJoinTable clause filtering criteria, and we will get just 2 Account entities, with the order_id values of 0 and 1.

Example 374. Traversing collections mapped with @FilterJoinTable
Client client = entityManager.find(Client.class, 1L);

entityManager
    .unwrap(Session.class)
    .enableFilter("firstAccounts")
    .setParameter("maxOrderId", 1);

assertEquals(2, client.getAccounts().size());
SELECT
    ca.Client_id as Client_i1_2_0_,
    ca.accounts_id as accounts2_2_0_,
    ca.order_id as order_id3_0_,
    a.id as id1_0_1_,
    a.amount as amount3_0_1_,
    a.rate as rate4_0_1_,
    a.account_type as account_5_0_1_
FROM
    Client_Account ca
INNER JOIN
    Account a
ON  ca.accounts_id=a.id
WHERE
    ca.order_id <= ?
    AND ca.Client_id = ?

-- binding parameter [1] as [INTEGER] - [1]
-- binding parameter [2] as [BIGINT] - [1]

5.10. Modifying managed/persistent state

Entities in managed/persistent state may be manipulated by the application, and any changes will be automatically detected and persisted when the persistence context is flushed. There is no need to call a particular method to make your modifications persistent.

Example 375. Modifying managed state with Jakarta Persistence
Person person = entityManager.find(Person.class, personId);
person.setName("John Doe");
entityManager.flush();
Example 376. Modifying managed state with Hibernate API
Person person = session.byId(Person.class).load(personId);
person.setName("John Doe");
session.flush();

By default, when you modify an entity, all columns but the identifier are being set during update.

Therefore, considering you have the following Product entity mapping:

Example 377. Product entity mapping
@Entity(name = "Product")
public static class Product {

	@Id
	private Long id;

	@Column
	private String name;

	@Column
	private String description;

	@Column(name = "price_cents")
	private Integer priceCents;

	@Column
	private Integer quantity;

	//Getters and setters are omitted for brevity

}

If you persist the following Product entity:

Example 378. Persisting a Product entity
Product book = new Product();
book.setId(1L);
book.setName("High-Performance Java Persistence");
book.setDescription("Get the most out of your persistence layer");
book.setPriceCents(29_99);
book.setQuantity(10_000);

entityManager.persist(book);

When you modify the Product entity, Hibernate generates the following SQL UPDATE statement:

Example 379. Modifying the Product entity
doInJPA(this::entityManagerFactory, entityManager -> {
	Product book = entityManager.find(Product.class, 1L);
	book.setPriceCents(24_99);
});
UPDATE
    Product
SET
    description = ?,
    name = ?,
    price_cents = ?,
    quantity = ?
WHERE
    id = ?

-- binding parameter [1] as [VARCHAR] - [Get the most out of your persistence layer]
-- binding parameter [2] as [VARCHAR] - [High-Performance Java Persistence]
-- binding parameter [3] as [INTEGER] - [2499]
-- binding parameter [4] as [INTEGER] - [10000]
-- binding parameter [5] as [BIGINT]  - [1]

The default UPDATE statement containing all columns has two advantages:

  • it allows you to better benefit from JDBC Statement caching.

  • it allows you to enable batch updates even if multiple entities modify different properties.

However, there is also one downside to including all columns in the SQL UPDATE statement. If you have multiple indexes, the database might update those redundantly even if you don’t actually modify all column values.

To fix this issue, you can use dynamic updates.

5.10.1. Dynamic updates

To enable dynamic updates, you need to annotate the entity with the @DynamicUpdate annotation:

Example 380. Product entity mapping
@Entity(name = "Product")
@DynamicUpdate
public static class Product {

	@Id
	private Long id;

	@Column
	private String name;

	@Column
	private String description;

	@Column(name = "price_cents")
	private Integer priceCents;

	@Column
	private Integer quantity;

	//Getters and setters are omitted for brevity

}

This time, when rerunning the previous test case, Hibernate generates the following SQL UPDATE statement:

Example 381. Modifying the Product entity with a dynamic update
UPDATE
    Product
SET
    price_cents = ?
WHERE
    id = ?

-- binding parameter [1] as [INTEGER] - [2499]
-- binding parameter [2] as [BIGINT]  - [1]

The dynamic update allows you to set just the columns that were modified in the associated entity.

5.11. Refresh entity state

You can reload an entity instance and its collections at any time.

Example 382. Refreshing entity state with Jakarta Persistence
Person person = entityManager.find(Person.class, personId);

entityManager.createQuery("update Person set name = UPPER(name)").executeUpdate();

entityManager.refresh(person);
assertEquals("JOHN DOE", person.getName());
Example 383. Refreshing entity state with Hibernate API
Person person = session.byId(Person.class).load(personId);

session.doWork(connection -> {
	try(Statement statement = connection.createStatement()) {
		statement.executeUpdate("UPDATE Person SET name = UPPER(name)");
	}
});

session.refresh(person);
assertEquals("JOHN DOE", person.getName());

One case where this is useful is when it is known that the database state has changed since the data was read. Refreshing allows the current database state to be pulled into the entity instance and the persistence context.

Another case where this might be useful is when database triggers are used to initialize some of the properties of the entity.

Only the entity instance and its value type collections are refreshed unless you specify REFRESH as a cascade style of any associations. However, please note that Hibernate has the capability to handle this automatically through its notion of generated properties. See the discussion of non-identifier generated attributes.

Traditionally, Hibernate allowed detached entities to be refreshed. Unfortunately, Jakarta Persistence prohibits this practice and specifies that an IllegalArgumentException should be thrown instead.

For this reason, when bootstrapping the Hibernate SessionFactory using the native API, the legacy detached entity refresh behavior is going to be preserved. On the other hand, when bootstrapping Hibernate through the Jakarta Persistence EntityManagerFactory building process, detached entities are not allowed to be refreshed by default.

However, this default behavior can be overwritten through the hibernate.allow_refresh_detached_entity configuration property. If this property is explicitly set to true, then you can refresh detached entities even when using the Jakarta Persistence bootstraps mechanism, therefore bypassing the Jakarta Persistence specification restriction.

For more about the hibernate.allow_refresh_detached_entity configuration property, check out the Configurations section as well.

5.11.1. Refresh gotchas

The refresh entity state transition is meant to overwrite the entity attributes according to the info currently contained in the associated database record.

However, you have to be very careful when cascading the refresh action to any transient entity.

For instance, consider the following example:

Example 384. Refreshing entity state gotcha
try {
	Person person = entityManager.find(Person.class, personId);

	Book book = new Book();
	book.setId(100L);
	book.setTitle("Hibernate User Guide");
	book.setAuthor(person);
	person.getBooks().add(book);

	entityManager.refresh(person);
}
catch (EntityNotFoundException expected) {
	log.info("Beware when cascading the refresh associations to transient entities!");
}

In the aforementioned example, an EntityNotFoundException is thrown because the Book entity is still in a transient state. When the refresh action is cascaded from the Person entity, Hibernate will not be able to locate the Book entity in the database.

For this reason, you should be very careful when mixing the refresh action with transient child entity objects.

5.12. Working with detached data

Detachment is the process of working with data outside the scope of any persistence context. Data becomes detached in a number of ways. Once the persistence context is closed, all data that was associated with it becomes detached. Clearing the persistence context has the same effect. Evicting a particular entity from the persistence context makes it detached. And finally, serialization will make the deserialized form be detached (the original instance is still managed).

Detached data can still be manipulated, however, the persistence context will no longer automatically know about these modifications, and the application will need to intervene to make the changes persistent again.

5.12.1. Reattaching detached data

Reattachment is the process of taking an incoming entity instance that is in the detached state and re-associating it with the current persistence context.

Jakarta Persistence does not support reattaching detached data. This is only available through Hibernate org.hibernate.Session.

Example 385. Reattaching a detached entity using lock
Person person = session.byId(Person.class).load(personId);
//Clear the Session so the person entity becomes detached
session.clear();
person.setName("Mr. John Doe");

session.lock(person, LockMode.NONE);
Example 386. Reattaching a detached entity using saveOrUpdate
Person person = session.byId(Person.class).load(personId);
//Clear the Session so the person entity becomes detached
session.clear();
person.setName("Mr. John Doe");

session.saveOrUpdate(person);

The method name update is a bit misleading here. It does not mean that an SQL UPDATE is immediately performed. It does, however, mean that an SQL UPDATE will be performed when the persistence context is flushed since Hibernate does not know its previous state against which to compare for changes. If the entity is mapped with select-before-update, Hibernate will pull the current state from the database and see if an update is needed.

Provided the entity is detached, update and saveOrUpdate operate exactly the same.

5.12.2. Merging detached data

Merging is the process of taking an incoming entity instance that is in the detached state and copying its data over onto a new managed instance.

Although not exactly per se, the following example is a good visualization of the merge operation internals.

Example 387. Visualizing merge
public Person merge(Person detached) {
	Person newReference = session.byId(Person.class).load(detached.getId());
	newReference.setName(detached.getName());
	return newReference;
}
Example 388. Merging a detached entity with Jakarta Persistence
Person person = entityManager.find(Person.class, personId);
//Clear the EntityManager so the person entity becomes detached
entityManager.clear();
person.setName("Mr. John Doe");

person = entityManager.merge(person);
Example 389. Merging a detached entity with Hibernate API
Person person = session.byId(Person.class).load(personId);
//Clear the Session so the person entity becomes detached
session.clear();
person.setName("Mr. John Doe");

person = (Person) session.merge(person);
Merging gotchas

For example, Hibernate throws IllegalStateException when merging a parent entity which has references to 2 detached child entities child1 and child2 (obtained from different sessions), and child1 and child2 represent the same persistent entity, Child.

A new configuration property, hibernate.event.merge.entity_copy_observer, controls how Hibernate will respond when multiple representations of the same persistent entity ("entity copy") is detected while merging.

The possible values are:

disallow (the default)

throws IllegalStateException if an entity copy is detected

allow

performs the merge operation on each entity copy that is detected

log

(provided for testing only) performs the merge operation on each entity copy that is detected and logs information about the entity copies. This setting requires DEBUG logging be enabled for org.hibernate.event.internal.EntityCopyAllowedLoggedObserver

In addition, the application may customize the behavior by providing an implementation of org.hibernate.event.spi.EntityCopyObserver and setting hibernate.event.merge.entity_copy_observer to the class name. When this property is set to allow or log, Hibernate will merge each entity copy detected while cascading the merge operation. In the process of merging each entity copy, Hibernate will cascade the merge operation from each entity copy to its associations with cascade=CascadeType.MERGE or CascadeType.ALL. The entity state resulting from merging an entity copy will be overwritten when another entity copy is merged.

Because cascade order is undefined, the order in which the entity copies are merged is undefined. As a result, if property values in the entity copies are not consistent, the resulting entity state will be indeterminate, and data will be lost from all entity copies except for the last one merged. Therefore, the last writer wins.

If an entity copy cascades the merge operation to an association that is (or contains) a new entity, that new entity will be merged (i.e., persisted and the merge operation will be cascaded to its associations according to its mapping), even if that same association is ultimately overwritten when Hibernate merges a different representation having a different value for its association.

If the association is mapped with orphanRemoval = true, the new entity will not be deleted because the semantics of orphanRemoval do not apply if the entity being orphaned is a new entity.

There are known issues when representations of the same persistent entity have different values for a collection. See HHH-9239 and HHH-9240 for more details. These issues can cause data loss or corruption.

By setting hibernate.event.merge.entity_copy_observer configuration property to allow or log, Hibernate will allow entity copies of any type of entity to be merged.

The only way to exclude particular entity classes or associations that contain critical data is to provide a custom implementation of org.hibernate.event.spi.EntityCopyObserver with the desired behavior, and setting hibernate.event.merge.entity_copy_observer to the class name.

Hibernate provides limited DEBUG logging capabilities that can help determine the entity classes for which entity copies were found. By setting hibernate.event.merge.entity_copy_observer to log and enabling DEBUG logging for org.hibernate.event.internal.EntityCopyAllowedLoggedObserver, the following will be logged each time an application calls EntityManager.merge( entity ) or
Session.merge( entity ):

  • number of times multiple representations of the same persistent entity was detected summarized by entity name;

  • details by entity name and ID, including output from calling toString() on each representation being merged as well as the merge result.

The log should be reviewed to determine if multiple representations of entities containing critical data are detected. If so, the application should be modified so there is only one representation, and a custom implementation of org.hibernate.event.spi.EntityCopyObserver should be provided to disallow entity copies for entities with critical data.

Using optimistic locking is recommended to detect if different representations are from different versions of the same persistent entity. If they are not from the same version, Hibernate will throw either the Jakarta Persistence OptimisticEntityLockException or the native StaleObjectStateException depending on your bootstrapping strategy.

5.13. Checking persistent state

An application can verify the state of entities and collections in relation to the persistence context.

Example 390. Verifying managed state with Jakarta Persistence
boolean contained = entityManager.contains(person);
Example 391. Verifying managed state with Hibernate API
boolean contained = session.contains(person);
Example 392. Verifying laziness with Jakarta Persistence
PersistenceUnitUtil persistenceUnitUtil = entityManager.getEntityManagerFactory().getPersistenceUnitUtil();

boolean personInitialized = persistenceUnitUtil.isLoaded(person);

boolean personBooksInitialized = persistenceUnitUtil.isLoaded(person.getBooks());

boolean personNameInitialized = persistenceUnitUtil.isLoaded(person, "name");
Example 393. Verifying laziness with Hibernate API
boolean personInitialized = Hibernate.isInitialized(person);

boolean personBooksInitialized = Hibernate.isInitialized(person.getBooks());

boolean personNameInitialized = Hibernate.isPropertyInitialized(person, "name");

In Jakarta Persistence there is an alternative means to check laziness using the following jakarta.persistence.PersistenceUtil pattern (which is recommended wherever possible).

Example 394. Alternative Jakarta Persistence means to verify laziness
PersistenceUtil persistenceUnitUtil = Persistence.getPersistenceUtil();

boolean personInitialized = persistenceUnitUtil.isLoaded(person);

boolean personBooksInitialized = persistenceUnitUtil.isLoaded(person.getBooks());

boolean personNameInitialized = persistenceUnitUtil.isLoaded(person, "name");

5.14. Evicting entities

When the flush() method is called, the state of the entity is synchronized with the database. If you do not want this synchronization to occur, or if you are processing a huge number of objects and need to manage memory efficiently, the evict() method can be used to remove the object and its collections from the first-level cache.

Example 395. Detaching an entity from the EntityManager
for(Person person : entityManager.createQuery("select p from Person p", Person.class)
		.getResultList()) {
	dtos.add(toDTO(person));
	entityManager.detach(person);
}
Example 396. Evicting an entity from the Hibernate Session
Session session = entityManager.unwrap(Session.class);
for(Person person : (List<Person>) session.createQuery("select p from Person p").list()) {
	dtos.add(toDTO(person));
	session.evict(person);
}

To detach all entities from the current persistence context, both the EntityManager and the Hibernate Session define a clear() method.

Example 397. Clearing the persistence context
entityManager.clear();

session.clear();

To verify if an entity instance is currently attached to the running persistence context, both the EntityManager and the Hibernate Session define a contains(Object entity) method.

Example 398. Verify if an entity is contained in a persistence context
entityManager.contains(person);

session.contains(person);

5.15. Cascading entity state transitions

Jakarta Persistence allows you to propagate the state transition from a parent entity to a child. For this purpose, the Jakarta Persistence jakarta.persistence.CascadeType defines various cascade types:

ALL

cascades all entity state transitions.

PERSIST

cascades the entity persist operation.

MERGE

cascades the entity merge operation.

REMOVE

cascades the entity remove operation.

REFRESH

cascades the entity refresh operation.

DETACH

cascades the entity detach operation.

Additionally, the CascadeType.ALL will propagate any Hibernate-specific operation, which is defined by the org.hibernate.annotations.CascadeType enum:

SAVE_UPDATE

cascades the entity saveOrUpdate operation.

REPLICATE

cascades the entity replicate operation.

LOCK

cascades the entity lock operation.

The following examples will explain some of the aforementioned cascade operations using the following entities:

@Entity
public class Person {

    @Id
    private Long id;

    private String name;

    @OneToMany(mappedBy = "owner", cascade = CascadeType.ALL)
    private List<Phone> phones = new ArrayList<>();

    //Getters and setters are omitted for brevity

    public void addPhone(Phone phone) {
        this.phones.add(phone);
        phone.setOwner(this);
    }
}


@Entity
public class Phone {

    @Id
    private Long id;

    @Column(name = "`number`")
    private String number;

    @ManyToOne(fetch = FetchType.LAZY)
    private Person owner;

    //Getters and setters are omitted for brevity
}

5.15.1. CascadeType.PERSIST

The CascadeType.PERSIST allows us to persist a child entity along with the parent one.

Example 399. CascadeType.PERSIST example
Person person = new Person();
person.setId(1L);
person.setName("John Doe");

Phone phone = new Phone();
phone.setId(1L);
phone.setNumber("123-456-7890");

person.addPhone(phone);

entityManager.persist(person);
INSERT INTO Person ( name, id )
VALUES ( 'John Doe', 1 )

INSERT INTO Phone ( `number`, person_id, id )
VALUE ( '123-456-7890', 1, 1 )

Even if just the Person parent entity was persisted, Hibernate has managed to cascade the persist operation to the associated Phone child entity as well.

5.15.2. CascadeType.MERGE

The CascadeType.MERGE allows us to merge a child entity along with the parent one.

Example 400. CascadeType.MERGE example
Phone phone = entityManager.find(Phone.class, 1L);
Person person = phone.getOwner();

person.setName("John Doe Jr.");
phone.setNumber("987-654-3210");

entityManager.clear();

entityManager.merge(person);
SELECT
    p.id as id1_0_1_,
    p.name as name2_0_1_,
    ph.owner_id as owner_id3_1_3_,
    ph.id as id1_1_3_,
    ph.id as id1_1_0_,
    ph."number" as number2_1_0_,
    ph.owner_id as owner_id3_1_0_
FROM
    Person p
LEFT OUTER JOIN
    Phone ph
        on p.id=ph.owner_id
WHERE
    p.id = 1

During merge, the current state of the entity is copied onto the entity version that was just fetched from the database. That’s the reason why Hibernate executed the SELECT statement which fetched both the Person entity along with its children.

5.15.3. CascadeType.REMOVE

The CascadeType.REMOVE allows us to remove a child entity along with the parent one. Traditionally, Hibernate called this operation delete, that’s why the org.hibernate.annotations.CascadeType provides a DELETE cascade option. However, CascadeType.REMOVE and org.hibernate.annotations.CascadeType.DELETE are identical.

Example 401. CascadeType.REMOVE example
Person person = entityManager.find(Person.class, 1L);

entityManager.remove(person);
DELETE FROM Phone WHERE id = 1

DELETE FROM Person WHERE id = 1

5.15.4. CascadeType.DETACH

CascadeType.DETACH is used to propagate the detach operation from a parent entity to a child.

Example 402. CascadeType.DETACH example
Person person = entityManager.find(Person.class, 1L);
assertEquals(1, person.getPhones().size());
Phone phone = person.getPhones().get(0);

assertTrue(entityManager.contains(person));
assertTrue(entityManager.contains(phone));

entityManager.detach(person);

assertFalse(entityManager.contains(person));
assertFalse(entityManager.contains(phone));

5.15.5. CascadeType.LOCK

Although unintuitively, CascadeType.LOCK does not propagate a lock request from a parent entity to its children. Such a use case requires the use of the PessimisticLockScope.EXTENDED value of the jakarta.persistence.lock.scope property.

However, CascadeType.LOCK allows us to reattach a parent entity along with its children to the currently running Persistence Context.

Example 403. CascadeType.LOCK example
Person person = entityManager.find(Person.class, 1L);
assertEquals(1, person.getPhones().size());
Phone phone = person.getPhones().get(0);

assertTrue(entityManager.contains(person));
assertTrue(entityManager.contains(phone));

entityManager.detach(person);

assertFalse(entityManager.contains(person));
assertFalse(entityManager.contains(phone));

entityManager.unwrap(Session.class)
		.buildLockRequest(new LockOptions(LockMode.NONE))
		.lock(person);

assertTrue(entityManager.contains(person));
assertTrue(entityManager.contains(phone));

5.15.6. CascadeType.REFRESH

The CascadeType.REFRESH is used to propagate the refresh operation from a parent entity to a child. The refresh operation will discard the current entity state, and it will override it using the one loaded from the database.

Example 404. CascadeType.REFRESH example
Person person = entityManager.find(Person.class, 1L);
Phone phone = person.getPhones().get(0);

person.setName("John Doe Jr.");
phone.setNumber("987-654-3210");

entityManager.refresh(person);

assertEquals("John Doe", person.getName());
assertEquals("123-456-7890", phone.getNumber());
SELECT
    p.id as id1_0_1_,
    p.name as name2_0_1_,
    ph.owner_id as owner_id3_1_3_,
    ph.id as id1_1_3_,
    ph.id as id1_1_0_,
    ph."number" as number2_1_0_,
    ph.owner_id as owner_id3_1_0_
FROM
    Person p
LEFT OUTER JOIN
    Phone ph
        ON p.id=ph.owner_id
WHERE
    p.id = 1

In the aforementioned example, you can see that both the Person and Phone entities are refreshed even if we only called this operation on the parent entity only.

5.15.7. CascadeType.REPLICATE

The CascadeType.REPLICATE is to replicate both the parent and the child entities. The replicate operation allows you to synchronize entities coming from different sources of data.

Example 405. CascadeType.REPLICATE example
Person person = new Person();
person.setId(1L);
person.setName("John Doe Sr.");

Phone phone = new Phone();
phone.setId(1L);
phone.setNumber("(01) 123-456-7890");
person.addPhone(phone);

entityManager.unwrap(Session.class).replicate(person, ReplicationMode.OVERWRITE);
SELECT
    id
FROM
    Person
WHERE
    id = 1

SELECT
    id
FROM
    Phone
WHERE
    id = 1

UPDATE
    Person
SET
    name = 'John Doe Sr.'
WHERE
    id = 1

UPDATE
    Phone
SET
    "number" = '(01) 123-456-7890',
    owner_id = 1
WHERE
    id = 1

As illustrated by the SQL statements being generated, both the Person and Phone entities are replicated to the underlying database rows.

5.15.8. @OnDelete cascade

While the previous cascade types propagate entity state transitions, the @OnDelete cascade is a DDL-level FK feature which allows you to remove a child record whenever the parent row is deleted.

So, when annotating the @ManyToOne association with @OnDelete( action = OnDeleteAction.CASCADE ), the automatic schema generator will apply the ON DELETE CASCADE SQL directive to the Foreign Key declaration, as illustrated by the following example.

Example 406. @OnDelete @ManyToOne mapping
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	//Getters and setters are omitted for brevity

}
@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	@Column(name = "`number`")
	private String number;

	@ManyToOne(fetch = FetchType.LAZY)
	@OnDelete(action = OnDeleteAction.CASCADE)
	private Person owner;

	//Getters and setters are omitted for brevity

}
create table Person (
    id bigint not null,
    name varchar(255),
    primary key (id)
)

create table Phone (
    id bigint not null,
    "number" varchar(255),
    owner_id bigint,
    primary key (id)
)

alter table Phone
    add constraint FK82m836qc1ss2niru7eogfndhl
    foreign key (owner_id)
    references Person
    on delete cascade

Now, you can just remove the Person entity, and the associated Phone entities are going to be deleted automatically via the Foreign Key cascade.

Example 407. @OnDelete @ManyToOne delete example
Person person = entityManager.find(Person.class, 1L);
entityManager.remove(person);
delete from Person where id = ?

-- binding parameter [1] as [BIGINT] - [1]

The @OnDelete annotation can also be placed on a collection, as illustrated in the following example.

Example 408. @OnDelete @OneToMany mapping
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String name;

	@OneToMany(mappedBy = "owner", cascade = CascadeType.ALL)
	@OnDelete(action = OnDeleteAction.CASCADE)
	private List<Phone> phones = new ArrayList<>();

	//Getters and setters are omitted for brevity

}
@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	@Column(name = "`number`")
	private String number;

	@ManyToOne(fetch = FetchType.LAZY)
	private Person owner;

	//Getters and setters are omitted for brevity

}

Now, when removing the Person entity, all the associated Phone child entities are deleted via the Foreign Key cascade even if the @OneToMany collection was using the CascadeType.ALL attribute.

Example 409. @OnDelete @ManyToOne delete example
Person person = entityManager.find(Person.class, 1L);
entityManager.remove(person);
delete from Person where id = ?

-- binding parameter [1] as [BIGINT] - [1]

Without the @OnDelete annotation, the @OneToMany association relies on the cascade attribute to propagate the remove entity state transition from the parent entity to its children. However, when the @OnDelete annotation is in place, Hibernate prevents the child entity DELETE statement from being executed while flushing the Persistence Context.

This way, only the parent entity gets deleted, and all the associated child records are removed by the database engine, instead of being deleted explicitly via DELETE statements.

5.16. Exception handling

If the Jakarta Persistence EntityManager or the Hibernate-specific Session throws an exception, including any JDBC SQLException, you have to immediately rollback the database transaction and close the current EntityManager or Session.

Certain methods of the Jakarta Persistence EntityManager or the Hibernate Session will not leave the Persistence Context in a consistent state. As a rule of thumb, no exception thrown by Hibernate can be treated as recoverable. Ensure that the Session will be closed by calling the close() method in a finally block.

Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. This means that the database state and the business objects will be out of sync. Usually, this is not a problem because exceptions are not recoverable and you will have to start over after rollback anyway.

The Jakarta Persistence PersistenceException or the HibernateException wraps most of the errors that can occur in a Hibernate persistence layer.

Both the PersistenceException and the HibernateException are runtime exceptions because, in our opinion, we should not force the application developer to catch an unrecoverable exception at a low layer. In most systems, unchecked and fatal exceptions are handled in one of the first frames of the method call stack (i.e., in higher layers) and either an error message is presented to the application user or some other appropriate action is taken. Note that Hibernate might also throw other unchecked exceptions that are not a HibernateException. These are not recoverable either, and appropriate action should be taken.

Hibernate wraps the JDBC SQLException, thrown while interacting with the database, in a JDBCException. In fact, Hibernate will attempt to convert the exception into a more meaningful subclass of JDBCException. The underlying SQLException is always available via JDBCException.getSQLException(). Hibernate converts the SQLException into an appropriate JDBCException subclass using the SQLExceptionConverter attached to the current SessionFactory.

By default, the SQLExceptionConverter is defined by the configured Hibernate Dialect via the buildSQLExceptionConversionDelegate method which is overridden by several database-specific Dialects.

The standard JDBCException subtypes are:

ConstraintViolationException

indicates some form of integrity constraint violation.

DataException

indicates that evaluation of the valid SQL statement against the given data resulted in some illegal operation, mismatched types, truncation or incorrect cardinality.

GenericJDBCException

a generic exception which did not fall into any of the other categories.

JDBCConnectionException

indicates an error with the underlying JDBC communication.

LockAcquisitionException

indicates an error acquiring a lock level necessary to perform the requested operation.

LockTimeoutException

indicates that the lock acquisition request has timed out.

PessimisticLockException

indicates that a lock acquisition request has failed.

QueryTimeoutException

indicates that the current executing query has timed out.

SQLGrammarException

indicates a grammar or syntax problem with the issued SQL.

Starting with Hibernate 5.2, the Hibernate Session extends the Jakarta Persistence EntityManager. For this reason, when a SessionFactory is built via Hibernate’s native bootstrapping, the HibernateException or SQLException can be wrapped in a Jakarta Persistence PersistenceException when thrown by Session methods that implement EntityManager methods (e.g., Session.merge(Object object), Session.flush()).

If your SessionFactory is built via Hibernate’s native bootstrapping, and you don’t want the Hibernate exceptions to be wrapped in the Jakarta Persistence PersistenceException, you need to set the hibernate.native_exception_handling_51_compliance configuration property to true. See the hibernate.native_exception_handling_51_compliance configuration property for more details.

6. Flushing

Flushing is the process of synchronizing the state of the persistence context with the underlying database. The EntityManager and the Hibernate Session expose a set of methods, through which the application developer can change the persistent state of an entity.

The persistence context acts as a transactional write-behind cache, queuing any entity state change. Like any write-behind cache, changes are first applied in-memory and synchronized with the database during the flush time. The flush operation takes every entity state change and translates it to an INSERT, UPDATE or DELETE statement.

Because DML statements are grouped together, Hibernate can apply batching transparently. See the Batching chapter for more information.

The flushing strategy is given by the flushMode of the current running Hibernate Session. Although Jakarta Persistence defines only two flushing strategies (AUTO and COMMIT), Hibernate has a much broader spectrum of flush types:

ALWAYS

Flushes the Session before every query.

AUTO

This is the default mode, and it flushes the Session only if necessary.

COMMIT

The Session tries to delay the flush until the current Transaction is committed, although it might flush prematurely too.

MANUAL

The Session flushing is delegated to the application, which must call Session.flush() explicitly in order to apply the persistence context changes.

6.1. AUTO flush

By default, Hibernate uses the AUTO flush mode which triggers a flush in the following circumstances:

  • prior to committing a Transaction

  • prior to executing a JPQL/HQL query that overlaps with the queued entity actions

  • before executing any native SQL query that has no registered synchronization

6.1.1. AUTO flush on commit

In the following example, an entity is persisted, and then the transaction is committed.

Example 410. Automatic flushing on commit
entityManager = entityManagerFactory().createEntityManager();
txn = entityManager.getTransaction();
txn.begin();

Person person = new Person("John Doe");
entityManager.persist(person);
log.info("Entity is in persisted state");

txn.commit();
--INFO: Entity is in persisted state
INSERT INTO Person (name, id) VALUES ('John Doe', 1)

Hibernate logs the message prior to inserting the entity because the flush only occurred during transaction commit.

This is valid for the SEQUENCE and TABLE identifier generators. The IDENTITY generator must execute the insert right after calling persist(). For more details, see the discussion of generators in Identifier generators.

6.1.2. AUTO flush on JPQL/HQL query

A flush may also be triggered when executing an entity query.

Example 411. Automatic flushing on JPQL/HQL
Person person = new Person("John Doe");
entityManager.persist(person);
entityManager.createQuery("select p from Advertisement p").getResultList();
entityManager.createQuery("select p from Person p").getResultList();
SELECT a.id AS id1_0_ ,
       a.title AS title2_0_
FROM   Advertisement a

INSERT INTO Person (name, id) VALUES ('John Doe', 1)

SELECT p.id AS id1_1_ ,
       p.name AS name2_1_
FROM   Person p

The reason why the Advertisement entity query didn’t trigger a flush is that there’s no overlapping between the Advertisement and the Person tables:

Example 412. Automatic flushing on JPQL/HQL entities
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	private String name;

	//Getters and setters are omitted for brevity

}

@Entity(name = "Advertisement")
public static class Advertisement {

	@Id
	@GeneratedValue
	private Long id;

	private String title;

	//Getters and setters are omitted for brevity

}

When querying for a Person entity, the flush is triggered prior to executing the entity query.

Example 413. Automatic flushing on JPQL/HQL
Person person = new Person("John Doe");
entityManager.persist(person);
entityManager.createQuery("select p from Person p").getResultList();
INSERT INTO Person (name, id) VALUES ('John Doe', 1)

SELECT p.id AS id1_1_ ,
       p.name AS name2_1_
FROM   Person p

This time, the flush was triggered by a JPQL query because the pending entity persisting action overlaps with the query being executed.

6.1.3. AUTO flush on native SQL query

When executing a native SQL query, a flush is always triggered when using the EntityManager API.

Example 414. Automatic flushing on native SQL using EntityManager
assertTrue(((Number) entityManager
		.createNativeQuery("select count(*) from Person")
		.getSingleResult()).intValue() == 0);

Person person = new Person("John Doe");
entityManager.persist(person);

assertTrue(((Number) entityManager
		.createNativeQuery("select count(*) from Person")
		.getSingleResult()).intValue() == 1);

If you bootstrap Hibernate natively, and not through Jakarta Persistence, by default, the Session API will trigger a flush automatically when executing a native query.

Example 415. Automatic flushing on native SQL using Session
assertTrue(((Number) session
		.createNativeQuery("select count(*) from Person")
		.getSingleResult()).intValue() == 0);

Person person = new Person("John Doe");
session.persist(person);

assertTrue(((Number) session
		.createNativeQuery("select count(*) from Person")
		.uniqueResult()).intValue() == 0);

To flush the Session, the query must use a synchronization:

Example 416. Automatic flushing on native SQL with Session synchronization
assertTrue(((Number) entityManager
		.createNativeQuery("select count(*) from Person")
		.getSingleResult()).intValue() == 0);

Person person = new Person("John Doe");
entityManager.persist(person);
Session session = entityManager.unwrap(Session.class);

assertTrue(((Number) session
		.createNativeQuery("select count(*) from Person")
		.addSynchronizedEntityClass(Person.class)
		.uniqueResult()).intValue() == 1);

6.2. COMMIT flush

Jakarta Persistence also defines a COMMIT flush mode, which is described as follows:

If FlushModeType.COMMIT is set, the effect of updates made to entities in the persistence context upon queries is unspecified.

— Section 3.10.8 of the Java Persistence 2.1 Specification

When executing a JPQL query, the persistence context is only flushed when the current running transaction is committed.

Example 417. COMMIT flushing on JPQL
Person person = new Person("John Doe");
entityManager.persist(person);

entityManager.createQuery("select p from Advertisement p")
    .setFlushMode(FlushModeType.COMMIT)
    .getResultList();

entityManager.createQuery("select p from Person p")
    .setFlushMode(FlushModeType.COMMIT)
    .getResultList();
SELECT a.id AS id1_0_ ,
       a.title AS title2_0_
FROM   Advertisement a

SELECT p.id AS id1_1_ ,
       p.name AS name2_1_
FROM   Person p

INSERT INTO Person (name, id) VALUES ('John Doe', 1)

Because the Jakarta Persistence doesn’t impose a strict rule on delaying flushing, when executing a native SQL query, the persistence context is going to be flushed.

Example 418. COMMIT flushing on native SQL
Person person = new Person("John Doe");
entityManager.persist(person);

assertTrue(((Number) entityManager
    .createNativeQuery("select count(*) from Person")
    .getSingleResult()).intValue() == 1);
INSERT INTO Person (name, id) VALUES ('John Doe', 1)

SELECT COUNT(*) FROM Person

6.3. ALWAYS flush

The ALWAYS is only available with the native Session API.

The ALWAYS flush mode triggers a persistence context flush even when executing a native SQL query against the Session API.

Example 419. COMMIT flushing on native SQL
Person person = new Person("John Doe");
entityManager.persist(person);

Session session = entityManager.unwrap(Session.class);
assertTrue(((Number) session
        .createNativeQuery("select count(*) from Person")
        .setHibernateFlushMode(FlushMode.ALWAYS)
        .uniqueResult()).intValue() == 1);
INSERT INTO Person (name, id) VALUES ('John Doe', 1)

SELECT COUNT(*) FROM Person

6.4. MANUAL flush

Both the EntityManager and the Hibernate Session define a flush() method that, when called, triggers a manual flush. Hibernate also provides a MANUAL flush mode so the persistence context can only be flushed manually.

Example 420. MANUAL flushing
Person person = new Person("John Doe");
entityManager.persist(person);

Session session = entityManager.unwrap(Session.class);
session.setHibernateFlushMode(FlushMode.MANUAL);

assertTrue(((Number) entityManager
    .createQuery("select count(id) from Person")
    .getSingleResult()).intValue() == 0);

assertTrue(((Number) session
    .createNativeQuery("select count(*) from Person")
    .uniqueResult()).intValue() == 0);
SELECT COUNT(p.id) AS col_0_0_
FROM   Person p

SELECT COUNT(*)
FROM   Person

The INSERT statement was not executed because there was no manual flush() call.

The MANUAL flush mode is useful when using multi-request logical transactions, and only the last request should flush the persistence context.

6.5. Flush operation order

From a database perspective, a row state can be altered using either an INSERT, an UPDATE or a DELETE statement. Because entity state changes are automatically converted to SQL statements, it’s important to know which entity actions are associated with a given SQL statement.

INSERT

The INSERT statement is generated either by the EntityInsertAction or EntityIdentityInsertAction. These actions are scheduled by the persist operation, either explicitly or through cascading the PersistEvent from a parent to a child entity.

DELETE

The DELETE statement is generated by the EntityDeleteAction or OrphanRemovalAction.

UPDATE

The UPDATE statement is generated by EntityUpdateAction during flushing if the managed entity has been marked modified. The dirty checking mechanism is responsible for determining if a managed entity has been modified since it was first loaded.

Hibernate does not execute the SQL statements in the order of their associated entity state operations.

To visualize how this works, consider the following example:

Example 421. Flush operation order
Person person = entityManager.find(Person.class, 1L);
entityManager.remove(person);

Person newPerson = new Person();
newPerson.setId(2L);
newPerson.setName("John Doe");
entityManager.persist(newPerson);
INSERT INTO Person (name, id)
VALUES ('John Doe', 2L)

DELETE FROM Person WHERE id = 1

Even if we removed the first entity and then persist a new one, Hibernate is going to execute the DELETE statement after the INSERT.

The order in which SQL statements are executed is given by the ActionQueue and not by the order in which entity state operations have been previously defined.

The ActionQueue executes all operations in the following order:

  1. OrphanRemovalAction

  2. EntityInsertAction or EntityIdentityInsertAction

  3. EntityUpdateAction

  4. QueuedOperationCollectionAction

  5. CollectionRemoveAction

  6. CollectionUpdateAction

  7. CollectionRecreateAction

  8. EntityDeleteAction

7. Database access

7.1. ConnectionProvider

As an ORM tool, probably the single most important thing you need to tell Hibernate is how to connect to your database so that it may connect on behalf of your application. This is ultimately the function of the org.hibernate.engine.jdbc.connections.spi.ConnectionProvider interface. Hibernate provides some out of the box implementations of this interface. ConnectionProvider is also an extension point so you can also use custom implementations from third parties or written yourself. The ConnectionProvider to use is defined by the hibernate.connection.provider_class setting. See the org.hibernate.cfg.AvailableSettings#CONNECTION_PROVIDER

Generally speaking, applications should not have to configure a ConnectionProvider explicitly if using one of the Hibernate-provided implementations. Hibernate will internally determine which ConnectionProvider to use based on the following algorithm:

  1. If hibernate.connection.provider_class is set, it takes precedence

  2. else if hibernate.connection.datasource is set → Using DataSources

  3. else if any setting prefixed by hibernate.c3p0. is set → Using c3p0

  4. else if any setting prefixed by hibernate.proxool. is set → Using Proxool

  5. else if any setting prefixed by hibernate.hikari. is set → Using HikariCP

  6. else if any setting prefixed by hibernate.vibur. is set → Using Vibur DBCP

  7. else if any setting prefixed by hibernate.agroal. is set → Using Agroal

  8. else if hibernate.connection.url is set → Using Hibernate’s built-in (and unsupported) pooling

  9. else → User-provided Connections

7.2. Using DataSources

Hibernate can integrate with a javax.sql.DataSource for obtaining JDBC Connections. Applications would tell Hibernate about the DataSource via the (required) hibernate.connection.datasource setting which can either specify a JNDI name or would reference the actual DataSource instance. For cases where a JNDI name is given, be sure to read JNDI.

For Jakarta Persistence applications, note that hibernate.connection.datasource corresponds to jakarta.persistence.jtaDataSource or jakarta.persistence.nonJtaDataSource.

The DataSource ConnectionProvider also (optionally) accepts the hibernate.connection.username and hibernate.connection.password. If specified, the DataSource#getConnection(String username, String password) will be used. Otherwise, the no-arg form is used.

7.3. Driver Configuration

hibernate.connection.driver_class

The name of the JDBC Driver class to use

hibernate.connection.url

The JDBC connection url

hibernate.connection.*

All such setting names (except the predefined ones) will have the hibernate.connection. prefix stripped. The remaining name and the original value will be passed to the driver as a JDBC connection property

Not all properties apply to all situations. For example, if you are providing a data source, hibernate.connection.driver_class setting will not be used.

7.4. Using c3p0

To use the c3p0 integration, the application must include the hibernate-c3p0 module jar (as well as its dependencies) on the classpath.

Hibernate also provides support for applications to use c3p0 connection pooling. When c3p0 support is enabled, a number of c3p0-specific configuration settings are recognized in addition to the general ones described in Driver Configuration.

Transaction isolation of the Connections is managed by the ConnectionProvider itself. See ConnectionProvider support for transaction isolation setting.

hibernate.c3p0.min_size or c3p0.minPoolSize

The minimum size of the c3p0 pool. See c3p0 minPoolSize

hibernate.c3p0.max_size or c3p0.maxPoolSize

The maximum size of the c3p0 pool. See c3p0 maxPoolSize

hibernate.c3p0.timeout or c3p0.maxIdleTime

The Connection idle time. See c3p0 maxIdleTime

hibernate.c3p0.max_statements or c3p0.maxStatements

Controls the c3p0 PreparedStatement cache size (if using). See c3p0 maxStatements

hibernate.c3p0.acquire_increment or c3p0.acquireIncrement

Number of connections c3p0 should acquire at a time when the pool is exhausted. See c3p0 acquireIncrement

hibernate.c3p0.idle_test_period or c3p0.idleConnectionTestPeriod

Idle time before a c3p0 pooled connection is validated. See c3p0 idleConnectionTestPeriod

hibernate.c3p0.initialPoolSize

The initial c3p0 pool size. If not specified, default is to use the min pool size. See c3p0 initialPoolSize

Any other settings prefixed with hibernate.c3p0.

Will have the hibernate. portion stripped and be passed to c3p0.

Any other settings prefixed with c3p0.

Get passed to c3p0 as is. See c3p0 configuration

7.5. Using Proxool

To use the Proxool integration, the application must include the hibernate-proxool module jar (as well as its dependencies) on the classpath.

Hibernate also provides support for applications to use Proxool connection pooling.

Transaction isolation of the Connections is managed by the ConnectionProvider itself. See ConnectionProvider support for transaction isolation setting.

7.5.1. Using existing Proxool pools

Controlled by the hibernate.proxool.existing_pool setting. If set to true, this ConnectionProvider will use an already existing Proxool pool by alias as indicated by the hibernate.proxool.pool_alias setting.

7.5.2. Configuring Proxool via XML

The hibernate.proxool.xml setting names a Proxool configuration XML file to be loaded as a classpath resource and loaded by Proxool’s JAXPConfigurator. See proxool configuration. hibernate.proxool.pool_alias must be set to indicate which pool to use.

7.5.3. Configuring Proxool via Properties

The hibernate.proxool.properties setting names a Proxool configuration properties file to be loaded as a classpath resource and loaded by Proxool’s PropertyConfigurator. See proxool configuration. hibernate.proxool.pool_alias must be set to indicate which pool to use.

7.6. Using HikariCP

To use the HikariCP this integration, the application must include the hibernate-hikari module jar (as well as its dependencies) on the classpath.

Hibernate also provides support for applications to use HikariCP connection pool.

Set all of your Hikari settings in Hibernate prefixed by hibernate.hikari. and this ConnectionProvider will pick them up and pass them along to Hikari. Additionally, this ConnectionProvider will pick up the following Hibernate-specific properties and map them to the corresponding Hikari ones (any hibernate.hikari. prefixed ones have precedence):

hibernate.connection.driver_class

Mapped to Hikari’s driverClassName setting

hibernate.connection.url

Mapped to Hikari’s jdbcUrl setting

hibernate.connection.username

Mapped to Hikari’s username setting

hibernate.connection.password

Mapped to Hikari’s password setting

hibernate.connection.isolation

Mapped to Hikari’s transactionIsolation setting. See ConnectionProvider support for transaction isolation setting. Note that Hikari only supports JDBC standard isolation levels (apparently).

hibernate.connection.autocommit

Mapped to Hikari’s autoCommit setting

7.7. Using Vibur DBCP

To use the Vibur DBCP integration, the application must include the hibernate-vibur module jar (as well as its dependencies) on the classpath.

Hibernate also provides support for applications to use Vibur DBCP connection pool.

Set all of your Vibur settings in Hibernate prefixed by hibernate.vibur. and this ConnectionProvider will pick them up and pass them along to Vibur DBCP. Additionally, this ConnectionProvider will pick up the following Hibernate-specific properties and map them to the corresponding Vibur ones (any hibernate.vibur. prefixed ones have precedence):

hibernate.connection.driver_class

Mapped to Vibur’s driverClassName setting

hibernate.connection.url

Mapped to Vibur’s jdbcUrl setting

hibernate.connection.username

Mapped to Vibur’s username setting

hibernate.connection.password

Mapped to Vibur’s password setting

hibernate.connection.isolation

Mapped to Vibur’s defaultTransactionIsolationValue setting. See ConnectionProvider support for transaction isolation setting.

hibernate.connection.autocommit

Mapped to Vibur’s defaultAutoCommit setting

7.8. Using Agroal

To use the Agroal integration, the application must include the hibernate-agroal module jar (as well as its dependencies) on the classpath.

Hibernate also provides support for applications to use Agroal connection pool.

Set all of your Agroal settings in Hibernate prefixed by hibernate.agroal. and this ConnectionProvider will pick them up and pass them along to Agroal connection pool. Additionally, this ConnectionProvider will pick up the following Hibernate-specific properties and map them to the corresponding Agroal ones (any hibernate.agroal. prefixed ones have precedence):

hibernate.connection.driver_class

Mapped to Agroal’s driverClassName setting

hibernate.connection.url

Mapped to Agroal’s jdbcUrl setting

hibernate.connection.username

Mapped to Agroal’s principal setting

hibernate.connection.password

Mapped to Agroal’s credential setting

hibernate.connection.isolation

Mapped to Agroal’s jdbcTransactionIsolation setting. See ConnectionProvider support for transaction isolation setting.

hibernate.connection.autocommit

Mapped to Agroal’s autoCommit setting

7.9. Using Hibernate’s built-in (and unsupported) pooling

The built-in connection pool is not supported for use in a production system.

This section is here just for completeness.

7.10. User-provided Connections

It is possible to use Hibernate by simply passing a Connection to use to the Session when the Session is opened. This usage is discouraged and not discussed here.

7.11. ConnectionProvider support for transaction isolation setting

All of the provided ConnectionProvider implementations, other than DataSourceConnectionProvider, support consistent setting of transaction isolation for all Connections obtained from the underlying pool. The value for hibernate.connection.isolation can be specified in one of 3 formats:

  • the integer value accepted at the JDBC level.

  • the name of the java.sql.Connection constant field representing the isolation you would like to use. For example, TRANSACTION_REPEATABLE_READ for java.sql.Connection#TRANSACTION_REPEATABLE_READ. Not that this is only supported for JDBC standard isolation levels, not for isolation levels specific to a particular JDBC driver.

  • a short-name version of the java.sql.Connection constant field without the TRANSACTION_ prefix. For example, REPEATABLE_READ for java.sql.Connection#TRANSACTION_REPEATABLE_READ. Again, this is only supported for JDBC standard isolation levels, not for isolation levels specific to a particular JDBC driver.

7.12. Connection handling

The connection handling mode is defined by the PhysicalConnectionHandlingMode enumeration which provides the following strategies:

IMMEDIATE_ACQUISITION_AND_HOLD

The Connection will be acquired as soon as the Session is opened and held until the Session is closed.

DELAYED_ACQUISITION_AND_HOLD

The Connection will be acquired as soon as it is needed and then held until the Session is closed.

DELAYED_ACQUISITION_AND_RELEASE_AFTER_STATEMENT

The Connection will be acquired as soon as it is needed and will be released after each statement is executed.

DELAYED_ACQUISITION_AND_RELEASE_AFTER_TRANSACTION

The Connection will be acquired as soon as it is needed and will be released after each transaction is completed.

If you don’t want to use the default connection handling mode, you can specify a connection handling mode via the hibernate.connection.handling_mode configuration property. For more details, check out the Database connection properties section.

7.12.1. Transaction type and connection handling

By default, the connection handling mode is given by the underlying transaction coordinator. There are two types of transactions: RESOURCE_LOCAL (which involves a single database Connection and the transaction is controlled via the commit and rollback Connection methods) and JTA (which may involve multiple resources including database connections, JMS queues, etc).

RESOURCE_LOCAL transaction connection handling

For RESOURCE_LOCAL transactions, the connection handling mode is DELAYED_ACQUISITION_AND_RELEASE_AFTER_TRANSACTION meaning that the database connection is acquired when needed and released after the current running transaction is either committed or rolled back.

However, because Hibernate needs to make sure that the default autocommit mode is disabled on the JDBC Connection when starting a new transaction, the Connection is acquired and the autocommit mode is set to false.

If you are using a connection pool DataSource that already disabled the autocommit mode for every pooled Connection, you should set the hibernate.connection.provider_disables_autocommit to true and the database connection acquisition will be, indeed, delayed until Hibernate needs to execute the first SQL statement.

JTA transaction connection handling

For JTA transactions, the connection handling mode is DELAYED_ACQUISITION_AND_RELEASE_AFTER_STATEMENT meaning that the database connection is acquired when needed and released after each statement execution.

The reason for releasing the database connection after statement execution is because some Java EE application servers report a connection leak when a method call goes from one EJB to another. However, even if the JDBC Connection is released to the pool, the Connection is still allocated to the current executing Thread, hence when executing a subsequent statement in the current running transaction, the same Connection object reference will be obtained from the pool.

If the Java EE application server or JTA transaction manager supports switching from one EJB to another while the transaction gets propagated from the outer EJB to the inner one, and no connection leak false positive is being reported, then you should consider switching to DELAYED_ACQUISITION_AND_RELEASE_AFTER_TRANSACTION via the hibernate.connection.handling_mode configuration property.

7.12.2. User-provided connections

If the current Session was created using the SessionBuilder and a JDBC Connection was provided via the SessionBuilder#connection method, then the user-provided Connection is going to be used, and the connection handling mode will be IMMEDIATE_ACQUISITION_AND_HOLD.

Therefore for user-provided connection, the connection is acquired right away and held until the current Session is closed, without being influenced by the Jakarta Persistence or Hibernate transaction context.

7.13. Database Dialect

Although SQL is relatively standardized, each database vendor uses a subset and superset of ANSI SQL defined syntax. This is referred to as the database’s dialect. Hibernate handles variations across these dialects through its org.hibernate.dialect.Dialect class and the various subclasses for each database vendor.

In most cases, Hibernate will be able to determine the proper Dialect to use by asking some questions of the JDBC Connection during bootstrap. For information on Hibernate’s ability to determine the proper Dialect to use (and your ability to influence that resolution), see Dialect resolution.

If for some reason it is not able to determine the proper one or you want to use a custom Dialect, you will need to set the hibernate.dialect setting.

Table 1. Provided Dialects
Dialect (short name) Remarks

Cache71

Support for the Caché database, version 2007.1.

CockroachDB192

Support for the CockroachDB database version 19.2.

CockroachDB201

Support for the CockroachDB database version 20.1.

CUBRID

Support for the CUBRID database, version 8.3. May work with later versions.

DB2

Support for the DB2 database, version 8.2.

DB297

Support for the DB2 database, version 9.7.

DB2390

Support for DB2 Universal Database for OS/390, also known as DB2/390.

DB2400

Support for DB2 Universal Database for iSeries, also known as DB2/400.

DB2400V7R3

Support for DB2 Universal Database for i, also known as DB2/400, version 7.3

DerbyTenFive

Support for the Derby database, version 10.5

DerbyTenSix

Support for the Derby database, version 10.6

DerbyTenSeven

Support for the Derby database, version 10.7

Firebird

Support for the Firebird database

FrontBase

Support for the Frontbase database

H2

Support for the H2 database

HANACloudColumnStore

Support for the SAP HANA Cloud database column store.

HANAColumnStore

Support for the SAP HANA database column store, version 2.x. This is the recommended dialect for the SAP HANA database. May work with SAP HANA, version 1.x

HANARowStore

Support for the SAP HANA database row store, version 2.x. May work with SAP HANA, version 1.x

HSQL

Support for the HSQL (HyperSQL) database

Informix

Support for the Informix database

Ingres

Support for the Ingres database, version 9.2

Ingres9

Support for the Ingres database, version 9.3. May work with newer versions

Ingres10

Support for the Ingres database, version 10. May work with newer versions

Interbase

Support for the Interbase database.

JDataStore

Support for the JDataStore database

McKoi

Support for the McKoi database

Mimer

Support for the Mimer database, version 9.2.1. May work with newer versions

MySQL5

Support for the MySQL database, version 5.x

MySQL5InnoDB

Support for the MySQL database, version 5.x preferring the InnoDB storage engine when exporting tables.

MySQL57InnoDB

Support for the MySQL database, version 5.7 preferring the InnoDB storage engine when exporting tables. May work with newer versions

MariaDB

Support for the MariaDB database. May work with newer versions

MariaDB53

Support for the MariaDB database, version 5.3 and newer.

Oracle8i

Support for the Oracle database, version 8i

Oracle9i

Support for the Oracle database, version 9i

Oracle10g

Support for the Oracle database, version 10g

Pointbase

Support for the Pointbase database

PostgresPlus

Support for the Postgres Plus database

PostgreSQL81

Support for the PostgrSQL database, version 8.1

PostgreSQL82

Support for the PostgreSQL database, version 8.2

PostgreSQL9

Support for the PostgreSQL database, version 9. May work with later versions.

Progress

Support for the Progress database, version 9.1C. May work with newer versions.

SAPDB

Support for the SAPDB/MAXDB database.

SQLServer

Support for the SQL Server 2000 database

SQLServer2005

Support for the SQL Server 2005 database

SQLServer2008

Support for the SQL Server 2008 database

Sybase11

Support for the Sybase database, up to version 11.9.2

SybaseAnywhere

Support for the Sybase Anywhere database

SybaseASE15

Support for the Sybase Adaptive Server Enterprise database, version 15

SybaseASE157

Support for the Sybase Adaptive Server Enterprise database, version 15.7. May work with newer versions.

Teradata

Support for the Teradata database

TimesTen

Support for the TimesTen database, version 5.1. May work with newer versions

8. Transactions and concurrency control

It is important to understand that the term transaction has many different yet related meanings in regards to persistence and Object/Relational Mapping. In most use-cases these definitions align, but that is not always the case.

  • It might refer to the physical transaction with the database.

  • It might refer to the logical notion of a transaction as related to a persistence context.

  • It might refer to the application notion of a Unit-of-Work, as defined by the archetypal pattern.

This documentation largely treats the physical and logic notions of a transaction as one-in-the-same.

8.1. Physical Transactions

Hibernate uses the JDBC API for persistence. In the world of Java, there are two well-defined mechanisms for dealing with transactions in JDBC: JDBC itself and JTA. Hibernate supports both mechanisms for integrating with transactions and allowing applications to manage physical transactions.

The transaction handling per Session is handled by the org.hibernate.resource.transaction.spi.TransactionCoordinator contract, which are built by the org.hibernate.resource.transaction.spi.TransactionCoordinatorBuilder service. TransactionCoordinatorBuilder represents a strategy for dealing with transactions whereas TransactionCoordinator represents one instance of that strategy related to a Session. Which TransactionCoordinatorBuilder implementation to use is defined by the hibernate.transaction.coordinator_class setting.

jdbc (the default for non-Jakarta Persistence applications)

Manages transactions via calls to java.sql.Connection

jta

Manages transactions via JTA. See Java EE bootstrapping

If a Jakarta Persistence application does not provide a setting for hibernate.transaction.coordinator_class, Hibernate will automatically build the proper transaction coordinator based on the transaction type for the persistence unit.

If a non-Jakarta Persistence application does not provide a setting for hibernate.transaction.coordinator_class, Hibernate will use jdbc as the default. This default will cause problems if the application actually uses JTA-based transactions. A non-Jakarta Persistence application that uses JTA-based transactions should explicitly set hibernate.transaction.coordinator_class=jta or provide a custom org.hibernate.resource.transaction.TransactionCoordinatorBuilder that builds a org.hibernate.resource.transaction.TransactionCoordinator that properly coordinates with JTA-based transactions.

For details on implementing a custom TransactionCoordinatorBuilder, or simply better understanding how it works, see the Integration Guide .

Hibernate uses JDBC connections and JTA resources directly, without adding any additional locking behavior. Hibernate does not lock objects in memory. The behavior defined by the isolation level of your database transactions does not change when you use Hibernate. The Hibernate Session acts as a transaction-scoped cache providing repeatable reads for lookup by identifier and queries that result in loading entities.

To reduce lock contention in the database, the physical database transaction needs to be as short as possible.

Long-running database transactions prevent your application from scaling to a highly-concurrent load. Do not hold a database transaction open during end-user-level work, but open it after the end-user-level work is finished.

This concept is referred to as transactional write-behind.

8.2. JTA configuration

Interaction with a JTA system is consolidated behind a single contract named org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform which exposes access to the javax.transaction.TransactionManager and javax.transaction.UserTransaction for that system as well as exposing the ability to register javax.transaction.Synchronization instances, check transaction status, etc.

Generally, JtaPlatform will need access to JNDI to resolve the JTA TransactionManager, UserTransaction, etc. See JNDI chapter for details on configuring access to JNDI.

Hibernate tries to discover the JtaPlatform it should use through the use of another service named org.hibernate.engine.transaction.jta.platform.spi.JtaPlatformResolver. If that resolution does not work, or if you wish to provide a custom implementation you will need to specify the hibernate.transaction.jta.platform setting. Hibernate provides many implementations of the JtaPlatform contract, all with short names:

Atomikos

JtaPlatform for Atomikos.

Borland

JtaPlatform for the Borland Enterprise Server.

Bitronix

JtaPlatform for Bitronix.

JBossAS

JtaPlatform for Arjuna/JBossTransactions/Narayana when used within the JBoss/WildFly Application Server.

JBossTS

JtaPlatform for Arjuna/JBossTransactions/Narayana when used standalone.

JOnAS

JtaPlatform for JOTM when used within JOnAS.

JOTM

JtaPlatform for JOTM when used standalone.

JRun4

JtaPlatform for the JRun 4 Application Server.

OC4J

JtaPlatform for Oracle’s OC4J container.

Orion

JtaPlatform for the Orion Application Server.

Resin

JtaPlatform for the Resin Application Server.

SapNetWeaver

JtaPlatform for the SAP NetWeaver Application Server.

SunOne

JtaPlatform for the SunOne Application Server.

Weblogic

JtaPlatform for the Weblogic Application Server.

WebSphere

JtaPlatform for older versions of the WebSphere Application Server.

WebSphereExtended

JtaPlatform for newer versions of the WebSphere Application Server.

8.3. Hibernate Transaction API

Hibernate provides an API for helping to isolate applications from the differences in the underlying physical transaction system in use. Based on the configured TransactionCoordinatorBuilder, Hibernate will simply do the right thing when this transaction API is used by the application. This allows your applications and components to be more portable to move around into different environments.

To use this API, you would obtain the org.hibernate.Transaction from the Session. Transaction allows for all the normal operations you’d expect: begin, commit and rollback, and it even exposes some cool methods like:

markRollbackOnly

that works in both JTA and JDBC.

getTimeout and setTimeout

that again work in both JTA and JDBC.

registerSynchronization

that allows you to register JTA Synchronizations even in non-JTA environments. In fact, in both JTA and JDBC environments, these Synchronizations are kept locally by Hibernate. In JTA environments, Hibernate will only ever register one single Synchronization with the TransactionManager to avoid ordering problems.

Additionally, it exposes a getStatus method that returns an org.hibernate.resource.transaction.spi.TransactionStatus enum. This method checks with the underlying transaction system if needed, so care should be taken to minimize its use; it can have a big performance impact in certain JTA setups.

Let’s take a look at using the Transaction API in the various environments.

Example 422. Using Transaction API in JDBC
StandardServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
		// "jdbc" is the default, but for explicitness
		.applySetting(AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jdbc")
		.build();

Metadata metadata = new MetadataSources(serviceRegistry)
		.addAnnotatedClass(Customer.class)
		.getMetadataBuilder()
		.build();

SessionFactory sessionFactory = metadata.getSessionFactoryBuilder()
		.build();

Session session = sessionFactory.openSession();
try {
	// calls Connection#setAutoCommit(false) to
	// signal start of transaction
	session.getTransaction().begin();

	session.createQuery("UPDATE customer set NAME = 'Sir. '||NAME")
			.executeUpdate();

	// calls Connection#commit(), if an error
	// happens we attempt a rollback
	session.getTransaction().commit();
}
catch (Exception e) {
	// we may need to rollback depending on
	// where the exception happened
	if (session.getTransaction().getStatus() == TransactionStatus.ACTIVE
			|| session.getTransaction().getStatus() == TransactionStatus.MARKED_ROLLBACK) {
		session.getTransaction().rollback();
	}
	// handle the underlying error
}
finally {
	session.close();
	sessionFactory.close();
}
Example 423. Using Transaction API in JTA (CMT)
StandardServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
		// "jdbc" is the default, but for explicitness
		.applySetting(AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jta")
		.build();

Metadata metadata = new MetadataSources(serviceRegistry)
		.addAnnotatedClass(Customer.class)
		.getMetadataBuilder()
		.build();

SessionFactory sessionFactory = metadata.getSessionFactoryBuilder()
		.build();

// Note: depending on the JtaPlatform used and some optional settings,
// the underlying transactions here will be controlled through either
// the JTA TransactionManager or UserTransaction

Session session = sessionFactory.openSession();
try {
	// Since we are in CMT, a JTA transaction would
	// already have been started.  This call essentially
	// no-ops
	session.getTransaction().begin();

	Number customerCount = (Number) session.createQuery("select count(c) from Customer c").uniqueResult();

	// Since we did not start the transaction (CMT),
	// we also will not end it.  This call essentially
	// no-ops in terms of transaction handling.
	session.getTransaction().commit();
}
catch (Exception e) {
	// again, the rollback call here would no-op (aside from
	// marking the underlying CMT transaction for rollback only).
	if (session.getTransaction().getStatus() == TransactionStatus.ACTIVE
			|| session.getTransaction().getStatus() == TransactionStatus.MARKED_ROLLBACK) {
		session.getTransaction().rollback();
	}
	// handle the underlying error
}
finally {
	session.close();
	sessionFactory.close();
}
Example 424. Using Transaction API in JTA (BMT)
StandardServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
		// "jdbc" is the default, but for explicitness
		.applySetting(AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jta")
		.build();

Metadata metadata = new MetadataSources(serviceRegistry)
		.addAnnotatedClass(Customer.class)
		.getMetadataBuilder()
		.build();

SessionFactory sessionFactory = metadata.getSessionFactoryBuilder()
		.build();

// Note: depending on the JtaPlatform used and some optional settings,
// the underlying transactions here will be controlled through either
// the JTA TransactionManager or UserTransaction

Session session = sessionFactory.openSession();
try {
	// Assuming a JTA transaction is not already active,
	// this call the TM/UT begin method.  If a JTA
	// transaction is already active, we remember that
	// the Transaction associated with the Session did
	// not "initiate" the JTA transaction and will later
	// nop-op the commit and rollback calls...
	session.getTransaction().begin();

	session.persist(new Customer());
	Customer customer = (Customer) session.createQuery("select c from Customer c").uniqueResult();

	// calls TM/UT commit method, assuming we are initiator.
	session.getTransaction().commit();
}
catch (Exception e) {
	// we may need to rollback depending on
	// where the exception happened
	if (session.getTransaction().getStatus() == TransactionStatus.ACTIVE
			|| session.getTransaction().getStatus() == TransactionStatus.MARKED_ROLLBACK) {
		// calls TM/UT commit method, assuming we are initiator;
		// otherwise marks the JTA transaction for rollback only
		session.getTransaction().rollback();
	}
	// handle the underlying error
}
finally {
	session.close();
	sessionFactory.close();
}

In the CMT case, we really could have omitted all of the Transaction calls. But the point of the examples was to show that the Transaction API really does insulate your code from the underlying transaction mechanism. In fact, if you strip away the comments and the single configuration setting supplied at bootstrap, the code is exactly the same in all 3 examples. In other words, we could develop that code and drop it, as-is, in any of the 3 transaction environments.

The Transaction API tries hard to make the experience consistent across all environments. To that end, it generally defers to the JTA specification when there are differences (for example automatically trying rollback on a failed commit).

8.4. Contextual sessions

Most applications using Hibernate need some form of contextual session, where a given session is in effect throughout the scope of a given context. However, across applications the definition of what constitutes a context is typically different; different contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grown ThreadLocal-based contextual sessions, helper classes such as HibernateUtil, or utilized third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession() method. Initially, this assumed usage of JTA transactions, where the JTA transaction defined both the scope and context of a current session. Given the maturity of the numerous stand-alone JTA TransactionManager implementations, most, if not all, applications should be using JTA transaction management, whether or not they are deployed into a J2EE container. Based on that, the JTA-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind SessionFactory.getCurrentSession() is now pluggable. To that end, a new extension interface, org.hibernate.context.spi.CurrentSessionContext, and a new configuration parameter, hibernate.current_session_context_class, have been added to allow pluggability of the scope and context of defining current sessions.

See the Javadocs for the org.hibernate.context.spi.CurrentSessionContext interface for a detailed discussion of its contract. It defines a single method, currentSession(), by which the implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate comes with three implementations of this interface:

org.hibernate.context.internal.JTASessionContext

current sessions are tracked and scoped by a JTA transaction. The processing here is exactly the same as in the older JTA-only approach.

org.hibernate.context.internal.ThreadLocalSessionContext

current sessions are tracked by thread of execution. See the Javadocs for more details.

org.hibernate.context.internal.ManagedSessionContext

current sessions are tracked by thread of execution. However, you are responsible to bind and unbind a Session instance with static methods on this class; it does not open, flush, or close a Session.

Typically, the value of this parameter would just name the implementation class to use. For the three out-of-the-box implementations, however, there are three corresponding short names: jta, thread, and managed.

The first two implementations provide a one session - one database transaction programming model. This is also known and used as session-per-request. The beginning and end of a Hibernate session is defined by the duration of a database transaction. If you use programmatic transaction demarcation in plain Java SE without JTA, you are advised to use the Hibernate Transaction API to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code.

The hibernate.current_session_context_class configuration parameter defines which org.hibernate.context.spi.CurrentSessionContext implementation should be used. For backward compatibility, if this configuration parameter is not set but a org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform is configured, Hibernate will use the org.hibernate.context.internal.JTASessionContext.

8.5. Transactional patterns (and anti-patterns)

8.5.1. Session-per-operation anti-pattern

This is an anti-pattern of opening and closing a Session for each database call in a single thread. It is also an anti-pattern in terms of database transactions. Group your database calls into a planned sequence. In the same way, do not auto-commit after every SQL statement in your application. Hibernate disables or expects the application server to disable, auto-commit mode immediately. Database transactions are never optional. All communication with a database must be encapsulated by a transaction. Avoid auto-commit behavior for reading data because many small transactions are unlikely to perform better than one clearly-defined unit of work, and are more difficult to maintain and extend.

Using auto-commit does not circumvent database transactions.

Instead, when in auto-commit mode, JDBC drivers simply perform each call in an implicit transaction call. It is as if your application called commit after each and every JDBC call.

8.5.2. Session-per-request pattern

This is the most common transaction pattern. The term request here relates to the concept of a system that reacts to a series of requests from a client/user. Web applications are a prime example of this type of system, though certainly not the only one. At the beginning of handling such a request, the application opens a Hibernate Session, starts a transaction, performs all data related work, ends the transaction and closes the Session. The crux of the pattern is the one-to-one relationship between the transaction and the Session.

Within this pattern, there is a common technique of defining a current session to simplify the need of passing this Session around to all the application components that may need access to it. Hibernate provides support for this technique through the getCurrentSession method of the SessionFactory. The concept of a current session has to have a scope that defines the bounds in which the notion of current is valid. This is the purpose of the org.hibernate.context.spi.CurrentSessionContext contract.

There are 2 reliable defining scopes:

  • First is a JTA transaction because it allows a callback hook to know when it is ending, which gives Hibernate a chance to close the Session and clean up. This is represented by the org.hibernate.context.internal.JTASessionContext implementation of the org.hibernate.context.spi.CurrentSessionContext contract. Using this implementation, a Session will be opened the first time getCurrentSession is called within that transaction.

  • Secondly is this application request cycle itself. This is best represented with the org.hibernate.context.internal.ManagedSessionContext implementation of the org.hibernate.context.spi.CurrentSessionContext contract. Here an external component is responsible for managing the lifecycle and scoping of a current session. At the start of such a scope, ManagedSessionContext#bind() method is called passing in the Session. In the end, its unbind() method is called. Some common examples of such external components include:

    • javax.servlet.Filter implementation

    • AOP interceptor with a pointcut on the service methods

    • A proxy/interception container

The getCurrentSession() method has one downside in a JTA environment. If you use it, after_statement connection release mode is also used by default. Due to a limitation of the JTA specification, Hibernate cannot automatically clean up any unclosed ScrollableResults or Iterator instances returned by scroll() or iterate(). Release the underlying database cursor by calling ScrollableResults#close() or Hibernate.close(Iterator) explicitly from a finally block.

8.5.3. Conversations (application-level transactions)

The session-per-request pattern is not the only valid way of designing units of work. Many business processes require a whole series of interactions with the user that are interleaved with database accesses. In web and enterprise applications, it is not acceptable for a database transaction to span a user interaction. Consider the following example:

The first screen of a dialog opens. The data seen by the user is loaded in a particular Session and database transaction. The user is free to modify the objects.

The user uses a UI element to save their work after five minutes of editing. The modifications are made persistent. The user also expects to have exclusive access to the data during the edit session.

Even though we have multiple databases access here, from the point of view of the user, this series of steps represents a single unit of work. There are many ways to implement this in your application.

A first naive implementation might keep the Session and database transaction open while the user is editing, using database-level locks to prevent other users from modifying the same data and to guarantee isolation and atomicity. This is an anti-pattern because lock contention is a bottleneck which will prevent scalability in the future.

Several database transactions are used to implement the conversation. In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier. A single conversation usually spans several database transactions. These multiple database accesses can only be atomic as a whole if only one of these database transactions (typically the last one) stores the updated data. All others only read data. A common way to receive this data is through a wizard-style dialog spanning several request/response cycles. Hibernate includes some features which make this easy to implement.

Automatic Versioning

Hibernate can perform automatic optimistic concurrency control for you. It can automatically detect (at the end of the conversation) if a concurrent modification occurred during user think time.

Detached Objects

If you decide to use the session-per-request pattern, all loaded instances will be in the detached state during user think time. Hibernate allows you to reattach the objects and persist the modifications. The pattern is called session-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications.

Extended Session

The Hibernate Session can be disconnected from the underlying JDBC connection after the database transaction has been committed and reconnected when a new client request occurs. This pattern is known as session-per-conversation and makes even reattachment unnecessary. Automatic versioning is used to isolate concurrent modifications, and the Session will not be allowed to flush automatically, only explicitly.

Session-per-request-with-detached-objects and session-per-conversation each have advantages and disadvantages.

8.5.4. Session-per-application anti-pattern

The session-per-application is also considered an anti-pattern. The Hibernate Session, like the Jakarta Persistence EntityManager, is not a thread-safe object and it is intended to be confined to a single thread at once. If the Session is shared among multiple threads, there will be race conditions as well as visibility issues, so beware of this.

An exception thrown by Hibernate means you have to rollback your database transaction and close the Session immediately. If your Session is bound to the application, you have to stop the application. Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. This means that the database state and the business objects will be out of sync. Usually, this is not a problem because exceptions are not recoverable and you will have to start over after rollback anyway.

For more details, check out the exception handling section in Persistence Context chapter.

The Session caches every object that is in a persistent state (watched and checked for dirty state by Hibernate). If you keep it open for a long time or simply load too much data, it will grow endlessly until you get an OutOfMemoryException. One solution is to call clear() and evict() to manage the Session cache, but you should consider a Stored Procedure if you need mass data operations. Some solutions are shown in the Batching chapter. Keeping a Session open for the duration of a user session also means a higher probability of stale data.

9. JNDI

Hibernate does optionally interact with JNDI on the application’s behalf. Generally, it does this when the application:

  • has asked the SessionFactory be bound to JNDI

  • has specified a DataSource to use by JNDI name

  • is using JTA transactions and the JtaPlatform needs to do JNDI lookups for TransactionManager, UserTransaction, etc

All of these JNDI calls route through a single service whose role is org.hibernate.engine.jndi.spi.JndiService. The standard JndiService accepts a number of configuration settings:

hibernate.jndi.class

names the javax.naming.InitialContext implementation class to use. See javax.naming.Context#INITIAL_CONTEXT_FACTORY

hibernate.jndi.url

names the JNDI InitialContext connection url. See javax.naming.Context.PROVIDER_URL

Any other settings prefixed with hibernate.jndi. will be collected and passed along to the JNDI provider.

The standard JndiService assumes that all JNDI calls are relative to the same InitialContext. If your application uses multiple naming servers for whatever reason, you will need a custom JndiService implementation to handle those details.

10. Locking

In a relational database, locking refers to actions taken to prevent data from changing between the time it is read and the time is used.

Your locking strategy can be either optimistic or pessimistic.

Optimistic

Optimistic locking assumes that multiple transactions can complete without affecting each other, and that therefore transactions can proceed without locking the data resources that they affect. Before committing, each transaction verifies that no other transaction has modified its data. If the check reveals conflicting modifications, the committing transaction rolls back.

Pessimistic

Pessimistic locking assumes that concurrent transactions will conflict with each other, and requires resources to be locked after they are read and only unlocked after the application has finished using the data.

Hibernate provides mechanisms for implementing both types of locking in your applications.

10.1. Optimistic

When your application uses long transactions or conversations that span several database transactions, you can store versioning data so that if the same entity is updated by two conversations, the last to commit changes is informed of the conflict, and does not override the other conversation’s work. This approach guarantees some isolation, but scales well and works particularly well in read-often-write-sometimes situations.

Hibernate provides two different mechanisms for storing versioning information, a dedicated version number or a timestamp.

A version or timestamp property can never be null for a detached instance. Hibernate detects any instance with a null version or timestamp as transient, regardless of other unsaved-value strategies that you specify. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate, especially useful if you use assigned identifiers or composite keys.

10.1.1. Mapping optimistic locking

Jakarta Persistence defines support for optimistic locking based on either a version (sequential numeric) or timestamp strategy. To enable this style of optimistic locking simply add the jakarta.persistence.Version to the persistent attribute that defines the optimistic locking value. According to Jakarta Persistence, the valid types for these attributes are limited to:

  • int or Integer

  • short or Short

  • long or Long

  • java.sql.Timestamp

However, Hibernate allows you to use even Java 8 Date/Time types, such as Instant.

Example 425. @Version annotation mapping
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`name`")
	private String name;

	@Version
	private long version;

	//Getters and setters are omitted for brevity

}
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`name`")
	private String name;

	@Version
	private Timestamp version;

	//Getters and setters are omitted for brevity

}
@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`name`")
	private String name;

	@Version
	private Instant version;

	//Getters and setters are omitted for brevity

}
Dedicated version number

The version number mechanism for optimistic locking is provided through a @Version annotation.

Example 426. @Version annotation
@Version
private long version;

Here, the version property is mapped to the version column, and the entity manager uses it to detect conflicting updates, and prevent the loss of updates that would otherwise be overwritten by a last-commit-wins strategy.

The version column can be any kind of type, as long as you define and implement the appropriate UserVersionType.

Your application is forbidden from altering the version number set by Hibernate. To artificially increase the version number, see the documentation for properties LockModeType.OPTIMISTIC_FORCE_INCREMENT or LockModeType.PESSIMISTIC_FORCE_INCREMENT check in the Hibernate Entity Manager reference documentation.

If the version number is generated by the database, such as a trigger, use the annotation @org.hibernate.annotations.Generated(GenerationTime.ALWAYS) on the version attribute.

Timestamp

Timestamps are a less reliable way of optimistic locking than version numbers but can be used by applications for other purposes as well. Timestamping is automatically used if you the @Version annotation on a Date or Calendar property type.

Example 427. Using timestamps for optimistic locking
@Version
private Date version;

Hibernate can retrieve the timestamp value from the database or the JVM, by reading the value you specify for the @org.hibernate.annotations.Source annotation. The value can be either org.hibernate.annotations.SourceType.DB or org.hibernate.annotations.SourceType.VM. The default behavior is to use the database, and database is also used if you don’t specify the annotation at all.

The timestamp can also be generated by the database instead of Hibernate if you use the @org.hibernate.annotations.Generated(GenerationTime.ALWAYS) or the @Source annotation.

Example 428. Database-generated version timestamp mapping
@Entity(name = "Person")
public static class Person {

	@Id
	private Long id;

	private String firstName;

	private String lastName;

	@Version
	@Source(value = SourceType.DB)
	private Date version;
}

Now, when persisting a Person entity, Hibernate calls the database-specific current timestamp retrieval function:

Example 429. Database-generated version timestamp example
Person person = new Person();
person.setId(1L);
person.setFirstName("John");
person.setLastName("Doe");
assertNull(person.getVersion());

entityManager.persist(person);
assertNotNull(person.getVersion());
CALL current_timestamp()

INSERT INTO
    Person
    (firstName, lastName, version, id)
VALUES
    (?, ?, ?, ?)

-- binding parameter [1] as [VARCHAR]   - [John]
-- binding parameter [2] as [VARCHAR]   - [Doe]
-- binding parameter [3] as [TIMESTAMP] - [2017-05-18 12:03:03.808]
-- binding parameter [4] as [BIGINT]    - [1]
Excluding attributes

By default, every entity attribute modification is going to trigger a version incrementation. If there is an entity property which should not bump up the entity version, then you need to annotate it with the Hibernate @OptimisticLock annotation, as illustrated in the following example.

Example 430. @OptimisticLock mapping example
@Entity(name = "Phone")
public static class Phone {

	@Id
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OptimisticLock(excluded = true)
	private long callCount;

	@Version
	private Long version;

	//Getters and setters are omitted for brevity

	public void incrementCallCount() {
		this.callCount++;
	}
}

This way, if one thread modifies the Phone number while a second thread increments the callCount attribute, the two concurrent transactions are not going to conflict as illustrated by the following example.

Example 431. @OptimisticLock exlude attribute example
doInJPA(this::entityManagerFactory, entityManager -> {
	Phone phone = entityManager.find(Phone.class, 1L);
	phone.setNumber("+123-456-7890");

	doInJPA(this::entityManagerFactory, _entityManager -> {
		Phone _phone = _entityManager.find(Phone.class, 1L);
		_phone.incrementCallCount();

		log.info("Bob changes the Phone call count");
	});

	log.info("Alice changes the Phone number");
});
-- Bob changes the Phone call count

update
    Phone
set
    callCount = 1,
    "number" = '123-456-7890',
    version = 0
where
    id = 1
    and version = 0

-- Alice changes the Phone number

update
    Phone
set
    callCount = 0,
    "number" = '+123-456-7890',
    version = 1
where
    id = 1
    and version = 0

When Bob changes the Phone entity callCount, the entity version is not bumped up. That’s why Alice’s UPDATE succeeds since the entity version is still 0, even if Bob has changed the record since Alice loaded it.

Although there is no conflict between Bob and Alice, Alice’s UPDATE overrides Bob’s change to the callCount attribute.

For this reason, you should only use this feature if you can accommodate lost updates on the excluded entity properties.

Versionless optimistic locking

Although the default @Version property optimistic locking mechanism is sufficient in many situations, sometimes, you need rely on the actual database row column values to prevent lost updates.

Hibernate supports a form of optimistic locking that does not require a dedicated "version attribute". This is also useful for use with modeling legacy schemas.

The idea is that you can get Hibernate to perform "version checks" using either all of the entity’s attributes or just the attributes that have changed. This is achieved through the use of the @OptimisticLocking annotation which defines a single attribute of type org.hibernate.annotations.OptimisticLockType.

There are 4 available OptimisticLockTypes:

NONE

optimistic locking is disabled even if there is a @Version annotation present

VERSION (the default)

performs optimistic locking based on a @Version as described above

ALL

performs optimistic locking based on all fields as part of an expanded WHERE clause restriction for the UPDATE/DELETE SQL statements

DIRTY

performs optimistic locking based on dirty fields as part of an expanded WHERE clause restriction for the UPDATE/DELETE SQL statements

Versionless optimistic locking using OptimisticLockType.ALL
Example 432. OptimisticLockType.ALL mapping example
@Entity(name = "Person")
@OptimisticLocking(type = OptimisticLockType.ALL)
@DynamicUpdate
public static class Person {

	@Id
	private Long id;

	@Column(name = "`name`")
	private String name;

	private String country;

	private String city;

	@Column(name = "created_on")
	private Timestamp createdOn;

	//Getters and setters are omitted for brevity
}

When you need to modify the Person entity above:

Example 433. OptimisticLockType.ALL update example
Person person = entityManager.find(Person.class, 1L);
person.setCity("Washington D.C.");
UPDATE
    Person
SET
    city=?
WHERE
    id=?
    AND city=?
    AND country=?
    AND created_on=?
    AND "name"=?

-- binding parameter [1] as [VARCHAR] - [Washington D.C.]
-- binding parameter [2] as [BIGINT] - [1]
-- binding parameter [3] as [VARCHAR] - [New York]
-- binding parameter [4] as [VARCHAR] - [US]
-- binding parameter [5] as [TIMESTAMP] - [2016-11-16 16:05:12.876]
-- binding parameter [6] as [VARCHAR] - [John Doe]

As you can see, all the columns of the associated database row are used in the WHERE clause. If any column has changed after the row was loaded, there won’t be any match, and a StaleStateException or an OptimisticEntityLockException is going to be thrown.

When using OptimisticLockType.ALL, you should also use @DynamicUpdate because the UPDATE statement must take into consideration all the entity property values.

Versionless optimistic locking using OptimisticLockType.DIRTY

The OptimisticLockType.DIRTY differs from OptimisticLockType.ALL in that it only takes into consideration the entity properties that have changed since the entity was loaded in the currently running Persistence Context.

Example 434. OptimisticLockType.DIRTY mapping example
@Entity(name = "Person")
@OptimisticLocking(type = OptimisticLockType.DIRTY)
@DynamicUpdate
@SelectBeforeUpdate
public static class Person {

	@Id
	private Long id;

	@Column(name = "`name`")
	private String name;

	private String country;

	private String city;

	@Column(name = "created_on")
	private Timestamp createdOn;

	//Getters and setters are omitted for brevity
}

When you need to modify the Person entity above:

Example 435. OptimisticLockType.DIRTY update example
Person person = entityManager.find(Person.class, 1L);
person.setCity("Washington D.C.");
UPDATE
    Person
SET
    city=?
WHERE
    id=?
    and city=?

-- binding parameter [1] as [VARCHAR] - [Washington D.C.]
-- binding parameter [2] as [BIGINT] - [1]
-- binding parameter [3] as [VARCHAR] - [New York]

This time, only the database column that has changed was used in the WHERE clause.

The main advantage of OptimisticLockType.DIRTY over OptimisticLockType.ALL and the default OptimisticLockType.VERSION used implicitly along with the @Version mapping, is that it allows you to minimize the risk of OptimisticEntityLockException across non-overlapping entity property changes.

When using OptimisticLockType.DIRTY, you should also use @DynamicUpdate because the UPDATE statement must take into consideration all the dirty entity property values, and also the @SelectBeforeUpdate annotation so that detached entities are properly handled by the Session#update(entity) operation.

10.2. Pessimistic

Typically, you only need to specify an isolation level for the JDBC connections and let the database handle locking issues. If you do need to obtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction, Hibernate gives you the tools you need.

Hibernate always uses the locking mechanism of the database, and never lock objects in memory.

10.3. LockMode and LockModeType

Long before Java Persistence 1.0, Hibernate already defined various explicit locking strategies through its LockMode enumeration. Jakarta Persistence comes with its own LockModeType enumeration which defines similar strategies as the Hibernate-native LockMode.

LockModeType LockMode Description

NONE

NONE

The absence of a lock. All objects switch to this lock mode at the end of a Transaction. Objects associated with the session via a call to update() or saveOrUpdate() also start out in this lock mode.

READ and OPTIMISTIC

READ

The entity version is checked towards the end of the currently running transaction.

WRITE and OPTIMISTIC_FORCE_INCREMENT

WRITE

The entity version is incremented automatically even if the entity has not changed.

PESSIMISTIC_FORCE_INCREMENT

PESSIMISTIC_FORCE_INCREMENT

The entity is locked pessimistically and its version is incremented automatically even if the entity has not changed.

PESSIMISTIC_READ

PESSIMISTIC_READ

The entity is locked pessimistically using a shared lock if the database supports such a feature. Otherwise, an explicit lock is used.

PESSIMISTIC_WRITE

PESSIMISTIC_WRITE, UPGRADE

The entity is locked using an explicit lock.

PESSIMISTIC_WRITE with a jakarta.persistence.lock.timeout setting of 0

UPGRADE_NOWAIT

The lock acquisition request fails fast if the row s already locked.

PESSIMISTIC_WRITE with a jakarta.persistence.lock.timeout setting of -2

UPGRADE_SKIPLOCKED

The lock acquisition request skips the already locked rows. It uses a SELECT …​ FOR UPDATE SKIP LOCKED in Oracle and PostgreSQL 9.5, or SELECT …​ with (rowlock, updlock, readpast) in SQL Server.

The explicit user request mentioned above occurs as a consequence of any of the following actions:

  • a call to Session.load(), specifying a LockMode.

  • a call to Session.lock().

  • a call to Query.setLockMode().

If you call Session.load() with option UPGRADE, UPGRADE_NOWAIT or UPGRADE_SKIPLOCKED, and the requested object is not already loaded by the session, the object is loaded using SELECT …​ FOR UPDATE.

If you call load() for an object that is already loaded with a less restrictive lock than the one you request, Hibernate calls lock() for that object.

Session.lock() performs a version number check if the specified lock mode is READ, UPGRADE, UPGRADE_NOWAIT or UPGRADE_SKIPLOCKED. In the case of UPGRADE, UPGRADE_NOWAIT or UPGRADE_SKIPLOCKED, the SELECT …​ FOR UPDATE syntax is used.

If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead of throwing an exception. This ensures that applications are portable.

10.4. Jakarta Persistence locking query hints

Jakarta Persistence defined two locking-related query hints:

jakarta.persistence.lock.timeout

it gives the number of milliseconds a lock acquisition request will wait before throwing an exception

jakarta.persistence.lock.scope

defines the scope of the lock acquisition request. The scope can either be NORMAL (default value) or EXTENDED. The EXTENDED scope will cause a lock acquisition request to be passed to other owned table structured (e.g. @Inheritance(strategy=InheritanceType.JOINED), @ElementCollection)

Example 436. jakarta.persistence.lock.timeout example
entityManager.find(
	Person.class, id, LockModeType.PESSIMISTIC_WRITE,
	Collections.singletonMap("jakarta.persistence.lock.timeout", 200)
);
SELECT explicitlo0_.id     AS id1_0_0_,
       explicitlo0_."name" AS name2_0_0_
FROM   person explicitlo0_
WHERE  explicitlo0_.id = 1
FOR UPDATE wait 2

Not all JDBC database drivers support setting a timeout value for a locking request. If not supported, the Hibernate dialect ignores this query hint.

The jakarta.persistence.lock.scope is not yet supported as specified by the Jakarta Persistence standard.

10.5. The buildLockRequest API

Traditionally, Hibernate offered the Session#lock() method for acquiring an optimistic or a pessimistic lock on a given entity. Because varying the locking options was difficult when using a single LockMode parameter, Hibernate has added the Session#buildLockRequest() method API.

The following example shows how to obtain a shared database lock without waiting for the lock acquisition request.

Example 437. buildLockRequest example
Person person = entityManager.find(Person.class, id);
Session session = entityManager.unwrap(Session.class);
session
	.buildLockRequest(LockOptions.NONE)
	.setLockMode(LockMode.PESSIMISTIC_READ)
	.setTimeOut(LockOptions.NO_WAIT)
	.lock(person);
SELECT p.id AS id1_0_0_ ,
       p.name AS name2_0_0_
FROM   Person p
WHERE  p.id = 1

SELECT id
FROM   Person
WHERE  id = 1
FOR    SHARE NOWAIT

10.6. Follow-on-locking

When using Oracle, the FOR UPDATE exclusive locking clause cannot be used with:

  • DISTINCT

  • GROUP BY

  • UNION

  • inlined views (derived tables), therefore, affecting the legacy Oracle pagination mechanism as well.

For this reason, Hibernate uses secondary selects to lock the previously fetched entities.

Example 438. Follow-on-locking example
List<Person> persons = entityManager.createQuery(
	"select DISTINCT p from Person p", Person.class)
.setLockMode(LockModeType.PESSIMISTIC_WRITE)
.getResultList();
SELECT DISTINCT p.id as id1_0_, p."name" as name2_0_
FROM Person p

SELECT id
FROM Person
WHERE id = 1 FOR UPDATE

SELECT id
FROM Person
WHERE id = 1 FOR UPDATE

To avoid the N+1 query problem, a separate query can be used to apply the lock using the associated entity identifiers.

Example 439. Secondary query entity locking
List<Person> persons = entityManager.createQuery(
	"select DISTINCT p from Person p", Person.class)
.getResultList();

entityManager.createQuery(
	"select p.id from Person p where p in :persons")
.setLockMode(LockModeType.PESSIMISTIC_WRITE)
.setParameter("persons", persons)
.getResultList();
SELECT DISTINCT p.id as id1_0_, p."name" as name2_0_
FROM Person p

SELECT p.id as col_0_0_
FROM Person p
WHERE p.id IN ( 1 , 2 )
FOR UPDATE

The lock request was moved from the original query to a secondary one which takes the previously fetched entities to lock their associated database records.

Prior to Hibernate 5.2.1, the follow-on-locking mechanism was applied uniformly to any locking query executing on Oracle. Since 5.2.1, the Oracle Dialect tries to figure out if the current query demands the follow-on-locking mechanism.

Even more important is that you can overrule the default follow-on-locking detection logic and explicitly enable or disable it on a per query basis.

Example 440. Disabling the follow-on-locking mechanism explicitly
List<Person> persons = entityManager.createQuery(
	"select p from Person p", Person.class)
.setMaxResults(10)
.unwrap(Query.class)
.setLockOptions(
	new LockOptions(LockMode.PESSIMISTIC_WRITE)
		.setFollowOnLocking(false))
.getResultList();
SELECT *
FROM (
    SELECT p.id as id1_0_, p."name" as name2_0_
    FROM Person p
)
WHERE rownum <= 10
FOR UPDATE

The follow-on-locking mechanism should be explicitly enabled only if the currently executing query fails because the FOR UPDATE clause cannot be applied, meaning that the Dialect resolving mechanism needs to be further improved.

11. Fetching

Fetching, essentially, is the process of grabbing data from the database and making it available to the application. Tuning how an application does fetching is one of the biggest factors in determining how an application will perform. Fetching too much data, in terms of width (values/columns) and/or depth (results/rows), adds unnecessary overhead in terms of both JDBC communication and ResultSet processing. Fetching too little data might cause additional fetching to be needed. Tuning how an application fetches data presents a great opportunity to influence the overall application performance.

11.1. The basics

The concept of fetching breaks down into two different questions.

  • When should the data be fetched? Now? Later?

  • How should the data be fetched?

"Now" is generally termed eager or immediate while "later" is generally termed lazy or delayed.

There are a number of scopes for defining fetching:

static

Static definition of fetching strategies is done in the mappings. The statically-defined fetch strategies are used in the absence of any dynamically defined strategies.

SELECT

Performs a separate SQL select to load the data. This can either be EAGER (the second select is issued immediately) or LAZY (the second select is delayed until the data is needed). This is the strategy generally termed N+1.

JOIN

Inherently an EAGER style of fetching. The data to be fetched is obtained through the use of an SQL outer join.

BATCH

Performs a separate SQL select to load a number of related data items using an IN-restriction as part of the SQL WHERE-clause based on a batch size. Again, this can either be EAGER (the second select is issued immediately) or LAZY (the second select is delayed until the data is needed).

SUBSELECT

Performs a separate SQL select to load associated data based on the SQL restriction used to load the owner. Again, this can either be EAGER (the second select is issued immediately) or LAZY (the second select is delayed until the data is needed).

dynamic (sometimes referred to as runtime)

The dynamic definition is really use-case centric. There are multiple ways to define dynamic fetching:

fetch profiles

defined in mappings, but can be enabled/disabled on the Session.

HQL / JPQL

both Hibernate and Jakarta Persistence Criteria queries have the ability to specify fetching, specific to said query.

entity graphs

using Jakarta Persistence EntityGraphs

fetch profile and entity graph are mutually exclusive. When both are present, entity graph would take effect and fetch profile would be ignored.

11.2. Direct fetching vs. entity queries

To see the difference between direct fetching and entity queries in regard to eagerly fetched associations, consider the following entities:

Example 441. Domain model
@Entity(name = "Department")
public static class Department {

	@Id
	private Long id;

	//Getters and setters omitted for brevity
}

@Entity(name = "Employee")
public static class Employee {

	@Id
	private Long id;

	@NaturalId
	private String username;

	@ManyToOne(fetch = FetchType.EAGER)
	private Department department;

	//Getters and setters omitted for brevity
}

The Employee entity has a @ManyToOne association to a Department which is fetched eagerly.

When issuing a direct entity fetch, Hibernate executed the following SQL query:

Example 442. Direct fetching example
Employee employee = entityManager.find(Employee.class, 1L);
select
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.username as username2_1_0_,
    d.id as id1_0_1_
from
    Employee e
left outer join
    Department d
        on e.department_id=d.id
where
    e.id = 1

The LEFT OUTER JOIN clause is added to the generated SQL query because this association is required to be fetched eagerly.

On the other hand, if you are using an entity query that does not contain a JOIN FETCH directive to the Department association:

Example 443. Entity query fetching example
Employee employee = entityManager.createQuery(
		"select e " +
		"from Employee e " +
		"where e.id = :id", Employee.class)
.setParameter("id", 1L)
.getSingleResult();
select
    e.id as id1_1_,
    e.department_id as departme3_1_,
    e.username as username2_1_
from
    Employee e
where
    e.id = 1

select
    d.id as id1_0_0_
from
    Department d
where
    d.id = 1

Hibernate uses a secondary select instead. This is because the entity query fetch policy cannot be overridden, so Hibernate requires a secondary select to ensure that the EAGER association is fetched prior to returning the result to the user.

If you forget to JOIN FETCH all EAGER associations, Hibernate is going to issue a secondary select for each and every one of those which, in turn, can lead to N + 1 query issue.

For this reason, you should prefer LAZY associations.

11.3. Applying fetch strategies

Let’s consider these topics as it relates to a sample domain model and a few use cases.

Example 444. Sample domain model
	@Entity(name = "Department")
	public static class Department {

		@Id
		private Long id;

		@OneToMany(mappedBy = "department")
		private List<Employee> employees = new ArrayList<>();

		//Getters and setters omitted for brevity
	}

	@Entity(name = "Employee")
	public static class Employee {

		@Id
		private Long id;

		@NaturalId
		private String username;

		@Column(name = "pswd")
		@ColumnTransformer(
			read = "decrypt('AES', '00', pswd )",
			write = "encrypt('AES', '00', ?)"
		)
// For H2 2.0.202+ one must use the varbinary DDL type
//		@Column(name = "pswd", columnDefinition = "varbinary")
//		@ColumnTransformer(
//			read = "trim(trailing u&'\\0000' from cast(decrypt('AES', '00', pswd ) as character varying))",
//			write = "encrypt('AES', '00', ?)"
//		)
		private String password;

		private int accessLevel;

		@ManyToOne(fetch = FetchType.LAZY)
		private Department department;

		@ManyToMany(mappedBy = "employees")
		private List<Project> projects = new ArrayList<>();

		//Getters and setters omitted for brevity
	}

	@Entity(name = "Project")
	public class Project {

		@Id
		private Long id;

		@ManyToMany
		private List<Employee> employees = new ArrayList<>();

		//Getters and setters omitted for brevity
	}

The Hibernate recommendation is to statically mark all associations lazy and to use dynamic fetching strategies for eagerness.

This is unfortunately at odds with the Jakarta Persistence specification which defines that all one-to-one and many-to-one associations should be eagerly fetched by default.

Hibernate, as a Jakarta Persistence provider, honors that default.

11.4. No fetching

For the first use case, consider the application login process for an Employee. Let’s assume that login only requires access to the Employee information, not Project nor Department information.

Example 445. No fetching example
Employee employee = entityManager.createQuery(
	"select e " +
	"from Employee e " +
	"where " +
	"	e.username = :username and " +
	"	e.password = :password",
	Employee.class)
.setParameter("username", username)
.setParameter("password", password)
.getSingleResult();

In this example, the application gets the Employee data. However, because all associations from Employee are declared as LAZY (Jakarta Persistence defines the default for collections as LAZY) no other data is fetched.

If the login process does not need access to the Employee information specifically, another fetching optimization here would be to limit the width of the query results.

Example 446. No fetching (scalar) example
Integer accessLevel = entityManager.createQuery(
	"select e.accessLevel " +
	"from Employee e " +
	"where " +
	"	e.username = :username and " +
	"	e.password = :password",
	Integer.class)
.setParameter("username", username)
.setParameter("password", password)
.getSingleResult();

11.5. Dynamic fetching via queries

For the second use case, consider a screen displaying the Projects for an Employee. Certainly access to the Employee is needed, as is the collection of Projects for that Employee. Information about Departments, other Employees or other Projects is not needed.

Example 447. Dynamic JPQL fetching example
Employee employee = entityManager.createQuery(
	"select e " +
	"from Employee e " +
	"left join fetch e.projects " +
	"where " +
	"	e.username = :username and " +
	"	e.password = :password",
	Employee.class)
.setParameter("username", username)
.setParameter("password", password)
.getSingleResult();
Example 448. Dynamic query fetching example
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<Employee> query = builder.createQuery(Employee.class);
Root<Employee> root = query.from(Employee.class);
root.fetch("projects", JoinType.LEFT);
query.select(root).where(
	builder.and(
		builder.equal(root.get("username"), username),
		builder.equal(root.get("password"), password)
	)
);
Employee employee = entityManager.createQuery(query).getSingleResult();

In this example we have an Employee and their Projects loaded in a single query shown both as an HQL query and a Jakarta Persistence Criteria query. In both cases, this resolves to exactly one database query to get all that information.

11.6. Dynamic fetching via Jakarta Persistence entity graph

Jakarta Persistence also supports a feature called EntityGraphs to provide the application developer has more control over fetch plans. It has two modes to choose from:

fetch graph

In this case, all attributes specified in the entity graph will be treated as FetchType.EAGER, and all attributes not specified will ALWAYS be treated as FetchType.LAZY.

load graph

In this case, all attributes specified in the entity graph will be treated as FetchType.EAGER, but attributes not specified use their static mapping specification.

Below is a fetch graph dynamic fetching example:

Example 449. Fetch graph example
@Entity(name = "Employee")
@NamedEntityGraph(name = "employee.projects",
	attributeNodes = @NamedAttributeNode("projects")
)
Employee employee = entityManager.find(
	Employee.class,
	userId,
	Collections.singletonMap(
		"jakarta.persistence.fetchgraph",
		entityManager.getEntityGraph("employee.projects")
	)
);

When executing a JPQL query, if an EAGER association is omitted, Hibernate will issue a secondary select for every association needed to be fetched eagerly, which can lead to N+1 query issues.

For this reason, it’s better to use LAZY associations, and only fetch them eagerly on a per-query basis.

An EntityGraph is the root of a "load plan" and must correspond to an EntityType.

11.6.1. Jakarta Persistence (key) subgraphs

A sub-graph is used to control the fetching of sub-attributes of the AttributeNode it is applied to. It is generally defined via the @NamedSubgraph annotation.

If we have a Project parent entity which has an employees child associations, and we’d like to fetch the department for the Employee child association.

Example 450. Fetch graph with a subgraph mapping
@Entity(name = "Project")
@NamedEntityGraph(name = "project.employees",
	attributeNodes = @NamedAttributeNode(
		value = "employees",
		subgraph = "project.employees.department"
	),
	subgraphs = @NamedSubgraph(
		name = "project.employees.department",
		attributeNodes = @NamedAttributeNode("department")
	)
)
public static class Project {

	@Id
	private Long id;

	@ManyToMany
	private List<Employee> employees = new ArrayList<>();

	//Getters and setters omitted for brevity
}

When fetching this entity graph, Hibernate generates the following SQL query:

Example 451. Fetch graph with a subgraph mapping
Project project = doInJPA(this::entityManagerFactory, entityManager -> {
	return entityManager.find(
		Project.class,
		1L,
		Collections.singletonMap(
			"jakarta.persistence.fetchgraph",
			entityManager.getEntityGraph("project.employees")
		)
	);
});
select
    p.id as id1_2_0_, e.id as id1_1_1_, d.id as id1_0_2_,
    e.accessLevel as accessLe2_1_1_,
    e.department_id as departme5_1_1_,
    decrypt( 'AES', '00', e.pswd  ) as pswd3_1_1_,
    e.username as username4_1_1_,
    p_e.projects_id as projects1_3_0__,
    p_e.employees_id as employee2_3_0__
from
    Project p
inner join
    Project_Employee p_e
        on p.id=p_e.projects_id
inner join
    Employee e
        on p_e.employees_id=e.id
inner join
    Department d
        on e.department_id=d.id
where
    p.id = ?

-- binding parameter [1] as [BIGINT] - [1]

Specifying a sub-graph is only valid for an attribute (or its "key") whose type is a ManagedType. So while an EntityGraph must correspond to an EntityType, a Subgraph is legal for any ManagedType. An attribute’s key is defined as either:

  • For a singular attribute, the attribute’s type must be an IdentifiableType and that IdentifiableType must have a composite identifier. The "key sub-graph" is applied to the identifier type. The non-key sub-graph applies to the attribute’s value, which must be a ManagedType.

  • For a plural attribute, the attribute must be a Map and the Map’s key value must be a ManagedType. The "key sub-graph" is applied to the Map’s key type. In this case, the non-key sub-graph applies to the plural attribute’s value/element.

11.6.2. Jakarta Persistence SubGraph sub-typing

SubGraphs can also be sub-type specific. Given an attribute whose value is an inheritance hierarchy, we can refer to attributes of a specific sub-type using the forms of sub-graph definition that accept the sub-type Class.

11.6.3. Creating and applying Jakarta Persistence graphs from text representations

Hibernate allows the creation of Jakarta Persistence fetch/load graphs by parsing a textual representation of the graph. Generally speaking, the textual representation of a graph is a comma-separated list of attribute names, optionally including any sub-graph specifications. org.hibernate.graph.EntityGraphParser is the starting point for such parsing operations.

Parsing a textual representation of a graph is not (yet) a part of the Jakarta Persistence specification. So the syntax described here is specific to Hibernate. We do hope to eventually make this syntax part of the Jakarta Persistence specification proper.

Example 452. Parsing a simple graph
final EntityGraph<Project> graph = GraphParser.parse(
		Project.class,
		"employees(department)",
		entityManager
);

This example actually functions exactly as Fetch graph with a subgraph mapping, just using a parsed graph rather than a named graph.

The syntax also supports defining "key sub-graphs". To specify a key sub-graph, .key is added to the end of the attribute name.

Example 453. Parsing an entity key graph
final EntityGraph<Movie> graph = GraphParser.parse(
		Movie.class,
		"cast.key(name)",
		entityManager
);
Example 454. Parsing a map key graph
final EntityGraph<Ticket> graph = GraphParser.parse(
		Ticket.class,
		"showing.key(movie(cast))",
		entityManager
);

Parsing can also handle sub-type specific sub-graphs. For example, given an entity hierarchy of LegalEntity ← (Corporation | Person | NonProfit) and an attribute named responsibleParty whose type is the LegalEntity base type we might have:

responsibleParty(Corporation: ceo)

We can even duplicate the attribute names to apply different sub-type sub-graphs:

responsibleParty(taxIdNumber), responsibleParty(Corporation: ceo), responsibleParty(NonProfit: sector)

The duplicated attribute names are handled according to the Jakarta Persistence specification which says that duplicate specification of the attribute node results in the originally registered AttributeNode to be re-used effectively merging the 2 AttributeNode specifications together. In other words, the above specification creates a single AttributeNode with 3 distinct SubGraphs. It is functionally the same as calling:

Class<Invoice> invoiceClass = ...;
jakarta.persistence.EntityGraph<Invoice> invoiceGraph = entityManager.createEntityGraph( invoiceClass );
invoiceGraph.addAttributeNode( "responsibleParty" );
invoiceGraph.addSubgraph( "responsibleParty" ).addAttributeNode( "taxIdNumber" );
invoiceGraph.addSubgraph( "responsibleParty", Corporation.class ).addAttributeNode( "ceo" );
invoiceGraph.addSubgraph( "responsibleParty", NonProfit.class ).addAttributeNode( "sector" );

11.6.4. Combining multiple Jakarta Persistence entity graphs into one

Multiple entity graphs can be combined into a single "super graph" that acts as a union. Graph from the previous example can also be built by combining separate aspect graphs into one, such as:

Example 455. Combining multiple graphs into one
final EntityGraph<Project> a = GraphParser.parse(
		Project.class, "employees(username)", entityManager
);

final EntityGraph<Project> b = GraphParser.parse(
		Project.class, "employees(password, accessLevel)", entityManager
);

final EntityGraph<Project> c = GraphParser.parse(
		Project.class, "employees(department(employees(username)))", entityManager
);

final EntityGraph<Project> all = EntityGraphs.merge(entityManager, Project.class, a, b, c);

11.7. Dynamic fetching via Hibernate profiles

Suppose we wanted to leverage loading by natural-id to obtain the Employee information in the "projects for and employee" use-case. Loading by natural-id uses the statically defined fetching strategies, but does not expose a means to define load-specific fetching. So we would leverage a fetch profile.

Example 456. Fetch profile example
@Entity(name = "Employee")
@FetchProfile(
	name = "employee.projects",
	fetchOverrides = {
		@FetchProfile.FetchOverride(
			entity = Employee.class,
			association = "projects",
			mode = FetchMode.JOIN
		)
	}
)
session.enableFetchProfile("employee.projects");
Employee employee = session.bySimpleNaturalId(Employee.class).load(username);

Here the Employee is obtained by natural-id lookup and the Employee’s Project data is fetched eagerly. If the Employee data is resolved from cache, the Project data is resolved on its own. However, if the Employee data is not resolved in cache, the Employee and Project data is resolved in one SQL query via join as we saw above.

11.8. Batch fetching

Hibernate offers the @BatchSize annotation, which can be used when fetching uninitialized entity proxies.

Considering the following entity mapping:

Example 457. @BatchSize mapping example
@Entity(name = "Department")
public static class Department {

	@Id
	private Long id;

	@OneToMany(mappedBy = "department")
	//@BatchSize(size = 5)
	private List<Employee> employees = new ArrayList<>();

	//Getters and setters omitted for brevity

}

@Entity(name = "Employee")
public static class Employee {

	@Id
	private Long id;

	@NaturalId
	private String name;

	@ManyToOne(fetch = FetchType.LAZY)
	private Department department;

	//Getters and setters omitted for brevity
}

Considering that we have previously fetched several Department entities, and now we need to initialize the employees entity collection for each particular Department, the @BatchSize annotations allows us to load multiple Employee entities in a single database roundtrip.

Example 458. @BatchSize fetching example
List<Department> departments = entityManager.createQuery(
	"select d " +
	"from Department d " +
	"inner join d.employees e " +
	"where e.name like 'John%'", Department.class)
.getResultList();

for (Department department : departments) {
	log.infof(
		"Department %d has {} employees",
		department.getId(),
		department.getEmployees().size()
	);
}
SELECT
    d.id as id1_0_
FROM
    Department d
INNER JOIN
    Employee employees1_
    ON d.id=employees1_.department_id

SELECT
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.name as name2_1_0_
FROM
    Employee e
WHERE
    e.department_id IN (
        0, 2, 3, 4, 5
    )

SELECT
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.name as name2_1_0_
FROM
    Employee e
WHERE
    e.department_id IN (
        6, 7, 8, 9, 1
    )

As you can see in the example above, there are only two SQL statements used to fetch the Employee entities associated with multiple Department entities.

Without @BatchSize, you’d run into a N + 1 query issue, so, instead of 2 SQL statements, there would be 10 queries needed for fetching the Employee child entities.

However, although @BatchSize is better than running into an N + 1 query issue, most of the time, a DTO projection or a JOIN FETCH is a much better alternative since it allows you to fetch all the required data with a single query.

11.9. The @Fetch annotation mapping

Besides the FetchType.LAZY or FetchType.EAGER Jakarta Persistence annotations, you can also use the Hibernate-specific @Fetch annotation that accepts one of the following FetchModes:

SELECT

The association is going to be fetched using a secondary select for each individual entity, collection, or join load. This mode can be used for either FetchType.EAGER or FetchType.LAZY.

JOIN

Use an outer join to load the related entities, collections or joins when using direct fetching. This mode can only be used for FetchType.EAGER.

SUBSELECT

Available for collections only. When accessing a non-initialized collection, this fetch mode will trigger loading all elements of all collections of the same role for all owners associated with the persistence context using a single secondary select.

11.10. FetchMode.SELECT

To demonstrate how FetchMode.SELECT works, consider the following entity mapping:

Example 459. FetchMode.SELECT mapping example
@Entity(name = "Department")
public static class Department {

	@Id
	private Long id;

	@OneToMany(mappedBy = "department", fetch = FetchType.LAZY)
	@Fetch(FetchMode.SELECT)
	private List<Employee> employees = new ArrayList<>();

	//Getters and setters omitted for brevity

}

@Entity(name = "Employee")
public static class Employee {

	@Id
	@GeneratedValue
	private Long id;

	@NaturalId
	private String username;

	@ManyToOne(fetch = FetchType.LAZY)
	private Department department;

	//Getters and setters omitted for brevity

}

Considering there are multiple Department entities, each one having multiple Employee entities, when executing the following test case, Hibernate fetches every uninitialized Employee collection using a secondary SELECT statement upon accessing the child collection for the first time:

Example 460. FetchMode.SELECT mapping example
List<Department> departments = entityManager.createQuery(
	"select d from Department d", Department.class)
.getResultList();

log.infof("Fetched %d Departments", departments.size());

for (Department department : departments) {
	assertEquals(3, department.getEmployees().size());
}
SELECT
    d.id as id1_0_
FROM
    Department d

-- Fetched 2 Departments

SELECT
    e.department_id as departme3_1_0_,
    e.id as id1_1_0_,
    e.id as id1_1_1_,
    e.department_id as departme3_1_1_,
    e.username as username2_1_1_
FROM
    Employee e
WHERE
    e.department_id = 1

SELECT
    e.department_id as departme3_1_0_,
    e.id as id1_1_0_,
    e.id as id1_1_1_,
    e.department_id as departme3_1_1_,
    e.username as username2_1_1_
FROM
    Employee e
WHERE
    e.department_id = 2

The more Department entities are fetched by the first query, the more secondary SELECT statements are executed to initialize the employees collections. Therefore, FetchMode.SELECT can lead to N + 1 query issue.

11.11. FetchMode.SUBSELECT

To demonstrate how FetchMode.SUBSELECT works, we are going to modify the FetchMode.SELECT mapping example to use FetchMode.SUBSELECT:

Example 461. FetchMode.SUBSELECT mapping example
@OneToMany(mappedBy = "department", fetch = FetchType.LAZY)
@Fetch(FetchMode.SUBSELECT)
private List<Employee> employees = new ArrayList<>();

Now, we are going to fetch all Department entities that match a given filtering predicate and then navigate their employees collections.

Hibernate is going to avoid the N + 1 query issue by generating a single SQL statement to initialize all employees collections for all Department entities that were previously fetched. Instead of passing all entity identifiers, Hibernate simply reruns the previous query that fetched the Department entities.

Example 462. FetchMode.SUBSELECT mapping example
List<Department> departments = entityManager.createQuery(
	"select d " +
	"from Department d " +
	"where d.name like :token", Department.class)
.setParameter("token", "Department%")
.getResultList();

log.infof("Fetched %d Departments", departments.size());

for (Department department : departments) {
	assertEquals(3, department.getEmployees().size());
}
SELECT
    d.id as id1_0_
FROM
    Department d
where
    d.name like 'Department%'

-- Fetched 2 Departments

SELECT
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.username as username2_1_0_
FROM
    Employee e
WHERE
    e.department_id in (
        SELECT
            fetchmodes0_.id
        FROM
            Department fetchmodes0_
        WHERE
            d.name like 'Department%'
    )

11.12. FetchMode.JOIN

To demonstrate how FetchMode.JOIN works, we are going to modify the FetchMode.SELECT mapping example to use FetchMode.JOIN instead:

Example 463. FetchMode.JOIN mapping example
@OneToMany(mappedBy = "department")
@Fetch(FetchMode.JOIN)
private List<Employee> employees = new ArrayList<>();

Now, we are going to fetch one Department and navigate its employees collections.

The reason why we are not using a JPQL query to fetch multiple Department entities is because the FetchMode.JOIN strategy would be overridden by the query fetching directive.

To fetch multiple relationships with a JPQL query, the JOIN FETCH directive must be used instead.

Therefore, FetchMode.JOIN is useful for when entities are fetched directly, via their identifier or natural-id.

Also, the FetchMode.JOIN acts as a FetchType.EAGER strategy. Even if we mark the association as FetchType.LAZY, the FetchMode.JOIN will load the association eagerly.

Hibernate is going to avoid the secondary query by issuing an OUTER JOIN for the employees collection.

Example 464. FetchMode.JOIN mapping example
Department department = entityManager.find(Department.class, 1L);

log.infof("Fetched department: %s", department.getId());

assertEquals(3, department.getEmployees().size());
SELECT
    d.id as id1_0_0_,
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_2_,
    e.department_id as departme3_1_2_,
    e.username as username2_1_2_
FROM
    Department d
LEFT OUTER JOIN
    Employee e
        on d.id = e.department_id
WHERE
    d.id = 1

-- Fetched department: 1

This time, there was no secondary query because the child collection was loaded along with the parent entity.

11.13. @LazyCollection

The @LazyCollection annotation is used to specify the lazy fetching behavior of a given collection. The possible values are given by the LazyCollectionOption enumeration:

TRUE

Load it when the state is requested.

FALSE

Eagerly load it.

EXTRA

Prefer extra queries over full collection loading.

The TRUE and FALSE values are deprecated since you should be using the Jakarta Persistence FetchType attribute of the @ElementCollection, @OneToMany, or @ManyToMany collection.

The EXTRA value has no equivalent in the Jakarta Persistence specification, and it’s used to avoid loading the entire collection even when the collection is accessed for the first time. Each element is fetched individually using a secondary query.

Example 465. LazyCollectionOption.EXTRA mapping example
@Entity(name = "Department")
public static class Department {

	@Id
	private Long id;

	@OneToMany(mappedBy = "department", cascade = CascadeType.ALL)
	@OrderColumn(name = "order_id")
	@LazyCollection(LazyCollectionOption.EXTRA)
	private List<Employee> employees = new ArrayList<>();

	//Getters and setters omitted for brevity

}

@Entity(name = "Employee")
public static class Employee {

	@Id
	private Long id;

	@NaturalId
	private String username;

	@ManyToOne(fetch = FetchType.LAZY)
	private Department department;

	//Getters and setters omitted for brevity

}

LazyCollectionOption.EXTRA only works for ordered collections, either List(s) that are annotated with @OrderColumn or Map(s).

For bags (e.g. regular List(s) of entities that do not preserve any certain ordering), the @LazyCollection(LazyCollectionOption.EXTRA) behaves like any other FetchType.LAZY collection (the collection is fetched entirely upon being accessed for the first time).

Now, considering we have the following entities:

Example 466. LazyCollectionOption.EXTRA Domain Model example
Department department = new Department();
department.setId(1L);
entityManager.persist(department);

for (long i = 1; i <= 3; i++) {
	Employee employee = new Employee();
	employee.setId(i);
	employee.setUsername(String.format("user_%d", i));
	department.addEmployee(employee);
}

When fetching the employee collection entries by their position in the List, Hibernate generates the following SQL statements:

Example 467. LazyCollectionOption.EXTRA fetching example
Department department = entityManager.find(Department.class, 1L);

int employeeCount = department.getEmployees().size();

for(int i = 0; i < employeeCount; i++) {
	log.infof("Fetched employee: %s", department.getEmployees().get(i).getUsername());
}
SELECT
    max(order_id) + 1
FROM
    Employee
WHERE
    department_id = ?

-- binding parameter [1] as [BIGINT] - [1]

SELECT
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.username as username2_1_0_
FROM
    Employee e
WHERE
    e.department_id=?
    AND e.order_id=?

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [INTEGER] - [0]

SELECT
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.username as username2_1_0_
FROM
    Employee e
WHERE
    e.department_id=?
    AND e.order_id=?

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [INTEGER] - [1]

SELECT
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.username as username2_1_0_
FROM
    Employee e
WHERE
    e.department_id=?
    AND e.order_id=?

-- binding parameter [1] as [BIGINT]  - [1]
-- binding parameter [2] as [INTEGER] - [2]

Therefore, the child entities were fetched one after the other without triggering a full collection initialization.

For this reason, caution is advised since accessing all elements using LazyCollectionOption.EXTRA can lead to N + 1 query issue.

12. Batching

12.1. JDBC batching

JDBC offers support for batching together SQL statements that can be represented as a single PreparedStatement. Implementation wise this generally means that drivers will send the batched operation to the server in one call, which can save on network calls to the database. Hibernate can leverage JDBC batching. The following settings control this behavior.

hibernate.jdbc.batch_size

Controls the maximum number of statements Hibernate will batch together before asking the driver to execute the batch. Zero or a negative number disables this feature.

hibernate.jdbc.batch_versioned_data

Some JDBC drivers return incorrect row counts when a batch is executed. If your JDBC driver falls into this category this setting should be set to false. Otherwise, it is safe to enable this which will allow Hibernate to still batch the DML for versioned entities and still use the returned row counts for optimistic lock checks. Since 5.0, it defaults to true. Previously (versions 3.x and 4.x), it used to be false.

hibernate.jdbc.batch.builder

Names the implementation class used to manage batching capabilities. It is almost never a good idea to switch from Hibernate’s default implementation. But if you wish to, this setting would name the org.hibernate.engine.jdbc.batch.spi.BatchBuilder implementation to use.

hibernate.order_updates

Forces Hibernate to order SQL updates by the entity type and the primary key value of the items being updated. This allows for more batching to be used. It will also result in fewer transaction deadlocks in highly concurrent systems. Comes with a performance hit, so benchmark before and after to see if this actually helps or hurts your application.

hibernate.order_inserts

Forces Hibernate to order inserts to allow for more batching to be used. Comes with a performance hit, so benchmark before and after to see if this actually helps or hurts your application.

Since version 5.2, Hibernate allows overriding the global JDBC batch size given by the hibernate.jdbc.batch_size configuration property on a per Session basis.

Example 468. Hibernate specific JDBC batch size configuration on a per Session basis
entityManager
	.unwrap(Session.class)
	.setJdbcBatchSize(10);

12.2. Session batching

The following example shows an anti-pattern for batch inserts.

Example 469. Naive way to insert 100 000 entities with Hibernate
EntityManager entityManager = null;
EntityTransaction txn = null;
try {
	entityManager = entityManagerFactory().createEntityManager();

	txn = entityManager.getTransaction();
	txn.begin();

	for (int i = 0; i < 100_000; i++) {
		Person Person = new Person(String.format("Person %d", i));
		entityManager.persist(Person);
	}

	txn.commit();
} catch (RuntimeException e) {
	if (txn != null && txn.isActive()) txn.rollback();
		throw e;
} finally {
	if (entityManager != null) {
		entityManager.close();
	}
}

There are several problems associated with this example:

  1. Hibernate caches all the newly inserted Customer instances in the session-level cache, so, when the transaction ends, 100 000 entities are managed by the persistence context. If the maximum memory allocated to the JVM is rather low, this example could fail with an OutOfMemoryException. The Java 1.8 JVM allocated either 1/4 of available RAM or 1Gb, which can easily accommodate 100 000 objects on the heap.

  2. long-running transactions can deplete a connection pool so other transactions don’t get a chance to proceed.

  3. JDBC batching is not enabled by default, so every insert statement requires a database roundtrip. To enable JDBC batching, set the hibernate.jdbc.batch_size property to an integer between 10 and 50.

Hibernate disables insert batching at the JDBC level transparently if you use an identity identifier generator.

12.2.1. Batch inserts

When you make new objects persistent, employ methods flush() and clear() to the session regularly, to control the size of the first-level cache.

Example 470. Flushing and clearing the Session
EntityManager entityManager = null;
EntityTransaction txn = null;
try {
	entityManager = entityManagerFactory().createEntityManager();

	txn = entityManager.getTransaction();
	txn.begin();

	int batchSize = 25;

	for (int i = 0; i < entityCount; i++) {
		if (i > 0 && i % batchSize == 0) {
			//flush a batch of inserts and release memory
			entityManager.flush();
			entityManager.clear();
		}

		Person Person = new Person(String.format("Person %d", i));
		entityManager.persist(Person);
	}

	txn.commit();
} catch (RuntimeException e) {
	if (txn != null && txn.isActive()) txn.rollback();
		throw e;
} finally {
	if (entityManager != null) {
		entityManager.close();
	}
}

12.2.2. Session scroll

When you retrieve and update data, flush() and clear() the session regularly. In addition, use method scroll() to take advantage of server-side cursors for queries that return many rows of data.

Example 471. Using scroll()
EntityManager entityManager = null;
EntityTransaction txn = null;
ScrollableResults scrollableResults = null;
try {
	entityManager = entityManagerFactory().createEntityManager();

	txn = entityManager.getTransaction();
	txn.begin();

	int batchSize = 25;

	Session session = entityManager.unwrap(Session.class);

	scrollableResults = session
		.createQuery("select p from Person p")
		.setCacheMode(CacheMode.IGNORE)
		.scroll(ScrollMode.FORWARD_ONLY);

	int count = 0;
	while (scrollableResults.next()) {
		Person Person = (Person) scrollableResults.get();
		processPerson(Person);
		if (++count % batchSize == 0) {
			//flush a batch of updates and release memory:
			entityManager.flush();
			entityManager.clear();
		}
	}

	txn.commit();
} catch (RuntimeException e) {
	if (txn != null && txn.isActive()) txn.rollback();
		throw e;
} finally {
	if (scrollableResults != null) {
		scrollableResults.close();
	}
	if (entityManager != null) {
		entityManager.close();
	}
}

If left unclosed by the application, Hibernate will automatically close the underlying resources (e.g. ResultSet and PreparedStatement) used internally by the ScrollableResults when the current transaction is ended (either commit or rollback).

However, it is good practice to close the ScrollableResults explicitly.

12.2.3. StatelessSession

StatelessSession is an alternative to Session and provides:

  • a command-oriented API

  • with no associated persistence context.

Thus, a stateless session is a slightly lower-level abstraction that’s closer to the underlying JDBC activity:

  • there’s no first-level cache,

  • the stateless session does not interact with any second-level or query cache, and

  • there’s no transactional write-behind or automatic dirty checking.

Instead, persistence operations occur synchronously when a method of StatelessSession is invoked, and entities returned by a stateless session are always detached.

A stateless session may be used to stream data to and from the database in the form of detached objects. With a stateless session, there’s no need to explicitly manage the size of the first-level cache by explicitly clearing the persistence context.

The StatelessSession API comes with certain limitations:

  • operations performed using a stateless session never cascade to associated instances,

  • collections are ignored by a stateless session,

  • lazy loading of associations is not transparent, and is only available via an explicit operation named fetch(), and

  • operations performed via a stateless session bypass Hibernate’s event model and interceptors.

Due to the lack of a first-level cache, stateless sessions are vulnerable to data aliasing effects.
Example 472. Using a StatelessSession
StatelessSession statelessSession = null;
Transaction txn = null;
ScrollableResults scrollableResults = null;
try {
	SessionFactory sessionFactory = entityManagerFactory().unwrap(SessionFactory.class);
	statelessSession = sessionFactory.openStatelessSession();

	txn = statelessSession.getTransaction();
	txn.begin();

	scrollableResults = statelessSession
		.createQuery("select p from Person p")
		.scroll(ScrollMode.FORWARD_ONLY);

	while (scrollableResults.next()) {
		Person Person = (Person) scrollableResults.get();
		processPerson(Person);
		statelessSession.update(Person);
	}

	txn.commit();
} catch (RuntimeException e) {
	if (txn != null && txn.getStatus() == TransactionStatus.ACTIVE) txn.rollback();
		throw e;
} finally {
	if (scrollableResults != null) {
		scrollableResults.close();
	}
	if (statelessSession != null) {
		statelessSession.close();
	}
}

The Customer instances returned by the query are immediately detached. They are never associated with any persistence context.

The insert(), update(), and delete() operations defined by the StatelessSession interface operate directly on database rows. They cause the corresponding SQL operations to be executed immediately. They have different semantics from the save(), saveOrUpdate(), and delete() operations defined by the Session interface.

12.3. Hibernate Query Language for DML

DML, or Data Manipulation Language, refers to SQL statements such as INSERT, UPDATE, and DELETE. Hibernate provides methods for bulk SQL-style DML statement execution, in the form of Hibernate Query Language (HQL).

12.3.1. HQL/JPQL for UPDATE and DELETE

Both the Hibernate native Query Language and JPQL (Java Persistence Query Language) provide support for bulk UPDATE and DELETE.

Example 473. Pseudo-syntax for UPDATE and DELETE statements using HQL
UPDATE FROM EntityName e WHERE e.name = ?

DELETE FROM EntityName e WHERE e.name = ?

Although the FROM and WHERE clauses are optional, it is good practice to declare them explicitly.

The FROM clause can only refer to a single entity, which can be aliased. If the entity name is aliased, any property references must be qualified using that alias. If the entity name is not aliased, then it is illegal for any property references to be qualified.

Joins, either implicit or explicit, are prohibited in a bulk HQL query. You can use sub-queries in the WHERE clause, and the sub-queries themselves can contain joins.

Example 474. Executing a JPQL UPDATE, using the Query.executeUpdate()
int updatedEntities = entityManager.createQuery(
	"update Person p " +
	"set p.name = :newName " +
	"where p.name = :oldName")
.setParameter("oldName", oldName)
.setParameter("newName", newName)
.executeUpdate();
Example 475. Executing an HQL UPDATE, using the Query.executeUpdate()
int updatedEntities = session.createQuery(
	"update Person " +
	"set name = :newName " +
	"where name = :oldName")
.setParameter("oldName", oldName)
.setParameter("newName", newName)
.executeUpdate();

In keeping with the EJB3 specification, HQL UPDATE statements, by default, do not effect the version or the timestamp property values for the affected entities. You can use a versioned update to force Hibernate to reset the version or timestamp property values, by adding the VERSIONED keyword after the UPDATE keyword.

Example 476. Updating the version of timestamp
int updatedEntities = session.createQuery(
	"update versioned Person " +
	"set name = :newName " +
	"where name = :oldName")
.setParameter("oldName", oldName)
.setParameter("newName", newName)
.executeUpdate();

If you use the VERSIONED statement, you cannot use custom version types that implement the org.hibernate.usertype.UserVersionType.

This feature is only available in HQL since it’s not standardized by Jakarta Persistence.

Example 477. A JPQL DELETE statement
int deletedEntities = entityManager.createQuery(
	"delete Person p " +
	"where p.name = :name")
.setParameter("name", name)
.executeUpdate();
Example 478. An HQL DELETE statement
int deletedEntities = session.createQuery(
	"delete Person " +
	"where name = :name")
.setParameter("name", name)
.executeUpdate();

Method Query.executeUpdate() returns an int value, which indicates the number of entities affected by the operation. This may or may not correlate to the number of rows affected in the database. A JPQL/HQL bulk operation might result in multiple SQL statements being executed, such as for joined-subclass. In the example of joined-subclass, a DELETE against one of the subclasses may actually result in deletes in the tables underlying the join, or further down the inheritance hierarchy.

12.3.2. HQL syntax for INSERT

Example 479. Pseudo-syntax for INSERT-SELECT statements
INSERT INTO EntityName
	properties_list
SELECT select_list
FROM ...

Alternatively one can also declare individual values

Example 480. Pseudo-syntax for INSERT-VALUES statements
INSERT INTO EntityName
	properties_list
VALUES values_list

The properties_list is analogous to the column specification in the SQL INSERT statement. Note that INSERT statements are inherently non-polymorphic, so it is not possible to use an EntityName which is abstract or refer to subclass properties.

The SELECT statement can be any valid HQL select query, but the return types must match the types expected by the INSERT. Hibernate verifies the return types during query compilation, instead of expecting the database to check it. Problems might result from Hibernate types which are equivalent, rather than equal. One such example is a mismatch between a property defined as an org.hibernate.type.StandardBasicTypes.DATE and a property defined as an org.hibernate.type.StandardBasicTypes.TIMESTAMP, even though the database may not make a distinction, or may be capable of handling the conversion.

If id property is not specified in the properties_list, Hibernate generates a value automatically. Automatic generation is only available if you use ID generators which operate on the database. Otherwise, Hibernate throws an exception during parsing. Available in-database generators implement org.hibernate.id.PostInsertIdentifierGenerator.

For properties mapped as either version or timestamp, the insert statement gives you two options. You can either specify the property in the properties_list, in which case its value is taken from the corresponding select expressions or omit it from the properties_list, in which case the seed value defined by the org.hibernate.type.descriptor.java.VersionJavaType is used.

Example 481. HQL INSERT statement
int insertedEntities = session.createQuery(
	"insert into Partner (id, name) " +
	"select p.id, p.name " +
	"from Person p ")
.executeUpdate();

This section is only a brief overview of HQL. For more information, see Hibernate Query Language.

12.3.3. Bulk mutation strategies

When a bulk mutation involves multiple tables, Hibernate has to issue individual DML statements to the respective tables. Since the mutation itself could have an effect on the conditions used in the statement, it’s generally not possible to simply execute parts of the DML statement against the respective tables. Instead, Hibernate has to temporarily remember which rows will be affected, and execute the DML statements based on these rows.

Usually, Hibernate will make use of local or global temporary tables to remember the primary keys of the rows. For some databases, currently only PostgreSQL and DB2, a more advanced strategy (CteMutationStrategy) is used, which makes use of DML in CTE support to execute the whole operation in one SQL statement.

The chosen strategy, unless overridden through the hibernate.query.mutation_strategy setting, is based on the Dialect support through org.hibernate.dialect.Dialect.getFallbackSqmMutationStrategy.

Class diagram

Considering we have the following entities:

Entity class diagram

The Person entity is the base class of this entity inheritance model, and is mapped as follows:

Example 482. Bulk mutation base class entity
@Entity(name = "Person")
@Inheritance(strategy = InheritanceType.JOINED)
public static class Person implements Serializable {

	@Id
	private Integer id;

	@Id
	private String companyName;

	private String name;

	private boolean employed;

	//Getters and setters are omitted for brevity

}

Both the Doctor and Engineer entity classes extend the Person base class:

Example 483. Bulk mutation subclass entities
@Entity(name = "Doctor")
public static class Doctor extends Person {
}

@Entity(name = "Engineer")
public static class Engineer extends Person {

	private boolean fellow;

	public boolean isFellow() {
		return fellow;
	}

	public void setFellow(boolean fellow) {
		this.fellow = fellow;
	}
}
Inheritance tree bulk processing

Now, when you try to execute a bulk entity delete query:

Example 484. Bulk mutation delete query example
int updateCount = session.createQuery(
	"delete from Person where employed = :employed" )
.setParameter( "employed", false )
.executeUpdate();
create temporary table
    HT_Person
(
    id int4 not null,
    companyName varchar(255) not null
)

insert
into
    HT_Person
    select
        p.id as id,
        p.companyName as companyName
    from
        Person p
    where
        p.employed = ?

delete
from
    Engineer
where
    (
        id, companyName
    ) IN (
        select
            id,
            companyName
        from
            HT_Person
    )

delete
from
    Doctor
where
    (
        id, companyName
    ) IN (
        select
            id,
            companyName
        from
            HT_Person
    )

delete
from
    Person
where
    (
        id, companyName
    ) IN (
        select
            id,
            companyName
        from
            HT_Person
    )

HT_Person is a temporary table that Hibernate creates to hold all the entity identifiers that are to be updated or deleted by the bulk operation. The temporary table can be either global or local, depending on the underlying database capabilities.

Non-temporary table bulk mutation strategies

When the temporary table strategy can not be used because the database user lacks privilege to create temporary tables, the InlineMutationStrategy must be used.

To use this strategy, you need to configure the following configuration property:

<property name="hibernate.query.mutation_strategy"
          value="org.hibernate.query.sqm.mutation.internal.inline.InlineMutationStrategy"
/>

Now, when running the previous test case, Hibernate generates the following SQL statements:

Example 485. InlineIdsInClauseBulkIdStrategy delete entity query example
select
    p.id as id,
    p.companyName as companyName
from
    Person p
where
    p.employed = ?

delete
from
    Engineer
where
        ( id, companyName )
    in (
        ( 1,'Red Hat USA' ),
        ( 3,'Red Hat USA' ),
        ( 1,'Red Hat Europe' ),
        ( 3,'Red Hat Europe' )
    )

delete
from
    Doctor
where
        ( id, companyName )
    in (
        ( 1,'Red Hat USA' ),
        ( 3,'Red Hat USA' ),
        ( 1,'Red Hat Europe' ),
        ( 3,'Red Hat Europe' )
    )

delete
from
    Person
where
        ( id, companyName )
    in (
        ( 1,'Red Hat USA' ),
        ( 3,'Red Hat USA' ),
        ( 1,'Red Hat Europe' ),
        ( 3,'Red Hat Europe' )
    )

So, the entity identifiers are selected first and used for each particular update or delete statement.

13. Caching

At runtime, Hibernate handles moving data into and out of the second-level cache in response to the operations performed by the Session, which acts as a transaction-level cache of persistent data. Once an entity becomes managed, that object is added to the internal cache of the current persistence context (EntityManager or Session). The persistence context is also called the first-level cache, and it’s enabled by default.

It is possible to configure a JVM-level (SessionFactory-level) or even a cluster cache on a class-by-class and collection-by-collection basis.

Be aware that Hibernate caches are not aware of changes made to the persistent store by other applications.

To address this limitation, you can configure a TTL (Time To Live) retention policy at the second-level cache region level so that the underlying cache entries expire regularly.

13.1. Configuring second-level caching

Hibernate can integrate with various caching providers for the purpose of caching data outside the context of a particular Session. This section defines the settings which control this behavior.

13.1.1. RegionFactory

org.hibernate.cache.spi.RegionFactory defines the integration between Hibernate and a pluggable caching provider. hibernate.cache.region.factory_class is used to declare the provider to use. Hibernate comes with built-in support for the Java caching standard JCache and also the popular caching library: Infinispan. Detailed information is provided later in this chapter.

13.1.2. Caching configuration properties

Besides provider specific configuration, there are a number of configurations options on the Hibernate side of the integration that control various caching behaviors:

hibernate.cache.use_second_level_cache

Enable or disable second level caching overall. By default, if the currently configured RegionFactory is not the NoCachingRegionFactory, then the second-level cache is going to be enabled. Otherwise, the second-level cache is disabled.

hibernate.cache.use_query_cache

Enable or disable second level caching of query results. The default is false.

hibernate.cache.query_cache_factory

Query result caching is handled by a special contract that deals with staleness-based invalidation of the results. The default implementation does not allow stale results at all. Use this for applications that would like to relax that. Names an implementation of org.hibernate.cache.spi.TimestampsCacheFactory.

hibernate.cache.use_minimal_puts

Optimizes second-level cache operations to minimize writes, at the cost of more frequent reads. Providers typically set this appropriately.

hibernate.cache.region_prefix

Defines a name to be used as a prefix to all second-level cache region names.

hibernate.cache.default_cache_concurrency_strategy

In Hibernate second-level caching, all regions can be configured differently including the concurrency strategy to use when accessing that particular region. This setting allows defining a default strategy to be used. This setting is very rarely required as the pluggable providers do specify the default strategy to use. Valid values include:

  • read-only,

  • read-write,

  • nonstrict-read-write,

  • transactional

hibernate.cache.use_structured_entries

If true, forces Hibernate to store data in the second-level cache in a more human-friendly format. Can be useful if you’d like to be able to "browse" the data directly in your cache, but does have a performance impact.

hibernate.cache.auto_evict_collection_cache

Enables or disables the automatic eviction of a bidirectional association’s collection cache entry when the association is changed just from the owning side. This is disabled by default, as it has a performance impact to track this state. However, if your application does not manage both sides of bidirectional association where the collection side is cached, the alternative is to have stale data in that collection cache.

hibernate.cache.use_reference_entries

Enable direct storage of entity references into the second level cache for read-only or immutable entities.

hibernate.cache.keys_factory

When storing entries into the second-level cache as a key-value pair, the identifiers can be wrapped into tuples <entity type, tenant, identifier> to guarantee uniqueness in case that second-level cache stores all entities in single space. These tuples are then used as keys in the cache. When the second-level cache implementation (incl. its configuration) guarantees that different entity types are stored separately and multi-tenancy is not used, you can omit this wrapping to achieve better performance. Currently, this property is only supported when Infinispan is configured as the second-level cache implementation. Valid values are:

  • default (wraps identitifers in the tuple)

  • simple (uses identifiers as keys without any wrapping)

  • fully qualified class name that implements org.hibernate.cache.spi.CacheKeysFactory

13.2. Configuring second-level cache mappings

The cache mappings can be configured via Jakarta Persistence annotations or XML descriptors or using the Hibernate-specific mapping files.

By default, entities are not part of the second level cache and we recommend you to stick to this setting. However, you can override this by setting the shared-cache-mode element in your persistence.xml file or by using the jakarta.persistence.sharedCache.mode property in your configuration file. The following values are possible:

ENABLE_SELECTIVE (Default and recommended value)

Entities are not cached unless explicitly marked as cacheable (with the @Cacheable annotation).

DISABLE_SELECTIVE

Entities are cached unless explicitly marked as non-cacheable.

ALL

Entities are always cached even if marked as non-cacheable.

NONE

No entity is cached even if marked as cacheable. This option can make sense to disable second-level cache altogether.

The cache concurrency strategy used by default can be set globally via the hibernate.cache.default_cache_concurrency_strategy configuration property. The values for this property are:

read-only

If your application needs to read, but not modify, instances of a persistent class, a read-only cache is the best choice. Application can still delete entities and these changes should be reflected in second-level cache so that the cache does not provide stale entities. Implementations may use performance optimizations based on the immutability of entities.

read-write

If the application needs to update data, a read-write cache might be appropriate. This strategy provides consistent access to single entity, but not a serializable transaction isolation level; e.g. when TX1 reads looks up an entity and does not find it, TX2 inserts the entity into cache and TX1 looks it up again, the new entity can be read in TX1.

nonstrict-read-write

Similar to read-write strategy but there might be occasional stale reads upon concurrent access to an entity. The choice of this strategy might be appropriate if the application rarely updates the same data simultaneously and strict transaction isolation is not required. Implementations may use performance optimizations that make use of the relaxed consistency guarantee.

transactional

Provides serializable transaction isolation level.

Rather than using a global setting, it is recommended to define the cache concurrency strategy on a per entity basis.

Use the @org.hibernate.annotations.Cache annotation for this purpose.

The @Cache annotation define three attributes:

usage

Defines the CacheConcurrencyStrategy

region

Defines a cache region where entries will be stored

include

If lazy properties should be included in the second level cache. The default value is all so lazy properties are cacheable. The other possible value is non-lazy so lazy properties are not cacheable.

13.3. Entity inheritance and second-level cache mapping

Traditionally, when using entity inheritance, Hibernate required an entity hierarchy to be either cached entirely or not cached at all. Therefore, if you wanted to cache a subclass belonging to a given entity hierarchy, the Jakarta Persistence @Cacheable and the Hibernate-specific @Cache annotations would have to be declared at the root-entity level only.

Although we still believe that all entities belonging to a given entity hierarchy should share the same caching semantics, the Jakarta Persistence specification says that the @Cacheable annotation could be overwritten by a subclass:

The value of the Cacheable annotation is inherited by subclasses; it can be overridden by specifying Cacheable on a subclass.

— Section 11.1.7 of the Jakarta Persistence

As of Hibernate ORM 5.3, you can now override a base class @Cacheable or @Cache definition at subclass level.

However, the Hibernate cache concurrency strategy (e.g. read-only, nonstrict-read-write, read-write, transactional) is still defined at the root entity level and cannot be overridden.

Nevertheless, the reasons why we advise you to have all entities belonging to an inheritance tree share the same caching definition can be summed as follows:

  • from a performance perspective, adding an additional check on a per entity type level slows the bootstrap process.

  • providing different caching semantics for subclasses would violate the Liskov substitution principle.

13.4. Entity cache

Example 486. Entity cache mapping
@Entity(name = "Phone")
@Cacheable
@org.hibernate.annotations.Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	private String mobile;

	@ManyToOne
	private Person person;

	@Version
	private int version;

	//Getters and setters are omitted for brevity

}

Hibernate stores cached entities in a dehydrated form, which is similar to the database representation. Aside from the foreign key column values of the @ManyToOne or @OneToOne child-side associations, entity relationships are not stored in the cache,

Once an entity is stored in the second-level cache, you can avoid a database hit and load the entity from the cache alone:

Example 487. Loading entity using Jakarta Persistence
Person person = entityManager.find(Person.class, 1L);
Example 488. Loading entity using Hibernate native API
Person person = session.get(Person.class, 1L);

The Hibernate second-level cache can also load entities by their natural id:

Example 489. Hibernate natural id entity mapping
@Entity(name = "Person")
@Cacheable
@org.hibernate.annotations.Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
   public static class Person {

       @Id
       @GeneratedValue(strategy = GenerationType.AUTO)
       private Long id;

       private String name;

	@NaturalId
	@Column(name = "code", unique = true)
	private String code;

	//Getters and setters are omitted for brevity

}
Example 490. Loading entity using Hibernate native natural id API
Person person = session
	.byNaturalId(Person.class)
	.using("code", "unique-code")
	.load();

13.5. Collection cache

Hibernate can also cache collections, and the @Cache annotation must be on added to the collection property.

If the collection is made of value types (basic or embeddables mapped with @ElementCollection), the collection is stored as such. If the collection contains other entities (@OneToMany or @ManyToMany), the collection cache entry will store the entity identifiers only.

Example 491. Collection cache mapping
@OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
@org.hibernate.annotations.Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
private List<Phone> phones = new ArrayList<>();

Collections are read-through, meaning they are cached upon being accessed for the first time:

Example 492. Collection cache usage
Person person = entityManager.find(Person.class, 1L);
person.getPhones().size();

Subsequent collection retrievals will use the cache instead of going to the database.

The collection cache is not write-through so any modification will trigger a collection cache entry invalidation. On a subsequent access, the collection will be loaded from the database and re-cached.

13.6. Query cache

Aside from caching entities and collections, Hibernate offers a query cache too. This is useful for frequently executed queries with fixed parameter values.

Caching of query results introduces some overhead in terms of your applications normal transactional processing. For example, if you cache results of a query against Person, Hibernate will need to keep track of when those results should be invalidated because changes have been committed against any Person entity.

That, coupled with the fact that most applications simply gain no benefit from caching query results, leads Hibernate to disable caching of query results by default.

To use query caching, you will first need to enable it with the following configuration property:

Example 493. Enabling query cache
<property
    name="hibernate.cache.use_query_cache"
    value="true" />

As mentioned above, most queries do not benefit from caching or their results. So by default, individual queries are not cached even after enabling query caching. Each particular query that needs to be cached must be manually set as cacheable. This way, the query looks for existing cache results or adds the query results to the cache when being executed.

Example 494. Caching query using Jakarta Persistence
List<Person> persons = entityManager.createQuery(
	"select p " +
	"from Person p " +
	"where p.name = :name", Person.class)
.setParameter("name", "John Doe")
.setHint("org.hibernate.cacheable", "true")
.getResultList();
Example 495. Caching query using Hibernate native API
List<Person> persons = session.createQuery(
	"select p " +
	"from Person p " +
	"where p.name = :name")
.setParameter("name", "John Doe")
.setCacheable(true)
.list();

For entity queries, the query cache does not cache the state of the actual entities. Instead, it stores the entity identifiers, and when the query result is fetched from the cache, the entity state is going to be loaded from the second-level cache entity regions.

Just as with collection caching, the query cache should always be used in conjunction with the second-level cache for those entities expected to be cached as part of a query result cache.

For projection queries, the query cache stores the dehydrated entity state (e.g. Object[]) associated with the underlying JDBC ResultSet.

13.6.1. Query cache regions

This setting creates two new cache regions:

default-query-results-region

Holding the cached query results.

default-update-timestamps-region

Holding timestamps of the most recent updates to queryable tables. These are used to validate the results as they are served from the query cache.

If you configure your underlying cache implementation to use expiration, it’s very important that the timeout of the underlying cache region for the default-update-timestamps-region is set to a higher value than the timeout setting of any of the query caches.

In fact, we recommend that the default-update-timestamps-region region is not configured for expiration (time-based) or eviction (size/memory-based) at all. Note that an LRU (Least Recently Used) cache eviction policy is never appropriate for this particular cache region.

If you require fine-grained control over query cache expiration policies, you can specify a named cache region for a particular query.

Example 496. Caching query in custom region using Jakarta Persistence
List<Person> persons = entityManager.createQuery(
		"select p " +
		"from Person p " +
		"where p.id > :id", Person.class)
		.setParameter("id", 0L)
		.setHint(HINT_CACHEABLE, "true")
		.setHint(HINT_CACHE_REGION, "query.cache.person")
		.getResultList();
Example 497. Caching query in custom region using Hibernate native API
List<Person> persons = session.createQuery(
	"select p " +
	"from Person p " +
	"where p.id > :id")
.setParameter("id", 0L)
.setCacheable(true)
.setCacheRegion("query.cache.person")
.list();

If you want to force the query cache to refresh one of its regions (disregarding any cached results it finds there), you can use custom cache modes.

Example 498. Using custom query cache mode with Jakarta Persistence
List<Person> persons = entityManager.createQuery(
	"select p " +
	"from Person p " +
	"where p.id > :id", Person.class)
.setParameter("id", 0L)
.setHint(HINT_CACHEABLE, "true")
.setHint(HINT_CACHE_REGION, "query.cache.person")
.setHint("jakarta.persistence.cache.storeMode", CacheStoreMode.REFRESH)
.getResultList();
Example 499. Using custom query cache mode with Hibernate native API
List<Person> persons = session.createQuery(
	"select p " +
	"from Person p " +
	"where p.id > :id")
.setParameter("id", 0L)
.setCacheable(true)
.setCacheRegion("query.cache.person")
.setCacheMode(CacheMode.REFRESH)
.list();

When using CacheStoreMode.REFRESH or CacheMode.REFRESH in conjunction with the region you have defined for the given query, Hibernate will selectively force the results cached in that particular region to be refreshed.

This behavior is particularly useful in cases when the underlying data may have been updated via a separate process and is a far more efficient alternative to the bulk eviction of the region via SessionFactory eviction which looks as follows:

session.getSessionFactory().getCache().evictQueryRegion("query.cache.person");

13.7. Managing the cached data

Traditionally, Hibernate defined the CacheMode enumeration to describe the ways of interactions with the cached data. Jakarta Persistence split cache modes by storage (CacheStoreMode) and retrieval (CacheRetrieveMode).

The relationship between Hibernate and Jakarta Persistence cache modes can be seen in the following table:

Table 2. Cache modes relationships
Hibernate Jakarta Persistence Description

CacheMode.NORMAL

CacheStoreMode.USE and CacheRetrieveMode.USE

Default. Reads/writes data from/into the cache

CacheMode.REFRESH

CacheStoreMode.REFRESH and CacheRetrieveMode.BYPASS

Doesn’t read from cache, but writes to the cache upon loading from the database

CacheMode.PUT

CacheStoreMode.USE and CacheRetrieveMode.BYPASS

Doesn’t read from cache, but writes to the cache as it reads from the database

CacheMode.GET

CacheStoreMode.BYPASS and CacheRetrieveMode.USE

Read from the cache, but doesn’t write to cache

CacheMode.IGNORE

CacheStoreMode.BYPASS and CacheRetrieveMode.BYPASS

Doesn’t read/write data from/into the cache

Setting the cache mode can be done either when loading entities directly or when executing a query.

Example 500. Using custom cache modes with Jakarta Persistence
Map<String, Object> hints = new HashMap<>();
hints.put("jakarta.persistence.cache.retrieveMode" , CacheRetrieveMode.USE);
hints.put("jakarta.persistence.cache.storeMode" , CacheStoreMode.REFRESH);
Person person = entityManager.find(Person.class, 1L , hints);
Example 501. Using custom cache modes with Hibernate native API
session.setCacheMode(CacheMode.REFRESH);
Person person = session.get(Person.class, 1L);

The custom cache modes can be set for queries as well:

Example 502. Using custom cache modes for queries with Jakarta Persistence
List<Person> persons = entityManager.createQuery(
	"select p from Person p", Person.class)
.setHint(HINT_CACHEABLE, "true")
.setHint("jakarta.persistence.cache.retrieveMode" , CacheRetrieveMode.USE)
.setHint("jakarta.persistence.cache.storeMode" , CacheStoreMode.REFRESH)
.getResultList();
Example 503. Using custom cache modes for queries with Hibernate native API
List<Person> persons = session.createQuery(
	"select p from Person p")
.setCacheable(true)
.setCacheMode(CacheMode.REFRESH)
.list();

13.7.1. Evicting cache entries

Because the second level cache is bound to the EntityManagerFactory or the SessionFactory, cache eviction must be done through these two interfaces.

Jakarta Persistence only supports entity eviction through the jakarta.persistence.Cache interface:

Example 504. Evicting entities with Jakarta Persistence
entityManager.getEntityManagerFactory().getCache().evict(Person.class);

Hibernate is much more flexible in this regard as it offers fine-grained control over what needs to be evicted. The org.hibernate.Cache interface defines various evicting strategies:

  • entities (by their class or region)

  • entities stored using the natural-id (by their class or region)

  • collections (by the region, and it might take the collection owner identifier as well)

  • queries (by region)

Example 505. Evicting entities with Hibernate native API
session.getSessionFactory().getCache().evictQueryRegion("query.cache.person");

13.8. Caching statistics

If you enable the hibernate.generate_statistics configuration property, Hibernate will expose a number of metrics via SessionFactory.getStatistics(). Hibernate can even be configured to expose these statistics via JMX.

This way, you can get access to the Statistics class which comprises all sort of second-level cache metrics.

Example 506. Caching statistics
Statistics statistics = session.getSessionFactory().getStatistics();
CacheRegionStatistics secondLevelCacheStatistics =
		statistics.getDomainDataRegionStatistics("query.cache.person");
long hitCount = secondLevelCacheStatistics.getHitCount();
long missCount = secondLevelCacheStatistics.getMissCount();
double hitRatio = (double) hitCount / (hitCount + missCount);

13.9. JCache

To use the built-in integration