Hibernate.orgCommunity Documentation
Table of Contents
Identifiers model the primary key of an entity. They are used to uniquely identify each specific entity.
Hibernate and JPA 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. This is more
a general guide, than a hard-fast rule as opinions vary. JPA defines the behavior of changing the
value of the identifier attribute to be undefined; Hibernate simply does not support that. 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 Chapter 7, Natural Ids.
Technically the identifier does not have to map to the column(s) physically defined as the entity table's 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 might be simple (single value) or composite (multiple values).
Simple identifiers map to a single basic attribute, and are denoted using the
javax.persistence.Id annotation.
According to JPA only the following types should be used 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
Any types used for identifier attributes beyond this list will not be portable.
Values for simple identifiers can be assigned, as we have seen in the examples above. The expectation for assigned identifier values is that the application assigns (sets them on the entity attribute) prior to calling save/persist.
Example 6.1. Simple assigned identifier
@Entity
public class MyEntity {
@Id
public Integer id;
...
}
Values for simple identifiers can be generated. To denote that an identifier attribute is
generated, it is annotated with javax.persistence.GeneratedValue
Example 6.2. Simple generated identifier
@Entity
public class MyEntity {
@Id
@GeneratedValue
public Integer id;
...
}
Additionally to the type restriction list above, JPA says that if using generated identifier values (see below) only integer types (short, int, long) will be portably supported.
The expectation for generated identifier values is that Hibernate will generate the value when the save/persist occurs.
Identifier value generations strategies are discussed in detail in Section 6.3, “Generated identifier values”.
Composite identifiers correspond to one or more persistent attributes. Here are the rules governing composite identifiers, as defined by the JPA specification.
The composite identifier must be represented by a "primary key class". The primary key class
may be defined using the javax.persistence.EmbeddedId annotation
(see Section 6.2.1, “Composite identifiers - aggregated (EmbeddedId)”) or defined using the
javax.persistence.IdClass annotation (see
Section 6.2.2, “Composite identifiers - non-aggregated (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 key is mapped.
The restriction that a composite identifier has to be represented by a "primary key class" is a JPA restriction. Hibernate does allow composite identifiers to be defined without a "primary key class", but use of that modeling technique is deprecated and not discussed here.
The attributes making up the composition can be either basic, composite, ManyToOne. Note especially that collections and one-to-ones are never appropriate.
Modelling a composite identifier using an EmbeddedId simply means defining an Embeddable to be a composition for the the one or more attributes making up the identifier and then exposing an attribute of that Embeddable type on the entity.
Example 6.3. Basic EmbeddedId
@Entity
public class Login {
@Embeddable
public static class PK implements Serializable {
private String system;
private String username;
...
}
@EmbeddedId
private PK pk;
...
}
As mentioned before, EmbeddedIds can even contain ManyToOne attributes.
Example 6.4. EmbeddedId with ManyToOne
@Entity
public class Login {
@Embeddable
public static class PK implements Serializable {
@ManyToOne
private System system;
private String username;
...
}
@EmbeddedId
private PK pk;
...
}
Hibernate supports directly modeling the ManyToOne in the PK class, whether EmbeddedId or IdClass. However that is not portably supported by the JPA specification. In JPA terms one would use "derived identifiers"; for details, see Section 6.4, “Derived Identifiers”.
Modelling 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 simply acts as a "shadow".
Example 6.5. Basic IdClass
@Entity
@IdClass(PK.class)
public class Login {
public static class PK implements Serializable {
private String system;
private String username;
...
}
@Id
private String system;
@Id
private String username;
...
}
Non-aggregated composite identifiers can also contain ManyToOne attributes as we saw with aggregated ones (still non-portably)
Example 6.6. IdClass with ManyToOne
@Entity
@IdClass(PK.class)
public class Login {
public static class PK implements Serializable {
private System system;
private String username;
...
}
@Id
@ManyToOne
private System system;
@Id
private String username;
...
}
With non-aggregated composite identifiers, Hibernate also supports "partial" generation of the composite values.
Example 6.7. IdClass with partial generation
@Entity
@IdClass(PK.class)
public class LogFile {
public static class PK implements Serializable {
private String name;
private LocalDate date;
private Integer uniqueStamp;
...
}
@Id
private String name;
@Id
private LocalDate date;
@Id
@GeneratedValue
private Integer uniqueStamp;
...
}
This feature exists because of a highly questionable interpretation of the JPA specification made by the SpecJ committee. Hibernate does not feel that JPA 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 JPA perspective.
For discussion of generated values for non-identifier attributes, see ???
Hibernate supports identifier value generation across a number of different types. Remember that JPA portably defines identifier value generation just for integer types.
Identifier value generation is indicates using the javax.persistence.GeneratedValue
annotation. The most important piece of information here is the specified
javax.persistence.GenerationType which indicates how values will be generated.
The discussions below assume that the application is using Hibernate's "new generator mappings" as
indicated by the hibernate.id.new_generator_mappings setting or
MetadataBuilder.enableNewIdentifierGeneratorSupport method during bootstrap.
This is set to true by default, however if applications set this to false the resolutions discussed
here will be very different. The rest of the discussion here assumes this setting is enabled (true).
GenerationTypes
AUTO (the default) - Indicates that the persistence provider (Hibernate) should
chose an appropriate generation strategy. See Section 6.3.1, “Interpreting AUTO”.
IDENTITY - Indicates that database IDENTITY columns will be used for
primary key value generation. See Section 6.3.3, “Using IDENTITY columns”.
SEQUENCE - Indicates that database sequence should be used for obtaining
primary key values. See Section 6.3.2, “Using sequences”.
TABLE - Indicates that a database table should be used for obtaining
primary key values. See Section 6.3.4, “Using identifier table”.
How a persistence provider interprets the AUTO generation type is left up to the provider. Hibernate interprets it in the following order:
If the given name matches the name for a javax.persistence.SequenceGenerator
annotation -> Section 6.3.2, “Using sequences”.
If the given name matches the name for a javax.persistence.TableGenerator
annotation -> Section 6.3.4, “Using identifier table”.
If the given name matches the name for a org.hibernate.annotations.GenericGenerator
annotation -> Section 6.3.6, “Using @GenericGenerator”.
The fallback is to consult with the pluggable org.hibernate.boot.model.IdGeneratorStrategyInterpreter
contract, which is covered in detail in the Hibernate Integrations Guide. The default
behavior is to look at the java type of the identifier attribute:
If it is UUID -> Section 6.3.5, “Using UUID generation”
Otherwise -> Section 6.3.2, “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 switching to a table as the underlying backing. This gives Hibernate a huge degree of portability
across databases while still maintaining consistent id generation behavior (versus say choosing
between sequence and IDENTITY). This backing storage is completely transparent to the user.
The preferred (and portable) way to configure this generator is using the JPA-defined
javax.persistence.SequenceGenerator annotation.
The simplest form is to simply request sequence generation; Hibernate will use a single, implicitly-named
sequence (hibernate_sequence) for all such unnamed definitions.
Example 6.8. Unnamed sequence
@Entity
public class MyEntity {
@Id
@GeneratedValue(generation=SEQUENCE)
public Integer id;
...
}
Or a specifically named sequence can be requested
Example 6.9. Named sequence
@Entity
public class MyEntity {
@Id
@GeneratedValue(generation=SEQUENCE, name="my_sequence")
public Integer id;
...
}
Use javax.persistence.SequenceGenerator to specify additional configuration.
Example 6.10. Configured sequence
@Entity
public class MyEntity {
@Id
@GeneratedValue(generation=SEQUENCE, name="my_sequence")
@SequenceGenerator( name = "my_sequence", schema = "globals", allocationSize = 30 )
public Integer id;
...
}
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 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 this 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 (conversations). Because of the runtime imposition/inconsistency Hibernate suggest other forms of identifier value generation be used.
There is yet another important runtime impact of choosing IDENTITY generation: Hibernate will not be able to JDBC batching for inserts of the entities that use IDENTITY generation. The importance of this depends on the application's specific use cases. If the application is not usually creating many new instances of a given type of entity that uses IDENTITY generation, then this is not an important impact since batching would not have been helpful anyway.
Hibernate achieves table-based identifier generation based on its
org.hibernate.id.enhanced.TableGenerator id generator which defines
a table capable of holding multiple named value segments for any number of entities.
Example 6.11. Table generator table structure
create table hibernate_sequences(
sequence_name VARCHAR NOT NULL,
next_val INTEGER NOT NULL
)
The basic idea is that a given table-generator table (hibernate_sequences for example)
can hold multiple segments of identifier generation values.
Example 6.12. Unnamed table generator
@Entity
public class MyEntity {
@Id
@GeneratedValue(generation=TABLE)
public Integer id;
...
}
If no table name is given Hibernate assumes an implicit name of hibernate_sequences.
Additionally, because no javax.persistence.TableGenerator#pkColumnValue is
specified, Hibernate will use the default segment (sequence_name='default') from the
hibernate_sequences table.
As mentioned above, Hibernate supports UUID identifier value generation. This is supported through its
org.hibernate.id.UUIDGenerator id generator.
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 6.13. Implicitly using the random UUID strategy
@Entity
public class MyEntity {
@Id
@GeneratedValue
public UUID id;
...
}
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 6.14. Implicitly using the random UUID strategy
@Entity
public class MyEntity {
@Id
@GeneratedValue( generator="uuid" )
@GenericGenerator(
name="uuid",
strategy="org.hibernate.id.UUIDGenerator",
parameters = {
@Parameter(
name="uuid_gen_strategy_class",
value="org.hibernate.id.uuid.CustomVersionOneStrategy"
)
}
)
public UUID id;
...
}
@GenericGenerator allows integration of any Hibernate org.hibernate.id.IdentifierGenerator
implementation, including any of the specific ones discussed here and any custom ones.
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, everytime 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.
No optimization is performed. We communicate with the database each and every time an identifier value is needed from the generator.
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-that-1 increment size. For example, consider a brand new sequence
defined as create sequence my_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.
Just like pooled-lo, except that here the value from the table/sequence is interpreted as the high end of the value pool.
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.