Preface

Hibernate 6 is a major redesign of the world’s most popular and feature-rich ORM solution. The redesign has touched almost every subsystem of Hibernate, including the APIs, mapping annotations, and the query language. This new Hibernate is more powerful, more robust, and more typesafe.

With so many improvements, it’s very difficult to summarize the significance of this work. But the following general themes stand out. Hibernate 6:

  • finally takes advantage of the advances in relational databases over the past decade, updating the query language to support a raft of new constructs in modern dialects of SQL,

  • exhibits much more consistent behavior across different databases, greatly improving portability, and generates much higher-quality DDL from dialect-independent code,

  • improves error reporting by more scrupulous validation of queries before access to the database,

  • improves the type-safety of O/R mapping annotations, clarifies the separation of API, SPI, and internal implementation, and fixes some long-standing architectural flaws,

  • removes or deprecates legacy APIs, laying the foundation for future evolution, and

  • makes far better use of Javadoc, putting much more information at the fingertips of developers.

Hibernate 6 and Hibernate Reactive are now core components of Quarkus 3, the most exciting new environment for cloud-native development in Java, and Hibernate remains the persistence solution of choice for almost every major Java framework or server.

Unfortunately, the changes in Hibernate 6 have obsoleted much of the information about Hibernate that’s available in books, in blog posts, and on stackoverflow.

This guide is an up-to-date, high-level discussion of the current feature set and recommended usage. It does not attempt to cover every feature and should be used in conjunction with other documentation:

The Hibernate User Guide includes detailed discussions of most aspects of Hibernate. But with so much information to cover, readability is difficult to achieve, and so it’s most useful as a reference. Where necessary, we’ll provide links to relevant sections of the User Guide.

1. Introduction

Hibernate is usually described as a library that makes it easy to map Java classes to relational database tables. But this formulation does no justice to the central role played by the relational data itself. So a better description might be:

Hibernate makes relational data visible to a program written in Java, in a natural and typesafe form,

  1. making it easy to write complex queries and work with their results,

  2. letting the program easily synchronize changes made in memory with the database, respecting the ACID properties of transactions, and

  3. allowing performance optimizations to be made after the basic persistence logic has already been written.

Here the relational data is the focus, along with the importance of typesafety. The goal of object/relational mapping (ORM) is to eliminate fragile and untypesafe code, and make large programs easier to maintain in the long run.

ORM takes the pain out of persistence by relieving the developer of the need to hand-write tedious, repetitive, and fragile code for flattening graphs of objects to database tables and rebuilding graphs of objects from flat SQL query result sets. Even better, ORM makes it much easier to tune performance later, after the basic persistence logic has already been written.

A perennial question is: should I use ORM, or plain SQL? The answer is usually: use both. JPA and Hibernate were designed to work in conjunction with handwritten SQL. You see, most programs with nontrivial data access logic will benefit from the use of ORM at least somewhere. But if Hibernate is making things more difficult, for some particularly tricky piece of data access logic, the only sensible thing to do is to use something better suited to the problem! Just because you’re using Hibernate for persistence doesn’t mean you have to use it for everything.

Developers often ask about the relationship between Hibernate and JPA, so let’s take a short detour into some history.

1.1. Hibernate and JPA

Hibernate was the inspiration behind the Java (now Jakarta) Persistence API, or JPA, and includes a complete implementation of the latest revision of this specification.

The early history of Hibernate and JPA

The Hibernate project began in 2001, when Gavin King’s frustration with Entity Beans in EJB 2 boiled over. It quickly overtook other open source and commercial contenders to become the most popular persistence solution for Java, and the book Hibernate in Action, written with Christian Bauer, was an influential bestseller.

In 2004, Gavin and Christian joined a tiny startup called JBoss, and other early Hibernate contributors soon followed: Max Rydahl Andersen, Emmanuel Bernard, Steve Ebersole, and Sanne Grinovero.

Soon after, Gavin joined the EJB 3 expert group and convinced the group to deprecate Entity Beans in favor of a brand-new persistence API modelled after Hibernate. Later, members of the TopLink team got involved, and the Java Persistence API evolved as a collaboration between—primarily—Sun, JBoss, Oracle, and Sybase, under the leadership of Linda Demichiel.

Over the intervening two decades, many talented people have contributed to the development of Hibernate. We’re all especially grateful to Steve, who has led the project for many years, since Gavin stepped back to focus in other work.

We can think of the API of Hibernate in terms of three basic elements:

  • an implementation of the JPA-defined APIs, most importantly, of the interfaces EntityManagerFactory and EntityManager, and of the JPA-defined O/R mapping annotations,

  • a native API exposing the full set of available functionality, centered around the interfaces SessionFactory, which extends EntityManagerFactory, and Session, which extends EntityManager, and

  • a set of mapping annotations which augment the O/R mapping annotations defined by JPA, and which may be used with the JPA-defined interfaces, or with the native API.

Hibernate also offers a range of SPIs for frameworks and libraries which extend or integrate with Hibernate, but we’re not interested in any of that stuff here.

API overview

As an application developer, you must decide whether to:

  • write your program in terms of Session and SessionFactory, or

  • maximize portability to other implementations of JPA by, wherever reasonable, writing code in terms of EntityManager and EntityManagerFactory, falling back to the native APIs only where necessary.

Whichever path you take, you will use the JPA-defined mapping annotations most of the time, and the Hibernate-defined annotations for more advanced mapping problems.

You might wonder if it’s possible to develop an application using only JPA-defined APIs, and, indeed, that’s possible in principle. JPA is a great baseline that really nails the basics of the object/relational mapping problem. But without the native APIs, and extended mapping annotations, you miss out on much of the power of Hibernate.

Since Hibernate existed before JPA, and since JPA was modelled on Hibernate, we unfortunately have some competition and duplication in naming between the standard and native APIs. For example:

Table 1. Examples of competing APIs with similar naming
Hibernate JPA

org.hibernate.annotations.CascadeType

javax.persistence.CascadeType

org.hibernate.FlushMode

javax.persistence.FlushModeType

org.hibernate.annotations.FetchMode

javax.persistence.FetchType

org.hibernate.query.Query

javax.persistence.Query

org.hibernate.Cache

javax.persistence.Cache

@org.hibernate.annotations.NamedQuery

@javax.persistence.NamedQuery

@org.hibernate.annotations.Cache

@javax.persistence.Cacheable

Typically, the Hibernate-native APIs offer something a little extra that’s missing in JPA, so this isn’t exactly a flaw. But it’s something to watch out for.

1.2. Writing Java code with Hibernate

If you’re completely new to Hibernate and JPA, you might already be wondering how the persistence-related code is structured.

Well, typically, our persistence-related code comes in two layers:

  1. a representation of our data model in Java, which takes the form of a set of annotated entity classes, and

  2. a larger number of functions which interact with Hibernate’s APIs to perform the persistence operations associated with your various transactions.

The first part, the data or "domain" model, is usually easier to write, but doing a great and very clean job of it will strongly affect your success in the second part.

Most people implement the domain model as a set of what we used to call "Plain Old Java Objects", that is, as simple Java classes with no direct dependencies on technical infrastructure, nor on application logic which deals with request processing, transaction management, communications, or interaction with the database.

Take your time with this code, and try to produce a Java model that’s as close as reasonable to the relational data model. Avoid using exotic or advanced mapping features when they’re not really needed. When in the slightest doubt, map a foreign key relationship using @ManyToOne with @OneToMany(mappedBy=…​) in preference to more complicated association mappings.

The second part of the code is much trickier to get right. This code must:

  • manage transactions and sessions,

  • interact with the database via the Hibernate session,

  • fetch and prepare data needed by the UI, and

  • handle failures.

Responsibility for transaction and session management, and for recovery from certain kinds of failure, is best handled in some sort of framework code.

We’re going to come back soon to the thorny question of how this persistence logic should be organized, and how it should fit into the rest of the system.

1.3. Hello, Hibernate

Before we get deeper into the weeds, we’ll quickly present a basic example program that will help you get started if you don’t already have Hibernate integrated into your project.

We begin with a simple gradle build file:

build.gradle
plugins {
    id 'java'
}

group = 'org.example'
version = '1.0-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    // the GOAT ORM
    implementation 'org.hibernate.orm:hibernate-core:7.0.0.Alpha3'

    // Hibernate Validator
    implementation 'org.hibernate.validator:hibernate-validator:8.0.0.Final'
    implementation 'org.glassfish:jakarta.el:4.0.2'

    // Agroal connection pool
    implementation 'org.hibernate.orm:hibernate-agroal:7.0.0.Alpha3'
    implementation 'io.agroal:agroal-pool:2.1'

    // logging via Log4j
    implementation 'org.apache.logging.log4j:log4j-core:2.20.0'

    // JPA Metamodel Generator
    annotationProcessor 'org.hibernate.orm:hibernate-processor:7.0.0.Alpha3'

    // Compile-time checking for HQL
    //implementation 'org.hibernate:query-validator:2.0-SNAPSHOT'
    //annotationProcessor 'org.hibernate:query-validator:2.0-SNAPSHOT'

    // H2 database
    runtimeOnly 'com.h2database:h2:2.1.214'
}

Only the first of these dependencies is absolutely required to run Hibernate.

Next, we’ll add a logging configuration file for log4j:

log4j2.properties
rootLogger.level = info
rootLogger.appenderRefs = console
rootLogger.appenderRef.console.ref = console

logger.hibernate.name = org.hibernate.SQL
logger.hibernate.level = info

appender.console.name = console
appender.console.type = Console
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %highlight{[%p]} %m%n

Now we need some Java code. We begin with our entity class:

Book.java
package org.hibernate.example;

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.validation.constraints.NotNull;

@Entity
class Book {
    @Id
    String isbn;

    @NotNull
    String title;

    Book() {}

    Book(String isbn, String title) {
        this.isbn = isbn;
        this.title = title;
    }
}

Finally, let’s see code which configures and instantiates Hibernate and asks it to persist and query the entity. Don’t worry if this makes no sense at all right now. It’s the job of this Introduction to make all this crystal clear.

Main.java
package org.hibernate.example;

import org.hibernate.cfg.Configuration;

import static java.lang.Boolean.TRUE;
import static java.lang.System.out;
import static org.hibernate.cfg.AvailableSettings.*;

public class Main {
    public static void main(String[] args) {
        var sessionFactory = new Configuration()
                .addAnnotatedClass(Book.class)
                // use H2 in-memory database
                .setProperty(URL, "jdbc:h2:mem:db1")
                .setProperty(USER, "sa")
                .setProperty(PASS, "")
                // use Agroal connection pool
                .setProperty("hibernate.agroal.maxSize", 20)
                // display SQL in console
                .setProperty(SHOW_SQL, true)
                .setProperty(FORMAT_SQL, true)
                .setProperty(HIGHLIGHT_SQL, true)
                .buildSessionFactory();

        // export the inferred database schema
        sessionFactory.getSchemaManager().exportMappedObjects(true);

        // persist an entity
        sessionFactory.inTransaction(session -> {
            session.persist(new Book("9781932394153", "Hibernate in Action"));
        });

        // query data using HQL
        sessionFactory.inSession(session -> {
            out.println(session.createSelectionQuery("select isbn||': '||title from Book").getSingleResult());
        });

        // query data using criteria API
        sessionFactory.inSession(session -> {
            var builder = sessionFactory.getCriteriaBuilder();
            var query = builder.createQuery(String.class);
            var book = query.from(Book.class);
            query.select(builder.concat(builder.concat(book.get(Book_.isbn), builder.literal(": ")),
                    book.get(Book_.title)));
            out.println(session.createSelectionQuery(query).getSingleResult());
        });
    }
}

Here we’ve used Hibernate’s native APIs. We could have used JPA-standard APIs to achieve the same thing.

1.4. Hello, JPA

If we limit ourselves to the use of JPA-standard APIs, we need to use XML to configure Hibernate.

META-INF/persistence.xml
<persistence xmlns="https://jakarta.ee/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd"
             version="3.0">

    <persistence-unit name="example">

        <class>org.hibernate.example.Book</class>

        <properties>

            <!-- H2 in-memory database -->
            <property name="jakarta.persistence.jdbc.url"
                      value="jdbc:h2:mem:db1"/>

            <!-- Credentials -->
            <property name="jakarta.persistence.jdbc.user"
                      value="sa"/>
            <property name="jakarta.persistence.jdbc.password"
                      value=""/>

            <!-- Agroal connection pool -->
            <property name="hibernate.agroal.maxSize"
                      value="20"/>

            <!-- display SQL in console -->
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.highlight_sql" value="true"/>

        </properties>

    </persistence-unit>
</persistence>

Note that our build.gradle and log4j2.properties files are unchanged.

Our entity class is also unchanged from what we had before.

Unfortunately, JPA doesn’t offer an inSession() method, so we’ll have to implement session and transaction management ourselves. We can put that logic in our own inSession() function, so that we don’t have to repeat it for every transaction. Again, you don’t need to understand any of this code right now.

Main.java (JPA version)
package org.hibernate.example;

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;

import java.util.Map;
import java.util.function.Consumer;

import static jakarta.persistence.Persistence.createEntityManagerFactory;
import static java.lang.System.out;
import static org.hibernate.cfg.AvailableSettings.JAKARTA_HBM2DDL_DATABASE_ACTION;
import static org.hibernate.tool.schema.Action.CREATE;

public class Main {
    public static void main(String[] args) {
        var factory = createEntityManagerFactory("example",
                // export the inferred database schema
                Map.of(JAKARTA_HBM2DDL_DATABASE_ACTION, CREATE));

        // persist an entity
        inSession(factory, entityManager -> {
            entityManager.persist(new Book("9781932394153", "Hibernate in Action"));
        });

        // query data using HQL
        inSession(factory, entityManager -> {
            out.println(entityManager.createQuery("select isbn||': '||title from Book").getSingleResult());
        });

        // query data using criteria API
        inSession(factory, entityManager -> {
            var builder = factory.getCriteriaBuilder();
            var query = builder.createQuery(String.class);
            var book = query.from(Book.class);
            query.select(builder.concat(builder.concat(book.get(Book_.isbn), builder.literal(": ")),
                    book.get(Book_.title)));
            out.println(entityManager.createQuery(query).getSingleResult());
        });
    }

    // do some work in a session, performing correct transaction management
    static void inSession(EntityManagerFactory factory, Consumer<EntityManager> work) {
        var entityManager = factory.createEntityManager();
        var transaction = entityManager.getTransaction();
        try {
            transaction.begin();
            work.accept(entityManager);
            transaction.commit();
        }
        catch (Exception e) {
            if (transaction.isActive()) transaction.rollback();
            throw e;
        }
        finally {
            entityManager.close();
        }
    }
}

In practice, we never access the database directly from a main() method. So now let’s talk about how to organize persistence logic in a real system. The rest of this chapter is not compulsory. If you’re itching for more details about Hibernate itself, you’re quite welcome to skip straight to the next chapter, and come back later.

1.5. Organizing persistence logic

In a real program, persistence logic like the code shown above is usually interleaved with other sorts of code, including logic:

  • implementing the rules of the business domain, or

  • for interacting with the user.

Therefore, many developers quickly—even too quickly, in our opinion—reach for ways to isolate the persistence logic into some sort of separate architectural layer. We’re going to ask you to suppress this urge for now.

The easiest way to use Hibernate is to call the Session or EntityManager directly. If you’re new to Hibernate, frameworks which wrap JPA are only going to make your life more difficult.

We prefer a bottom-up approach to organizing our code. We like to start thinking about methods and functions, not about architectural layers and container-managed objects. To illustrate the sort of approach to code organization that we advocate, let’s consider a service which queries the database using HQL or SQL.

We might start with something like this, a mix of UI and persistence logic:

@Path("/") @Produces("application/json")
public class BookResource {
    @GET @Path("book/{isbn}")
    public Book getBook(String isbn) {
        var book = sessionFactory.fromTransaction(session -> session.find(Book.class, isbn));
        return book == null ? Response.status(404).build() : book;
    }
}

Indeed, we might also finish with something like that—it’s quite hard to identify anything concretely wrong with the code above, and for such a simple case it seems really difficult to justify making this code more complicated by introducing additional objects.

One very nice aspect of this code, which we wish to draw your attention to, is that session and transaction management is handled by generic "framework" code, just as we already recommended above. In this case, we’re using the fromTransaction() method, which happens to come built in to Hibernate. But you might prefer to use something else, for example:

  • in a container environment like Jakarta EE or Quarkus, container-managed transactions and container-managed persistence contexts, or

  • something you write yourself.

The important thing is that calls like createEntityManager() and getTransaction().begin() don’t belong in regular program logic, because it’s tricky and tedious to get the error handling correct.

Let’s now consider a slightly more complicated case.

@Path("/") @Produces("application/json")
public class BookResource {
    private static final RESULTS_PER_PAGE = 20;

    @GET @Path("books/{titlePattern}/{page:\\d+}")
    public List<Book> findBooks(String titlePattern, int page) {
        var books = sessionFactory.fromTransaction(session -> {
            return session.createSelectionQuery("from Book where title like ?1 order by title", Book.class)
                    .setParameter(1, titlePattern)
                    .setPage(Page.page(RESULTS_PER_PAGE, page))
                    .getResultList();
        });
        return books.isEmpty() ? Response.status(404).build() : books;
    }

}

This is fine, and we won’t complain if you prefer to leave the code exactly as it appears above. But there’s one thing we could perhaps improve. We love super-short methods with single responsibilities, and there looks to be an opportunity to introduce one here. Let’s hit the code with our favorite thing, the Extract Method refactoring. We obtain:

static List<Book> findBooksByTitleWithPagination(Session session,
                                                 String titlePattern, Page page) {
    return session.createSelectionQuery("from Book where title like ?1 order by title", Book.class)
            .setParameter(1, titlePattern)
            .setPage(page)
            .getResultList();
}

This is an example of a query method, a function which accepts arguments to the parameters of a HQL or SQL query, and executes the query, returning its results to the caller. And that’s all it does; it doesn’t orchestrate additional program logic, and it doesn’t perform transaction or session management.

It’s even better to specify the query string using the @NamedQuery annotation, so that Hibernate can validate the query it at startup time, that is, when the SessionFactory is created, instead of when the query is first executed. Indeed, since we included the Metamodel Generator in our Gradle build, the query can even be validated at compile time.

We need a place to put the annotation, so lets move our query method to a new class:

@CheckHQL // validate named queries at compile time
@NamedQuery(name="findBooksByTitle",
            query="from Book where title like :title order by title")
class Queries {

    static List<Book> findBooksByTitleWithPagination(Session session,
                                                     String titlePattern, Page page) {
        return session.createNamedQuery("findBooksByTitle", Book.class)
                .setParameter("title", titlePattern)
                .setPage(page)
                .getResultList();
    }
}

Notice that our query method doesn’t attempt to hide the EntityManager from its clients. Indeed, the client code is responsible for providing the EntityManager or Session to the query method. This is a quite distinctive feature of our whole approach.

The client code may:

  • obtain an EntityManager or Session by calling inTransaction() or fromTransaction(), as we saw above, or,

  • in an environment with container-managed transactions, it might obtain it via dependency injection.

Whatever the case, the code which orchestrates a unit of work usually just calls the Session or EntityManager directly, passing it along to helper methods like our query method if necessary.

@GET
@Path("books/{titlePattern}")
public List<Book> findBooks(String titlePattern) {
    var books = sessionFactory.fromTransaction(session ->
            Queries.findBooksByTitleWithPagination(session, titlePattern,
                    Page.page(RESULTS_PER_PAGE, page));
    return books.isEmpty() ? Response.status(404).build() : books;
}

You might be thinking that our query method looks a bit boilerplatey. That’s true, perhaps, but we’re much more concerned that it’s not very typesafe. Indeed, for many years, the lack of compile-time checking for HQL queries and code which binds arguments to query parameters was our number one source of discomfort with Hibernate.

Fortunately, there’s now a solution to both problems: as an incubating feature of Hibernate 6.3, we now offer the possibility to have the Metamodel Generator fill in the implementation of such query methods for you. This facility is the topic of a whole chapter of this introduction, so for now we’ll just leave you with one simple example.

Suppose we simplify Queries to just the following:

interface Queries {
    @HQL("where title like :title order by title")
    List<Book> findBooksByTitleWithPagination(String title, Page page);
}

Then the Metamodel Generator automatically produces an implementation of the method annotated @HQL in a class named Queries_. We can call it just like we called our handwritten version:

@GET
@Path("books/{titlePattern}")
public List<Book> findBooks(String titlePattern) {
    var books = sessionFactory.fromTransaction(session ->
            Queries_.findBooksByTitleWithPagination(session, titlePattern,
                    Page.page(RESULTS_PER_PAGE, page));
    return books.isEmpty() ? Response.status(404).build() : books;
}

In this case, the quantity of code eliminated is pretty trivial. The real value is in improved type safety. We now find out about errors in assignments of arguments to query parameters at compile time.

At this point, we’re certain you’re full of doubts about this idea. And quite rightly so. We would love to answer your objections right here, but that will take us much too far off track. So we ask you to file away these thoughts for now. We promise to make it make sense when we properly address this topic later. And, after that, if you still don’t like this approach, please understand that it’s completely optional. Nobody’s going to come around to your house to force it down your throat.

Now that we have a rough picture of what our persistence logic might look like, it’s natural to ask how we should test our code.

1.6. Testing persistence logic

When we write tests for our persistence logic, we’re going to need:

  1. a database, with

  2. an instance of the schema mapped by our persistent entities, and

  3. a set of test data, in a well-defined state at the beginning of each test.

It might seem obvious that we should test against the same database system that we’re going to use in production, and, indeed, we should certainly have at least some tests for this configuration. But on the other hand, tests which perform I/O are much slower than tests which don’t, and most databases can’t be set up to run in-process.

So, since most persistence logic written using Hibernate 6 is extremely portable between databases, it often makes good sense to test against an in-memory Java database. (H2 is the one we recommend.)

We do need to be careful here if our persistence code uses native SQL, or if it uses concurrency-management features like pessimistic locks.

Whether we’re testing against our real database, or against an in-memory Java database, we’ll need to export the schema at the beginning of a test suite. We usually do this when we create the Hibernate SessionFactory or JPA EntityManager, and so traditionally we’ve used a configuration property for this.

The JPA-standard property is jakarta.persistence.schema-generation.database.action. For example, if we’re using Configuration to configure Hibernate, we could write:

configuration.setProperty(AvailableSettings.JAKARTA_HBM2DDL_DATABASE_ACTION,
                          Action.SPEC_ACTION_DROP_AND_CREATE);

Alternatively, in Hibernate 6, we may use the new SchemaManager API to export the schema, just as we did above.

sessionFactory.getSchemaManager().exportMappedObjects(true);

Since executing DDL statements is very slow on many databases, we don’t want to do this before every test. Instead, to ensure that each test begins with the test data in a well-defined state, we need to do two things before each test:

  1. clean up any mess left behind by the previous test, and then

  2. reinitialize the test data.

We may truncate all the tables, leaving an empty database schema, using the SchemaManager.

sessionFactory.getSchemaManager().truncateMappedObjects();

After truncating tables, we might need to initialize our test data. We may specify test data in a SQL script, for example:

/import.sql
insert into Books (isbn, title) values ('9781932394153', 'Hibernate in Action')
insert into Books (isbn, title) values ('9781932394887', 'Java Persistence with Hibernate')
insert into Books (isbn, title) values ('9781617290459', 'Java Persistence with Hibernate, Second Edition')

If we name this file import.sql, and place it in the root classpath, that’s all we need to do.

Otherwise, we need to specify the file in the configuration property jakarta.persistence.sql-load-script-source. If we’re using Configuration to configure Hibernate, we could write:

configuration.setProperty(AvailableSettings.JAKARTA_HBM2DDL_LOAD_SCRIPT_SOURCE,
                          "/org/example/test-data.sql");

The SQL script will be executed every time exportMappedObjects() or truncateMappedObjects() is called.

There’s another sort of mess a test can leave behind: cached data in the second-level cache. We recommend disabling Hibernate’s second-level cache for most sorts of testing. Alternatively, if the second-level cache is not disabled, then before each test we should call:

sessionFactory.getCache().evictAllRegions();

Now, suppose you’ve followed our advice, and written your entities and query methods to minimize dependencies on "infrastructure", that is, on libraries other than JPA and Hibernate, on frameworks, on container-managed objects, and even on bits of your own system which are hard to instantiate from scratch. Then testing persistence logic is now straightforward!

You’ll need to:

  • bootstrap Hibernate and create a SessionFactory or EntityManagerFactory and the beginning of your test suite (we’ve already seen how to do that), and

  • create a new Session or EntityManager inside each @Test method, using inTransaction(), for example.

Actually, some tests might require multiple sessions. But be careful not to leak a session between different tests.

Another important test we’ll need is one which validates our O/R mapping annotations against the actual database schema. This is again the job of the schema management tooling, either:

configuration.setProperty(AvailableSettings.JAKARTA_HBM2DDL_DATABASE_ACTION,
                          Action.ACTION_VALIDATE);

Or:

sessionFactory.getSchemaManager().validateMappedObjects();

This "test" is one which many people like to run even in production, when the system starts up.

1.7. Architecture and the persistence layer

Let’s now consider a different approach to code organization, one we treat with suspicion.

In this section, we’re going to give you our opinion. If you’re only interested in facts, or if you prefer not to read things that might undermine the opinion you currently hold, please feel free to skip straight to the next chapter.

Hibernate is an architecture-agnostic library, not a framework, and therefore integrates comfortably with a wide range of Java frameworks and containers. Consistent with our place within the ecosystem, we’ve historically avoided giving out much advice on architecture. This is a practice we’re now perhaps inclined to regret, since the resulting vacuum has come to be filled with advice from people advocating architectures, design patterns, and extra frameworks which we suspect make Hibernate a bit less pleasant to use than it should be.

In particular, frameworks which wrap JPA seem to add bloat while subtracting some of the fine-grained control over data access that Hibernate works so hard to provide. These frameworks don’t expose the full feature set of Hibernate, and so the program is forced to work with a less powerful abstraction.

The stodgy, dogmatic, conventional wisdom, which we hesitate to challenge for simple fear of pricking ourselves on the erect hackles that inevitably accompany such dogma-baiting is:

Code which interacts with the database belongs in a separate persistence layer.

We lack the courage—perhaps even the conviction—to tell you categorically to not follow this recommendation. But we do ask you to consider the cost in boilerplate of any architectural layer, and whether the benefits this cost buys are really worth it in the context of your system.

To add a little background texture to this discussion, and at the risk of our Introduction degenerating into a rant at such an early stage, we’re going ask you to humor us while talk a little more about ancient history.

An epic tale of DAOs and Repositories

Back in the dark days of Java EE 4, before the standardization of Hibernate, and subsequent ascendance of JPA in Java enterprise development, it was common to hand-code the messy JDBC interactions that Hibernate takes care of today. In those terrible times, a pattern arose that we used to call Data Access Objects (DAOs). A DAO gave you a place to put all that nasty JDBC code, leaving the important program logic cleaner.

When Hibernate arrived suddenly on the scene in 2001, developers loved it. But Hibernate implemented no specification, and many wished to reduce or at least localize the dependence of their project logic on Hibernate. An obvious solution was to keep the DAOs around, but to replace the JDBC code inside them with calls to the Hibernate Session.

We partly blame ourselves for what happened next.

Back in 2002 and 2003 this really seemed like a pretty reasonable thing to do. In fact, we contributed to the popularity of this approach by recommending—or at least not discouraging—the use of DAOs in Hibernate in Action. We hereby apologize for this mistake, and for taking much too long to recognize it.

Eventually, some folks came to believe that their DAOs shielded their program from depending in a hard way on ORM, allowing them to "swap out" Hibernate, and replace it with JDBC, or with something else. In fact, this was never really true—there’s quite a deep difference between the programming model of JDBC, where every interaction with the database is explicit and synchronous, and the programming model of stateful sessions in Hibernate, where updates are implicit, and SQL statements are executed asynchronously.

But then the whole landscape for persistence in Java changed in April 2006, when the final draft of JPA 1.0 was approved. Java now had a standard way to do ORM, with multiple high-quality implementations of the standard API. This was the end of the line for the DAOs, right?

Well, no. It wasn’t. DAOs were rebranded "repositories", and continue to enjoy a sort-of zombie afterlife as a front-end to JPA. But are they really pulling their weight, or are they just unnecessary extra complexity and bloat? An extra layer of indirection that makes stack traces harder to read and code harder to debug?

Our considered view is that they’re mostly just bloat. The JPA EntityManager is a "repository", and it’s a standard repository with a well-defined specification written by people who spend all day thinking about persistence. If these repository frameworks offered anything actually useful—and not obviously foot-shooty—over and above what EntityManager provides, we would have already added it to EntityManager decades ago.

Ultimately, we’re not sure you need a separate persistence layer at all. At least consider the possibility that it might be OK to call the EntityManager directly from your business logic.

API overview

We can already hear you hissing at our heresy. But before slamming shut the lid of your laptop and heading off to fetch garlic and a pitchfork, take a couple of hours to weigh what we’re proposing.

OK, so, look, if it makes you feel better, one way to view EntityManager is to think of it as a single generic "repository" that works for every entity in your system. From this point of view, JPA is your persistence layer. And there’s few good reasons to wrap this abstraction in a second abstraction that’s less generic.

Even where a distinct persistence layer is appropriate, DAO-style repositories aren’t the unambiguously most-correct way to factorize the equation:

  • most nontrivial queries touch multiple entities, and so it’s often quite ambiguous which repository such a query belongs to,

  • most queries are extremely specific to a particular fragment of program logic, and aren’t reused in different places across the system, and

  • the various operations of a repository rarely interact or share common internal implementation details.

Indeed, repositories, by nature, exhibit very low cohesion. A layer of repository objects might make sense if you have multiple implementations of each repository, but in practice almost nobody ever does. That’s because they’re also extremely highly coupled to their clients, with a very large API surface. And, on the contrary, a layer is only easily replaceable if it has a very narrow API.

Some people do indeed use mock repositories for testing, but we really struggle to see any value in this. If we don’t want to run our tests against our real database, it’s usually very easy to "mock" the database itself by running tests against an in-memory Java database like H2. This works even better in Hibernate 6 than in older versions of Hibernate, since HQL is now much more portable between platforms.

Phew, let’s move on.

1.8. Overview

It’s now time to begin our journey toward actually understanding the code we saw earlier.

This introduction will guide you through the basic tasks involved in developing a program that uses Hibernate for persistence:

  1. configuring and bootstrapping Hibernate, and obtaining an instance of SessionFactory or EntityManagerFactory,

  2. writing a domain model, that is, a set of entity classes which represent the persistent types in your program, and which map to tables of your database,

  3. customizing these mappings when the model maps to a pre-existing relational schema,

  4. using the Session or EntityManager to perform operations which query the database and return entity instances, or which update the data held in the database,

  5. using the Hibernate Metamodel Generator to improve compile-time type-safety,

  6. writing complex queries using the Hibernate Query Language (HQL) or native SQL, and, finally

  7. tuning performance of the data access logic.

Naturally, we’ll start at the top of this list, with the least-interesting topic: configuration.

2. Configuration and bootstrap

We would love to make this section short. Unfortunately, there’s several distinct ways to configure and bootstrap Hibernate, and we’re going to have to describe at least two of them in detail.

The four basic ways to obtain an instance of Hibernate are shown in the following table:

Using the standard JPA-defined XML, and the operation Persistence.createEntityManagerFactory()

Usually chosen when portability between JPA implementations is important.

Using the Configuration class to construct a SessionFactory

When portability between JPA implementations is not important, this option is quicker, adds some flexibility and saves a typecast.

Using the more complex APIs defined in org.hibernate.boot

Used primarily by framework integrators, this option is outside the scope of this document.

By letting the container take care of the bootstrap process and of injecting the SessionFactory or EntityManagerFactory

Used in a container environment like WildFly or Quarkus.

Here we’ll focus on the first two options.

Hibernate in containers

Actually, the last option is extremely popular, since every major Java application server and microservice framework comes with built-in support for Hibernate. Such container environments typically also feature facilities to automatically manage the lifecycle of an EntityManager or Session and its association with container-managed transactions.

To learn how to configure Hibernate in such a container environment, you’ll need to refer to the documentation of your chosen container. For Quarkus, here’s the relevant documentation.

If you’re using Hibernate outside of a container environment, you’ll need to:

  • include Hibernate ORM itself, along with the appropriate JDBC driver, as dependencies of your project, and

  • configure Hibernate with information about your database, by specifying configuration properties.

2.1. Including Hibernate in your project build

First, add the following dependency to your project:

org.hibernate.orm:hibernate-core:{version}

Where {version} is the version of Hibernate you’re using.

You’ll also need to add a dependency for the JDBC driver for your database.

Table 2. JDBC driver dependencies
Database Driver dependency

PostgreSQL or CockroachDB

org.postgresql:postgresql:{version}

MySQL or TiDB

com.mysql:mysql-connector-j:{version}

MariaDB

org.mariadb.jdbc:mariadb-java-client:{version}

DB2

com.ibm.db2:jcc:{version}

SQL Server

com.microsoft.sqlserver:mssql-jdbc:${version}

Oracle

com.oracle.database.jdbc:ojdbc11:${version}

H2

com.h2database:h2:{version}

HSQLDB

org.hsqldb:hsqldb:{version}

Where {version} is the latest version of the JDBC driver for your databse.

2.2. Optional dependencies

Optionally, you might also add any of the following additional features:

Table 3. Optional dependencies
Optional feature Dependencies

An SLF4J logging implementation

org.apache.logging.log4j:log4j-core
or org.slf4j:slf4j-jdk14

A JDBC connection pool, for example, Agroal

org.hibernate.orm:hibernate-agroal
and io.agroal:agroal-pool

The Hibernate Metamodel Generator, especially if you’re using the JPA criteria query API

org.hibernate.orm:hibernate-processor

The Query Validator, for compile-time checking of HQL

org.hibernate:query-validator

Hibernate Validator, an implementation of Bean Validation

org.hibernate.validator:hibernate-validator
and org.glassfish:jakarta.el

Local second-level cache support via JCache and EHCache

org.hibernate.orm:hibernate-jcache
and org.ehcache:ehcache

Local second-level cache support via JCache and Caffeine

org.hibernate.orm:hibernate-jcache
and com.github.ben-manes.caffeine:jcache

Distributed second-level cache support via Infinispan

org.infinispan:infinispan-hibernate-cache-v60

A JSON serialization library for working with JSON datatypes, for example, Jackson or Yasson

com.fasterxml.jackson.core:jackson-databind
or org.eclipse:yasson

Hibernate Spatial

org.hibernate.orm:hibernate-spatial

Envers, for auditing historical data

org.hibernate.orm:hibernate-envers

You might also add the Hibernate bytecode enhancer to your Gradle build if you want to use field-level lazy fetching.

2.3. Configuration using JPA XML

Sticking to the JPA-standard approach, we would provide a file named persistence.xml, which we usually place in the META-INF directory of a persistence archive, that is, of the .jar file or directory which contains our entity classes.

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd"
             version="2.0">

    <persistence-unit name="org.hibernate.example">

        <class>org.hibernate.example.Book</class>
        <class>org.hibernate.example.Author</class>

        <properties>
            <!-- PostgreSQL -->
            <property name="jakarta.persistence.jdbc.url"
                      value="jdbc:postgresql://localhost/example"/>

            <!-- Credentials -->
            <property name="jakarta.persistence.jdbc.user"
                      value="gavin"/>
            <property name="jakarta.persistence.jdbc.password"
                      value="hibernate"/>

            <!-- Automatic schema export -->
            <property name="jakarta.persistence.schema-generation.database.action"
                      value="drop-and-create"/>

            <!-- SQL statement logging -->
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.highlight_sql" value="true"/>

        </properties>

    </persistence-unit>

</persistence>

The <persistence-unit> element defines a named persistence unit, that is:

  • a collection of associated entity types, along with

  • a set of default configuration settings, which may be augmented or overridden at runtime.

Each <class> element specifies the fully-qualified name of an entity class.

Scanning for entity classes

In some container environments, for example, in any EE container, the <class> elements are unnecessary, since the container will scan the archive for annotated classes, and automatically recognize any class annotated @Entity.

Each <property> element specifies a configuration property and its value. Note that:

  • the configuration properties in the jakarta.persistence namespace are standard properties defined by the JPA spec, and

  • properties in the hibernate namespace are specific to Hibernate.

We may obtain an EntityManagerFactory by calling Persistence.createEntityManagerFactory():

EntityManagerFactory entityManagerFactory =
    Persistence.createEntityManagerFactory("org.hibernate.example");

If necessary, we may override configuration properties specified in persistence.xml:

EntityManagerFactory entityManagerFactory =
    Persistence.createEntityManagerFactory("org.hibernate.example",
            Map.of(AvailableSettings.JAKARTA_JDBC_PASSWORD, password));

2.4. Configuration using Hibernate API

Alternatively, the venerable class Configuration allows an instance of Hibernate to be configured in Java code.

SessionFactory sessionFactory =
        new Configuration()
            .addAnnotatedClass(Book.class)
            .addAnnotatedClass(Author.class)
            // PostgreSQL
            .setProperty(AvailableSettings.JAKARTA_JDBC_URL, "jdbc:postgresql://localhost/example")
            // Credentials
            .setProperty(AvailableSettings.JAKARTA_JDBC_USER, user)
            .setProperty(AvailableSettings.JAKARTA_JDBC_PASSWORD, password)
            // Automatic schema export
            .setProperty(AvailableSettings.JAKARTA_HBM2DDL_DATABASE_ACTION,
                         Action.SPEC_ACTION_DROP_AND_CREATE)
            // SQL statement logging
            .setProperty(AvailableSettings.SHOW_SQL, true)
            .setProperty(AvailableSettings.FORMAT_SQL, true)
            .setProperty(AvailableSettings.HIGHLIGHT_SQL, true)
            // Create a new SessionFactory
            .buildSessionFactory();

The Configuration class has survived almost unchanged since the very earliest (pre-1.0) versions of Hibernate, and so it doesn’t look particularly modern. On the other hand, it’s very easy to use, and exposes some options that persistence.xml doesn’t support.

Advanced configuration options

Actually, the Configuration class is just a very simple facade for the more modern, much more powerful—but more complex—API defined in the package org.hibernate.boot. This API is useful if you have very advanced requirements, for example, if you’re writing a framework or implementing a container. You’ll find more information in the User Guide, and in the package-level documentation of org.hibernate.boot.

2.5. Configuration using Hibernate properties file

If we’re using the Hibernate Configuration API, but we don’t want to put certain configuration properties directly in the Java code, we can specify them in a file named hibernate.properties, and place the file in the root classpath.

# PostgreSQL
jakarta.persistence.jdbc.url=jdbc:postgresql://localhost/example
# Credentials
jakarta.persistence.jdbc.user=hibernate
jakarta.persistence.jdbc.password=zAh7mY$2MNshzAQ5

# SQL statement logging
hibernate.show_sql=true
hibernate.format_sql=true
hibernate.highlight_sql=true

2.6. Basic configuration settings

The class AvailableSettings enumerates all the configuration properties understood by Hibernate.

Of course, we’re not going to cover every useful configuration setting in this chapter. Instead, we’ll mention the ones you need to get started, and come back to some other important settings later, especially when we talk about performance tuning.

Hibernate has many—too many—switches and toggles. Please don’t go crazy messing about with these settings; most of them are rarely needed, and many only exist to provide backward compatibility with older versions of Hibernate. With rare exception, the default behavior of every one of these settings was carefully chosen to be the behavior we recommend.

The properties you really do need to get started are these three:

Table 4. JDBC connection settings
Configuration property name Purpose

jakarta.persistence.jdbc.url

JDBC URL of your database

jakarta.persistence.jdbc.user and jakarta.persistence.jdbc.password

Your database credentials

In Hibernate 6, you don’t need to specify hibernate.dialect. The correct Hibernate SQL Dialect will be determined for you automatically. The only reason to specify this property is if you’re using a custom user-written Dialect class.

Similarly, neither hibernate.connection.driver_class nor jakarta.persistence.jdbc.driver is needed when working with one of the supported databases.

In some environments it’s useful to be able to start Hibernate without accessing the database. In this case, we must explicitly specify not only the database platform, but also the version of the database, using the standard JPA configuration properties.

# disable use of JDBC database metadata
hibernate.boot.allow_jdbc_metadata_access=false

# explicitly specify database and version
jakarta.persistence.database-product-name=PostgreSQL
jakarta.persistence.database-major-version=15
jakarta.persistence.database-minor-version=7

The product name is the value returned by java.sql.DatabaseMetaData.getDatabaseProductName(), for example, PostgreSQL, MySQL H2, Oracle, EnterpriseDB, MariaDB, or Microsoft SQL Server.

Table 5. Settings needed when database is inaccessible at startup
Configuration property name Purpose

hibernate.boot.allow_jdbc_metadata_access

Set to false to disallow access to the database at startup

jakarta.persistence.database-product-name

The database product name, according to the JDBC driver

jakarta.persistence.database-major-version and jakarta.persistence.database-minor-version

The major and minor versions of the database

Pooling JDBC connections is an extremely important performance optimization. You can set the size of Hibernate’s built-in connection pool using this property:

Table 6. Built-in connection pool size
Configuration property name Purpose

hibernate.connection.pool_size

The size of the built-in connection pool

By default, Hibernate uses a simplistic built-in connection pool. This pool is not meant for use in production, and later, when we discuss performance, we’ll see how to select a more robust implementation.

Alternatively, in a container environment, you’ll need at least one of these properties:

Table 7. Transaction management settings
Configuration property name Purpose

jakarta.persistence.transactionType

(Optional, defaults to JTA) Determines if transaction management is via JTA or resource-local transactions. Specify RESOURCE_LOCAL if JTA should not be used.

jakarta.persistence.jtaDataSource

JNDI name of a JTA datasource

jakarta.persistence.nonJtaDataSource

JNDI name of a non-JTA datasource

In this case, Hibernate obtains pooled JDBC database connections from a container-managed DataSource.

2.7. Automatic schema export

You can have Hibernate infer your database schema from the mapping annotations you’ve specified in your Java code, and export the schema at initialization time by specifying one or more of the following configuration properties:

Table 8. Schema management settings
Configuration property name Purpose

jakarta.persistence.schema-generation.database.action

  • If drop-and-create, first drop the schema and then export tables, sequences, and constraints

  • If create, export tables, sequences, and constraints, without attempting to drop them first

  • If create-drop, drop the schema and recreate it on SessionFactory startup Additionally, drop the schema on SessionFactory shutdown

  • If drop, drop the schema on SessionFactory shutdown

  • If validate, validate the database schema without changing it

  • If update, only export what’s missing in the schema

jakarta.persistence.create-database-schemas

(Optional) If true, automatically create schemas and catalogs

jakarta.persistence.schema-generation.create-source

(Optional) If metadata-then-script or script-then-metadata, execute an additional SQL script when exported tables and sequences

jakarta.persistence.schema-generation.create-script-source

(Optional) The name of a SQL DDL script to be executed

jakarta.persistence.sql-load-script-source

(Optional) The name of a SQL DML script to be executed

This feature is extremely useful for testing.

The easiest way to pre-initialize a database with test or "reference" data is to place a list of SQL insert statements in a file named, for example, import.sql, and specify the path to this file using the property jakarta.persistence.sql-load-script-source. We’ve already seen an example of this approach, which is cleaner than writing Java code to instantiate entity instances and calling persist() on each of them.

As we mentioned earlier, it can also be useful to control schema export programmatically.

The SchemaManager API allows programmatic control over schema export:

sessionFactory.getSchemaManager().exportMappedObjects(true);

JPA has a more limited and less ergonomic API:

Persistence.generateSchema("org.hibernate.example",
                           Map.of(JAKARTA_HBM2DDL_DATABASE_ACTION, CREATE))

2.8. Logging the generated SQL

To see the generated SQL as it’s sent to the database, you have two options.

One way is to set the property hibernate.show_sql to true, and Hibernate will log SQL direct to the console. You can make the output much more readable by enabling formatting or highlighting. These settings really help when troubleshooting the generated SQL statements.

Table 9. Settings for SQL logging to the console
Configuration property name Purpose

hibernate.show_sql

If true, log SQL directly to the console

hibernate.format_sql

If true, log SQL in a multiline, indented format

hibernate.highlight_sql

If true, log SQL with syntax highlighting via ANSI escape codes

Alternatively, you can enable debug-level logging for the category org.hibernate.SQL using your preferred SLF4J logging implementation.

For example, if you’re using Log4J 2 (as above in Optional dependencies), add these lines to your log4j2.properties file:

# SQL execution
logger.hibernate.name = org.hibernate.SQL
logger.hibernate.level = debug

# JDBC parameter binding
logger.jdbc-bind.name=org.hibernate.orm.jdbc.bind
logger.jdbc-bind.level=trace
# JDBC result set extraction
logger.jdbc-extract.name=org.hibernate.orm.jdbc.extract
logger.jdbc-extract.level=trace

But with this approach we miss out on the pretty highlighting.

2.9. Minimizing repetitive mapping information

The following properties are very useful for minimizing the amount of information you’ll need to explicitly specify in @Table and @Column annotations, which we’ll discuss below in Object/relational mapping:

Table 10. Settings for minimizing explicit mapping information
Configuration property name Purpose

hibernate.default_schema

A default schema name for entities which do not explicitly declare one

hibernate.default_catalog

A default catalog name for entities which do not explicitly declare one

hibernate.physical_naming_strategy

A PhysicalNamingStrategy implementing your database naming standards

hibernate.implicit_naming_strategy

An ImplicitNamingStrategy which specifies how "logical" names of relational objects should be inferred when no name is specified in annotations

Writing your own PhysicalNamingStrategy and/or ImplicitNamingStrategy is an especially good way to reduce the clutter of annotations on your entity classes, and to implement your database naming conventions, and so we think you should do it for any nontrivial data model. We’ll have more to say about them in Naming strategies.

2.10. Nationalized character data in SQL Server

By default, SQL Server’s char and varchar types don’t accommodate Unicode data. But a Java string may contain any Unicode character. So, if you’re working with SQL Server, you might need to force Hibernate to use the nchar and nvarchar column types.

Table 11. Setting the use of nationalized character data
Configuration property name Purpose

hibernate.use_nationalized_character_data

Use nchar and nvarchar instead of char and varchar

On the other hand, if only some columns store nationalized data, use the @Nationalized annotation to indicate fields of your entities which map these columns.

Alternatively, you can configure SQL Server to use the UTF-8 enabled collation _UTF8.

3. Entities

An entity is a Java class which represents data in a relational database table. We say that the entity maps or maps to the table. Much less commonly, an entity might aggregate data from multiple tables, but we’ll get to that later.

An entity has attributes—properties or fields—which map to columns of the table. In particular, every entity must have an identifier or id, which maps to the primary key of the table. The id allows us to uniquely associate a row of the table with an instance of the Java class, at least within a given persistence context.

We’ll explore the idea of a persistence context later. For now, think of it as a one-to-one mapping between ids and entity instances.

An instance of a Java class cannot outlive the virtual machine to which it belongs. But we may think of an entity instance having a lifecycle which transcends a particular instantiation in memory. By providing its id to Hibernate, we may re-materialize the instance in a new persistence context, as long as the associated row is present in the database. Therefore, the operations persist() and remove() may be thought of as demarcating the beginning and end of the lifecycle of an entity, at least with respect to persistence.

Thus, an id represents the persistent identity of an entity, an identity that outlives a particular instantiation in memory. And this is an important difference between entity class itself and the values of its attributes—the entity has a persistent identity, and a well-defined lifecycle with respect to persistence, whereas a String or List representing one of its attribute values doesn’t.

An entity usually has associations to other entities. Typically, an association between two entities maps to a foreign key in one of the database tables. A group of mutually associated entities is often called a domain model, though data model is also a perfectly good term.

3.1. Entity classes

An entity must:

  • be a non-final class,

  • with a non-private constructor with no parameters.

On the other hand, the entity class may be either concrete or abstract, and it may have any number of additional constructors.

An entity class may be a static inner class.

Every entity class must be annotated @Entity.

@Entity
class Book {
    Book() {}
    ...
}

Alternatively, the class may be identified as an entity type by providing an XML-based mapping for the class.

Mapping entities using XML

When XML-based mappings are used, the <entity> element is used to declare an entity class:

<entity-mappings>
    <package>org.hibernate.example</package>

    <entity class="Book">
        <attributes> ... </attributes>
    </entity>

    ...
</entity-mappings>

Since the orm.xml mapping file format defined by the JPA specification was modelled closely on the annotation-based mappings, it’s usually easy to go back and forth between the two options.

We won’t have much more to say about XML-based mappings in this Introduction, since it’s not our preferred way to do things.

"Dynamic" models

We love representing entities as classes because the classes give us a type-safe model of our data. But Hibernate also has the ability to represent entities as detyped instances of java.util.Map. There’s information in the User Guide, if you’re curious.

This must sound like a weird feature for a project that places importance on type-safety. Actually, it’s a useful capability for a very particular sort of generic code. For example, Hibernate Envers is a great auditing/versioning system for Hibernate entities. Envers makes use of maps to represent its versioned model of the data.

3.2. Access types

Each entity class has a default access type, either:

  • direct field access, or

  • property access.

Hibernate automatically determines the access type from the location of attribute-level annotations. Concretely:

  • if a field is annotated @Id, field access is used, or

  • if a getter method is annotated @Id, property access is used.

Back when Hibernate was just a baby, property access was quite popular in the Hibernate community. Today, however, field access is much more common.

The default access type may be specified explicitly using the @Access annotation, but we strongly discourage this, since it’s ugly and never necessary.

Mapping annotations should be placed consistently:

  • if @Id annotates a field, the other mapping annotations should also be applied to fields, or,

  • if @Id annotates a getter, the other mapping annotations should be applied to getters.

It is in principle possible to mix field and property access using explicit @Access annotations at the attribute level. We don’t recommend doing this.

An entity class like Book, which does not extend any other entity class, is called a root entity. Every root entity must declare an identifier attribute.

3.3. Entity class inheritance

An entity class may extend another entity class.

@Entity
class AudioBook extends Book {
    AudioBook() {}
    ...
}

A subclass entity inherits every persistent attribute of every entity it extends.

A root entity may also extend another class and inherit mapped attributes from the other class. But in this case, the class which declares the mapped attributes must be annotated @MappedSuperclass.

@MappedSuperclass
class Versioned {
    ...
}

@Entity
class Book extends Versioned {
    ...
}

A root entity class must declare an attribute annotated @Id, or inherit one from a @MappedSuperclass. A subclass entity always inherits the identifier attribute of the root entity. It may not declare its own @Id attribute.

3.4. Identifier attributes

An identifier attribute is usually a field:

@Entity
class Book {
    Book() {}

    @Id
    Long id;

    ...
}

But it may be a property:

@Entity
class Book {
    Book() {}

    private Long id;

    @Id
    Long getId() { return id; }
    void setId(Long id) { this.id = id; }

    ...
}

An identifier attribute must be annotated @Id or @EmbeddedId.

Identifier values may be:

  • assigned by the application, that is, by your Java code, or

  • generated and assigned by Hibernate.

We’ll discuss the second option first.

3.5. Generated identifiers

An identifier is often system-generated, in which case it should be annotated @GeneratedValue:

@Id @GeneratedValue
Long id;

System-generated identifiers, or surrogate keys make it easier to evolve or refactor the relational data model. If you have the freedom to define the relational schema, we recommend the use of surrogate keys. On the other hand, if, as is more common, you’re working with a pre-existing database schema, you might not have the option.

JPA defines the following strategies for generating ids, which are enumerated by GenerationType:

Table 12. Standard id generation strategies
Strategy Java type Implementation

GenerationType.UUID

UUID or String

A Java UUID

GenerationType.IDENTITY

Long or Integer

An identity or autoincrement column

GenerationType.SEQUENCE

Long or Integer

A database sequence

GenerationType.TABLE

Long or Integer

A database table

GenerationType.AUTO

Long or Integer

Selects SEQUENCE, TABLE, or UUID based on the identifier type and capabilities of the database

For example, this UUID is generated in Java code:

@Id @GeneratedValue UUID id;  // AUTO strategy selects UUID based on the field type

This id maps to a SQL identity, auto_increment, or bigserial column:

@Id @GeneratedValue(strategy=IDENTITY) Long id;

The @SequenceGenerator and @TableGenerator annotations allow further control over SEQUENCE and TABLE generation respectively.

Consider this sequence generator:

@SequenceGenerator(name="bookSeq", sequenceName="seq_book", initialValue = 5, allocationSize=10)

Values are generated using a database sequence defined as follows:

create sequence seq_book start with 5 increment by 10

Notice that Hibernate doesn’t have to go to the database every time a new identifier is needed. Instead, a given process obtains a block of ids, of size allocationSize, and only needs to hit the database each time the block is exhausted. Of course, the downside is that generated identifiers are not contiguous.

If you let Hibernate export your database schema, the sequence definition will have the right start with and increment values. But if you’re working with a database schema managed outside Hibernate, make sure the initialValue and allocationSize members of @SequenceGenerator match the start with and increment specified in the DDL.

Any identifier attribute may now make use of the generator named bookSeq:

@Id
@GeneratedValue(strategy=SEQUENCE, generator="bookSeq")  // reference to generator defined elsewhere
Long id;

Actually, it’s extremely common to place the @SequenceGenerator annotation on the @Id attribute that makes use of it:

@Id
@GeneratedValue(strategy=SEQUENCE, generator="bookSeq")  // reference to generator defined below
@SequenceGenerator(name="bookSeq", sequenceName="seq_book", initialValue = 5, allocationSize=10)
Long id;

JPA id generators may be shared between entities. A @SequenceGenerator or @TableGenerator must have a name, and may be shared between multiple id attributes. This fits somewhat uncomfortably with the common practice of annotating the @Id attribute which makes use of the generator!

As you can see, JPA provides quite adequate support for the most common strategies for system-generated ids. However, the annotations themselves are a bit more intrusive than they should be, and there’s no well-defined way to extend this framework to support custom strategies for id generation. Nor may @GeneratedValue be used on a property not annotated @Id. Since custom id generation is a rather common requirement, Hibernate provides a very carefully-designed framework for user-defined Generators, which we’ll discuss in User-defined generators.

3.6. Natural keys as identifiers

Not every identifier attribute maps to a (system-generated) surrogate key. Primary keys which are meaningful to the user of the system are called natural keys.

When the primary key of a table is a natural key, we don’t annotate the identifier attribute @GeneratedValue, and it’s the responsibility of the application code to assign a value to the identifier attribute.

@Entity
class Book {
    @Id
    String isbn;

    ...
}

Of particular interest are natural keys which comprise more than one database column, and such natural keys are called composite keys.

3.7. Composite identifiers

If your database uses composite keys, you’ll need more than one identifier attribute. There are two ways to map composite keys in JPA:

  • using an @IdClass, or

  • using an @EmbeddedId.

Perhaps the most immediately-natural way to represent this in an entity class is with multiple fields annotated @Id, for example:

@Entity
@IdClass(BookId.class)
class Book {
    Book() {}

    @Id
    String isbn;

    @Id
    int printing;

    ...
}

But this approach comes with a problem: what object can we use to identify a Book and pass to methods like find() which accept an identifier?

The solution is to write a separate class with fields that match the identifier attributes of the entity. Every such id class must override equals() and hashCode(). Of course, the easiest way to satisfy these requirements is to declare the id class as a record.

record BookId(String isbn, int printing) {}

The @IdClass annotation of the Book entity identifies BookId as the id class to use for that entity.

This is not our preferred approach. Instead, we recommend that the BookId class be declared as an @Embeddable type:

@Embeddable
record BookId(String isbn, int printing) {}

We’ll learn more about Embeddable objects below.

Now the entity class may reuse this definition using @EmbeddedId, and the @IdClass annotation is no longer required:

@Entity
class Book {
    Book() {}

    @EmbeddedId
    BookId bookId;

    ...
}

This second approach eliminates some duplicated code.

Either way, we may now use BookId to obtain instances of Book:

Book book = session.find(Book.class, new BookId(isbn, printing));

3.8. Version attributes

An entity may have an attribute which is used by Hibernate for optimistic lock checking. A version attribute is usually of type Integer, Short, Long, LocalDateTime, OffsetDateTime, ZonedDateTime, or Instant.

@Version
LocalDateTime lastUpdated;

Version attributes are automatically assigned by Hibernate when an entity is made persistent, and automatically incremented or updated each time the entity is updated.

If an entity doesn’t have a version number, which often happens when mapping legacy data, we can still do optimistic locking. The @OptimisticLocking annotation lets us specify that optimistic locks should be checked by validating the values of ALL fields, or only the DIRTY fields of the entity. And the @OptimisticLock annotation lets us selectively exclude certain fields from optimistic locking.

The @Id and @Version attributes we’ve already seen are just specialized examples of basic attributes.

3.9. Natural id attributes

Even when an entity has a surrogate key, it should always be possible to write down a combination of fields which uniquely identifies an instance of the entity, from the point of view of the user of the system. This combination of fields is its natural key. Above, we considered the case where the natural key coincides with the primary key. Here, the natural key is a second unique key of the entity, distinct from its surrogate primary key.

If you can’t identify a natural key, it might be a sign that you need to think more carefully about some aspect of your data model. If an entity doesn’t have a meaningful unique key, then it’s impossible to say what event or object it represents in the "real world" outside your program.

Since it’s extremely common to retrieve an entity based on its natural key, Hibernate has a way to mark the attributes of the entity which make up its natural key. Each attribute must be annotated @NaturalId.

@Entity
class Book {
    Book() {}

    @Id @GeneratedValue
    Long id; // the system-generated surrogate key

    @NaturalId
    String isbn; // belongs to the natural key

    @NaturalId
    int printing; // also belongs to the natural key

    ...
}

Hibernate automatically generates a UNIQUE constraint on the columns mapped by the annotated fields.

Consider using the natural id attributes to implement equals() and hashCode().

The payoff for doing this extra work, as we will see much later, is that we can take advantage of optimized natural id lookups that make use of the second-level cache.

Note that even when you’ve identified a natural key, we still recommend the use of a generated surrogate key in foreign keys, since this makes your data model much easier to change.

3.10. Basic attributes

A basic attribute of an entity is a field or property which maps to a single column of the associated database table. The JPA specification defines a quite limited set of basic types:

Table 13. JPA-standard basic attribute types
Classification Package Types

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, BigDecimal

Date/time types

java.time

LocalDate, LocalTime, LocalDateTime, OffsetDateTime, Instant

Deprecated date/time types 💀

java.util

Date, Calendar

Deprecated JDBC date/time types 💀

java.sql

Date, Time, Timestamp

Binary and character arrays

byte[], char[]

UUIDs

java.util

UUID

Enumerated types

Any enum

Serializable types

Any type which implements java.io.Serializable

We’re begging you to use types from the java.time package instead of anything which inherits java.util.Date.

Serializing a Java object and storing its binary representation in the database is usually wrong. As we’ll soon see in Embeddable objects, Hibernate has much better ways to handle complex Java objects.

Hibernate slightly extends this list with the following types:

Table 14. Additional basic attribute types in Hibernate
Classification Package Types

Additional date/time types

java.time

Duration, ZoneId, ZoneOffset, Year, and even ZonedDateTime

JDBC LOB types

java.sql

Blob, Clob, NClob

Java class object

java.lang

Class

Miscellaneous types

java.util

Currency, URL, TimeZone

The @Basic annotation explicitly specifies that an attribute is basic, but it’s often not needed, since attributes are assumed basic by default. On the other hand, if a non-primitively-typed attribute cannot be null, use of @Basic(optional=false) is highly recommended.

@Basic(optional=false) String firstName;
@Basic(optional=false) String lastName;
String middleName; // may be null

Note that primitively-typed attributes are inferred NOT NULL by default.

How to make a column not null in JPA

There are two standard ways to add a NOT NULL constraint to a mapped column in JPA:

  • using @Basic(optional=false), or

  • using @Column(nullable=false).

You might wonder what the difference is.

Well, it’s perhaps not obvious to a casual user of the JPA annotations, but they actually come in two "layers":

  • annotations like @Entity, @Id, and @Basic belong to the logical layer, the subject of the current chapter—they specify the semantics of your Java domain model, whereas

  • annotations like @Table and @Column belong to the mapping layer, the topic of the next chapter—they specify how elements of the domain model map to objects in the relational database.

Information may be inferred from the logical layer down to the mapping layer, but is never inferred in the opposite direction.

Now, the @Column annotation, to whom we’ll be properly introduced a bit later, belongs to the mapping layer, and so its nullable member only affects schema generation (resulting in a not null constraint in the generated DDL). On the other hand, the @Basic annotation belongs to the logical layer, and so an attribute marked optional=false is checked by Hibernate before it even writes an entity to the database. Note that:

  • optional=false implies nullable=false, but

  • nullable=false does not imply optional=false.

Therefore, we prefer @Basic(optional=false) to @Column(nullable=false).

But wait! An even better solution is to use the @NotNull annotation from Bean Validation. Just add Hibernate Validator to your project build, as described in Optional dependencies.

3.11. Enumerated types

We included Java enums on the list above. An enumerated type is considered a sort of basic type, but since most databases don’t have a native ENUM type, JPA provides a special @Enumerated annotation to specify how the enumerated values should be represented in the database:

  • by default, an enum is stored as an integer, the value of its ordinal() member, but

  • if the attribute is annotated @Enumerated(STRING), it will be stored as a string, the value of its name() member.

//here, an ORDINAL encoding makes sense
@Enumerated
@Basic(optional=false)
DayOfWeek dayOfWeek;

//but usually, a STRING encoding is better
@Enumerated(EnumType.STRING)
@Basic(optional=false)
Status status;

In Hibernate 6, an enum annotated @Enumerated(STRING) is mapped to:

  • a VARCHAR column type with a CHECK constraint on most databases, or

  • an ENUM column type on MySQL.

Any other enum is mapped to a TINYINT column with a CHECK constraint.

JPA picks the wrong default here. In most cases, storing an integer encoding of the enum value makes the relational data harder to interpret.

Even considering DayOfWeek, the encoding to integers is ambiguous. If you check java.time.DayOfWeek, you’ll notice that SUNDAY is encoded as 6. But in the country I was born, SUNDAY is the first day of the week!

So we prefer @Enumerated(STRING) for most enum attributes.

An interesting special case is PostgreSQL. Postgres supports named ENUM types, which must be declared using a DDL CREATE TYPE statement. Sadly, these ENUM types aren’t well-integrated with the language nor well-supported by the Postgres JDBC driver, so Hibernate doesn’t use them by default. But if you would like to use a named enumerated type on Postgres, just annotate your enum attribute like this:

@JdbcTypeCode(SqlTypes.NAMED_ENUM)
@Basic(optional=false)
Status status;

The limited set of pre-defined basic attribute types can be stretched a bit further by supplying a converter.

3.12. Converters

A JPA AttributeConverter is responsible for:

  • converting a given Java type to one of the types listed above, and/or

  • perform any other sort of pre- and post-processing you might need to perform on a basic attribute value before writing and reading it to or from the database.

Converters substantially widen the set of attribute types that can be handled by JPA.

There are two ways to apply a converter:

  • the @Convert annotation applies an AttributeConverter to a particular entity attribute, or

  • the @Converter annotation (or, alternatively, the @ConverterRegistration annotation) registers an AttributeConverter for automatic application to all attributes of a given type.

For example, the following converter will be automatically applied to any attribute of type BitSet, and takes care of persisting the BitSet to a column of type varbinary:

@Converter(autoApply = true)
public static class EnumSetConverter
        // converts Java values of type EnumSet<DayOfWeek> to integers for storage in an INT column
        implements AttributeConverter<EnumSet<DayOfWeek>,Integer> {
    @Override
    public Integer convertToDatabaseColumn(EnumSet<DayOfWeek> enumSet) {
        int encoded = 0;
        var values = DayOfWeek.values();
        for (int i = 0; i<values.length; i++) {
            if (enumSet.contains(values[i])) {
                encoded |= 1<<i;
            }
        }
        return encoded;
    }

    @Override
    public EnumSet<DayOfWeek> convertToEntityAttribute(Integer encoded) {
        var set = EnumSet.noneOf(DayOfWeek.class);
        var values = DayOfWeek.values();
        for (int i = 0; i<values.length; i++) {
            if (((1<<i) & encoded) != 0) {
                set.add(values[i]);
            }
        }
        return set;
    }
}

On the other hand, if we don’t set autoapply=true, then we must explicitly apply the converter using the @Convert annotation:

@Convert(converter = BitSetConverter.class)
@Basic(optional = false)
BitSet bitset;

All this is nice, but it probably won’t surprise you that Hibernate goes beyond what is required by JPA.

3.13. Compositional basic types

Hibernate considers a "basic type" to be formed by the marriage of two objects:

  • a JavaType, which models the semantics of a certain Java class, and

  • a JdbcType, representing a SQL type which is understood by JDBC.

When mapping a basic attribute, we may explicitly specify a JavaType, a JdbcType, or both.

JavaType

An instance of org.hibernate.type.descriptor.java.JavaType represents a particular Java class. It’s able to:

  • compare instances of the class to determine if an attribute of that class type is dirty (modified),

  • produce a useful hash code for an instance of the class,

  • coerce values to other types, and, in particular,

  • convert an instance of the class to one of several other equivalent Java representations at the request of its partner JdbcType.

For example, IntegerJavaType knows how to convert an Integer or int value to the types Long, BigInteger, and String, among others.

We may explicitly specify a Java type using the @JavaType annotation, but for the built-in JavaTypes this is never necessary.

@JavaType(LongJavaType.class)  // not needed, this is the default JavaType for long
long currentTimeMillis;

For a user-written JavaType, the annotation is more useful:

@JavaType(BitSetJavaType.class)
BitSet bitSet;

Alternatively, the @JavaTypeRegistration annotation may be used to register BitSetJavaType as the default JavaType for BitSet.

JdbcType

A org.hibernate.type.descriptor.jdbc.JdbcType is able to read and write a single Java type from and to JDBC.

For example, VarcharJdbcType takes care of:

  • writing Java strings to JDBC PreparedStatements by calling setString(), and

  • reading Java strings from JDBC ResultSets using getString().

By pairing LongJavaType with VarcharJdbcType in holy matrimony, we produce a basic type which maps Longs and primitive longss to the SQL type VARCHAR.

We may explicitly specify a JDBC type using the @JdbcType annotation.

@JdbcType(VarcharJdbcType.class)
long currentTimeMillis;

Alternatively, we may specify a JDBC type code:

@JdbcTypeCode(Types.VARCHAR)
long currentTimeMillis;

The @JdbcTypeRegistration annotation may be used to register a user-written JdbcType as the default for a given SQL type code.

JDBC types and JDBC type codes

The types defined by the JDBC specification are enumerated by the integer type codes in the class java.sql.Types. Each JDBC type is an abstraction of a commonly-available type in SQL. For example, Types.VARCHAR represents the SQL type VARCHAR (or VARCHAR2 on Oracle).

Since Hibernate understands more SQL types than JDBC, there’s an extended list of integer type codes in the class org.hibernate.type.SqlTypes. For example, SqlTypes.GEOMETRY represents the spatial data type GEOMETRY.

AttributeConverter

If a given JavaType doesn’t know how to convert its instances to the type required by its partner JdbcType, we must help it out by providing a JPA AttributeConverter to perform the conversion.

For example, to form a basic type using LongJavaType and TimestampJdbcType, we would provide an AttributeConverter<Long,Timestamp>.

@JdbcType(TimestampJdbcType.class)
@Convert(converter = LongToTimestampConverter.class)
long currentTimeMillis;

Let’s abandon our analogy right here, before we start calling this basic type a "throuple".

3.14. Embeddable objects

An embeddable object is a Java class whose state maps to multiple columns of a table, but which doesn’t have its own persistent identity. That is, it’s a class with mapped attributes, but no @Id attribute.

An embeddable object can only be made persistent by assigning it to the attribute of an entity. Since the embeddable object does not have its own persistent identity, its lifecycle with respect to persistence is completely determined by the lifecycle of the entity to which it belongs.

An embeddable class must be annotated @Embeddable instead of @Entity.

@Embeddable
class Name {

    @Basic(optional=false)
    String firstName;

    @Basic(optional=false)
    String lastName;

    String middleName;

    Name() {}

    Name(String firstName, String middleName, String lastName) {
        this.firstName = firstName;
        this.middleName = middleName;
        this.lastName = lastName;
    }

    ...
}

An embeddable class must satisfy the same requirements that entity classes satisfy, with the exception that an embeddable class has no @Id attribute. In particular, it must have a constructor with no parameters.

Alternatively, an embeddable type may be defined as a Java record type:

@Embeddable
record Name(String firstName, String middleName, String lastName) {}

In this case, the requirement for a constructor with no parameters is relaxed.

We may now use our Name class (or record) as the type of an entity attribute:

@Entity
class Author {
    @Id @GeneratedValue
    Long id;

    Name name;

    ...
}

Embeddable types can be nested. That is, an @Embeddable class may have an attribute whose type is itself a different @Embeddable class.

JPA provides an @Embedded annotation to identify an attribute of an entity that refers to an embeddable type. This annotation is completely optional, and so we don’t usually use it.

On the other hand a reference to an embeddable type is never polymorphic. One @Embeddable class F may inherit a second @Embeddable class E, but an attribute of type E will always refer to an instance of that concrete class E, never to an instance of F.

Usually, embeddable types are stored in a "flattened" format. Their attributes map columns of the table of their parent entity. Later, in Mapping embeddable types to UDTs or to JSON, we’ll see a couple of different options.

An attribute of embeddable type represents a relationship between a Java object with a persistent identity, and a Java object with no persistent identity. We can think of it as a whole/part relationship. The embeddable object belongs to the entity, and can’t be shared with other entity instances. And it exists for only as long as its parent entity exists.

Next we’ll discuss a different kind of relationship: a relationship between Java objects which each have their own distinct persistent identity and persistence lifecycle.

3.15. Associations

An association is a relationship between entities. We usually classify associations based on their multiplicity. If E and F are both entity classes, then:

  • a one-to-one association relates at most one unique instance E with at most one unique instance of F,

  • a many-to-one association relates zero or more instances of E with a unique instance of F, and

  • a many-to-many association relates zero or more instances of E with zero or more instance of F.

An association between entity classes may be either:

  • unidirectional, navigable from E to F but not from F to E, or

  • bidirectional, and navigable in either direction.

In this example data model, we can see the sorts of associations which are possible:

Example data model

An astute observer of the diagram above might notice that the relationship we’ve presented as a unidirectional one-to-one association could reasonably be represented in Java using subtyping. This is quite normal. A one-to-one association is the usual way we implement subtyping in a fully-normalized relational model. It’s related to the JOINED inheritance mapping strategy.

There are three annotations for mapping associations: @ManyToOne, @OneToMany, and @ManyToMany. They share some common annotation members:

Table 15. Association-defining annotation members
Member Interpretation Default value

cascade

Persistence operations which should cascade to the associated entity; a list of CascadeTypes

{}

fetch

Whether the association is eagerly fetched or may be proxied

  • LAZY for @OneToMany and @ManyToMany

  • EAGER for @ManyToOne 💀💀💀

targetEntity

The associated entity class

Determined from the attribute type declaration

optional

For a @ManyToOne or @OneToOne association, whether the association can be null

true

mappedBy

For a bidirectional association, an attribute of the associated entity which maps the association

By default, the association is assumed unidirectional

We’ll explain the effect of these members as we consider the various types of association mapping.

Let’s begin with the most common association multiplicity.

3.16. Many-to-one

A many-to-one association is the most basic sort of association we can imagine. It maps completely naturally to a foreign key in the database. Almost all the associations in your domain model are going to be of this form.

Later, we’ll see how to map a many-to-one association to an association table.

The @ManyToOne annotation marks the "to one" side of the association, so a unidirectional many-to-one association looks like this:

class Book {
    @Id @GeneratedValue
    Long id;

    @ManyToOne(fetch=LAZY)
    Publisher publisher;

    ...
}

Here, the Book table has a foreign key column holding the identifier of the associated Publisher.

A very unfortunate misfeature of JPA is that @ManyToOne associations are fetched eagerly by default. This is almost never what we want. Almost all associations should be lazy. The only scenario in which fetch=EAGER makes sense is if we think there’s always a very high probability that the associated object will be found in the second-level cache. Whenever this isn’t the case, remember to explicitly specify fetch=LAZY.

Most of the time, we would like to be able to easily navigate our associations in both directions. We do need a way to get the Publisher of a given Book, but we would also like to be able to obtain all the Books belonging to a given publisher.

To make this association bidirectional, we need to add a collection-valued attribute to the Publisher class, and annotate it @OneToMany.

Hibernate needs to proxy unfetched associations at runtime. Therefore, the many-valued side must be declared using an interface type like Set or List, and never using a concrete type like HashSet or ArrayList.

To indicate clearly that this is a bidirectional association, and to reuse any mapping information already specified in the Book entity, we must use the mappedBy annotation member to refer back to Book.publisher.

@Entity
class Publisher {
    @Id @GeneratedValue
    Long id;

    @OneToMany(mappedBy="publisher")
    Set<Book> books;

    ...
}

The Publisher.books field is called the unowned side of the association.

Now, we passionately hate the stringly-typed mappedBy reference to the owning side of the association. Thankfully, the Metamodel Generator gives us a way to make it a bit more typesafe:

@OneToMany(mappedBy=Book_.PUBLISHER)  // get used to doing it this way!
Set<Book> books;

We’re going to use this approach for the rest of the Introduction.

To modify a bidirectional association, we must change the owning side.

Changes made to the unowned side of an association are never synchronized to the database. If we desire to change an association in the database, we must change it from the owning side. Here, we must set Book.publisher.

In fact, it’s often necessary to change both sides of a bidirectional association. For example, if the collection Publisher.books was stored in the second-level cache, we must also modify the collection, to ensure that the second-level cache remains synchronized with the database.

That said, it’s not a hard requirement to update the unowned side, at least if you’re sure you know what you’re doing.

In principle Hibernate does allow you to have a unidirectional one-to-many, that is, a @OneToMany with no matching @ManyToOne on the other side. In practice, this mapping is unnatural, and just doesn’t work very well. Avoid it.

Here we’ve used Set as the type of the collection, but Hibernate also allows the use of List or Collection here, with almost no difference in semantics. In particular, the List may not contain duplicate elements, and its order will not be persistent.

@OneToMany(mappedBy=Book_.PUBLISHER)
Collection<Book> books;

We’ll see how to map a collection with a persistent order much later.

Set, List, or Collection?

A one-to-many association mapped to a foreign key can never contain duplicate elements, so Set seems like the most semantically correct Java collection type to use here, and so that’s the conventional practice in the Hibernate community.

The catch associated with using a set is that we must carefully ensure that Book has a high-quality implementation of equals() and hashCode(). Now, that’s not necessarily a bad thing, since a quality equals() is independently useful.

But what if we used Collection or List instead? Then our code would be much less sensitive to how equals() and hashCode() were implemented.

In the past, we were perhaps too dogmatic in recommending the use of Set. Now? I guess we’re happy to let you guys decide. In hindsight, we could have done more to make clear that this was always a viable option.

3.17. One-to-one (first way)

The simplest sort of one-to-one association is almost exactly like a @ManyToOne association, except that it maps to a foreign key column with a UNIQUE constraint.

Later, we’ll see how to map a one-to-one association to an association table.

A one-to-one association must be annotated @OneToOne:

@Entity
class Author {
    @Id @GeneratedValue
    Long id;

    @OneToOne(optional=false, fetch=LAZY)
    Person author;

    ...
}

Here, the Author table has a foreign key column holding the identifier of the associated Person.

A one-to-one association often models a "type of" relationship. In our example, an Author is a type of Person. An alternative—and often more natural—way to represent "type of" relationships in Java is via entity class inheritance.

We can make this association bidirectional by adding a reference back to the Author in the Person entity:

@Entity
class Person {
    @Id @GeneratedValue
    Long id;

    @OneToOne(mappedBy = Author_.PERSON)
    Author author;

    ...
}

Person.author is the unowned side, because it’s the side marked mappedBy.

Lazy fetching for one-to-one associations

Notice that we did not declare the unowned end of the association fetch=LAZY. That’s because:

  1. not every Person has an associated Author, and

  2. the foreign key is held in the table mapped by Author, not in the table mapped by Person.

Therefore, Hibernate can’t tell if the reference from Person to Author is null without fetching the associated Author.

On the other hand, if every Person was an Author, that is, if the association were non-optional, we would not have to consider the possibility of null references, and we would map it like this:

@OneToOne(optional=false, mappedBy = Author_.PERSON, fetch=LAZY)
Author author;

This is not the only sort of one-to-one association.

3.18. One-to-one (second way)

An arguably more elegant way to represent such a relationship is to share a primary key between the two tables.

To use this approach, the Author class must be annotated like this:

@Entity
class Author {
    @Id
    Long id;

    @OneToOne(optional=false, fetch=LAZY)
    @MapsId
    Person author;

    ...
}

Notice that, compared with the previous mapping:

  • the @Id attribute is no longer a @GeneratedValue and,

  • instead, the author association is annotated @MapsId.

This lets Hibernate know that the association to Person is the source of primary key values for Author.

Here, there’s no extra foreign key column in the Author table, since the id column holds the identifier of Person. That is, the primary key of the Author table does double duty as the foreign key referring to the Person table.

The Person class doesn’t change. If the association is bidirectional, we annotate the unowned side @OneToOne(mappedBy = Author_.PERSON) just as before.

3.19. Many-to-many

A unidirectional many-to-many association is represented as a collection-valued attribute. It always maps to a separate association table in the database.

It tends to happen that a many-to-many association eventually turns out to be an entity in disguise.

Suppose we start with a nice clean many-to-many association between Author and Book. Later on, it’s quite likely that we’ll discover some additional information which comes attached to the association, so that the association table needs some extra columns.

For example, imagine that we needed to report the percentage contribution of each author to a book. That information naturally belongs to the association table. We can’t easily store it as an attribute of Book, nor as an attribute of Author.

When this happens, we need to change our Java model, usually introducing a new entity class which maps the association table directly. In our example, we might call this entity something like BookAuthorship, and it would have @OneToMany associations to both Author and Book, along with the contribution attribute.

We can evade the disruption occasioned by such "discoveries" by simply avoiding the use of @ManyToMany right from the start. There’s little downside to representing every—or at least almost every—logical many-to-many association using an intermediate entity.

A many-to-many association must be annotated @ManyToMany:

@Entity
class Book {
    @Id @GeneratedValue
    Long id;

    @ManyToMany
    Set<Author> authors;

    ...
}

If the association is bidirectional, we add a very similar-looking attribute to Book, but this time we must specify mappedBy to indicate that this is the unowned side of the association:

@Entity
class Book {
    @Id @GeneratedValue
    Long id;

    @ManyToMany(mappedBy=Author_.BOOKS)
    Set<Author> authors;

    ...
}

Remember, if we wish to the modify the collection we must change the owning side.

We’ve again used Sets to represent the association. As before, we have the option to use Collection or List. But in this case it does make a difference to the semantics of the association.

A many-to-many association represented as a Collection or List may contain duplicate elements. However, as before, the order of the elements is not persistent. That is, the collection is a bag, not a set.

3.20. Collections of basic values and embeddable objects

We’ve now seen the following kinds of entity attribute:

Kind of entity attribute Kind of reference Multiplicity Examples

Single-valued attribute of basic type

Non-entity

At most one

@Basic String name

Single-valued attribute of embeddable type

Non-entity

At most one

@Embedded Name name

Single-valued association

Entity

At most one

@ManyToOne Publisher publisher
@OneToOne Person person

Many-valued association

Entity

Zero or more

@OneToMany Set<Book> books
@ManyToMany Set<Author> authors

Scanning this taxonomy, you might ask: does Hibernate have multivalued attributes of basic or embeddable type?

Well, actually, we’ve already seen that it does, at least in two special cases. So first, lets recall that JPA treats byte[] and char[] arrays as basic types. Hibernate persists a byte[] or char[] array to a VARBINARY or VARCHAR column, respectively.

But in this section we’re really concerned with cases other than these two special cases. So then, apart from byte[] and char[], does Hibernate have multivalued attributes of basic or embeddable type?

And the answer again is that it does. Indeed, there are two different ways to handle such a collection, by mapping it:

  • to a column of SQL ARRAY type (assuming the database has an ARRAY type), or

  • to a separate table.

So we may expand our taxonomy with:

Kind of entity attribute Kind of reference Multiplicity Examples

byte[] and char[] arrays

Non-entity

Zero or more

byte[] image
char[] text

Collection of basic-typed elements

Non-entity

Zero or more

@Array String[] names
@ElementCollection Set<String> names

Collection of embeddable elements

Non-entity

Zero or more

@ElementCollection Set<Name> names

There’s actually two new kinds of mapping here: @Array mappings, and @ElementCollection mappings.

These sorts of mappings are overused.

There are situations where we think it’s appropriate to use a collection of basic-typed values in our entity class. But such situations are rare. Almost every many-valued relationship should map to a foreign key association between separate tables. And almost every table should be mapped by an entity class.

The features we’re about to meet in the next two subsections are used much more often by beginners than they’re used by experts. So if you’re a beginner, you’ll save yourself same hassle by staying away from these features for now.

We’ll talk about @Array mappings first.

3.21. Collections mapped to SQL arrays

Let’s consider a calendar event which repeats on certain days of the week. We might represent this in our Event entity as an attribute of type DayOfWeek[] or List<DayOfWeek>. Since the number of elements of this array or list is upper bounded by 7, this is a reasonable case for the use of an ARRAY-typed column. It’s hard to see much value in storing this collection in a separate table.

Learning to not hate SQL arrays

For a long time, we thought arrays were a kind of weird and warty thing to add to the relational model, but recently we’ve come to realize that this view was overly closed-minded. Indeed, we might choose to view SQL ARRAY types as a generalization of VARCHAR and VARBINARY to generic "element" types. And from this point of view, SQL arrays look quite attractive, at least for certain problems. If we’re comfortable mapping byte[] to VARBINARY(255), why would we shy away from mapping DayOfWeek[] to TINYINT ARRAY[7]?

Unfortunately, JPA doesn’t define a standard way to map SQL arrays, but here’s how we can do it in Hibernate:

@Entity
class Event {
    @Id @GeneratedValue
    Long id;
    ...
    @Array(length=7)
    DayOfWeek[] daysOfWeek;  // stored as a SQL ARRAY type
    ...
}

The @Array annotation is optional, but it’s important to limit the amount of storage space the database allocates to the ARRAY column. By writing @Array(length=7) here, we specified that DDL should be generated with the column type TINYINT ARRAY[7].

Just for fun, we used an enumerated type in the code above, but the array element time may be almost any basic type. For example, the Java array types String[], UUID[], double[], BigDecimal[], LocalDate[], and OffsetDateTime[] are all allowed, mapping to the SQL types VARCHAR(n) ARRAY, UUID ARRAY, FLOAT(53) ARRAY, NUMERIC(p,s) ARRAY, DATE ARRAY, and TIMESTAMP(p) WITH TIME ZONE ARRAY, respectively.

Now for the gotcha: not every database has a SQL ARRAY type, and some that do have an ARRAY type don’t allow it to be used as a column type.

In particular, neither DB2 nor SQL Server have array-typed columns. On these databases, Hibernate falls back to something much worse: it uses Java serialization to encode the array to a binary representation, and stores the binary stream in a VARBINARY column. Quite clearly, this is terrible. You can ask Hibernate to do something slightly less terrible by annotating the attribute @JdbcTypeCode(SqlTypes.JSON), so that the array is serialized to JSON instead of binary format. But at this point it’s better to just admit defeat and use an @ElementCollection instead.

Alternatively, we could store this array or list in a separate table.

3.22. Collections mapped to a separate table

JPA does define a standard way to map a collection to an auxiliary table: the @ElementCollection annotation.

@Entity
class Event {
    @Id @GeneratedValue
    Long id;
    ...
    @ElementCollection
    DayOfWeek[] daysOfWeek;  // stored in a dedicated table
    ...
}

Actually, we shouldn’t use an array here, since array types can’t be proxied, and so the JPA specification doesn’t even say they’re supported. Instead, we should use Set, List, or Map.

@Entity
class Event {
    @Id @GeneratedValue
    Long id;
    ...
    @ElementCollection
    List<DayOfWeek> daysOfWeek;  // stored in a dedicated table
    ...
}

Here, each collection element is stored as a separate row of the auxiliary table. By default, this table has the following definition:

create table Event_daysOfWeek (
    Event_id bigint not null,
    daysOfWeek tinyint check (daysOfWeek between 0 and 6),
    daysOfWeek_ORDER integer not null,
    primary key (Event_id, daysOfWeek_ORDER)
)

Which is fine, but it’s still a mapping we prefer to avoid.

@ElementCollection is one of our least-favorite features of JPA. Even the name of the annotation is bad.

The code above results in a table with three columns:

  • a foreign key of the Event table,

  • a TINYINT encoding the enum, and

  • an INTEGER encoding the ordering of elements in the array.

Instead of a surrogate primary key, it has a composite key comprising the foreign key of Event and the order column.

When—inevitably—we find that we need to add a fourth column to that table, our Java code must change completely. Most likely, we’ll realize that we need to add a separate entity after all. So this mapping isn’t very robust in the face of minor changes to our data model.

There’s much more we could say about "element collections", but we won’t say it, because we don’t want to hand you the gun you’ll shoot your foot with.

3.23. Summary of annotations

Let’s pause to remember the annotations we’ve met so far.

Table 16. Declaring entities and embeddable types
Annotation Purpose JPA-standard

@Entity

Declare an entity class

@MappedSuperclass

Declare a non-entity class with mapped attributes inherited by an entity

@Embeddable

Declare an embeddable type

@IdClass

Declare the identifier class for an entity with multiple @Id attributes

Table 17. Declaring basic and embedded attributes
Annotation Purpose JPA-standard

@Id

Declare a basic-typed identifier attribute

@Version

Declare a version attribute

@Basic

Declare a basic attribute

Default

@EmbeddedId

Declare an embeddable-typed identifier attribute

@Embedded

Declare an embeddable-typed attribute

Inferred

@Enumerated

Declare an enum-typed attribute and specify how it is encoded

Inferred

@Array

Declare that an attribute maps to a SQL ARRAY, and specify the length

Inferred

@ElementCollection

Declare that a collection is mapped to a dedicated table

Table 18. Converters and compositional basic types
Annotation Purpose JPA-standard

@Converter

Register an AttributeConverter

@Convert

Apply a converter to an attribute

@JavaType

Explicitly specify an implementation of JavaType for a basic attribute

@JdbcType

Explicitly specify an implementation of JdbcType for a basic attribute

@JdbcTypeCode

Explicitly specify a JDBC type code used to determine the JdbcType for a basic attribute

@JavaTypeRegistration

Register a JavaType for a given Java type

@JdbcTypeRegistration

Register a JdbcType for a given JDBC type code

Table 19. System-generated identifiers
Annotation Purpose JPA-standard

@GeneratedValue

Specify that an identifier is system-generated

@SequenceGenerator

Define an id generated backed by on a database sequence

@TableGenerator

Define an id generated backed by a database table

@IdGeneratorType

Declare an annotation that associates a custom Generator with each @Id attribute it annotates

@ValueGenerationType

Declare an annotation that associates a custom Generator with each @Basic attribute it annotates

Table 20. Declaring entity associations
Annotation Purpose JPA-standard

@ManyToOne

Declare the single-valued side of a many-to-one association (the owning side)

@OneToMany

Declare the many-valued side of a many-to-one association (the unowned side)

@ManyToMany

Declare either side of a many-to-many association

@OneToOne

Declare either side of a one-to-one association

@MapsId

Declare that the owning side of a @OneToOne association maps the primary key column

Phew! That’s already a lot of annotations, and we have not even started with the annotations for O/R mapping!

3.24. equals() and hashCode()

Entity classes should override equals() and hashCode(), especially when associations are represented as sets.

People new to Hibernate or JPA are often confused by exactly which fields should be included in the hashCode(). And people with more experience often argue quite religiously that one or another approach is the only right way. The truth is, there’s no unique right way to do it, but there are some constraints. So please keep the following principles in mind:

  • You should not include a mutable field in the hashcode, since that would require rehashing every collection containing the entity whenever the field is mutated.

  • It’s not completely wrong to include a generated identifier (surrogate key) in the hashcode, but since the identifier is not generated until the entity instance is made persistent, you must take great care to not add it to any hashed collection before the identifier is generated. We therefore advise against including any database-generated field in the hashcode.

It’s OK to include any immutable, non-generated field in the hashcode.

We therefore recommend identifying a natural key for each entity, that is, a combination of fields that uniquely identifies an instance of the entity, from the perspective of the data model of the program. The natural key should correspond to a unique constraint on the database, and to the fields which are included in equals() and hashCode().

In this example, the equals() and hashCode() methods agree with the @NaturalId annotation:

@Entity
class Book {

    @Id @GeneratedValue
    Long id;

    @NaturalId
    @Basic(optional=false)
    String isbn;

    String getIsbn() {
        return isbn;
    }

    ...

    @Override
    public boolean equals(Object other) {
        return other instanceof Book                   // check type with instanceof, not getClass()
            && ((Book) other).getIsbn().equals(isbn);  // compare natural ids
    }
    @Override
    public int hashCode() {
        return isbn.hashCode();  // hashcode based on the natural id
    }
}

That said, an implementation of equals() and hashCode() based on the generated identifier of the entity can work if you’re careful.

Your implementation of equals() must be written to accommodate the possibility that the object passed to the equals() might be a proxy. Therefore, you should use instanceof, not getClass() to check the type of the argument, and should access fields of the passed entity via its accessor methods.

4. Object/relational mapping

Given a domain model—that is, a collection of entity classes decorated with all the fancy annotations we just met in the previous chapter—Hibernate will happily go away and infer a complete relational schema, and even export it to your database if you ask politely.

The resulting schema will be entirely sane and reasonable, though if you look closely, you’ll find some flaws. For example, every VARCHAR column will have the same length, VARCHAR(255).

But the process I just described—which we call top down mapping—simply doesn’t fit the most common scenario for the use of O/R mapping. It’s only rarely that the Java classes precede the relational schema. Usually, we already have a relational schema, and we’re constructing our domain model around the schema. This is called bottom up mapping.

Developers often refer to a pre-existing relational database as "legacy" data. This tends to conjure images of bad old "legacy apps" written in COBOL or something. But legacy data is valuable, and learning to work with it is important.

Especially when mapping bottom up, we often need to customize the inferred object/relational mappings. This is a somewhat tedious topic, and so we don’t want to spend too many words on it. Instead, we’ll quickly skim the most important mapping annotations.

Hibernate SQL case convention

Computers have had lowercase letters for rather a long time now. Most developers learned long ago that text written in MixedCase, camelCase, or even snake_case is easier to read than text written in SHOUTYCASE. This is just as true of SQL as it is of any other language.

Therefore, for over twenty years, the convention on the Hibernate project has been that:

  • query language identifiers are written in lowercase,

  • table names are written in MixedCase, and

  • column names are written in camelCase.

That is to say, we simply adopted Java’s excellent conventions and applied them to SQL.

Now, there’s no way we can force you to follow this convention, even if we wished to. Hell, you can easily write a PhysicalNamingStrategy which makes table and column names ALL UGLY AND SHOUTY LIKE THIS IF YOU PREFER. But, by default, it’s the convention Hibernate follows, and it’s frankly a pretty reasonable one.

4.1. Mapping entity inheritance hierarchies

In Entity class inheritance we saw that entity classes may exist within an inheritance hierarchy. There’s three basic strategies for mapping an entity hierarchy to relational tables. Let’s put them in a table, so we can more easily compare the points of difference between them.

Table 21. Entity inheritance mapping strategies
Strategy Mapping Polymorphic queries Constraints Normalization When to use it

SINGLE_TABLE

Map every class in the hierarchy to the same table, and uses the value of a discriminator column to determine which concrete class each row represents.

To retrieve instances of a given class, we only need to query the one table.

Attributes declared by subclasses map to columns without NOT NULL constraints. 💀

Any association may have a FOREIGN KEY constraint. 🤓

Subclass data is denormalized. 🧐

Works well when subclasses declare few or no additional attributes.

JOINED

Map every class in the hierarchy to a separate table, but each table only maps the attributes declared by the class itself.

Optionally, a discriminator column may be used.

To retrieve instances of a given class, we must JOIN the table mapped by the class with:

  • all tables mapped by its superclasses and

  • all tables mapped by its subclasses.

Any attribute may map to a column with a NOT NULL constraint. 🤓

Any association may have a FOREIGN KEY constraint. 🤓

The tables are normalized. 🤓

The best option when we care a lot about constraints and normalization.

TABLE_PER_CLASS

Map every concrete class in the hierarchy to a separate table, but denormalize all inherited attributes into the table.

To retrieve instances of a given class, we must take a UNION over the table mapped by the class and the tables mapped by its subclasses.

Associations targeting a superclass cannot have a corresponding FOREIGN KEY constraint in the database. 💀💀

Any attribute may map to a column with a NOT NULL constraint. 🤓

Superclass data is denormalized. 🧐

Not very popular.

From a certain point of view, competes with @MappedSuperclass.

The three mapping strategies are enumerated by InheritanceType. We specify an inheritance mapping strategy using the @Inheritance annotation.

For mappings with a discriminator column, we should:

  • specify the discriminator column name and type by annotating the root entity @DiscriminatorColumn, and

  • specify the values of this discriminator by annotating each entity in the hierarchy @DiscriminatorValue.

For single table inheritance we always need a discriminator:

@Entity
@DiscriminatorColumn(discriminatorType=CHAR, name="kind")
@DiscriminatorValue('P')
class Person { ... }

@Entity
@DiscriminatorValue('A')
class Author { ... }

We don’t need to explicitly specify @Inheritance(strategy=SINGLE_TABLE), since that’s the default.

For JOINED inheritance we don’t need a discriminator:

@Entity
@Inheritance(strategy=JOINED)
class Person { ... }

@Entity
class Author { ... }

However, we can add a discriminator column if we like, and in that case the generated SQL for polymorphic queries will be slightly simpler.

Similarly, for TABLE_PER_CLASS inheritance we have:

@Entity
@Inheritance(strategy=TABLE_PER_CLASS)
class Person { ... }

@Entity
class Author { ... }

Hibernate doesn’t allow discriminator columns for TABLE_PER_CLASS inheritance mappings, since they would make no sense, and offer no advantage.

Notice that in this last case, a polymorphic association like:

@ManyToOne Person person;

is a bad idea, since it’s impossible to create a foreign key constraint that targets both mapped tables.

4.2. Mapping to tables

The following annotations specify exactly how elements of the domain model map to tables of the relational model:

Table 22. Annotations for mapping tables
Annotation Purpose

@Table

Map an entity class to its primary table

@SecondaryTable

Define a secondary table for an entity class

@JoinTable

Map a many-to-many or many-to-one association to its association table

@CollectionTable

Map an @ElementCollection to its table

The first two annotations are used to map an entity to its primary table and, optionally, one or more secondary tables.

4.3. Mapping entities to tables

By default, an entity maps to a single table, which may be specified using @Table:

@Entity
@Table(name="People")
class Person { ... }

However, the @SecondaryTable annotation allows us to spread its attributes across multiple secondary tables.

@Entity
@Table(name="Books")
@SecondaryTable(name="Editions")
class Book { ... }

The @Table annotation can do more than just specify a name:

Table 23. @Table annotation members
Annotation member Purpose

name

The name of the mapped table

schema 💀

The schema to which the table belongs

catalog 💀

The catalog to which the table belongs

uniqueConstraints

One or more @UniqueConstraint annotations declaring multi-column unique constraints

indexes

One or more @Index annotations each declaring an index

It only makes sense to explicitly specify the schema in annotations if the domain model is spread across multiple schemas.

Otherwise, it’s a bad idea to hardcode the schema (or catalog) in a @Table annotation. Instead:

  • set the configuration property hibernate.default_schema (or hibernate.default_catalog), or

  • simply specify the schema in the JDBC connection URL.

The @SecondaryTable annotation is even more interesting:

Table 24. @SecondaryTable annotation members
Annotation member Purpose

name

The name of the mapped table

schema 💀

The schema to which the table belongs

catalog 💀

The catalog to which the table belongs

uniqueConstraints

One or more @UniqueConstraint annotations declaring multi-column unique constraints

indexes

One or more @Index annotations each declaring an index

pkJoinColumns

One or more @PrimaryKeyJoinColumn annotations, specifying primary key column mappings

foreignKey

An @ForeignKey annotation specifying the name of the FOREIGN KEY constraint on the @PrimaryKeyJoinColumns

Using @SecondaryTable on a subclass in a SINGLE_TABLE entity inheritance hierarchy gives us a sort of mix of SINGLE_TABLE with JOINED inheritance.

4.4. Mapping associations to tables

The @JoinTable annotation specifies an association table, that is, a table holding foreign keys of both associated entities. This annotation is usually used with @ManyToMany associations:

@Entity
class Book {
    ...

    @ManyToMany
    @JoinTable(name="BooksAuthors")
    Set<Author> authors;

    ...
}

But it’s even possible to use it to map a @ManyToOne or @OneToOne association to an association table.

@Entity
class Book {
    ...

    @ManyToOne(fetch=LAZY)
    @JoinTable(name="BookPublisher")
    Publisher publisher;

    ...
}

Here, there should be a UNIQUE constraint on one of the columns of the association table.

@Entity
class Author {
    ...

    @OneToOne(optional=false, fetch=LAZY)
    @JoinTable(name="AuthorPerson")
    Person author;

    ...
}

Here, there should be a UNIQUE constraint on both columns of the association table.

Table 25. @JoinTable annotation members
Annotation member Purpose

name

The name of the mapped association table

schema 💀

The schema to which the table belongs

catalog 💀

The catalog to which the table belongs

uniqueConstraints

One or more @UniqueConstraint annotations declaring multi-column unique constraints

indexes

One or more @Index annotations each declaring an index

joinColumns

One or more @JoinColumn annotations, specifying foreign key column mappings to the table of the owning side

inverseJoinColumns

One or more @JoinColumn annotations, specifying foreign key column mappings to the table of the unowned side

foreignKey

An @ForeignKey annotation specifying the name of the FOREIGN KEY constraint on the joinColumnss

inverseForeignKey

An @ForeignKey annotation specifying the name of the FOREIGN KEY constraint on the inverseJoinColumnss

To better understand these annotations, we must first discuss column mappings in general.

4.5. Mapping to columns

These annotations specify how elements of the domain model map to columns of tables in the relational model:

Table 26. Annotations for mapping columns
Annotation Purpose

@Column

Map an attribute to a column

@JoinColumn

Map an association to a foreign key column

@PrimaryKeyJoinColumn

Map the primary key used to join a secondary table with its primary, or a subclass table in JOINED inheritance with its root class table

@OrderColumn

Specifies a column that should be used to maintain the order of a List.

@MapKeyColumn

Specified a column that should be used to persist the keys of a Map.

We use the @Column annotation to map basic attributes.

4.6. Mapping basic attributes to columns

The @Column annotation is not only useful for specifying the column name.

Table 27. @Column annotation members
Annotation member Purpose

name

The name of the mapped column

table

The name of the table to which this column belongs

length

The length of a VARCHAR, CHAR, or VARBINARY column type

precision

The decimal digits of precision of a FLOAT, DECIMAL, NUMERIC, or TIME, or TIMESTAMP column type

scale

The scale of a DECIMAL or NUMERIC column type, the digits of precision that occur to the right of the decimal point

unique

Whether the column has a UNIQUE constraint

nullable

Whether the column has a NOT NULL constraint

insertable

Whether the column should appear in generated SQL INSERT statements

updatable

Whether the column should appear in generated SQL UPDATE statements

columnDefinition 💀

A DDL fragment that should be used to declare the column

We no longer recommend the use of columnDefinition since it results in unportable DDL. Hibernate has much better ways to customize the generated DDL using techniques that result in portable behavior across different databases.

Here we see four different ways to use the @Column annotation:

@Entity
@Table(name="Books")
@SecondaryTable(name="Editions")
class Book {
    @Id @GeneratedValue
    @Column(name="bookId") // customize column name
    Long id;

    @Column(length=100, nullable=false) // declare column as VARCHAR(100) NOT NULL
    String title;

    @Column(length=17, unique=true, nullable=false) // declare column as VARCHAR(17) NOT NULL UNIQUE
    String isbn;

    @Column(table="Editions", updatable=false) // column belongs to the secondary table, and is never updated
    int edition;
}

We don’t use @Column to map associations.

4.7. Mapping associations to foreign key columns

The @JoinColumn annotation is used to customize a foreign key column.

Table 28. @JoinColumn annotation members
Annotation member Purpose

name

The name of the mapped foreign key column

table

The name of the table to which this column belongs

referencedColumnName

The name of the column to which the mapped foreign key column refers

unique

Whether the column has a UNIQUE constraint

nullable

Whether the column has a NOT NULL constraint

insertable

Whether the column should appear in generated SQL INSERT statements

updatable

Whether the column should appear in generated SQL UPDATE statements

columnDefinition 💀

A DDL fragment that should be used to declare the column

foreignKey

A @ForeignKey annotation specifying the name of the FOREIGN KEY constraint

A foreign key column doesn’t necessarily have to refer to the primary key of the referenced table. It’s quite acceptable for the foreign key to refer to any other unique key of the referenced entity, even to a unique key of a secondary table.

Here we see how to use @JoinColumn to define a @ManyToOne association mapping a foreign key column which refers to the @NaturalId of Book:

@Entity
@Table(name="Items")
class Item {
    ...

    @ManyToOne(optional=false)  // implies nullable=false
    @JoinColumn(name = "bookIsbn", referencedColumnName = "isbn",  // a reference to a non-PK column
                foreignKey = @ForeignKey(name="ItemsToBooksBySsn")) // supply a name for the FK constraint
    Book book;

    ...
}

In case this is confusing:

  • bookIsbn is the name of the foreign key column in the Items table,

  • it refers to a unique key isbn in the Books table, and

  • it has a foreign key constraint named ItemsToBooksBySsn.

Note that the foreignKey member is completely optional and only affects DDL generation.

If you don’t supply an explicit name using @ForeignKey, Hibernate will generate a quite ugly name. The reason for this is that the maximum length of foreign key names on some databases is extremely constrained, and we need to avoid collisions. To be fair, this is perfectly fine if you’re only using the generated DDL for testing.

For composite foreign keys we might have multiple @JoinColumn annotations:

@Entity
@Table(name="Items")
class Item {
    ...

    @ManyToOne(optional=false)
    @JoinColumn(name = "bookIsbn", referencedColumnName = "isbn")
    @JoinColumn(name = "bookPrinting", referencedColumnName = "printing")
    Book book;

    ...
}

If we need to specify the @ForeignKey, this starts to get a bit messy:

@Entity
@Table(name="Items")
class Item {
    ...

    @ManyToOne(optional=false)
    @JoinColumns(value = {@JoinColumn(name = "bookIsbn", referencedColumnName = "isbn"),
                          @JoinColumn(name = "bookPrinting", referencedColumnName = "printing")},
                 foreignKey = @ForeignKey(name="ItemsToBooksBySsn"))
    Book book;

    ...
}

For associations mapped to a @JoinTable, fetching the association requires two joins, and so we must declare the @JoinColumns inside the @JoinTable annotation:

@Entity
class Book {
    @Id @GeneratedValue
    Long id;

    @ManyToMany
    @JoinTable(joinColumns=@JoinColumn(name="bookId"),
               inverseJoinColumns=@joinColumn(name="authorId"),
               foreignKey=@ForeignKey(name="BooksToAuthors"))
    Set<Author> authors;

    ...
}

Again, the foreignKey member is optional.

For mapping a @OneToOne association to a primary key with @MapsId, Hibernate lets us use either @JoinColumn or @PrimaryKeyJoinColumn.

@Entity
class Author {
    @Id
    Long id;

    @OneToOne(optional=false, fetch=LAZY)
    @MapsId
    @PrimaryKeyJoinColumn(name="personId")
    Person author;

    ...
}

Arguably, the use of @PrimaryKeyJoinColumn is clearer.

4.8. Mapping primary key joins between tables

The @PrimaryKeyJoinColumn is a special-purpose annotation for mapping:

  • the primary key column of a @SecondaryTable—which is also a foreign key referencing the primary table, or

  • the primary key column of the primary table mapped by a subclass in a JOINED inheritance hierarchy—which is also a foreign key referencing the primary table mapped by the root entity.

Table 29. @PrimaryKeyJoinColumn annotation members
Annotation member Purpose

name

The name of the mapped foreign key column

referencedColumnName

The name of the column to which the mapped foreign key column refers

columnDefinition 💀

A DDL fragment that should be used to declare the column

foreignKey

A @ForeignKey annotation specifying the name of the FOREIGN KEY constraint

When mapping a subclass table primary key, we place the @PrimaryKeyJoinColumn annotation on the entity class:

@Entity
@Table(name="People")
@Inheritance(strategy=JOINED)
class Person { ... }

@Entity
@Table(name="Authors")
@PrimaryKeyJoinColumn(name="personId") // the primary key of the Authors table
class Author { ... }

But to map a secondary table primary key, the @PrimaryKeyJoinColumn annotation must occur inside the @SecondaryTable annotation:

@Entity
@Table(name="Books")
@SecondaryTable(name="Editions",
                pkJoinColumns = @PrimaryKeyJoinColumn(name="bookId")) // the primary key of the Editions table
class Book {
    @Id @GeneratedValue
    @Column(name="bookId") // the name of the primary key of the Books table
    Long id;

    ...
}

4.9. Column lengths and adaptive column types

Hibernate automatically adjusts the column type used in generated DDL based on the column length specified by the @Column annotation. So we don’t usually need to explicitly specify that a column should be of type TEXT or CLOB—or worry about the parade of TINYTEXT, MEDIUMTEXT, TEXT, LONGTEXT types on MySQL—because Hibernate will automatically select one of those types if required to accommodate a string of the length we specify.

The constant values defined in the class Length are very helpful here:

Table 30. Predefined column lengths
Constant Value Description

DEFAULT

255

The default length of a VARCHAR or VARBINARY column when none is explicitly specified

LONG

32600

The largest column length for a VARCHAR or VARBINARY that is allowed on every database Hibernate supports

LONG16

32767

The maximum length that can be represented using 16 bits (but this length is too large for a VARCHAR or VARBINARY column on for some database)

LONG32

2147483647

The maximum length for a Java string

We can use these constants in the @Column annotation:

@Column(length=LONG)
String text;

@Column(length=LONG32)
byte[] binaryData;

This is usually all you need to do to make use of large object types in Hibernate.

4.10. LOBs

JPA provides a @Lob annotation which specifies that a field should be persisted as a BLOB or CLOB.

Semantics of the @Lob annotation

What the spec actually says is that the field should be persisted

…​as a large object to a database-supported large object type.

It’s quite unclear what this means, and the spec goes on to say that

…​the treatment of the Lob annotation is provider-dependent…​

which doesn’t help much.

Hibernate interprets this annotation in what we think is the most reasonable way. In Hibernate, an attribute annotated @Lob will be written to JDBC using the setClob() or setBlob() method of PreparedStatement, and will be read from JDBC using the getClob() or getBlob() method of ResultSet.

Now, the use of these JDBC methods is usually unnecessary! JDBC drivers are perfectly capable of converting between String and CLOB or between byte[] and BLOB. So unless you specifically need to use these JDBC LOB APIs, you don’t need the @Lob annotation.

Instead, as we just saw in Column lengths and adaptive column types, all you need is to specify a large enough column length to accommodate the data you plan to write to that column.

You should usually write this:

@Column(length=LONG32) // good, correct column type inferred
String text;

instead of this:

@Lob // almost always unnecessary
String text;

This is particularly true for PostgreSQL.

Unfortunately, the driver for PostgreSQL doesn’t allow BYTEA or TEXT columns to be read via the JDBC LOB APIs.

This limitation of the Postgres driver has resulted in a whole cottage industry of bloggers and stackoverflow question-answerers recommending convoluted ways to hack the Hibernate Dialect for Postgres to allow an attribute annotated @Lob to be written using setString() and read using getString().

But simply removing the @Lob annotation has exactly the same effect.

Conclusion:

  • on PostgreSQL, @Lob always means the OID type,

  • @Lob should never be used to map columns of type BYTEA or TEXT, and

  • please don’t believe everything you read on stackoverflow.

Finally, as an alternative, Hibernate lets you declare an attribute of type java.sql.Blob or java.sql.Clob.

@Entity
class Book {
    ...
    Clob text;
    Blob coverArt;
    ....
}

The advantage is that a java.sql.Clob or java.sql.Blob can in principle index up to 263 characters or bytes, much more data than you can fit in a Java String or byte[] array (or in your computer).

To assign a value to these fields, we’ll need to use a LobHelper. We can get one from the Session:

LobHelper helper = session.getLobHelper();
book.text = helper.createClob(text);
book.coverArt = helper.createBlob(image);

In principle, the Blob and Clob objects provide efficient ways to read or stream LOB data from the server.

Book book = session.find(Book.class, bookId);
String text = book.text.getSubString(1, textLength);
InputStream bytes = book.images.getBinaryStream();

Of course, the behavior here depends very much on the JDBC driver, and so we really can’t promise that this is a sensible thing to do on your database.

4.11. Mapping embeddable types to UDTs or to JSON

There’s a couple of alternative ways to represent an embeddable type on the database side.

Embeddables as UDTs

First, a really nice option, at least in the case of Java record types, and for databases which support user-defined types (UDTs), is to define a UDT which represents the record type. Hibernate 6 makes this really easy. Just annotate the record type, or the attribute which holds a reference to it, with the new @Struct annotation:

@Embeddable
@Struct(name="PersonName")
record Name(String firstName, String middleName, String lastName) {}
@Entity
class Person {
    ...
    Name name;
    ...
}

This results in the following UDT:

create type PersonName as (firstName varchar(255), middleName varchar(255), lastName varchar(255))

And the name column of the Author table will have the type PersonName.

Embeddables to JSON

A second option that’s available is to map the embeddable type to a JSON (or JSONB) column. Now, this isn’t something we would exactly recommend if you’re defining a data model from scratch, but it’s at least useful for mapping pre-existing tables with JSON-typed columns. Since embeddable types are nestable, we can map some JSON formats this way, and even query JSON properties using HQL.

At this time, JSON arrays are not supported!

To map an attribute of embeddable type to JSON, we must annotate the attribute @JdbcTypeCode(SqlTypes.JSON), instead of annotating the embeddable type. But the embeddable type Name should still be annotated @Embeddable if we want to query its attributes using HQL.

@Embeddable
record Name(String firstName, String middleName, String lastName) {}
@Entity
class Person {
    ...
    @JdbcTypeCode(SqlTypes.JSON)
    Name name;
    ...
}

We also need to add Jackson or an implementation of JSONB—for example, Yasson—to our runtime classpath. To use Jackson we could add this line to our Gradle build:

runtimeOnly 'com.fasterxml.jackson.core:jackson-databind:{jacksonVersion}'

Now the name column of the Author table will have the type jsonb, and Hibernate will automatically use Jackson to serialize a Name to and from JSON format.

4.12. Summary of SQL column type mappings

So, as we’ve seen, there are quite a few annotations that affect the mapping of Java types to SQL column types in DDL. Here we summarize the ones we’ve just seen in the second half of this chapter, along with some we already mentioned in earlier chapters.

Table 31. Annotations for mapping SQL column types
Annotation Interpretation

@Enumerated

Specify how an enum type should be persisted

@Nationalized

Use a nationalized character type: NCHAR, NVARCHAR, or NCLOB

@Lob 💀

Use JDBC LOB APIs to read and write the annotated attribute

@Array

Map a collection to a SQL ARRAY type of the specified length

@Struct

Map an embeddable to a SQL UDT with the given name

@TimeZoneStorage

Specify how the time zone information should be persisted

@JdbcType or @JdbcTypeCode

Use an implementation of JdbcType to map an arbitrary SQL type

@Collate

Specify a collation for a column

In addition, there are some configuration properties which have a global affect on how basic types map to SQL column types:

Table 32. Type mapping settings
Configuration property name Purpose

hibernate.use_nationalized_character_data

Enable use of nationalized character types by default

hibernate.type.preferred_boolean_jdbc_type

Specify the default SQL column type for mapping boolean

hibernate.type.preferred_uuid_jdbc_type

Specify the default SQL column type for mapping UUID

hibernate.type.preferred_duration_jdbc_type

Specify the default SQL column type for mapping Duration

hibernate.type.preferred_instant_jdbc_type

Specify the default SQL column type for mapping Instant

hibernate.timezone.default_storage

Specify the default strategy for storing time zone information

These are global settings and thus quite clumsy. We recommend against messing with any of these settings unless you have a really good reason for it.

There’s one more topic we would like to cover in this chapter.

4.13. Mapping to formulas

Hibernate lets us map an attribute of an entity to a SQL formula involving columns of the mapped table. Thus, the attribute is a sort of "derived" value.

Table 33. Annotations for mapping formulas
Annotation Purpose

@Formula

Map an attribute to a SQL formula

@JoinFormula

Map an association to a SQL formula

@DiscriminatorFormula

Use a SQL formula as the discriminator in single table inheritance.

For example:

@Entity
class Order {
    ...
    @Column(name = "sub_total", scale=2, precision=8)
    BigDecimal subTotal;

    @Column(name = "tax", scale=4, precision=4)
    BigDecimal taxRate;

    @Formula("sub_total * (1.0 + tax)")
    BigDecimal totalWithTax;
    ...
}

4.14. Derived Identity

An entity has a derived identity if it inherits part of its primary key from an associated "parent" entity. We’ve already met a kind of degenerate case of derived identity when we talked about one-to-one associations with a shared primary key.

But a @ManyToOne association may also form part of a derived identity. That is to say, there could be a foreign key column or columns included as part of the composite primary key. There’s three different ways to represent this situation on the Java side of things:

  • using @IdClass without @MapsId,

  • using @IdClass with @MapsId, or

  • using @EmbeddedId with @MapsId.

Let’s suppose we have a Parent entity class defined as follows:

@Entity
class Parent {
    @Id
    Long parentId;

    ...
}

The parentId field holds the primary key of the Parent table, which will also form part of the composite primary key of every Child belonging to the Parent.

First way

In the first, slightly simpler approach, we define an @IdClass to represent the primary key of Child:

class DerivedId {
    Long parent;
    String childId;

    // constructors, equals, hashcode, etc
    ...
}

And a Child entity class with a @ManyToOne association annotated @Id:

@Entity
@IdClass(DerivedId.class)
class Child {
    @Id
    String childId;

    @Id @ManyToOne
    @JoinColumn(name="parentId")
    Parent parent;

    ...
}

Then the primary key of the Child table comprises the columns (childId,parentId).

Second way

This is fine, but sometimes it’s nice to have a field for each element of the primary key. We may use the @MapsId annotation we met earlier:

@Entity
@IdClass(DerivedId.class)
class Child {
    @Id
    Long parentId;
    @Id
    String childId;

    @ManyToOne
    @MapsId(Child_.PARENT_ID) // typesafe reference to Child.parentId
    @JoinColumn(name="parentId")
    Parent parent;

    ...
}

We’re using the approach we saw previously to refer to the parentId property of Child in a typesafe way.

Note that we must place column mapping information on the association annotated @MapsId, not on the @Id field.

We must slightly modify our @IdClass so that field names align:

class DerivedId {
    Long parentId;
    String childId;

    // constructors, equals, hashcode, etc
    ...
}

Third way

The third alternative is to redefine our @IdClass as an @Embeddable. We don’t actually need to change the DerivedId class, but we do need to add the annotation.

@Embeddable
class DerivedId {
    Long parentId;
    String childId;

    // constructors, equals, hashcode, etc
    ...
}

Then we may use @EmbeddedId in Child:

@Entity
class Child {
    @EmbeddedId
    DerivedId id;

    @ManyToOne
    @MapsId(DerivedId_.PARENT_ID) // typesafe reference to DerivedId.parentId
    @JoinColumn(name="parentId")
    Parent parent;

    ...
}

The choice between @IdClass and @EmbeddedId boils down to taste. The @EmbeddedId is perhaps a little DRYer.

4.15. Adding constraints

Database constraints are important. Even if you’re sure that your program has no bugs 🧐, it’s probably not the only program with access to the database. Constraints help ensure that different programs (and human administrators) play nicely with each other.

Hibernate adds certain constraints to generated DDL automatically: primary key constraints, foreign key constraints, and some unique constraints. But it’s common to need to:

  • add additional unique constraints,

  • add check constraints, or

  • customize the name of a foreign key constraint.

We’ve already seen how to use @ForeignKey to specify the name of a foreign key constraint.

There’s two ways to add a unique constraint to a table:

  • using @Column(unique=true) to indicate a single-column unique key, or

  • using the @UniqueConstraint annotation to define a uniqueness constraint on a combination of columns.

@Entity
@Table(uniqueConstraints=@UniqueConstraint(columnNames={"title", "year", "publisher_id"}))
class Book { ... }

This annotation looks a bit ugly perhaps, but it’s actually useful even as documentation.

The @Check annotation adds a check constraint to the table.

@Entity
@Check(name="ValidISBN", constraints="length(isbn)=13")
class Book { ... }

The @Check annotation is commonly used at the field level:

@Id @Check(constraints="length(isbn)=13")
String isbn;

5. Interacting with the database

To interact with the database, that is, to execute queries, or to insert, update, or delete data, we need an instance of one of the following objects:

  • a JPA EntityManager,

  • a Hibernate Session, or

  • a Hibernate StatelessSession.

The Session interface extends EntityManager, and so the only difference between the two interfaces is that Session offers a few more operations.

Actually, in Hibernate, every EntityManager is a Session, and you can narrow it like this:

Session session = entityManager.unwrap(Session.class);

An instance of Session (or of EntityManager) is a stateful session. It mediates the interaction between your program and the database via a operations on a persistence context.

In this chapter, we’re not going to talk much about StatelessSession. We’ll come back to this very useful API when we talk about performance. What you need to know for now is that a stateless session doesn’t have a persistence context.

Still, we should let you know that some people prefer to use StatelessSession everywhere. It’s a simpler programming model, and lets the developer interact with the database more directly.

Stateful sessions certainly have their advantages, but they’re more difficult to reason about, and when something goes wrong, the error messages can be more difficult to understand.

5.1. Persistence Contexts

A persistence context is a sort of cache; we sometimes call it the "first-level cache", to distinguish it from the second-level cache. For every entity instance read from the database within the scope of a persistence context, and for every new entity made persistent within the scope of the persistence context, the context holds a unique mapping from the identifier of the entity instance to the instance itself.

Thus, an entity instance may be in one of three states with respect to a given persistence context:

  1. transient — never persistent, and not associated with the persistence context,

  2. persistent — currently associated with the persistence context, or

  3. detached — previously persistent in another session, but not currently associated with this persistence context.

Entity lifecycle

At any given moment, an instance may be associated with at most one persistence context.

The lifetime of a persistence context usually corresponds to the lifetime of a transaction, though it’s possible to have a persistence context that spans several database-level transactions that form a single logical unit of work.

A persistence context—that is, a Session or EntityManager—absolutely positively must not be shared between multiple threads or between concurrent transactions.

If you accidentally leak a session across threads, you will suffer.

Container-managed persistence contexts

In a container environment, the lifecycle of a persistence context scoped to the transaction will usually be managed for you.

There are several reasons we like persistence contexts.

  1. They help avoid data aliasing: if we modify an entity in one section of code, then other code executing within the same persistence context will see our modification.

  2. They enable automatic dirty checking: after modifying an entity, we don’t need to perform any explicit operation to ask Hibernate to propagate that change back to the database. Instead, the change will be automatically synchronized with the database when the session is flushed.

  3. They can improve performance by avoiding a trip to the database when a given entity instance is requested repeatedly in a given unit of work.

  4. They make it possible to transparently batch together multiple database operations.

A persistence context also allows us to detect circularities when performing operations on graphs of entities. (Even in a stateless session, we need some sort of temporary cache of the entity instances we’ve visited while executing a query.)

On the other hand, stateful sessions come with some very important restrictions, since:

  • persistence contexts aren’t threadsafe, and can’t be shared across threads, and

  • a persistence context can’t be reused across unrelated transactions, since that would break the isolation and atomicity of the transactions.

Furthermore, a persistence context holds a hard references to all its entities, preventing them from being garbage collected. Thus, the session must be discarded once a unit of work is complete.

If you don’t completely understand the previous passage, go back and re-read it until you do. A great deal of human suffering has resulted from users mismanaging the lifecycle of the Hibernate Session or JPA EntityManager.

We’ll conclude by noting that whether a persistence context helps or harms the performance of a given unit of work depends greatly on the nature of the unit of work. For this reason Hibernate provides both stateful and stateless sessions.

5.2. Creating a session

Sticking with standard JPA-defined APIs, we saw how to obtain an EntityManagerFactory in Configuration using JPA XML. It’s quite unsurprising that we may use this object to create an EntityManager:

EntityManager entityManager = entityManagerFactory.createEntityManager();

When we’re finished with the EntityManager, we should explicitly clean it up:

entityManager.close();

On the other hand, if we’re starting from a SessionFactory, as described in Configuration using Hibernate API, we may use:

Session session = sessionFactory.openSession();

But we still need to clean up:

session.close();
Injecting the EntityManager

If you’re writing code for some sort of container environment, you’ll probably obtain the EntityManager by some sort of dependency injection. For example, in Java (or Jakarta) EE you would write:

@PersistenceContext EntityManager entityManager;

In Quarkus, injection is handled by CDI:

@Inject EntityManager entityManager;

Outside a container environment, we’ll also have to write code to manage database transactions.

5.3. Managing transactions

Using JPA-standard APIs, the EntityTransaction interface allows us to control database transactions. The idiom we recommend is the following:

EntityManager entityManager = entityManagerFactory.createEntityManager();
EntityTransaction tx = entityManager.getTransaction();
try {
    tx.begin();
    //do some work
    ...
    tx.commit();
}
catch (Exception e) {
    if (tx.isActive()) tx.rollback();
    throw e;
}
finally {
    entityManager.close();
}

Using Hibernate’s native APIs we might write something really similar, but since this sort of code is extremely tedious, we have a much nicer option:

sessionFactory.inTransaction(session -> {
    //do the work
    ...
});
Container-managed transactions

In a container environment, the container itself is usually responsible for managing transactions. In Java EE or Quarkus, you’ll probably indicate the boundaries of the transaction using the @Transactional annotation.

JPA doesn’t have a standard way to set the transaction timeout, but Hibernate does:

session.getTransaction().setTimeout(30); // 30 seconds

5.4. Operations on the persistence context

Of course, the main reason we need an EntityManager is to do stuff to the database. The following important operations let us interact with the persistence context and schedule modifications to the data:

Table 34. Methods for modifying data and managing the persistence context
Method name and parameters Effect

persist(Object)

Make a transient object persistent and schedule a SQL insert statement for later execution

remove(Object)

Make a persistent object transient and schedule a SQL delete statement for later execution

merge(Object)

Copy the state of a given detached object to a corresponding managed persistent instance and return the persistent object

detach(Object)

Disassociate a persistent object from a session without affecting the database

clear()

Empty the persistence context and detach all its entities

flush()

Detect changes made to persistent objects association with the session and synchronize the database state with the state of the session by executing SQL insert, update, and delete statements

Notice that persist() and remove() have no immediate effect on the database, and instead simply schedule a command for later execution. Also notice that there’s no update() operation for a stateful session. Modifications are automatically detected when the session is flushed.

On the other hand, except for getReference(), the following operations all result in immediate access to the database:

Table 35. Methods for reading and locking data
Method name and parameters Effect

find(Class,Object)

Obtain a persistent object given its type and its id

find(Class,Object,LockModeType)

Obtain a persistent object given its type and its id, requesting the given optimistic or pessimistic lock mode

getReference(Class,id)

Obtain a reference to a persistent object given its type and its id, without actually loading its state from the database

getReference(Object)

Obtain a reference to a persistent object with the same identity as the given detached instance, without actually loading its state from the database

refresh(Object)

Refresh the persistent state of an object using a new SQL select to retrieve its current state from the database

refresh(Object,LockModeType)

Refresh the persistent state of an object using a new SQL select to retrieve its current state from the database, requesting the given optimistic or pessimistic lock mode

lock(Object, LockModeType)

Obtain an optimistic or pessimistic lock on a persistent object

Any of these operations might throw an exception. Now, if an exception occurs while interacting with the database, there’s no good way to resynchronize the state of the current persistence context with the state held in database tables.

Therefore, a session is considered to be unusable after any of its methods throws an exception.

The persistence context is fragile. If you receive an exception from Hibernate, you should immediately close and discard the current session. Open a new session if you need to, but throw the bad one away first.

Each of the operations we’ve seen so far affects a single entity instance passed as an argument. But there’s a way to set things up so that an operation will propagate to associated entities.

5.5. Cascading persistence operations

It’s quite often the case that the lifecycle of a child entity is completely dependent on the lifecycle of some parent. This is especially common for many-to-one and one-to-one associations, though it’s very rare for many-to-many associations.

For example, it’s quite common to make an Order and all its Items persistent in the same transaction, or to delete a Project and its Filess at once. This sort of relationship is sometimes called a whole/part-type relationship.

Cascading is a convenience which allows us to propagate one of the operations listed in Operations on the persistence context from a parent to its children. To set up cascading, we specify the cascade member of one of the association mapping annotations, usually @OneToMany or @OneToOne.

@Entity
class Order {
    ...
    @OneToMany(mappedby=Item_.ORDER,
               // cascade persist(), remove(), and refresh() from Order to Item
               cascade={PERSIST,REMOVE,REFRESH},
               // also remove() orphaned Items
               orphanRemoval=true)
    private Set<Item> items;
    ...
}

Orphan removal indicates that an Item should be automatically deleted if it is removed from the set of items belonging to its parent Order.

5.6. Proxies and lazy fetching

Our data model is a set of interconnected entities, and in Java our whole dataset would be represented as an enormous interconnected graph of objects. It’s possible that this graph is disconnected, but more likely it’s connected, or composed of a relatively small number of connected subgraphs.

Therefore, when we retrieve on object belonging to this graph from the database and instantiate it in memory, we simply can’t recursively retrieve and instantiate all its associated entities. Quite aside from the waste of memory on the VM side, this process would involve a huge number of round trips to the database server, or a massive multidimensional cartesian product of tables, or both. Instead, we’re forced to cut the graph somewhere.

Hibernate solves this problem using proxies and lazy fetching. A proxy is an object that masquerades as a real entity or collection, but doesn’t actually hold any state, because that state has not yet been fetched from the database. When you call a method of the proxy, Hibernate will detect the call and fetch the state from the database before allowing the invocation to proceed to the real entity object or collection.

Now for the gotchas:

  1. Hibernate will only do this for an entity which is currently associated with a persistence context. Once the session ends, and the persistence context is cleaned up, the proxy is no longer fetchable, and instead its methods throw the hated LazyInitializationException.

  2. A round trip to the database to fetch the state of a single entity instance is just about the least efficient way to access data. It almost inevitably leads to the infamous N+1 selects problem we’ll discuss later when we talk about how to optimize association fetching.

We’re getting a bit ahead of ourselves here, but let’s quickly mention the general strategy we recommend to navigate past these gotchas:

  • All associations should be set fetch=LAZY to avoid fetching extra data when it’s not needed. As we mentioned earlier, this setting is not the default for @ManyToOne associations, and must be specified explicitly.

  • But strive to avoid writing code which triggers lazy fetching. Instead, fetch all the data you’ll need upfront at the beginning of a unit of work, using one of the techniques described in Association fetching, usually, using join fetch in HQL or an EntityGraph.

It’s important to know that some operations which may be performed with an unfetched proxy don’t require fetching its state from the database. First, we’re always allowed to obtain its identifier:

var pubId = entityManager.find(Book.class, bookId).getPublisher().getId(); // does not fetch publisher

Second, we may create an association to a proxy:

book.setPublisher(entityManager.getReference(Publisher.class, pubId)); // does not fetch publisher

Sometimes it’s useful to test whether a proxy or collection has been fetched from the database. JPA lets us do this using the PersistenceUnitUtil:

boolean authorsFetched = entityManagerFactory.getPersistenceUnitUtil().isLoaded(book.getAuthors());

Hibernate has a slightly easier way to do it:

boolean authorsFetched = Hibernate.isInitialized(book.getAuthors());

But the static methods of the Hibernate class let us do a lot more, and it’s worth getting a bit familiar with them.

Of particular interest are the operations which let us work with unfetched collections without fetching their state from the database. For example, consider this code:

Book book = session.find(Book.class, bookId);  // fetch just the Book, leaving authors unfetched
Author authorRef = session.getReference(Author.class, authorId);  // obtain an unfetched proxy
boolean isByAuthor = Hibernate.contains(book.getAuthors(), authorRef); // no fetching

This code fragment leaves both the set book.authors and the proxy authorRef unfetched.

Finally, Hibernate.initialize() is a convenience method that force-fetches a proxy or collection:

Book book = session.find(Book.class, bookId);  // fetch just the Book, leaving authors unfetched
Hibernate.initialize(book.getAuthors());  // fetch the Authors

But of course, this code is very inefficient, requiring two trips to the database to obtain data that could in principle be retrieved with just one query.

It’s clear from the discussion above that we need a way to request that an association be eagerly fetched using a database join, thus protecting ourselves from the infamous N+1 selects. One way to do this is by passing an EntityGraph to find().

5.7. Entity graphs and eager fetching

When an association is mapped fetch=LAZY, it won’t, by default, be fetched when we call the find() method. We may request that an association be fetched eagerly (immediately) by passing an EntityGraph to find().

The JPA-standard API for this is a bit unwieldy:

var graph = entityManager.createEntityGraph(Book.class);
graph.addSubgraph(Book_.publisher);
Book book = entityManager.find(Book.class, bookId, Map.of(SpecHints.HINT_SPEC_FETCH_GRAPH, graph));

This is untypesafe and unnecessarily verbose. Hibernate has a better way:

var graph = session.createEntityGraph(Book.class);
graph.addSubgraph(Book_.publisher);
Book book = session.byId(Book.class).withFetchGraph(graph).load(bookId);

This code adds a left outer join to our SQL query, fetching the associated Publisher along with the Book.

We may even attach additional nodes to our EntityGraph:

var graph = session.createEntityGraph(Book.class);
graph.addSubgraph(Book_.publisher);
graph.addPluralSubgraph(Book_.authors).addSubgraph(Author_.person);
Book book = session.byId(Book.class).withFetchGraph(graph).load(bookId);

This results in a SQL query with four left outer joins.

In the code examples above, The classes Book_ and Author_ are generated by the JPA Metamodel Generator we saw earlier. They let us refer to attributes of our model in a completely type-safe way. We’ll use them again, below, when we talk about Criteria queries.

JPA specifies that any given EntityGraph may be interpreted in two different ways.

  • A fetch graph specifies exactly the associations that should be eagerly loaded. Any association not belonging to the entity graph is proxied and loaded lazily only if required.

  • A load graph specifies that the associations in the entity graph are to be fetched in addition to the associations mapped fetch=EAGER.

You’re right, the names make no sense. But don’t worry, if you take our advice, and map your associations fetch=LAZY, there’s no difference between a "fetch" graph and a "load" graph, so the names don’t matter.

JPA even specifies a way to define named entity graphs using annotations. But the annotation-based API is so verbose that it’s just not worth using.

5.8. Flushing the session

From time to time, a flush operation is triggered, and the session synchronizes dirty state held in memory—that is, modifications to the state of entities associated with the persistence context—with persistent state held in the database. Of course, it does this by executing SQL INSERT, UPDATE, and DELETE statements.

By default, a flush is triggered:

  • when the current transaction commits, for example, when Transaction.commit() is called,

  • before execution of a query whose result would be affected by the synchronization of dirty state held in memory, or

  • when the program directly calls flush().

Notice that SQL statements are not usually executed synchronously by methods of the Session interface like persist() and remove(). If synchronous execution of SQL is desired, the StatelessSession allows this.

This behavior can be controlled by explicitly setting the flush mode. For example, to disable flushes that occur before query execution, call:

entityManager.setFlushMode(FlushModeType.COMMIT);

Hibernate allows greater control over the flush mode than JPA:

session.setHibernateFlushMode(FlushMode.MANUAL);

Since flushing is a somewhat expensive operation (the session must dirty-check every entity in the persistence context), setting the flush mode to COMMIT can occasionally be a useful optimization.

Table 36. Flush modes
Hibernate FlushMode JPA FlushModeType Interpretation

MANUAL

Never flush automatically

COMMIT

COMMIT

Flush before transaction commit

AUTO

AUTO

Flush before transaction commit, and before execution of a query whose results might be affected by modifications held in memory

ALWAYS

Flush before transaction commit, and before execution of every query

A second way to reduce the cost of flushing is to load entities in read-only mode:

  • Session.setDefaultReadOnly(false) specifies that all entities loaded by a given session should be loaded in read-only mode by default,

  • SelectionQuery.setReadOnly(false) specifies that every entity returned by a given query should be loaded in read-only mode, and

  • Session.setReadOnly(Object, false) specifies that a given entity already loaded by the session should be switched to read-only mode.

It’s not necessary to dirty-check an entity instance in read-only mode.

5.9. Queries

Hibernate features three complementary ways to write queries:

  • the Hibernate Query Language, an extremely powerful superset of JPQL, which abstracts most of the features of modern dialects of SQL,

  • the JPA criteria query API, along with extensions, allowing almost any HQL query to be constructed programmatically via a typesafe API, and, of course

  • for when all else fails, native SQL queries.

5.10. HQL queries

A full discussion of the query language would require almost as much text as the rest of this Introduction. Fortunately, HQL is already described in exhaustive (and exhausting) detail in A Guide to Hibernate Query Language. It doesn’t make sense to repeat that information here.

Here we want to see how to execute a query via the Session or EntityManager API. The method we call depends on what kind of query it is:

  • selection queries return a result list, but do not modify the data, but

  • mutation queries modify data, and return the number of modified rows.

Selection queries usually start with the keyword select or from, whereas mutation queries begin with the keyword insert, update, or delete.

Table 37. Executing HQL
Kind Session method EntityManager method Query execution method

Selection

createSelectionQuery(String,Class)

createQuery(String,Class)

getResultList(), getSingleResult(), or getSingleResultOrNull()

Mutation

createMutationQuery(String)

createQuery(String)

executeUpdate()

So for the Session API we would write:

List<Book> matchingBooks =
        session.createSelectionQuery("from Book where title like :titleSearchPattern", Book.class)
            .setParameter("titleSearchPattern", titleSearchPattern)
            .getResultList();

Or, if we’re sticking to the JPA-standard APIs:

List<Book> matchingBooks =
        entityManager.createQuery("select b from Book b where b.title like :titleSearchPattern", Book.class)
            .setParameter("titleSearchPattern", titleSearchPattern)
            .getResultList();

The only difference between createSelectionQuery() and createQuery() is that createSelectionQuery() throws an exception if passed an insert, delete, or update.

In the query above, :titleSearchPattern is called a named parameter. We may also identify parameters by a number. These are called ordinal parameters.

List<Book> matchingBooks =
        session.createSelectionQuery("from Book where title like ?1", Book.class)
            .setParameter(1, titleSearchPattern)
            .getResultList();

When a query has multiple parameters, named parameters tend to be easier to read, even if slightly more verbose.

Never concatenate user input with HQL and pass the concatenated string to createSelectionQuery(). This would open up the possibility for an attacker to execute arbitrary code on your database server.

If we’re expecting a query to return a single result, we can use getSingleResult().

Book book =
        session.createSelectionQuery("from Book where isbn = ?1", Book.class)
            .setParameter(1, isbn)
            .getSingleResult();

Or, if we’re expecting it to return at most one result, we can use getSingleResultOrNull().

Book bookOrNull =
        session.createSelectionQuery("from Book where isbn = ?1", Book.class)
            .setParameter(1, isbn)
            .getSingleResultOrNull();

The difference, of course, is that getSingleResult() throws an exception if there’s no matching row in the database, whereas getSingleResultOrNull() just returns null.

By default, Hibernate dirty checks entities in the persistence context before executing a query, in order to determine if the session should be flushed. If there are many entities association with the persistence context, then this can be an expensive operation.

To disable this behavior, set the flush mode to COMMIT or MANUAL:

Book bookOrNull =
        session.createSelectionQuery("from Book where isbn = ?1", Book.class)
            .setParameter(1, isbn)
            .setHibernateFlushMode(MANUAL)
            .getSingleResult();

Setting the flush mode to COMMIT or MANUAL might cause the query to return stale results.

Occasionally we need to build a query at runtime, from a set of optional conditions. For this, JPA offers an API which allows programmatic construction of a query.

5.11. Criteria queries

Imagine we’re implementing some sort of search screen, where the user of our system is offered several different ways to constrain the query result set. For example, we might let them search for books by title and/or the author name. Of course, we could construct a HQL query by string concatenation, but this is a bit fragile, so it’s quite nice to have an alternative.

HQL is implemented in terms of criteria objects

Actually, in Hibernate 6, every HQL query is compiled to a criteria query before being translated to SQL. This ensures that the semantics of HQL and criteria queries are identical.

First we need an object for building criteria queries. Using the JPA-standard APIs, this would be a CriteriaBuilder, and we get it from the EntityManagerFactory:

CriteriaBuilder builder = entityManagerFactory.getCriteriaBuilder();

But if we have a SessionFactory, we get something much better, a HibernateCriteriaBuilder:

HibernateCriteriaBuilder builder = sessionFactory.getCriteriaBuilder();

The HibernateCriteriaBuilder extends CriteriaBuilder and adds many operations that JPQL doesn’t have.

If you’re using EntityManagerFactory, don’t despair, you have two perfectly good ways to obtain the HibernateCriteriaBuilder associated with that factory. Either:

HibernateCriteriaBuilder builder =
        entityManagerFactory.unwrap(SessionFactory.class).getCriteriaBuilder();

Or simply:

HibernateCriteriaBuilder builder =
        (HibernateCriteriaBuilder) entityManagerFactory.getCriteriaBuilder();

We’re ready to create a criteria query.

CriteriaQuery<Book> query = builder.createQuery(Book.class);
Root<Book> book = query.from(Book.class);
Predicate where = builder.conjunction();
if (titlePattern != null) {
    where = builder.and(where, builder.like(book.get(Book_.title), titlePattern));
}
if (namePattern != null) {
    Join<Book,Author> author = book.join(Book_.author);
    where = builder.and(where, builder.like(author.get(Author_.name), namePattern));
}
query.select(book).where(where)
    .orderBy(builder.asc(book.get(Book_.title)));

Here, as before, the classes Book_ and Author_ are generated by Hibernate’s JPA Metamodel Generator.

Notice that we didn’t bother treating titlePattern and namePattern as parameters. That’s safe because, by default, Hibernate automatically and transparently treats strings passed to the CriteriaBuilder as JDBC parameters.

Execution of a criteria query works almost exactly like execution of HQL.

Table 38. Executing criteria queries
Kind Session method EntityManager method Query execution method

Selection

createSelectionQuery(CriteriaQuery)

createQuery(CriteriaQuery)

getResultList(), getSingleResult(), or getSingleResultOrNull()

Mutation

createMutationQuery(CriteriaUpdate) or createQuery(CriteriaDelete)

createQuery(CriteriaUpdate) or createQuery(CriteriaDelte)

executeUpdate()

For example:

List<Book> matchingBooks =
        session.createSelectionQuery(query)
            .getResultList();

Update, insert, and delete queries work similarly:

CriteriaDelete<Book> delete = builder.createCriteriaDelete(Book.class);
Root<Book> book = delete.from(Book.class);
delete.where(builder.lt(builder.year(book.get(Book_.publicationDate)), 2000));
session.createMutationQuery(delete).executeUpdate();

It’s even possible to transform a HQL query string to a criteria query, and modify the query programmatically before execution:

HibernateCriteriaBuilder builder = sessionFactory.getCriteriaBuilder();
var query = builder.createQuery("from Book where year(publicationDate) > 2000", Book.class);
var root = (Root<Book>) query.getRootList().get(0);
query.where(builder.like(root.get(Book_.title), builder.literal("Hibernate%")));
query.orderBy(builder.asc(root.get(Book_.title)), builder.desc(root.get(Book_.isbn)));
List<Book> matchingBooks = session.createSelectionQuery(query).getResultList();

Do you find some of the code above a bit too verbose? We do.

5.12. A more comfortable way to write criteria queries

Actually, what makes the JPA criteria API less ergonomic than it should be is the need to call all operations of the CriteriaBuilder as instance methods, instead of having them as static functions. The reason it works this way is that each JPA provider has its own implementation of CriteriaBuilder.

Hibernate 6.3 introduces the helper class CriteriaDefinition to reduce the verbosity of criteria queries. Our example looks like this:

CriteriaQuery<Book> query =
        new CriteriaDefinition(entityManagerFactory, Book.class) {{
            select(book);
            if (titlePattern != null) {
                restrict(like(book.get(Book_.title), titlePattern));
            }
            if (namePattern != null) {
                var author = book.join(Book_.author);
                restrict(like(author.get(Author_.name), namePattern));
            }
            orderBy(asc(book.get(Book_.title)));
        }};

When all else fails, and sometimes even before that, we’re left with the option of writing a query in SQL.

5.13. Native SQL queries

HQL is a powerful language which helps reduce the verbosity of SQL, and significantly increases portability of queries between databases. But ultimately, the true value of ORM is not in avoiding SQL, but in alleviating the pain involved in dealing with SQL result sets once we get them back to our Java program. As we said right up front, Hibernate’s generated SQL is meant to be used in conjunction with handwritten SQL, and native SQL queries are one of the facilities we provide to make that easy.

Table 39. Executing SQL
Kind Session method EntityManager method Query execution method

Selection

createNativeQuery(String,Class)

createNativeQuery(String,Class)

getResultList(), getSingleResult(), or getSingleResultOrNull()

Mutation

createNativeMutationQuery(String)

createNativeQuery(String)

executeUpdate()

Stored procedure

createStoredProcedureCall(String)

createStoredProcedureQuery(String)

execute()

For the most simple cases, Hibernate can infer the shape of the result set:

Book book =
        session.createNativeQuery("select * from Books where isbn = ?1", Book.class)
            .getSingleResult();

String title =
        session.createNativeQuery("select title from Books where isbn = ?1", String.class)
            .getSingleResult();

However, in general, there isn’t enough information in the JDBC ResultSetMetaData to infer the mapping of columns to entity objects. So for more complicated cases, you’ll need to use the @SqlResultSetMapping annotation to define a named mapping, and pass the name to createNativeQuery(). This gets fairly messy, so we don’t want to hurt your eyes by showing you an example of it.

By default, Hibernate doesn’t flush the session before execution of a native query. That’s because the session is unaware of which modifications held in memory would affect the results of the query.

So if there are any unflushed changes to Books, this query might return stale data:

List<Book> books =
        session.createNativeQuery("select * from Books", Book.class)
            .getResultList()

There’s two ways to ensure the persistence context is flushed before this query is executed.

Either, we could simply force a flush by calling flush() or by setting the flush mode to ALWAYS:

List<Book> books =
        session.createNativeQuery("select * from Books", Book.class)
            .setHibernateFlushMode(ALWAYS)
            .getResultList()

Or, alternatively, we could tell Hibernate which modified state affects the results of the query:

List<Book> books =
        session.createNativeQuery("select * from Books", Book.class)
            .addSynchronizedEntityClass(Book.class)
            .getResultList()

You can call stored procedures using createStoredProcedureQuery() or createStoredProcedureCall().

5.14. Limits, pagination, and ordering

If a query might return more results than we can handle at one time, we may specify:

  • a limit on the maximum number of rows returned, and,

  • optionally, an offset, the first row of an ordered result set to return.

The offset is used to paginate query results.

There’s two ways to add a limit or offset to a HQL or native SQL query:

  • using the syntax of the query language itself, for example, offset 10 rows fetch next 20 rows only, or

  • using the methods setFirstResult() and setMaxResults() of the SelectionQuery interface.

If the limit or offset is parameterized, the second option is simpler. For example, this:

List<Book> books =
        session.createSelectionQuery("from Book where title like ?1 order by title", Book.class)
            .setParameter(1, titlePattern)
            .setMaxResults(MAX_RESULTS)
            .getResultList();

is simpler than:

// a worse way to do pagination
List<Book> books =
        session.createSelectionQuery("from Book where title like ?1 order by title fetch first ?2 rows only", Book.class)
            .setParameter(1, titlePattern)
            .setParameter(2, MAX_RESULTS)
            .getResultList();

Hibernate’s SelectionQuery has a slightly different way to paginate the query results:

List<Book> books =
        session.createSelectionQuery("from Book where title like ?1 order by title", Book.class)
            .setParameter(1, titlePattern)
            .setPage(Page.first(MAX_RESULTS))
            .getResultList();

The getResultCount() method is useful for displaying the number of pages of results:

SelectionQuery<Book> query =
        session.createSelectionQuery("from Book where title like ?1 order by title", Book.class)
            .setParameter(1, titlePattern);
long pages = query.getResultCount() / MAX_RESULTS;
List<Book> books = query.setMaxResults(MAX_RESULTS).getResultList();

A closely-related issue is ordering. It’s quite common for pagination to be combined with the need to order query results by a field that’s determined at runtime. So, as an alternative to the HQL order by clause, SelectionQuery offers the ability to specify that the query results should be ordered by one or more fields of the entity type returned by the query:

List<Book> books =
        session.createSelectionQuery("from Book where title like ?1", Book.class)
            .setParameter(1, titlePattern)
            .setOrder(List.of(Order.asc(Book_.title), Order.asc(Book_.isbn)))
            .setMaxResults(MAX_RESULTS)
            .getResultList();

Unfortunately, there’s no way to do this using JPA’s TypedQuery interface.

Table 40. Methods for query limits, pagination, and ordering
Method name Purpose JPA-standard

setMaxResults()

Set a limit on the number of results returned by a query

setFirstResult()

Set an offset on the results returned by a query

setPage()

Set the limit and offset by specifying a Page object

setOrder()

Specify how the query results should be ordered

getResultCount()

Determine how many results the query would return in the absence of any limit or offset

The approach to pagination we’ve just seen is sometimes called offset-based pagination. Since Hibernate 6.5, there’s an alternative approach, which offers some advantages, though it’s a little more difficult to use.

5.15. Key-based pagination

Key-based pagination aims to reduce the likelihood of missed or duplicate results when data is modified between page requests. It’s most easily illustrated with an example:

String QUERY = "from Book where publicationDate > :minDate";

// obtain the first page of results
KeyedResultList<Book> first =
        session.createSelectionQuery(QUERY, Book.class)
                .setParameter("minDate", minDate)
                .getKeyedResultList(Page.first(25)
                        .keyedBy(Order.asc(Book_.isbn)));
List<Book> firstPage = first.getResultList();
...

if (!firstPage.isLastPage()) {
    // obtain the second page of results
    KeyedResultList<Book> second =
            session.createSelectionQuery(QUERY, Book.class)
                    .setParameter("minDate", minDate))
                    .getKeyedResultList(firstPage.getNextPage());
    List<Book> secondPage = second.getResultList();
    ...
}

The "key" in key-based pagination refers to a unique key of the result set which determines a total order on the query results. In this example, Book.isbn is the key.

Since this code is a little bit fiddly, key-based pagination works best with generated query or finder methods.

5.16. Representing projection lists

A projection list is the list of things that a query returns, that is, the list of expressions in the select clause. Since Java has no tuple types, representing query projection lists in Java has always been a problem for JPA and Hibernate. Traditionally, we’ve just used Object[] most of the time:

var results =
        session.createSelectionQuery("select isbn, title from Book", Object[].class)
            .getResultList();

for (var result : results) {
    var isbn = (String) result[0];
    var title = (String) result[1];
    ...
}

This is really a bit ugly. Java’s record types now offer an interesting alternative:

record IsbnTitle(String isbn, String title) {}

var results =
        session.createSelectionQuery("select isbn, title from Book", IsbnTitle.class)
            .getResultList();

for (var result : results) {
    var isbn = result.isbn();
    var title = result.title();
    ...
}

Notice that we’re able to declare the record right before the line which executes the query.

Now, this is only superficially more typesafe, since the query itself is not checked statically, and so we can’t say it’s objectively better. But perhaps you find it more aesthetically pleasing. And if we’re going to be passing query results around the system, the use of a record type is much better.

The criteria query API offers a much more satisfying solution to the problem. Consider the following code:

var builder = sessionFactory.getCriteriaBuilder();
var query = builder.createTupleQuery();
var book = query.from(Book.class);
var bookTitle = book.get(Book_.title);
var bookIsbn = book.get(Book_.isbn);
var bookPrice = book.get(Book_.price);
query.select(builder.tuple(bookTitle, bookIsbn, bookPrice));
var resultList = session.createSelectionQuery(query).getResultList();
for (var result: resultList) {
    String title = result.get(bookTitle);
    String isbn = result.get(bookIsbn);
    BigDecimal price = result.get(bookPrice);
    ...
}

This code is manifestly completely typesafe, and much better than we can hope to do with HQL.

5.17. Named queries

The @NamedQuery annotation lets us define a HQL query that is compiled and checked as part of the bootstrap process. This means we find out about errors in our queries earlier, instead of waiting until the query is actually executed. We can place the @NamedQuery annotation on any class, even on an entity class.

@NamedQuery(name="10BooksByTitle",
            query="from Book where title like :titlePattern order by title fetch first 10 rows only")
class BookQueries {}

We have to make sure that the class with the @NamedQuery annotation will be scanned by Hibernate, either:

  • by adding <class>org.hibernate.example.BookQueries</class> to persistence.xml, or

  • by calling configuration.addClass(BookQueries.class).

Unfortunately, JPA’s @NamedQuery annotation can’t be placed on a package descriptor. Therefore, Hibernate provides a very similar annotation, @org.hibernate.annotations.NamedQuery which can be specified at the package level. If we declare a named query at the package level, we must call:

configuration.addPackage("org.hibernate.example")

so that Hibernate knows where to find it.

The @NamedNativeQuery annotation lets us do the same for native SQL queries. There’s much less advantage to using @NamedNativeQuery, because there is very little that Hibernate can do to validate the correctness of a query written in the native SQL dialect of your database.

Table 41. Executing named queries
Kind Session method EntityManager method Query execution method

Selection

createNamedSelectionQuery(String,Class)

createNamedQuery(String,Class)

getResultList(), getSingleResult(), or getSingleResultOrNull()

Mutation

createNamedMutationQuery(String)

createNamedQuery(String)

executeUpdate()

We execute our named query like this:

List<Book> books =
        entityManager.createNamedQuery(BookQueries_.QUERY_10_BOOKS_BY_TITLE)
            .setParameter("titlePattern", titlePattern)
            .getResultList()

Here, BookQueries_.QUERY_10_BOOKS_BY_TITLE is a constant with value "10BooksByTitle", generated by the Metamodel Generator.

Note that the code which executes the named query is not aware of whether the query was written in HQL or in native SQL, making it slightly easier to change and optimize the query later.

It’s nice to have our queries checked at startup time. It’s even better to have them checked at compile time. In Organizing persistence logic, we mentioned that the Metamodel Generator can do that for us, with the help of the @CheckHQL annotation, and we presented that as a reason to use @NamedQuery.

But actually, Hibernate has a separate Query Validator capable of performing compile-time validation of HQL query strings that occur as arguments to createQuery() and friends. If we use the Query Validator, there’s not much advantage to the use of named queries.

5.18. Controlling lookup by id

We can do almost anything via HQL, criteria, or native SQL queries. But when we already know the identifier of the entity we need, a query can feel like overkill. And queries don’t make efficient use of the second level cache.

We met the find() method earlier. It’s the most basic way to perform a lookup by id. But as we also already saw, it can’t quite do everything. Therefore, Hibernate has some APIs that streamline certain more complicated lookups:

Table 42. Operations for lookup by id
Method name Purpose

byId()

Lets us specify association fetching via an EntityGraph, as we saw; also lets us specify some additional options, including how the lookup interacts with the second level cache, and whether the entity should be loaded in read-only mode

byMultipleIds()

Lets us load a batch of ids at the same time

Batch loading is very useful when we need to retrieve multiple instances of the same entity class by id:

var graph = session.createEntityGraph(Book.class);
graph.addSubgraph(Book_.publisher);

List<Book> books =
        session.byMultipleIds(Book.class)
            .withFetchGraph(graph)  // control association fetching
            .withBatchSize(20)      // specify an explicit batch size
            .with(CacheMode.GET)    // control interaction with the cache
            .multiLoad(bookIds);

The given list of bookIds will be broken into batches, and each batch will be fetched from the database in a single select. If we don’t specify the batch size explicitly, a batch size will be chosen automatically.

We also have some operations for working with lookups by natural id:

Method name Purpose

bySimpleNaturalId()

For an entity with just one attribute is annotated @NaturalId

byNaturalId()

For an entity with multiple attributes are annotated @NaturalId

byMultipleNaturalId()

Lets us load a batch of natural ids at the same time

Here’s how we can retrieve an entity by its composite natural id:

Book book =
        session.byNaturalId(Book.class)
            .using(Book_.isbn, isbn)
            .using(Book_.printing, printing)
            .load();

Notice that this code fragment is completely typesafe, again thanks to the Metamodel Generator.

5.19. Interacting directly with JDBC

From time to time we run into the need to write some code that calls JDBC directly. Unfortunately, JPA offers no good way to do this, but the Hibernate Session does.

session.doWork(connection -> {
    try (var callable = connection.prepareCall("{call myproc(?)}")) {
        callable.setLong(1, argument);
        callable.execute();
    }
});

The Connection passed to the work is the same connection being used by the session, and so any work performed using that connection occurs in the same transaction context.

If the work returns a value, use doReturningWork() instead of doWork().

In a container environment where transactions and database connections are managed by the container, this might not be the easiest way to obtain the JDBC connection.

5.20. What to do when things go wrong

Object/relational mapping has been called the "Vietnam of computer science". The person who made this analogy is American, and so one supposes that he meant to imply some kind of unwinnable war. This is quite ironic, since at the very moment he made this comment, Hibernate was already on the brink of winning the war.

Today, Vietnam is a peaceful country with exploding per-capita GDP, and ORM is a solved problem. That said, Hibernate is complex, and ORM still presents many pitfalls for the inexperienced, even occasionally for the experienced. Sometimes things go wrong.

In this section we’ll quickly sketch some general strategies for avoiding "quagmires".

  • Understand SQL and the relational model. Know the capabilities of your RDBMS. Work closely with the DBA if you’re lucky enough to have one. Hibernate is not about "transparent persistence" for Java objects. It’s about making two excellent technologies work smoothly together.

  • Log the SQL executed by Hibernate. You cannot know that your persistence logic is correct until you’ve actually inspected the SQL that’s being executed. Even when everything seems to be "working", there might be a lurking N+1 selects monster.

  • Be careful when modifying bidirectional associations. In principle, you should update both ends of the association. But Hibernate doesn’t strictly enforce that, since there are some situations where such a rule would be too heavy-handed. Whatever the case, it’s up to you to maintain consistency across your model.

  • Never leak a persistence context across threads or concurrent transactions. Have a strategy or framework to guarantee this never happens.

  • When running queries that return large result sets, take care to consider the size of the session cache. Consider using a stateless session.

  • Think carefully about the semantics of the second-level cache, and how the caching policies impact transaction isolation.

  • Avoid fancy bells and whistles you don’t need. Hibernate is incredibly feature-rich, and that’s a good thing, because it serves the needs of a huge number of users, many of whom have one or two very specialized needs. But nobody has all those specialized needs. In all probability, you have none of them. Write your domain model in the simplest way that’s reasonable, using the simplest mapping strategies that make sense.

  • When something isn’t behaving as you expect, simplify. Isolate the problem. Find the absolute minimum test case which reproduces the behavior, before asking for help online. Most of the time, the mere act of isolating the problem will suggest an obvious solution.

  • Avoid frameworks and libraries that "wrap" JPA. If there’s any one criticism of Hibernate and ORM that sometimes does ring true, it’s that it takes you too far from direct control over JDBC. An additional layer just takes you even further.

  • Avoid copy/pasting code from random bloggers or stackoverflow reply guys. Many of the suggestions you’ll find online just aren’t the simplest solution, and many aren’t correct for Hibernate 6. Instead, understand what you’re doing; study the Javadoc of the APIs you’re using; read the JPA specification; follow the advice we give in this document; go direct to the Hibernate team on Zulip. (Sure, we can be a bit cantankerous at times, but we do always want you to be successful.)

  • Always consider other options. You don’t have to use Hibernate for everything.

6. Compile-time tooling

The Metamodel Generator is a standard part of JPA. We’ve actually already seen its handiwork in the code examples earlier: it’s the author of the class Book_, which contains the static metamodel of the entity class Book.

The Metamodel Generator

Hibernate’s Metamodel Generator is an annotation processor that produces what JPA calls a static metamodel. That is, it produces a typed model of the persistent classes in our program, giving us a type-safe way to refer to their attributes in Java code. In particular, it lets us specify entity graphs and criteria queries in a completely type-safe way.

The history behind this thing is quite interesting. Back when Java’s annotation processing API was brand spankin' new, the static metamodel for JPA was proposed by Gavin King for inclusion in JPA 2.0, as a way to achieve type safety in the nascent criteria query API. It’s fair to say that, back in 2010, this API was not a runaway success. Tools did not, at the time, feature robust support for annotation processors. And all the explicit generic types made user code quite verbose and difficult to read. (The need for an explicit reference to a CriteriaBuilder instance also contributed verbosity to the criteria API.) For years, Gavin counted this as one of his more embarrassing missteps.

But time has been kind to the static metamodel. In 2023, all Java compilers, build tools, and IDEs have robust support for annotation processing, and Java’s local type inference (the var keyword) eliminates the verbose generic types. JPA’s CriteriaBuilder and EntityGraph APIs are still not quite perfect, but the imperfections aren’t related to static type safety or annotation processing. The static metamodel itself is undeniably useful and elegant.

And so now, in Hibernate 6.3, we’re finally ready to go new places with the Metamodel Generator. And it turns out that there’s quite a lot of unlocked potential there.

Now, you still don’t have to use the Metamodel Generator with Hibernate—the APIs we just mentioned still also accept plain strings—but we find that it works well with Gradle and integrates smoothly with our IDE, and the advantage in type-safety is compelling.

We’ve already seen how to set up the annotation processor in the Gradle build we saw earlier. For more details on how to integrate the Metamodel Generator, check out the Static Metamodel Generator section in the User Guide.

Here’s an example of the sort of code that’s generated for an entity class, as mandated by the JPA specification:

Generated Code
@StaticMetamodel(Book.class)
public abstract class Book_ {

    /**
     * @see org.example.Book#isbn
     **/
    public static volatile SingularAttribute<Book, String> isbn;

    /**
     * @see org.example.Book#text
     **/
    public static volatile SingularAttribute<Book, String> text;

    /**
     * @see org.example.Book#title
     **/
    public static volatile SingularAttribute<Book, String> title;

    /**
     * @see org.example.Book#type
     **/
    public static volatile SingularAttribute<Book, Type> type;

    /**
     * @see org.example.Book#publicationDate
     **/
    public static volatile SingularAttribute<Book, LocalDate> publicationDate;

    /**
     * @see org.example.Book#publisher
     **/
    public static volatile SingularAttribute<Book, Publisher> publisher;

    /**
     * @see org.example.Book#authors
     **/
    public static volatile SetAttribute<Book, Author> authors;

    public static final String ISBN = "isbn";
    public static final String TEXT = "text";
    public static final String TITLE = "title";
    public static final String TYPE = "type";
    public static final String PUBLICATION_DATE = "publicationDate";
    public static final String PUBLISHER = "publisher";
    public static final String AUTHORS = "authors";

}

For each attribute of the entity, the Book_ class has:

  1. a String-valued constant like TITLE , and

  2. a typesafe reference like title to a metamodel object of type Attribute.

We’ve already been using metamodel references like Book_.authors and Book.AUTHORS in the previous chapters. So now let’s see what else the Metamodel Generator can do for us.

The Metamodel Generator provides statically-typed access to elements of the JPA Metamodel. But the Metamodel is also accessible in a "reflective" way, via the EntityManagerFactory.

EntityType<Book> book = entityManagerFactory.getMetamodel().entity(Book.class);
SingularAttribute<Book,Long> id = book.getDeclaredId(Long.class)

This is very useful for writing generic code in frameworks or libraries. For example, you could use it to create your own criteria query API.

Automatic generation of finder methods and query methods is a new feature of Hibernate’s implementation of the Metamodel Generator, and an extension to the functionality defined by the JPA specification. In this chapter, we’re going to explore these features.

The functionality described in the rest of this chapter depends on the use of the annotations described in Entities. The Metamodel Generator is not currently able to generate finder methods and query methods for entities declared completely in XML, and it’s not able to validate HQL which queries such entities. (On the other hand, the O/R mappings may be specified in XML, since they’re not needed by the Metamodel Generator.)

We’re going to meet three different kinds of generated method:

  • a named query method has its signature and implementation generated directly from a @NamedQuery annotation,

  • a query method has a signature that’s explicitly declared, and a generated implementation which executes a HQL or SQL query specified via a @HQL or @SQL annotation, and

  • a finder method annotated @Find has a signature that’s explicitly declared, and a generated implementation inferred from the parameter list.

We’re also going to see two ways that these methods can be called:

To whet our appetites, let’s see how this works for a @NamedQuery.

6.1. Named queries and the Metamodel Generator

The very simplest way to generate a query method is to put a @NamedQuery annotation anywhere we like, with a name beginning with the magical character #.

Let’s just stick it on the Book class:

@CheckHQL // validate the query at compile time
@NamedQuery(name = "#findByTitleAndType",
            query = "select book from Book book where book.title like :title and book.type = :type")
@Entity
public class Book { ... }

Now the Metamodel Generator adds the following method declaration to the metamodel class Book_.

Generated Code
/**
 * Execute named query {@value #QUERY_FIND_BY_TITLE_AND_TYPE} defined by annotation of {@link Book}.
 **/
public static List<Book> findByTitleAndType(@Nonnull EntityManager entityManager, String title, Type type) {
    return entityManager.createNamedQuery(QUERY_FIND_BY_TITLE_AND_TYPE)
            .setParameter("title", title)
            .setParameter("type", type)
            .getResultList();
}

We can easily call this method from wherever we like, as long as we have access to an EntityManager:

List<Book> books =
        Book_.findByTitleAndType(entityManager, titlePattern, Type.BOOK);

Now, this is quite nice, but it’s a bit inflexible in various ways, and so this probably isn’t the best way to generate a query method.

6.2. Generated query methods

The principal problem with generating the query method straight from the @NamedQuery annotation is that it doesn’t let us explicitly specify the return type or parameter list. In the case we just saw, the Metamodel Generator does a reasonable job of inferring the query return type and parameter types, but we’re often going to need a bit more control.

The solution is to write down the signature of the query method explicitly, as an abstract method in Java. We’ll need a place to put this method, and since our Book entity isn’t an abstract class, we’ll just introduce a new interface for this purpose:

interface Queries {
    @HQL("where title like :title and type = :type")
    List<Book> findBooksByTitleAndType(String title, String type);
}

Instead of @NamedQuery, which is a type-level annotation, we specify the HQL query using the new @HQL annotation, which we place directly on the query method. This results in the following generated code in the Queries_ class:

Generated Code
@StaticMetamodel(Queries.class)
public abstract class Queries_ {

    /**
     * Execute the query {@value #FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type}.
     *
     * @see org.example.Queries#findBooksByTitleAndType(String,Type)
     **/
    public static List<Book> findBooksByTitleAndType(@Nonnull EntityManager entityManager, String title, Type type) {
        return entityManager.createQuery(FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type, Book.class)
                .setParameter("title", title)
                .setParameter("type", type)
                .getResultList();
    }

    static final String FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type =
            "where title like :title and type = :type";

}

Notice that the signature differs just slightly from the one we wrote down in the Queries interface: the Metamodel Generator has prepended a parameter accepting EntityManager to the parameter list.

If we want to explicitly specify the name and type of this parameter, we may declare it explicitly:

interface Queries {
    @HQL("where title like :title and type = :type")
    List<Book> findBooksByTitleAndType(StatelessSession session, String title, String type);
}

The Metamodel Generator defaults to using EntityManager as the session type, but other types are allowed:

  • Session,

  • StatelessSession, or

  • Mutiny.Session from Hibernate Reactive.

The real value of all this is in the checks which can now be done at compile time. The Metamodel Generator verifies that the parameters of our abstract method declaration match the parameters of the HQL query, for example:

  • for a named parameter :alice, there must be a method parameter named alice with exactly the same type, or

  • for an ordinal parameter ?2, the second method parameter must have exactly the same type.

The query must also be syntactically legal and semantically well-typed, that is, the entities, attributes, and functions referenced in the query must actually exist and have compatible types. The Metamodel Generator determines this by inspecting the annotations of the entity classes at compile time.

The @CheckHQL annotation which instructs Hibernate to validate named queries is not necessary for query methods annotated @HQL.

The @HQL annotation has a friend named @SQL which lets us specify a query written in native SQL instead of in HQL. In this case there’s a lot less the Metamodel Generator can do to check that the query is legal and well-typed.

We imagine you’re wondering whether a static method is really the right thing to use here.

6.3. Generating query methods as instance methods

One thing not to like about what we’ve just seen is that we can’t transparently replace a generated static function of the Queries_ class with an improved handwritten implementation without impacting clients. Now, if our query is only called in one place, which is quite common, this isn’t going to be a big issue, and so we’re inclined to think the static function is fine.

But if this function is called from many places, it’s probably better to promote it to an instance method of some class or interface. Fortunately, this is straightforward.

All we need to do is add an abstract getter method for the session object to our Queries interface. (And remove the session from the method parameter list.) We may call this method anything we like:

interface Queries {
    EntityManager entityManager();

    @HQL("where title like :title and type = :type")
    List<Book> findBooksByTitleAndType(String title, String type);
}

Here we’ve used EntityManager as the session type, but other types are allowed, as we saw above.

Now the Metamodel Generator does something a bit different:

Generated Code
@StaticMetamodel(Queries.class)
public class Queries_ implements Queries {

    private final @Nonnull EntityManager entityManager;

    public Queries_(@Nonnull EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    public @Nonnull EntityManager entityManager() {
        return entityManager;
    }

    /**
     * Execute the query {@value #FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type}.
     *
     * @see org.example.Queries#findBooksByTitleAndType(String,Type)
     **/
    @Override
    public List<Book> findBooksByTitleAndType(String title, Type type) {
        return entityManager.createQuery(FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type, Book.class)
                .setParameter("title", title)
                .setParameter("type", type)
                .getResultList();
    }

    static final String FIND_BOOKS_BY_TITLE_AND_TYPE_String_Type =
            "where title like :title and type = :type";

}

The generated class Queries_ now implements the Queries interface, and the generated query method implements our abstract method directly.

Of course, the protocol for calling the query method has to change:

Queries queries = new Queries_(entityManager);
List<Book> books = queries.findByTitleAndType(titlePattern, Type.BOOK);

If we ever need to swap out the generated query method with one we write by hand, without impacting clients, all we need to do is replace the abstract method with a default method of the Queries interface. For example:

interface Queries {
    EntityManager entityManager();

    // handwritten method replacing previous generated implementation
    default List<Book> findBooksByTitleAndType(String title, String type) {
        entityManager()
                .createQuery("where title like :title and type = :type", Book.class)
                        .setParameter("title", title)
                        .setParameter("type", type)
                        .setFlushMode(COMMIT)
                        .setMaxResults(100)
                        .getResultList();
    }
}

What if we would like to inject a Queries object instead of calling its constructor directly?

As you recall, we don’t think these things really need to be container-managed objects. But if you want them to be—if you’re allergic to calling constructors, for some reason—then:

  • placing jakarta.inject on the build path will cause an @Inject annotation to be added to the constructor of Queries_, and

  • placing jakarta.enterprise.context on the build path will cause a @Dependent annotation to be added to the Queries_ class.

Thus, the generated implementation of Queries will be a perfectly functional CDI bean with no extra work to be done.

Is the Queries interface starting to look a lot like a DAO-style repository object? Well, perhaps. You can certainly decide to use this facility to create a BookRepository if that’s what you prefer. But unlike a repository, our Queries interface:

  • doesn’t attempt to hide the EntityManager from its clients,

  • doesn’t implement or extend any framework-provided interface or abstract class, at least not unless you want to create such a framework yourself, and

  • isn’t restricted to service a particular entity class.

We can have as many or as few interfaces with query methods as we like. There’s no one-one-correspondence between these interfaces and entity types. This approach is so flexible that we don’t even really know what to call these "interfaces with query methods".

6.4. Generated finder methods

At this point, one usually begins to question whether it’s even necessary to write a query at all. Would it be possible to just infer the query from the method signature?

In some simple cases it’s indeed possible, and this is the purpose of finder methods. A finder method is a method annotated @Find. For example:

@Find
Book getBook(String isbn);

A finder method may have multiple parameters:

@Find
List<Book> getBooksByTitle(String title, Type type);

The name of the finder method is arbitrary and carries no semantics. But:

  • the return type determines the entity class to be queried, and

  • the parameters of the method must match the fields of the entity class exactly, by both name and type.

Considering our first example, Book has a persistent field String isbn, so this finder method is legal. If there were no field named isbn in Book, or if it had a different type, this method declaration would be rejected with a meaningful error at compile time. Similarly, the second example is legal, since Book has fields String title and Type type.

You might notice that our solution to this problem is very different from the approach taken by others. In DAO-style repository frameworks, you’re asked to encode the semantics of the finder method into the name of the method. This idea came to Java from Ruby, and we think it doesn’t belong here. It’s completely unnatural in Java, and by almost any measure other than counting characters it’s objectively worse than just writing the query in a string literal. At least string literals accommodate whitespace and punctuation characters. Oh and, you know, it’s pretty useful to be able to rename a finder method without changing its semantics. 🙄

The code generated for this finder method depends on what kind of fields match the method parameters:

@Id field

Uses EntityManager.find()

All @NaturalId fields

Uses Session.byNaturalId()

Other persistent fields, or a mix of field types

Uses a criteria query

The generated code also depends on what kind of session we have, since the capabilities of stateless sessions, and of reactive sessions, differ slightly from the capabilities of regular stateful sessions.

With EntityManager as the session type, we obtain:

/**
 * Find {@link Book} by {@link Book#isbn isbn}.
 *
 * @see org.example.Dao#getBook(String)
 **/
@Override
public Book getBook(@Nonnull String isbn) {
	return entityManager.find(Book.class, isbn);
}

/**
 * Find {@link Book} by {@link Book#title title} and {@link Book#type type}.
 *
 * @see org.example.Dao#getBooksByTitle(String,Type)
 **/
@Override
public List<Book> getBooksByTitle(String title, Type type) {
	var builder = entityManager.getEntityManagerFactory().getCriteriaBuilder();
	var query = builder.createQuery(Book.class);
	var entity = query.from(Book.class);
	query.where(
			title==null
				? entity.get(Book_.title).isNull()
				: builder.equal(entity.get(Book_.title), title),
			type==null
				? entity.get(Book_.type).isNull()
				: builder.equal(entity.get(Book_.type), type)
	);
	return entityManager.createQuery(query).getResultList();
}

It’s even possible to match a parameter of a finder method against a property of an associated entity or embeddable. The natural syntax would be a parameter declaration like String publisher.name, but because that’s not legal Java, we can write it as String publisher$name, taking advantage of a legal Java identifier character that nobody ever uses for anything else:

@Find
List<Book> getBooksByPublisherName(String publisher$name);

The @Pattern annotation may be applied to a parameter of type String, indicating that the argument is a wildcarded pattern which will be compared using like.

@Find
List<Book> getBooksByTitle(@Pattern String title, Type type);

A finder method may specify fetch profiles, for example:

@Find(namedFetchProfiles=Book_.FETCH_WITH_AUTHORS)
Book getBookWithAuthors(String isbn);

This lets us declare which associations of Book should be pre-fetched by annotating the Book class.

6.5. Paging and ordering

Optionally, a query method—​or a finder method which returns multiple results—​may have additional "magic" parameters which do not map to query parameters:

Parameter type Purpose Example argument

Page

Specifies a page of query results

Page.first(20)

Order<? super E>

Specifies an entity attribute to order by, if E is the entity type returned by the query

Order.asc(Book_.title)

List<Order? super E>
(or varargs)

Specifies entity attributes to order by, if E is the entity type returned by the query

List.of(Order.asc(Book_.title), Order.asc(Book_.isbn))

Order<Object[]>

Specifies a column to order by, if the query returns a projection list

Order.asc(1)

List<Object[]>
(or varargs)

Specifies columns to order by, if the query returns a projection list

List.of(Order.asc(1), Order.desc(2))

Thus, if we redefine our earlier query method as follows:

interface Queries {
    @HQL("from Book where title like :title and type = :type")
    List<Book> findBooksByTitleAndType(String title, Type type,
                                       Page page, Order<? super Book>... order);
}

Then we can call it like this:

List<Book> books =
        Queries_.findBooksByTitleAndType(entityManager, titlePattern, Type.BOOK,
                Page.page(RESULTS_PER_PAGE, page), Order.asc(Book_.isbn));

Alternatively, we could have written this query method as a finder method:

interface Queries {
    @Find
    List<Book> getBooksByTitle(String title, Type type,
                               Page page, Order<? super Book>... order);
}

This gives some dynamic control over query execution, but what if would like direct control over the Query object? Well, let’s talk about the return type.

6.6. Key-based pagination

A generated query or finder method can make use of key-based pagination.

@Query("where publicationDate > :minDate")
KeyedResultList<Book> booksFromDate(Session session, LocalDate minDate, KeyedPage<Book> page);

Note that this method:

  • accepts a KeyedPage, and

  • returns KeyedResultList.

Such a method may be used like this:

// obtain the first page of results
KeyedResultList<Book> first =
        Queries_.booksFromDate(session, minDate,
                Page.first(25).keyedBy(Order.asc(Book_.isbn)));
List<Book> firstPage = first.getResultList();
...

if (!firstPage.isLastPage()) {
    // obtain the second page of results
    KeyedResultList<Book> second =
            Queries_.booksFromDate(session, minDate,
                    firstPage.getNextPage());
    List<Book> secondPage = second.getResultList();
    ...
}

6.7. Query and finder method return types

A query method doesn’t need to return List. It might return a single Book.

@HQL("where isbn = :isbn")
Book findBookByIsbn(String isbn);

For a query with a projection list, Object[] or List<Object[]> is permitted:

@HQL("select isbn, title from Book where isbn = :isbn")
Object[] findBookAttributesByIsbn(String isbn);

But when there’s just one item in the select list, the type of that item should be used:

@HQL("select title from Book where isbn = :isbn")
String getBookTitleByIsbn(String isbn);
@HQL("select local datetime")
LocalDateTime getServerDateTime();

A query which returns a selection list may have a query method which repackages the result as a record, as we saw in Representing projection lists.

record IsbnTitle(String isbn, String title) {}

@HQL("select isbn, title from Book")
List<IsbnTitle> listIsbnAndTitleForEachBook(Page page);

A query method might even return TypedQuery or SelectionQuery:

@HQL("where title like :title")
SelectionQuery<Book> findBooksByTitle(String title);

This is extremely useful at times, since it allows the client to further manipulate the query:

List<Book> books =
        Queries_.findBooksByTitle(entityManager, titlePattern)
            .setOrder(Order.asc(Book_.title))                   // order the results
            .setPage(Page.page(RESULTS_PER_PAGE, page))         // return the given page of results
            .setFlushMode(FlushModeType.COMMIT)                 // don't flush session before query execution
            .setReadOnly(true)                                  // load the entities in read-only mode
            .setCacheStoreMode(CacheStoreMode.BYPASS)           // don't cache the results
            .setComment("Hello world!")                         // add a comment to the generated SQL
            .getResultList();

An insert, update, or delete query must return int, boolean, or void.

@HQL("delete from Book")
int deleteAllBooks();
@HQL("update Book set discontinued = true where discontinued = false and isbn = :isbn")
boolean discontinueBook(String isbn);
@HQL("update Book set discontinued = true where isbn = :isbn")
void discontinueBook(String isbn);

On the other hand, finder methods are currently much more limited. A finder method must return an entity type like Book, or a list of the entity type, List<Book>, for example.

As you might expect, for a reactive session, all query methods and finder methods must return Uni.

6.8. An alternative approach

What if you just don’t like the ideas we’ve presented in this chapter, preferring to call the Session or EntityManager directly, but you still want compile-time validation for HQL? Or what if you do like the ideas, but you’re working on a huge existing codebase full of code you don’t want to change?

Well, there’s a solution for you, too. The Query Validator is a separate annotation processor that’s capable of type-checking HQL strings, not only in annotations, but even when they occur as arguments to createQuery(), createSelectionQuery(), or createMutationQuery(). It’s even able to check calls to setParameter(), with some restrictions.

The Query Validator works in javac, Gradle, Maven, and the Eclipse Java Compiler.

Unlike the Metamodel Generator, which is a completely bog-standard Java annotation processor based on only standard Java APIs, the Query Validator makes use of internal compiler APIs in javac and ecj. This means it can’t be guaranteed to work in every Java compiler. The current release is known to work in JDK 11 and above, though JDK 15 or above is preferred.

7. Tuning and performance

Once you have a program up and running using Hibernate to access the database, it’s inevitable that you’ll find places where performance is disappointing or unacceptable.

Fortunately, most performance problems are relatively easy to solve with the tools that Hibernate makes available to you, as long as you keep a couple of simple principles in mind.

First and most important: the reason you’re using Hibernate is that it makes things easier. If, for a certain problem, it’s making things harder, stop using it. Solve this problem with a different tool instead.

Just because you’re using Hibernate in your program doesn’t mean you have to use it everywhere.

Second: there are two main potential sources of performance bottlenecks in a program that uses Hibernate:

  • too many round trips to the database, and

  • memory consumption associated with the first-level (session) cache.

So performance tuning primarily involves reducing the number of accesses to the database, and/or controlling the size of the session cache.

But before we get to those more advanced topics, we should start by tuning the connection pool.

7.1. Tuning the connection pool

The connection pool built in to Hibernate is suitable for testing, but isn’t intended for use in production. Instead, Hibernate supports a range of different connection pools, including our favorite, Agroal.

To select and configure Agroal, you’ll need to set some extra configuration properties, in addition to the settings we already saw in Basic configuration settings. Properties with the prefix hibernate.agroal are passed through to Agroal:

# configure Agroal connection pool
hibernate.agroal.maxSize 20
hibernate.agroal.minSize 10
hibernate.agroal.acquisitionTimeout PT1s
hibernate.agroal.reapTimeout PT10s

As long as you set at least one property with the prefix hibernate.agroal, the AgroalConnectionProvider will be selected automatically. There’s many to choose from:

Table 43. Settings for configuring Agroal
Configuration property name Purpose

hibernate.agroal.maxSize

The maximum number of connections present on the pool

hibernate.agroal.minSize

The minimum number of connections present on the pool

hibernate.agroal.initialSize

The number of connections added to the pool when it is started

hibernate.agroal.maxLifetime

The maximum amount of time a connection can live, after which it is removed from the pool

hibernate.agroal.acquisitionTimeout

The maximum amount of time a thread can wait for a connection, after which an exception is thrown instead

hibernate.agroal.reapTimeout

The duration for eviction of idle connections

hibernate.agroal.leakTimeout

The duration of time a connection can be held without causing a leak to be reported

hibernate.agroal.idleValidationTimeout

A foreground validation is executed if a connection has been idle on the pool for longer than this duration

hibernate.agroal.validationTimeout

The interval between background validation checks

hibernate.agroal.initialSql

A SQL command to be executed when a connection is created

The following settings are common to all connection pools supported by Hibernate:

Table 44. Common settings for connection pools

hibernate.connection.autocommit

The default autocommit mode

hibernate.connection.isolation

The default transaction isolation level

Container-managed datasources

In a container environment, you usually don’t need to configure a connection pool through Hibernate. Instead, you’ll use a container-managed datasource, as we saw in Basic configuration settings.

7.2. Enabling statement batching

An easy way to improve performance of some transactions, with almost no work at all, is to turn on automatic DML statement batching. Batching only helps in cases where a program executes many inserts, updates, or deletes against the same table in a single transaction.

All we need to do is set a single property:

Table 45. Enabling JDBC batching
Configuration property name Purpose Alternative

hibernate.jdbc.batch_size

Maximum batch size for SQL statement batching

setJdbcBatchSize()

Even better than DML statement batching is the use of HQL update or delete queries, or even native SQL that calls a stored procedure!

7.3. Association fetching

Achieving high performance in ORM means minimizing the number of round trips to the database. This goal should be uppermost in your mind whenever you’re writing data access code with Hibernate. The most fundamental rule of thumb in ORM is:

  • explicitly specify all the data you’re going to need right at the start of a session/transaction, and fetch it immediately in one or two queries,

  • and only then start navigating associations between persistent entities.

Fetching process

Without question, the most common cause of poorly-performing data access code in Java programs is the problem of N+1 selects. Here, a list of N rows is retrieved from the database in an initial query, and then associated instances of a related entity are fetched using N subsequent queries.

This isn’t a bug or limitation of Hibernate; this problem even affects typical handwritten JDBC code behind DAOs. Only you, the developer, can solve this problem, because only you know ahead of time what data you’re going to need in a given unit of work. But that’s OK. Hibernate gives you all the tools you need.

In this section we’re going to discuss different ways to avoid such "chatty" interaction with the database.

Hibernate provides several strategies for efficiently fetching associations and avoiding N+1 selects:

  • outer join fetching—where an association is fetched using a left outer join,

  • batch fetching—where an association is fetched using a subsequent select with a batch of primary keys, and

  • subselect fetching—where an association is fetched using a subsequent select with keys re-queried in a subselect.

Of these, you should almost always use outer join fetching. But let’s consider the alternatives first.

7.4. Batch fetching and subselect fetching

Consider the following code:

List<Book> books =
        session.createSelectionQuery("from Book order by isbn", Book.class)
            .getResultList();
books.forEach(book -> book.getAuthors().forEach(author -> out.println(book.title + " by " + author.name)));

This code is very inefficient, resulting, by default, in the execution of N+1 select statements, where N is the number of Books.

Let’s see how we can improve on that.

SQL for batch fetching

With batch fetching enabled, Hibernate might execute the following SQL on PostgreSQL:

/* initial query for Books */
select b1_0.isbn,b1_0.price,b1_0.published,b1_0.publisher_id,b1_0.title
from Book b1_0
order by b1_0.isbn

/* first batch of associated Authors */
select a1_0.books_isbn,a1_1.id,a1_1.bio,a1_1.name
from Book_Author a1_0
    join Author a1_1 on a1_1.id=a1_0.authors_id
where a1_0.books_isbn = any (?)

/* second batch of associated Authors */
select a1_0.books_isbn,a1_1.id,a1_1.bio,a1_1.name
from Book_Author a1_0
    join Author a1_1 on a1_1.id=a1_0.authors_id
where a1_0.books_isbn = any (?)

The first select statement queries and retrieves Books. The second and third queries fetch the associated Authors in batches. The number of batches required depends on the configured batch size. Here, two batches were required, so two SQL statements were executed.

The SQL for batch fetching looks slightly different depending on the database. Here, on PostgreSQL, Hibernate passes a batch of primary key values as a SQL ARRAY.

SQL for subselect fetching

On the other hand, with subselect fetching, Hibernate would execute this SQL:

/* initial query for Books */
select b1_0.isbn,b1_0.price,b1_0.published,b1_0.publisher_id,b1_0.title
from Book b1_0
order by b1_0.isbn

/* fetch all associated Authors */
select a1_0.books_isbn,a1_1.id,a1_1.bio,a1_1.name
from Book_Author a1_0
    join Author a1_1 on a1_1.id=a1_0.authors_id
where a1_0.books_isbn in (select b1_0.isbn from Book b1_0)

Notice that the first query is re-executed in a subselect in the second query. The execution of the subselect is likely to be relatively inexpensive, since the data should already be cached by the database. Clever, huh?

Enabling the use of batch or subselect fetching

Both batch fetching and subselect fetching are disabled by default, but we may enable one or the other globally using properties.

Table 46. Configuration settings to enable batch and subselect fetching
Configuration property name Property value Alternatives

hibernate.default_batch_fetch_size

A sensible batch size >1 to enable batch fetching

@BatchSize(), setFetchBatchSize()

hibernate.use_subselect_fetch

true to enable subselect fetching

@Fetch(SUBSELECT), setSubselectFetchingEnabled()

Alternatively, we can enable one or the other in a given session:

session.setFetchBatchSize(5);
session.setSubselectFetchingEnabled(true);

We may request subselect fetching more selectively by annotating a collection or many-valued association with the @Fetch annotation.

@ManyToMany @Fetch(SUBSELECT)
Set<Author> authors;

Note that @Fetch(SUBSELECT) has the same effect as @Fetch(SELECT), except after execution of a HQL or criteria query. But after query execution, @Fetch(SUBSELECT) is able to much more efficiently fetch associations.

Later, we’ll see how we can use fetch profiles to do this even more selectively.

That’s all there is to it. Too easy, right?

Sadly, that’s not the end of the story. While batch fetching might mitigate problems involving N+1 selects, it won’t solve them. The truly correct solution is to fetch associations using joins. Batch fetching (or subselect fetching) can only be the best solution in rare cases where outer join fetching would result in a cartesian product and a huge result set.

But batch fetching and subselect fetching have one important characteristic in common: they can be performed lazily. This is, in principle, pretty convenient. When we query data, and then navigate an object graph, lazy fetching saves us the effort of planning ahead. It turns out that this is a convenience we’re going to have to surrender.

7.5. Join fetching

Outer join fetching is usually the best way to fetch associations, and it’s what we use most of the time. Unfortunately, by its very nature, join fetching simply can’t be lazy. So to make use of join fetching, we must plan ahead. Our general advice is:

Avoid the use of lazy fetching, which is often the source of N+1 selects.

Now, we’re not saying that associations should be mapped for eager fetching by default! That would be a terrible idea, resulting in simple session operations that fetch almost the entire database. Therefore:

Most associations should be mapped for lazy fetching by default.

It sounds as if this tip is in contradiction to the previous one, but it’s not. It’s saying that you must explicitly specify eager fetching for associations precisely when and where they are needed.

If we need eager join fetching in some particular transaction, we have four different ways to specify that.

Passing a JPA EntityGraph

We’ve already seen this in Entity graphs and eager fetching

Specifying a named fetch profile

We’ll discuss this approach later in Named fetch profiles

Using left join fetch in HQL/JPQL

See A Guide to Hibernate Query Language for details

Using From.fetch() in a criteria query

Same semantics as join fetch in HQL

Typically, a query is the most convenient option. Here’s how we can ask for join fetching in HQL:

List<Book> booksWithJoinFetchedAuthors =
        session.createSelectionQuery("from Book join fetch authors order by isbn")
            .getResultList();

And this is the same query, written using the criteria API:

var builder = sessionFactory.getCriteriaBuilder();
var query = builder.createQuery(Book.class);
var book = query.from(Book.class);
book.fetch(Book_.authors);
query.select(book);
query.orderBy(builder.asc(book.get(Book_.isbn)));
List<Book> booksWithJoinFetchedAuthors =
        session.createSelectionQuery(query).getResultList();

Either way, a single SQL select statement is executed:

select b1_0.isbn,a1_0.books_isbn,a1_1.id,a1_1.bio,a1_1.name,b1_0.price,b1_0.published,b1_0.publisher_id,b1_0.title
from Book b1_0
    join (Book_Author a1_0 join Author a1_1 on a1_1.id=a1_0.authors_id)
        on b1_0.isbn=a1_0.books_isbn
order by b1_0.isbn

Much better!

Join fetching, despite its non-lazy nature, is clearly more efficient than either batch or subselect fetching, and this is the source of our recommendation to avoid the use of lazy fetching.

There’s one interesting case where join fetching becomes inefficient: when we fetch two many-valued associations in parallel. Imagine we wanted to fetch both Author.books and Author.royaltyStatements in some unit of work. Joining both collections in a single query would result in a cartesian product of tables, and a large SQL result set. Subselect fetching comes to the rescue here, allowing us to fetch books using a join, and royaltyStatements using a single subsequent select.

Of course, an alternative way to avoid many round trips to the database is to cache the data we need in the Java client. If we’re expecting to find the associated data in a local cache, we probably don’t need join fetching at all.

But what if we can’t be certain that all associated data will be in the cache? In that case, we might be able to reduce the cost of cache misses by enabling batch fetching.

7.6. The second-level cache

A classic way to reduce the number of accesses to the database is to use a second-level cache, allowing data cached in memory to be shared between sessions.

By nature, a second-level cache tends to undermine the ACID properties of transaction processing in a relational database. We don’t use a distributed transaction with two-phase commit to ensure that changes to the cache and database happen atomically. So a second-level cache is often by far the easiest way to improve the performance of a system, but only at the cost of making it much more difficult to reason about concurrency. And so the cache is a potential source of bugs which are difficult to isolate and reproduce.

Therefore, by default, an entity is not eligible for storage in the second-level cache. We must explicitly mark each entity that will be stored in the second-level cache with the @Cache annotation from org.hibernate.annotations.

But that’s still not enough. Hibernate does not itself contain an implementation of a second-level cache, so it’s necessary to configure an external cache provider.

Caching is disabled by default. To minimize the risk of data loss, we force you to stop and think before any entity goes into the cache.

Hibernate segments the second-level cache into named regions, one for each:

  • mapped entity hierarchy or

  • collection role.

For example, there might be separate cache regions for Author, Book, Author.books, and Book.authors.

Each region is permitted its own policies for expiry, persistence, and replication. These policies must be configured externally to Hibernate.

The appropriate policies depend on the kind of data an entity represents. For example, a program might have different caching policies for "reference" data, for transactional data, and for data used for analytics. Ordinarily, the implementation of those policies is the responsibility of the underlying cache implementation.

7.7. Specifying which data is cached

By default, no data is eligible for storage in the second-level cache.

An entity hierarchy or collection role may be assigned a region using the @Cache annotation. If no region name is explicitly specified, the region name is just the name of the entity class or collection role.

@Entity
@Cache(usage=NONSTRICT_READ_WRITE, region="Publishers")
class Publisher {
    ...

    @Cache(usage=READ_WRITE, region="PublishedBooks")
    @OneToMany(mappedBy=Book_.PUBLISHER)
    Set<Book> books;

    ...
}

The cache defined by a @Cache annotation is automatically utilized by Hibernate to:

  • retrieve an entity by id when find() is called, or

  • to resolve an association by id.

The @Cache annotation must be specified on the root class of an entity inheritance hierarchy. It’s an error to place it on a subclass entity.

The @Cache annotation always specifies a CacheConcurrencyStrategy, a policy governing access to the second-level cache by concurrent transactions.

Table 47. Cache concurrency
Concurrency policy Interpretation Explanation

READ_ONLY

  • Immutable data

  • Read-only access

Indicates that the cached object is immutable, and is never updated. If an entity with this cache concurrency is updated, an exception is thrown.

This is the simplest, safest, and best-performing cache concurrency strategy. It’s particularly suitable for so-called "reference" data.

NONSTRICT_READ_WRITE

  • Concurrent updates are extremely improbable

  • Read/write access with no locking

Indicates that the cached object is sometimes updated, but that it’s extremely unlikely that two transactions will attempt to update the same item of data at the same time.

This strategy does not use locks. When an item is updated, the cache is invalidated both before and after completion of the updating transaction. But without locking, it’s impossible to completely rule out the possibility of a second transaction storing or retrieving stale data in or from the cache during the completion process of the first transaction.

READ_WRITE

  • Concurrent updates are possible but not common

  • Read/write access using soft locks

Indicates a non-vanishing likelihood that two concurrent transactions attempt to update the same item of data simultaneously.

This strategy uses "soft" locks to prevent concurrent transactions from retrieving or storing a stale item from or in the cache during the transaction completion process. A soft lock is simply a marker entry placed in the cache while the updating transaction completes.

  • A second transaction may not read the item from the cache while the soft lock is present, and instead simply proceeds to read the item directly from the database, exactly as if a regular cache miss had occurred.

  • Similarly, the soft lock also prevents this second transaction from storing a stale item to the cache when it returns from its round trip to the database with something that might not quite be the latest version.

TRANSACTIONAL

  • Concurrent updates are frequent

  • Transactional access

Indicates that concurrent writes are common, and the only way to maintain synchronization between the second-level cache and the database is via the use of a fully transactional cache provider. In this case, the cache and the database must cooperate via JTA or the XA protocol, and Hibernate itself takes on little responsibility for maintaining the integrity of the cache.

Which policies make sense may also depend on the underlying second-level cache implementation.

JPA has a similar annotation, named @Cacheable. Unfortunately, it’s almost useless to us, since:

  • it provides no way to specify any information about the nature of the cached entity and how its cache should be managed, and

  • it may not be used to annotate associations, and so we can’t even use it to mark collection roles as eligible for storage in the second-level cache.

7.8. Caching by natural id

If our entity has a natural id, we can enable an additional cache, which holds cross-references from natural id to primary id, by annotating the entity @NaturalIdCache. By default, the natural id cache is stored in a dedicated region of the second-level cache, separate from the cached entity data.

@Entity
@Cache(usage=READ_WRITE, region="Book")
@NaturalIdCache(region="BookIsbn")
class Book {
    ...
    @NaturalId
    String isbn;

    @NaturalId
    int printing;
    ...
}

This cache is utilized when the entity is retrieved using one of the operations of Session which performs lookup by natural id.

Since the natural id cache doesn’t contain the actual state of the entity, it doesn’t make sense to annotate an entity @NaturalIdCache unless it’s already eligible for storage in the second-level cache, that is, unless it’s also annotated @Cache.

It’s worth noticing that, unlike the primary identifier of an entity, a natural id might be mutable.

We must now consider a subtlety that often arises when we have to deal with so-called "reference data", that is, data which fits easily in memory, and doesn’t change much.

7.9. Caching and association fetching

Let’s consider again our Publisher class:

@Cache(usage=NONSTRICT_READ_WRITE, region="Publishers")
@Entity
class Publisher { ... }

Data about publishers doesn’t change very often, and there aren’t so many of them. Suppose we’ve set everything up so that the publishers are almost always available in the second-level cache.

Then in this case we need to think carefully about associations of type Publisher.

@ManyToOne
Publisher publisher;

There’s no need for this association to be lazily fetched, since we’re expecting it to be available in memory, so we won’t set it fetch=LAZY. But on the other hand, if we leave it marked for eager fetching then, by default, Hibernate will often fetch it using a join. This places completely unnecessary load on the database.

The solution is the @Fetch annotation:

@ManyToOne @Fetch(SELECT)
Publisher publisher;

By annotating the association @Fetch(SELECT), we suppress join fetching, giving Hibernate a chance to find the associated Publisher in the cache.

Therefore, we arrive at this rule of thumb:

Many-to-one associations to "reference data", or to any other data that will almost always be available in the cache, should be mapped EAGER,SELECT.

Other associations, as we’ve already made clear, should be LAZY.

Once we’ve marked an entity or collection as eligible for storage in the second-level cache, we still need to set up an actual cache.

7.10. Configuring the second-level cache provider

Configuring a second-level cache provider is a rather involved topic, and quite outside the scope of this document. But in case it helps, we often test Hibernate with the following configuration, which uses EHCache as the cache implementation, as above in Optional dependencies:

Table 48. EHCache configuration
Configuration property name Property value

hibernate.cache.region.factory_class

jcache

hibernate.javax.cache.uri

/ehcache.xml

If you’re using EHCache, you’ll also need to include an ehcache.xml file that explicitly configures the behavior of each cache region belonging to your entities and collections. You’ll find more information about configuring EHCache here.

We may use any other implementation of JCache, such as Caffeine. JCache automatically selects whichever implementation it finds on the classpath. If there are multiple implementations on the classpath, we must disambiguate using:

Table 49. Disambiguating the JCache implementation
Configuration property name Property value

hibernate.javax.cache.provider

The implementation of javax.cache.spiCachingProvider, for example:

org.ehcache.jsr107.EhcacheCachingProvider

for EHCache

com.github.benmanes.caffeine.jcache.spi.CaffeineCachingProvider

for Caffeine

Alternatively, to use Infinispan as the cache implementation, the following settings are required:

Table 50. Infinispan provider configuration
Configuration property name Property value

hibernate.cache.region.factory_class

infinispan

hibernate.cache.infinispan.cfg

Path to infinispan configuration file, for example:

org/infinispan/hibernate/cache/commons/builder/infinispan-configs.xml

for a distributed cache

org/infinispan/hibernate/cache/commons/builder/infinispan-configs-local.xml

to test with local cache

Infinispan is usually used when distributed caching is required. There’s more about using Infinispan with Hibernate here.

Finally, there’s a way to globally disable the second-level cache:

Table 51. Setting to disable caching
Configuration property name Property value

hibernate.cache.use_second_level_cache

true to enable caching, or false to disable it

When hibernate.cache.region.factory_class is set, this property defaults to true.

This setting lets us easily disable the second-level cache completely when troubleshooting or profiling performance.

You can find much more information about the second-level cache in the User Guide.

7.11. Caching query result sets

The caches we’ve described above are only used to optimize lookups by id or by natural id. Hibernate also has a way to cache the result sets of queries, though this is only rarely an efficient thing to do.

The query cache must be enabled explicitly:

Table 52. Setting to enable the query cache
Configuration property name Property value

hibernate.cache.use_query_cache

true to enable the query cache

To cache the results of a query, call SelectionQuery.setCacheable(true):

session.createQuery("from Product where discontinued = false")
    .setCacheable(true)
    .getResultList();

By default, the query result set is stored in a cache region named default-query-results-region. Since different queries should have different caching policies, it’s common to explicitly specify a region name:

session.createQuery("from Product where discontinued = false")
    .setCacheable(true)
    .setCacheRegion("ProductCatalog")
    .getResultList();

A result set is cached together with a logical timestamp. By "logical", we mean that it doesn’t actually increase linearly with time, and in particular it’s not the system time.

When a Product is updated, Hibernate does not go through the query cache and invalidate every cached result set that’s affected by the change. Instead, there’s a special region of the cache which holds a logical timestamp of the most-recent update to each table. This is called the update timestamps cache, and it’s kept in the region default-update-timestamps-region.

It’s your responsibility to ensure that this cache region is configured with appropriate policies. In particular, update timestamps should never expire or be evicted.

When a query result set is read from the cache, Hibernate compares its timestamp with the timestamp of each of the tables that affect the results of the query, and only returns the result set if the result set isn’t stale. If the result set is stale, Hibernate goes ahead and re-executes the query against the database and updates the cached result set.

As is generally the case with any second-level cache, the query cache can break the ACID properties of transactions.

7.12. Second-level cache management

For the most part, the second-level cache is transparent. Program logic which interacts with the Hibernate session is unaware of the cache, and is not impacted by changes to caching policies.

At worst, interaction with the cache may be controlled by specifying of an explicit CacheMode:

session.setCacheMode(CacheMode.IGNORE);

Or, using JPA-standard APIs:

entityManager.setCacheRetrieveMode(CacheRetrieveMode.BYPASS);
entityManager.setCacheStoreMode(CacheStoreMode.BYPASS);

The JPA-defined cache modes come in two flavors: CacheRetrieveMode and CacheStoreMode.

Table 53. JPA-defined cache retrieval modes
Mode Interpretation

CacheRetrieveMode.USE

Read data from the cache if available

CacheRetrieveMode.BYPASS

Don’t read data from the cache; go direct to the database

We might select CacheRetrieveMode.BYPASS if we’re concerned about the possibility of reading stale data from the cache.

Table 54. JPA-defined cache storage modes
Mode Interpretation

CacheStoreMode.USE

Write data to the cache when read from the database or when modified; do not update already-cached items when reading

CacheStoreMode.REFRESH

Write data to the cache when read from the database or when modified; always update cached items when reading

CacheStoreMode.BYPASS

Don’t write data to the cache

We should select CacheStoreMode.BYPASS if we’re querying data that doesn’t need to be cached.

It’s a good idea to set the CacheStoreMode to BYPASS just before running a query which returns a large result set full of data that we don’t expect to need again soon. This saves work, and prevents the newly-read data from pushing out the previously cached data.

In JPA we would use this idiom:

entityManager.setCacheStoreMode(CacheStoreMode.BYPASS);
List<Publisher> allpubs =
        entityManager.createQuery("from Publisher", Publisher.class)
            .getResultList();
entityManager.setCacheStoreMode(CacheStoreMode.USE);

But Hibernate has a better way:

List<Publisher> allpubs =
        session.createSelectionQuery("from Publisher", Publisher.class)
            .setCacheStoreMode(CacheStoreMode.BYPASS)
            .getResultList();

A Hibernate CacheMode packages a CacheRetrieveMode with a CacheStoreMode.

Table 55. Hibernate cache modes and JPA equivalents
Hibernate CacheMode Equivalent JPA modes

NORMAL

CacheRetrieveMode.USE, CacheStoreMode.USE

IGNORE

CacheRetrieveMode.BYPASS, CacheStoreMode.BYPASS

GET

CacheRetrieveMode.USE, CacheStoreMode.BYPASS

PUT

CacheRetrieveMode.BYPASS, CacheStoreMode.USE

REFRESH

CacheRetrieveMode.REFRESH, CacheStoreMode.BYPASS

There’s no particular reason to prefer Hibernate’s CacheMode over the JPA equivalents. This enumeration only exists because Hibernate had cache modes long before they were added to JPA.

For "reference" data, that is, for data which is expected to always be found in the second-level cache, it’s a good idea to prime the cache at startup. There’s a really easy way to do this: just execute a query immediately after obtaining the EntityManager or SessionFactory.

SessionFactory sessionFactory =
        setupHibernate(new Configuration())
            .buildSessionFactory();
// prime the second-level cache
sessionFactory.inSession(session -> {
    session.createSelectionQuery("from Country"))
        .setReadOnly(true)
        .getResultList();
    session.createSelectionQuery("from Product where discontinued = false"))
        .setReadOnly(true)
        .getResultList();
});

Very occasionally, it’s necessary or advantageous to control the cache explicitly, for example, to evict some data that we know to be stale. The Cache interface allows programmatic eviction of cached items.

sessionFactory.getCache().evictEntityData(Book.class, bookId);

Second-level cache management via the Cache interface is not transaction-aware. None of the operations of Cache respect any isolation or transactional semantics associated with the underlying caches. In particular, eviction via the methods of this interface causes an immediate "hard" removal outside any current transaction and/or locking scheme.

Ordinarily, however, Hibernate automatically evicts or updates cached data after modifications, and, in addition, cached data which is unused will eventually be expired according to the configured policies.

This is quite different to what happens with the first-level cache.

7.13. Session cache management

Entity instances aren’t automatically evicted from the session cache when they’re no longer needed. Instead, they stay pinned in memory until the session they belong to is discarded by your program.

The methods detach() and clear() allow you to remove entities from the session cache, making them available for garbage collection. Since most sessions are rather short-lived, you won’t need these operations very often. And if you find yourself thinking you do need them in a certain situation, you should strongly consider an alternative solution: a stateless session.

7.14. Stateless sessions

An arguably-underappreciated feature of Hibernate is the StatelessSession interface, which provides a command-oriented, more bare-metal approach to interacting with the database.

You may obtain a stateless session from the SessionFactory:

StatelessSession ss = getSessionFactory().openStatelessSession();

A stateless session:

  • doesn’t have a first-level cache (persistence context), nor does it interact with any second-level caches, and

  • doesn’t implement transactional write-behind or automatic dirty checking, so all operations are executed immediately when they’re explicitly called.

For a stateless session, we’re always working with detached objects. Thus, the programming model is a bit different:

Table 56. Important methods of the StatelessSession
Method name and parameters Effect

get(Class, Object)

Obtain a detached object, given its type and its id, by executing a select

fetch(Object)

Fetch an association of a detached object

refresh(Object)

Refresh the state of a detached object by executing a select

insert(Object)

Immediately insert the state of the given transient object into the database

update(Object)

Immediately update the state of the given detached object in the database

delete(Object)

Immediately delete the state of the given detached object from the database

upsert(Object)

Immediately insert or update the state of the given detached object using a SQL merge into statement

The operations of a stateless session have no corresponding CascadeTypes, and so these operations never cascade to associated entity instances.
There’s no flush() operation, and so update() is always explicit.

In certain circumstances, this makes stateless sessions easier to work with and simpler to reason about, but with the caveat that a stateless session is much more vulnerable to data aliasing effects, since it’s easy to get two non-identical Java objects which both represent the same row of a database table.

If we use fetch() in a stateless session, we can very easily obtain two objects representing the same database row!

In particular, the absence of a persistence context means that we can safely perform bulk-processing tasks without allocating huge quantities of memory. Use of a StatelessSession alleviates the need to call:

  • clear() or detach() to perform first-level cache management, and

  • setCacheMode() to bypass interaction with the second-level cache.

Stateless sessions can be useful, but for bulk operations on huge datasets, Hibernate can’t possibly compete with stored procedures!

7.15. Optimistic and pessimistic locking

Finally, an aspect of behavior under load that we didn’t mention above is row-level data contention. When many transactions try to read and update the same data, the program might become unresponsive with lock escalation, deadlocks, and lock acquisition timeout errors.

There’s two basic approaches to data concurrency in Hibernate:

  • optimistic locking using @Version columns, and

  • database-level pessimistic locking using the SQL for update syntax (or equivalent).

In the Hibernate community it’s much more common to use optimistic locking, and Hibernate makes that incredibly easy.

Where possible, in a multiuser system, avoid holding a pessimistic lock across a user interaction. Indeed, the usual practice is to avoid having transactions that span user interactions. For multiuser systems, optimistic locking is king.

That said, there is also a place for pessimistic locks, which can sometimes reduce the probability of transaction rollbacks.

Therefore, the find(), lock(), and refresh() methods of the reactive session accept an optional LockMode. We can also specify a LockMode for a query. The lock mode can be used to request a pessimistic lock, or to customize the behavior of optimistic locking:

Table 57. Optimistic and pessimistic lock modes
LockMode type Meaning

READ

An optimistic lock obtained implicitly whenever an entity is read from the database using select

OPTIMISTIC

An optimistic lock obtained when an entity is read from the database, and verified using a select to check the version when the transaction completes

OPTIMISTIC_FORCE_INCREMENT

An optimistic lock obtained when an entity is read from the database, and enforced using an update to increment the version when the transaction completes

WRITE

A pessimistic lock obtained implicitly whenever an entity is written to the database using update or insert

PESSIMISTIC_READ

A pessimistic for share lock

PESSIMISTIC_WRITE

A pessimistic for update lock

PESSIMISTIC_FORCE_INCREMENT

A pessimistic lock enforced using an immediate update to increment the version

NONE

No lock; assigned when an entity is read from the second-level cache

Note that an OPTIMISTIC lock is always verified at the end of the transaction, even when the entity has not been modified. This is slightly different to what most people mean when they talk about an "optimistic lock". It’s never necessary to request an OPTIMISTIC lock on a modified entity, since the version number is always verified when a SQL update is executed.

JPA has its own LockModeType, which enumerates most of the same modes. However, JPA’s LockModeType.READ is a synonym for OPTIMISTIC — it’s not the same as Hibernate’s LockMode.READ. Similarly, LockModeType.WRITE is a synonym for OPTIMISTIC_FORCE_INCREMENT and is not the same as LockMode.WRITE.

7.16. Collecting statistics

We may ask Hibernate to collect statistics about its activity by setting this configuration property:

Configuration property name Property value

hibernate.generate_statistics

true to enable collection of statistics

The statistics are exposed by the Statistics object:

long failedVersionChecks =
        sessionFactory.getStatistics()
            .getOptimisticFailureCount();

long publisherCacheMissCount =
        sessionFactory.getStatistics()
            .getEntityStatistics(Publisher.class.getName())
                .getCacheMissCount()

Hibernate’s statistics enable observability. Both Micrometer and SmallRye Metrics are capable of exposing these metrics.

7.17. Tracking down slow queries

When a poorly-performing SQL query is discovered in production, it can sometimes be hard to track down exactly where in the Java code the query originates. Hibernate offers two configuration properties that can make it easier to identify a slow query and find its source.

Table 58. Settings for tracking slow queries
Configuration property name Purpose Property value

hibernate.log_slow_query

Log slow queries at the INFO level

The minimum execution time, in milliseconds, which characterizes a "slow" query

hibernate.use_sql_comments

Prepend comments to the executed SQL

true or false

When hibernate.use_sql_comments is enabled, the text of the HQL query is prepended as a comment to the generated SQL, which usually makes it easy to find the HQL in the Java code.

The comment text may be customized:

  • by calling Query.setComment(comment) or Query.setHint(AvailableHints.HINT_COMMENT,comment), or

  • via the @NamedQuery annotation.

Once you’ve identified a slow query, one of the best ways to make it faster is to actually go and talk to someone who is an expert at making queries go fast. These people are called "database administrators", and if you’re reading this document you probably aren’t one. Database administrators know lots of stuff that Java developers don’t. So if you’re lucky enough to have a DBA about, you don’t need to Dunning-Kruger your way out of a slow query.

An expertly-defined index might be all you need to fix a slow query.

7.18. Adding indexes

The @Index annotation may be used to add an index to a table:

@Entity
@Table(indexes=@Index(columnList="title, year, publisher_id"))
class Book { ... }

It’s even possible to specify an ordering for an indexed column, or that the index should be case-insensitive:

@Entity
@Table(indexes=@Index(columnList="(lower(title)), year desc, publisher_id"))
class Book { ... }

This lets us create a customized index for a particular query.

Note that SQL expressions like lower(title) must be enclosed in parentheses in the columnList of the index definition.

It’s not clear that information about indexes belongs in annotations of Java code. Indexes are usually maintained and modified by a database administrator, ideally by an expert in tuning the performance of one particular RDBMS. So it might be better to keep the definition of indexes in a SQL DDL script that your DBA can easily read and modify. Remember, we can ask Hibernate to execute a DDL script using the property javax.persistence.schema-generation.create-script-source.

7.19. Dealing with denormalized data

A typical relational database table in a well-normalized schema has a relatively small number of columns, and so there’s little to be gained by selectively querying columns and populating only certain fields of an entity class.

But occasionally, we hear from someone asking how to map a table with a hundred columns or more! This situation can arise when:

  • data is intentionally denormalized for performance,

  • the results of a complicated analytic query are exposed via a view, or

  • someone has done something crazy and wrong.

Let’s suppose that we’re not dealing with the last possibility. Then we would like to be able to query the monster table without returning all of its columns. At first glance, Hibernate doesn’t offer a perfect bottled solution to this problem. This first impression is misleading. Actually, Hibernate features more than one way to deal with this situation, and the real problem is deciding between the ways. We could:

  1. map multiple entity classes to the same table or view, being careful about "overlaps" where a mutable column is mapped to more than one of the entities,

  2. use HQL or native SQL queries returning results into record types instead of retrieving entity instances, or

  3. use the bytecode enhancer and @LazyGroup for attribute-level lazy fetching.

Some other ORM solutions push the third option as the recommended way to handle huge tables, but this has never been the preference of the Hibernate team or Hibernate community. It’s much more typesafe to use one of the first two options.

7.20. Reactive programming with Hibernate

Finally, many systems which require high scalability now make use of reactive programming and reactive streams. Hibernate Reactive brings O/R mapping to the world of reactive programming. You can learn much more about Hibernate Reactive from its Reference Documentation.

Hibernate Reactive may be used alongside vanilla Hibernate in the same program, and can reuse the same entity classes. This means you can use the reactive programming model exactly where you need it—perhaps only in one or two places in your system. You don’t need to rewrite your whole program using reactive streams.

8. Advanced Topics

In the last chapter of this Introduction, we turn to some topics that don’t really belong in an introduction. Here we consider some problems, and solutions, that you’re probably not going to run into immediately if you’re new to Hibernate. But we do want you to know about them, so that when the time comes, you’ll know what tool to reach for.

8.1. Filters

Filters are one of the nicest and under-usedest features of Hibernate, and we’re quite proud of them. A filter is a named, globally-defined, parameterized restriction on the data that is visible in a given session.

Examples of well-defined filters might include:

  • a filter that restricts the data visible to a given user according to row-level permissions,

  • a filter which hides data which has been soft-deleted,

  • in a versioned database, a filter that displays versions which were current at a given instant in the past, or

  • a filter that restricts to data associated with a certain geographical region.

A filter must be declared somewhere. A package descriptor is as good a place as any for a @FilterDef:

@FilterDef(name = "ByRegion",
           parameters = @ParamDef(name = "region", type = String.class))
package org.hibernate.example;

This filter has one parameter. Fancier filters might in principle have multiple parameters, though we admit this must be quite rare.

If you add annotations to a package descriptor, and you’re using Configuration to configure Hibernate, make sure you call Configuration.addPackage() to let Hibernate know that the package descriptor is annotated.

Typically, but not necessarily, a @FilterDef specifies a default restriction:

@FilterDef(name = "ByRegion",
           parameters = @ParamDef(name = "region", type = String.class),
           defaultCondition = "region = :region")
package org.hibernate.example;

The restriction must contain a reference to the parameter of the filter, specified using the usual syntax for named parameters.

Any entity or collection which is affected by a filter must be annotated @Filter:

@Entity
@Filter(name = example_.BY_REGION)
class User {

    @Id String username;

    String region;

    ...
}

Here, as usual, example_.BY_REGION is generated by the Metamodel Generator, and is just a constant with the value "ByRegion".

If the @Filter annotation does not explicitly specify a restriction, the default restriction given by the @FilterDef will be applied to the entity. But an entity is free to override the default condition.

@Entity
@Filter(name = example_.FILTER_BY_REGION, condition = "name = :region")
class Region {

    @Id String name;

    ...
}

Note that the restriction specified by the condition or defaultCondition is a native SQL expression.

Table 59. Annotations for defining filters
Annotation Purpose

@FilterDef

Defines a filter and declares its name (exactly one per filter)

@Filter

Specifies how a filter applies to a given entity or collection (many per filter)

A filter condition may not specify joins to other tables, but it may contain a subquery.

@Filter(name="notDeleted"
        condition="(select r.deletionTimestamp from Record r where r.id = record_id) is not null")

Only unqualified column names like record_id in this example are interpreted as belonging to the table of the filtered entity.

By default, a new session comes with every filter disabled. A filter may be explicitly enabled in a given session by calling enableFilter() and assigning arguments to the parameters of the filter using the returned instance of Filter. You should do this right at the start of the session.

sessionFactory.inTransaction(session -> {
    session.enableFilter(example_.FILTER_BY_REGION)
        .setParameter("region", "es")
        .validate();

    ...
});

Now, any queries executed within the session will have the filter restriction applied. Collections annotated @Filter will also have their members correctly filtered.

On the other hand, filters are not applied to @ManyToOne associations, nor to find(). This is completely by design and is not in any way a bug.

More than one filter may be enabled in a given session.

Alternatively, since Hibernate 6.5, a filter may be declared as autoEnabled in every session. In this case, the argument to a filter parameter must be obtained from a Supplier.

@FilterDef(name = "ByRegion",
           autoEnabled = true,
           parameters = @ParamDef(name = "region", type = String.class,
                                  resolver = RegionSupplier.class),
           defaultCondition = "region = :region")
package org.hibernate.example;

It’s not necessary to call enableFilter() for a filter declared autoEnabled = true.

When we only need to filter rows by a static condition with no parameters, we don’t need a filter, since @SQLRestriction provides a much simpler way to do that.

We’ve mentioned that a filter can be used to implement versioning, and to provide historical views of the data. Being such a general-purpose construct, filters provide a lot of flexibility here. But if you’re after a more focused/opinionated solution to this problem, you should definitely check out Envers.

Using Envers for auditing historical data

Envers is an add-on to Hibernate ORM which keeps a historical record of each versioned entity in a separate audit table, and allows past revisions of the data to be viewed and queried. A full introduction to Envers would require a whole chapter, so we’ll just give you a quick taste here.

First, we must mark an entity as versioned, using the @Audited annotation:

@Audited @Entity
@Table(name="CurrentDocument")
@AuditTable("DocumentRevision")
class Document { ... }

The @AuditTable annotation is optional, and it’s better to set either org.hibernate.envers.audit_table_prefix or org.hibernate.envers.audit_table_suffix and let the audit table name be inferred.

The AuditReader interface exposes operations for retrieving and querying historical revisions. It’s really easy to get hold of one of these:

AuditReader reader = AuditReaderFactory.get(entityManager);

Envers tracks revisions of the data via a global revision number. We may easily find the revision number which was current at a given instant:

Number revision = reader.getRevisionNumberForDate(datetime);

We can use the revision number to ask for the version of our entity associated with the given revision number:

Document doc = reader.find(Document.class, id, revision);

Alternatively, we can directly ask for the version which was current at a given instant:

Document doc = reader.find(Document.class, id, datetime);

We can even execute queries to obtain lists of entities current at the given revision number:

List documents =
        reader.createQuery()
            .forEntitiesAtRevision(Document.class, revision)
            .getResultList();

For much more information, see the User Guide.

Historically, filters where often used to implement soft-delete. But, since 6.4, Hibernate now comes with soft-delete built in.

8.2. Soft-delete

Even when we don’t need complete historical versioning, we often prefer to "delete" a row by marking it as obsolete using a SQL update, rather than by executing an actual SQL delete and removing the row from the database completely.

The @SoftDelete annotation controls how this works:

@Entity
@SoftDelete(columnName = "deleted",
            converter = TrueFalseConverter.class)
class Draft {

    ...
}

The columnName specifies a column holding the deletion status, and the converter is responsible for converting a Java Boolean to the type of that column. In this example, TrueFalseConverter sets the column to the character 'F' initially, and to 'T' when the row is deleted. Any JPA AttributeConverter for the Java Boolean type may be used here. Built-in options include NumericBooleanConverter and YesNoConverter.

Much more information about soft delete is available in the User Guide.

Another feature that you could use filters for, but now don’t need to, is multi-tenancy.

8.3. Multi-tenancy

A multi-tenant database is one where the data is segregated by tenant. We don’t need to actually define what a "tenant" really represents here; all we care about at this level of abstraction is that each tenant may be distinguished by a unique identifier. And that there’s a well-defined current tenant in each session.

We may specify the current tenant when we open a session:

var session =
        sessionFactory.withOptions()
            .tenantIdentifier(tenantId)
            .openSession();

Or, when using JPA-standard APIs:

var entityManager =
        entityManagerFactory.createEntityManager(Map.of(HibernateHints.HINT_TENANT_ID, tenantId));

However, since we often don’t have this level of control over creation of the session, it’s more common to supply an implementation of CurrentTenantIdentifierResolver to Hibernate.

There are three common ways to implement multi-tenancy:

  1. each tenant has its own database,

  2. each tenant has its own schema, or

  3. tenants share tables in a single schema, and rows are tagged with the tenant id.

From the point of view of Hibernate, there’s little difference between the first two options. Hibernate will need to obtain a JDBC connection with permissions on the database and schema owned by the current tenant.

Therefore, we must implement a MultiTenantConnectionProvider which takes on this responsibility:

  • from time to time, Hibernate will ask for a connection, passing the id of the current tenant, and then we must create an appropriate connection or obtain one from a pool, and return it to Hibernate, and

  • later, Hibernate will release the connection and ask us to destroy it or return it to the appropriate pool.

The third option is quite different. In this case we don’t need a MultiTenantConnectionProvider, but we will need a dedicated column holding the tenant id mapped by each of our entities.

@Entity
class Account {
    @Id String id;
    @TenantId String tenantId;

    ...
}

The @TenantId annotation is used to indicate an attribute of an entity which holds the tenant id. Within a given session, our data is automatically filtered so that only rows tagged with the tenant id of the current tenant are visible in that session.

Native SQL queries are not automatically filtered by tenant id; you’ll have to do that part yourself.

To make use of multi-tenancy, we’ll usually need to set at least one of these configuration properties:

Table 60. Multi-tenancy configuration
Configuration property name Purpose

hibernate.tenant_identifier_resolver

Specifies the CurrentTenantIdentifierResolver

hibernate.multi_tenant_connection_provider

Specifies the MultiTenantConnectionProvider

A longer discussion of multi-tenancy may be found in the User Guide.

8.4. Using custom-written SQL

We’ve already discussed how to run queries written in SQL, but occasionally that’s not enough. Sometimes—but much less often than you might expect—we would like to customize the SQL used by Hibernate to perform basic CRUD operations for an entity or collection.

For this we can use @SQLInsert and friends:

@Entity
@SQLInsert(sql = "insert into person (name, id, valid) values (?, ?, true)",
           verify = Expectation.RowCount.class)
@SQLUpdate(sql = "update person set name = ? where id = ?")
@SQLDelete(sql = "update person set valid = false where id = ?")
@SQLSelect(sql = "select id, name from person where id = ? and valid = true")
public static class Person { ... }
Table 61. Annotations for overriding generated SQL
Annotation Purpose

@SQLSelect

Overrides a generated SQL select statement

@SQLInsert

Overrides a generated SQL insert statement

@SQLUpdate

Overrides a generated SQL update statement

@SQDelete

Overrides a generated SQL delete statement a single rows

@SQDeleteAll

Overrides a generated SQL delete statement for multiple rows

@SQLRestriction

Adds a restriction to generated SQL

@SQLOrder

Adds an ordering to generated SQL

If the custom SQL should be executed via a CallableStatement, just specify callable=true.

Any SQL statement specified by one of these annotations must have exactly the number of JDBC parameters that Hibernate expects, that is, one for each column mapped by the entity, in the exact order Hibernate expects. In particular, the primary key columns must come last.

However, the @Column annotation does lend some flexibility here:

  • if a column should not be written as part of the custom insert statement, and has no corresponding JDBC parameter in the custom SQL, map it @Column(insertable=false), or

  • if a column should not be written as part of the custom update statement, and has no corresponding JDBC parameter in the custom SQL, map it @Column(updatable=false).

The verify member of these annotations specifies a class implementing Expectation, allowing customized logic for checking the success of an operation executed via JDBC. There are three built-in implementations:

  • Expectation.None, which performs no checks,

  • Expectation.RowCount, which is what Hibernate usually uses when executing its own generated SQL,

  • and Expectation.OutParameter, which is useful for checking an output parameter of a stored procedure.

You can write your own implementation of Expectation if none of these options is suitable.

If you need custom SQL, but are targeting multiple dialects of SQL, you can use the annotations defined in DialectOverride. For example, this annotation lets us override the custom insert statement just for PostgreSQL:

@DialectOverride.SQLInsert(dialect = PostgreSQLDialect.class,
    override = @SQLInsert(sql="insert into person (name,id) values (?,gen_random_uuid())"))

It’s even possible to override the custom SQL for specific versions of a database.

Sometimes a custom insert or update statement assigns a value to a mapped column which is calculated when the statement is executed on the database. For example, the value might be obtained by calling a SQL function:

@SQLInsert(sql = "insert into person (name, id) values (?, gen_random_uuid())")

But the entity instance which represents the row being inserted or updated won’t be automatically populated with that value. And so our persistence context loses synchronization with the database. In situations like this, we may use the @Generated annotation to tell Hibernate to reread the state of the entity after each insert or update.

8.5. Handling database-generated columns

Sometimes, a column value is assigned or mutated by events that happen in the database, and aren’t visible to Hibernate. For example:

  • a table might have a column value populated by a trigger,

  • a mapped column might have a default value defined in DDL, or

  • a custom SQL insert or update statement might assign a value to a mapped column, as we saw in the previous subsection.

One way to deal with this situation is to explicitly call refresh() at appropriate moments, forcing the session to reread the state of the entity. But this is annoying.

The @Generated annotation relieves us of the burden of explicitly calling refresh(). It specifies that the value of the annotated entity attribute is generated by the database, and that the generated value should be automatically retrieved using a SQL returning clause, or separate select after it is generated.

A useful example is the following mapping:

@Entity
class Entity {
    @Generated @Id
    @ColumnDefault("gen_random_uuid()")
    UUID id;
}

The generated DDL is:

create table Entity (
    id uuid default gen_random_uuid() not null,
    primary key (uuid)
)

So here the value of id is defined by the column default clause, by calling the PostgreSQL function gen_random_uuid().

When a column value is generated during updates, use @Generated(event=UPDATE). When a value is generated by both inserts and updates, use @Generated(event={INSERT,UPDATE}).

For columns which should be generated using a SQL generated always as clause, prefer the @GeneratedColumn annotation, so that Hibernate automatically generates the correct DDL.

Actually, the @Generated and @GeneratedColumn annotations are defined in terms of a more generic and user-extensible framework for handling attribute values generated in Java, or by the database. So let’s drop down a layer, and see how that works.

8.6. User-defined generators

JPA doesn’t define a standard way to extend the set of id generation strategies, but Hibernate does:

  • the Generator hierarchy of interfaces in the package org.hibernate.generator lets you define new generators, and

  • the @IdGeneratorType meta-annotation from the package org.hibernate.annotations lets you write an annotation which associates a Generator type with identifier attributes.

Furthermore, the @ValueGenerationType meta-annotation lets you write an annotation which associates a Generator type with a non-@Id attribute.

These APIs are new in Hibernate 6, and supersede the classic IdentifierGenerator interface and @GenericGenerator annotation from older versions of Hibernate. However, the older APIs are still available and custom IdentifierGenerators written for older versions of Hibernate continue to work in Hibernate 6.

Hibernate has a range of built-in generators which are defined in terms of this new framework.

Table 62. Built-in generators
Annotation Implementation Purpose

@Generated

GeneratedGeneration

Generically handles database-generated values

@GeneratedColumn

GeneratedAlwaysGeneration

Handles values generated using generated always

@CurrentTimestamp

CurrentTimestampGeneration

Generic support for database or in-memory generation of creation or update timestamps

@CreationTimestamp

CurrentTimestampGeneration

A timestamp generated when an entity is first made persistent

@UpdateTimestamp

CurrentTimestampGeneration

A timestamp generated when an entity is made persistent, and regenerated every time the entity is modified

@UuidGenerator

UuidGenerator

A more flexible generator for RFC 4122 UUIDs

Furthermore, support for JPA’s standard id generation strategies is also defined in terms of this framework.

As an example, let’s look at how @UuidGenerator is defined:

@IdGeneratorType(org.hibernate.id.uuid.UuidGenerator.class)
@ValueGenerationType(generatedBy = org.hibernate.id.uuid.UuidGenerator.class)
@Retention(RUNTIME)
@Target({ FIELD, METHOD })
public @interface UuidGenerator { ... }

@UuidGenerator is meta-annotated both @IdGeneratorType and @ValueGenerationType because it may be used to generate both ids and values of regular attributes. Either way, this Generator class does the hard work:

public class UuidGenerator
        // this generator produced values before SQL is executed
        implements BeforeExecutionGenerator {

    // constructors accept an instance of the @UuidGenerator
    // annotation, allowing the generator to be "configured"

    // called to create an id generator
    public UuidGenerator(
            org.hibernate.annotations.UuidGenerator config,
            Member idMember,
            GeneratorCreationContext creationContext) {
        this(config, idMember);
    }

    // called to create a generator for a regular attribute
    public UuidGenerator(
            org.hibernate.annotations.UuidGenerator config,
            Member member,
            GeneratorCreationContext creationContext) {
        this(config, idMember);
    }

    ...

    @Override
    public EnumSet<EventType> getEventTypes() {
        // UUIDs are only assigned on insert, and never regenerated
        return INSERT_ONLY;
    }

    @Override
    public Object generate(SharedSessionContractImplementor session, Object owner, Object currentValue, EventType eventType) {
        // actually generate a UUID and transform it to the required type
        return valueTransformer.transform( generator.generateUuid( session ) );
    }
}

You can find out more about custom generators from the Javadoc for @IdGeneratorType and for org.hibernate.generator.

8.7. Naming strategies

When working with a pre-existing relational schema, it’s usual to find that the column and table naming conventions used in the schema don’t match Java’s naming conventions.

Of course, the @Table and @Column annotations let us explicitly specify a mapped table or column name. But we would prefer to avoid scattering these annotations across our whole domain model.

Therefore, Hibernate lets us define a mapping between Java naming conventions, and the naming conventions of the relational schema. Such a mapping is called a naming strategy.

First, we need to understand how Hibernate assigns and processes names.

  • Logical naming is the process of applying naming rules to determine the logical names of objects which were not explicitly assigned names in the O/R mapping. That is, when there’s no @Table or @Column annotation.

  • Physical naming is the process of applying additional rules to transform a logical name into an actual "physical" name that will be used in the database. For example, the rules might include things like using standardized abbreviations, or trimming the length of identifiers.

Thus, there’s two flavors of naming strategy, with slightly different responsibilities. Hibernate comes with default implementations of these interfaces:

Flavor Default implementation

An ImplicitNamingStrategy is responsible for assigning a logical name when none is specified by an annotation

A default strategy which implements the rules defined by JPA

A PhysicalNamingStrategy is responsible for transforming a logical name and producing the name used in the database

A trivial implementation which does no processing

We happen to not much like the naming rules defined by JPA, which specify that mixed case and camel case identifiers should be concatenated using underscores. We bet you could easily come up with a much better ImplicitNamingStrategy than that! (Hint: it should always produce legit mixed case identifiers.)

A popular PhysicalNamingStrategy produces snake case identifiers.

Custom naming strategies may be enabled using the configuration properties we already mentioned without much explanation back in Minimizing repetitive mapping information.

Table 63. Naming strategy configuration
Configuration property name Purpose

hibernate.implicit_naming_strategy

Specifies the ImplicitNamingStrategy

hibernate.physical_naming_strategy

Specifies the PhysicalNamingStrategy

8.8. Spatial datatypes

Hibernate Spatial augments the built-in basic types with a set of Java mappings for OGC spatial types.

  • Geolatte-geom defines a set of Java types implementing the OGC spatial types, and codecs for translating to and from database-native spatial datatypes.

  • Hibernate Spatial itself supplies integration with Hibernate.

To use Hibernate Spatial, we must add it as a dependency, as described in Optional dependencies.

Then we may immediately use Geolatte-geom and JTS types in our entities. No special annotations are needed:

import org.locationtech.jts.geom.Point;
import jakarta.persistence.*;

@Entity
class Event {
    Event() {}

    Event(String name, Point location) {
        this.name = name;
        this.location = location;
    }

    @Id @GeneratedValue
    Long id;

    String name;

    Point location;

}

The generated DDL uses geometry as the type of the column mapped by location:

create table Event (
    id bigint not null,
    location geometry,
    name varchar(255),
    primary key (id)
)

Hibernate Spatial lets us work with spatial types just as we would with any of the built-in basic attribute types.

var geometryFactory = new GeometryFactory();
...

Point point = geometryFactory.createPoint(new Coordinate(10, 5));
session.persist(new Event("Hibernate ORM presentation", point));

But what makes this powerful is that we may write some very fancy queries involving functions of spatial types:

Polygon triangle =
        geometryFactory.createPolygon(
                new Coordinate[] {
                        new Coordinate(9, 4),
                        new Coordinate(11, 4),
                        new Coordinate(11, 20),
                        new Coordinate(9, 4)
                }
        );
Point event =
        session.createQuery("select location from Event where within(location, :zone) = true", Point.class)
                .setParameter("zone", triangle)
                .getSingleResult();

Here, within() is one of the functions for testing spatial relations defined by the OpenGIS specification. Other such functions include touches(), intersects(), distance(), boundary(), etc. Not every spatial relation function is supported on every database. A matrix of support for spatial relation functions may be found in the User Guide.

If you want to play with spatial functions on H2, run the following code first:

sessionFactory.inTransaction(session -> {
    session.doWork(connection -> {
        try (var statement = connection.createStatement()) {
            statement.execute("create alias if not exists h2gis_spatial for \"org.h2gis.functions.factory.H2GISFunctions.load\"");
            statement.execute("call h2gis_spatial()");
        }
    });
} );

8.9. Ordered and sorted collections and map keys

Java lists and maps don’t map very naturally to foreign key relationships between tables, and so we tend to avoid using them to represent associations between our entity classes. But if you feel like you really need a collection with a fancier structure than Set, Hibernate does have options.

The first three options let us map the index of a List or key of a Map to a column, and are usually used with a @ElementCollection, or on the owning side of an association:

Table 64. Annotations for mapping lists and maps
Annotation Purpose JPA-standard

@OrderColumn

Specifies the column used to maintain the order of a list

@ListIndexBase

The column value for the first element of the list (zero by default)

@MapKeyColumn

Specifies the column used to persist the keys of a map (used when the key is of basic type)

@MapKeyJoinColumn

Specifies the column used to persist the keys of a map (used when the key is an entity)

@ManyToMany
@OrderColumn // order of list is persistent
List<Author> authors = new ArrayList<>();
@ElementCollection
@OrderColumn(name="tag_order") @ListIndexBase(1) // order column and base value
List<String> tags;
@ElementCollection
@CollectionTable(name = "author_bios",                 // table name
        joinColumns = @JoinColumn(name = "book_isbn")) // column holding foreign key of owner
@Column(name="bio")                                    // column holding map values
@MapKeyJoinColumn(name="author_ssn")                   // column holding map keys
Map<Author,String> biographies;

For a Map representing an unowned @OneToMany association, the column must also be mapped on the owning side, usually by an attribute of the target entity. In this case we usually use a different annotation:

Table 65. Annotation for mapping an entity attribute to a map key
Annotation Purpose JPA-standard

@MapKey

Specifies an attribute of the target entity which acts as the key of the map

@OneToMany(mappedBy = Book_.PUBLISHER)
@MapKey(name = Book_.TITLE) // the key of the map is the title of the book
Map<String,Book> booksByTitle = new HashMap<>();

Now, let’s introduce a little distinction:

  • an ordered collection is one with an ordering maintained in the database, and

  • a sorted collection is one which is sorted in Java code.

These annotations allow us to specify how the elements of a collection should be ordered as they are read from the database:

Table 66. Annotations for ordered collections
Annotation Purpose JPA-standard

@OrderBy

Specifies a fragment of JPQL used to order the collection

@SQLOrder

Specifies a fragment of SQL used to order the collection

On the other hand, the following annotations specify how a collection should be sorted in memory, and are used for collections of type SortedSet or SortedMap:

Table 67. Annotations for sorted collections
Annotation Purpose JPA-standard

@SortNatural

Specifies that the elements of a collection are Comparable

@SortComparator

Specifies a Comparator used to sort the collection

Under the covers, Hibernate uses a TreeSet or TreeMap to maintain the collection in sorted order.

8.10. Any mappings

An @Any mapping is a sort of polymorphic many-to-one association where the target entity types are not related by the usual entity inheritance. The target type is distinguished using a discriminator value stored on the referring side of the relationship.

This is quite different to discriminated inheritance where the discriminator is held in the tables mapped by the referenced entity hierarchy.

For example, consider an Order entity containing Payment information, where a Payment might be a CashPayment or a CreditCardPayment:

interface Payment { ... }

@Entity
class CashPayment { ... }

@Entity
class CreditCardPayment { ... }

In this example, Payment is not be declared as an entity type, and is not annotated @Entity. It might even be an interface, or at most just a mapped superclass, of CashPayment and CreditCardPayment. So in terms of the object/relational mappings, CashPayment and CreditCardPayment would not be considered to participate in the same entity inheritance hierarchy.

On the other hand, CashPayment and CreditCardPayment do have the same identifier type. This is important.

An @Any mapping would store the discriminator value identifying the concrete type of Payment along with the state of the associated Order, instead of storing it in the table mapped by Payment.

@Entity
class Order {
    ...

    @Any
    @AnyKeyJavaClass(UUID.class)   //the foreign key type
    @JoinColumn(name="payment_id") // the foreign key column
    @Column(name="payment_type")   // the discriminator column
    // map from discriminator values to target entity types
    @AnyDiscriminatorValue(discriminator="CASH", entity=CashPayment.class)
    @AnyDiscriminatorValue(discriminator="CREDIT", entity=CreditCardPayment.class)
    Payment payment;

    ...
}

It’s reasonable to think of the "foreign key" in an @Any mapping as a composite value made up of the foreign key and discriminator taken together. Note, however, that this composite foreign key is only conceptual and cannot be declared as a physical constraint on the relational database table.

There are a number of annotations which are useful to express this sort of complicated and unnatural mapping:

Table 68. Annotations for @Any mappings
Annotations Purpose

@Any

Declares that an attribute is a discriminated polymorphic association mapping

@AnyDiscriminator

Specify the Java type of the discriminator

@JdbcType or @JdbcTypeCode

Specify the JDBC type of the discriminator

@AnyDiscriminatorValue

Specifies how discriminator values map to entity types

@Column or @Formula

Specify the column or formula in which the discriminator value is stored

@AnyKeyJavaType or @AnyKeyJavaClass

Specify the Java type of the foreign key (that is, of the ids of the target entities)

@AnyKeyJdbcType or @AnyKeyJdbcTypeCode

Specify the JDBC type of the foreign key

@JoinColumn

Specifies the foreign key column

Of course, @Any mappings are disfavored, except in extremely special cases, since it’s much more difficult to enforce referential integrity at the database level.

There’s also currently some limitations around querying @Any associations in HQL. This is allowed:

from Order ord
    join CashPayment cash
        on id(ord.payment) = cash.id

Polymorphic association joins for @Any mappings are not currently implemented.

Further information may be found in the User Guide.

8.11. Selective column lists in inserts and updates

By default, Hibernate generates insert and update statements for each entity during boostrap, and reuses the same insert statement every time an instance of the entity is made persistent, and the same update statement every time an instance of the entity is modified.

This means that:

  • if an attribute is null when the entity is made persistent, its mapped column is redundantly included in the SQL insert, and

  • worse, if a certain attribute is unmodified when other attributes are changed, the column mapped by that attribute is redundantly included in the SQL update.

Most of the time, this just isn’t an issue worth worrying about. The cost of interacting with the database is usually dominated by the cost of a round trip, not by the number of columns in the insert or update. But in cases where it does become important, there are two ways to be more selective about which columns are included in the SQL.

The JPA-standard way is to indicate statically which columns are eligible for inclusion via the @Column annotation. For example, if an entity is always created with an immutable creationDate, and with no completionDate, then we would write:

@Column(updatable=false) LocalDate creationDate;
@Column(insertable=false) LocalDate completionDate;

This approach works quite well in many cases, but often breaks down for entities with more than a handful of updatable columns.

An alternative solution is to ask Hibernate to generate SQL dynamically each time an insert or update is executed. We do this by annotating the entity class.

Table 69. Annotations for dynamic SQL generation
Annotation Purpose

@DynamicInsert

Specifies that an insert statement should be generated each time an entity is made persistent

@DynamicUpdate

Specifies that an update statement should be generated each time an entity is modified

It’s important to realize that, while @DynamicInsert has no impact on semantics, the more useful @DynamicUpdate annotation does have a subtle side effect.

The wrinkle is that if an entity has no version property, @DynamicUpdate opens the possibility of two optimistic transactions concurrently reading and selectively updating a given instance of the entity. In principle, this might lead to a row with inconsistent column values after both optimistic transactions commit successfully.

Of course, this consideration doesn’t arise for entities with a @Version attribute.

But there’s a solution! Well-designed relational schemas should have constraints to ensure data integrity. That’s true no matter what measures we take to preserve integrity in our program logic. We may ask Hibernate to add a check constraint to our table using the @Check annotation. Check constraints and foreign key constraints can help ensure that a row never contains inconsistent column values.

8.12. Using the bytecode enhancer

Hibernate’s bytecode enhancer enables the following features:

  • attribute-level lazy fetching for basic attributes annotated @Basic(fetch=LAZY) and for lazy non-polymorphic associations,

  • interception-based—instead of the usual snapshot-based—detection of modifications.

To use the bytecode enhancer, we must add the Hibernate plugin to our gradle build:

plugins {
    id "org.hibernate.orm" version "7.0.0.Alpha3"
}

hibernate { enhancement }

Consider this field:

@Entity
class Book {
    ...

    @Basic(optional = false, fetch = LAZY)
    @Column(length = LONG32)
    String fullText;

    ...
}

The fullText field maps to a clob or text column, depending on the SQL dialect. Since it’s expensive to retrieve the full book-length text, we’ve mapped the field fetch=LAZY, telling Hibernate not to read the field until it’s actually used.

  • Without the bytecode enhancer, this instruction is ignored, and the field is always fetched immediately, as part of the initial select that retrieves the Book entity.

  • With bytecode enhancement, Hibernate is able to detect access to the field, and lazy fetching is possible.

By default, Hibernate fetches all lazy fields of a given entity at once, in a single select, when any one of them is accessed. Using the @LazyGroup annotation, it’s possible to assign fields to distinct "fetch groups", so that different lazy fields may be fetched independently.

Similarly, interception lets us implement lazy fetching for non-polymorphic associations without the need for a separate proxy object. However, if an association is polymorphic, that is, if the target entity type has subclasses, then a proxy is still required.

Interception-based change detection is a nice performance optimization with a slight cost in terms of correctness.

  • Without the bytecode enhancer, Hibernate keeps a snapshot of the state of each entity after reading from or writing to the database. When the session flushes, the snapshot state is compared to the current state of the entity to determine if the entity has been modified. Maintaining these snapshots does have an impact on performance.

  • With bytecode enhancement, we may avoid this cost by intercepting writes to the field and recording these modifications as they happen.

This optimization isn’t completely transparent, however.

Interception-based change detection is less accurate than snapshot-based dirty checking. For example, consider this attribute:

byte[] image;

Interception is able to detect writes to the image field, that is, replacement of the whole array. It’s not able to detect modifications made directly to the elements of the array, and so such modifications may be lost.

8.13. Named fetch profiles

We’ve already seen two different ways to override the default fetching strategy for an association:

  • JPA entity graphs, and

  • the join fetch clause in HQL, or, equivalently, the method From.fetch() in the criteria query API.

A third way is to define a named fetch profile. First, we must declare the profile, by annotating a class or package @FetchProfile:

@FetchProfile(name = "EagerBook")
@Entity
class Book { ... }

Note that even though we’ve placed this annotation on the Book entity, a fetch profile—unlike an entity graph—isn’t "rooted" at any particular entity.

We may specify association fetching strategies using the fetchOverrides member of the @FetchProfile annotation, but frankly it looks so messy that we’re embarrassed to show it to you here.

Similarly, a JPA entity graph may be defined using @NamedEntityGraph. But the format of this annotation is even worse than @FetchProfile(fetchOverrides=…​), so we can’t recommend it. 💀

A better way is to annotate an association with the fetch profiles it should be fetched in:

@FetchProfile(name = "EagerBook")
@Entity
class Book {
    ...

    @ManyToOne(fetch = LAZY)
    @FetchProfileOverride(profile = Book_.PROFILE_EAGER_BOOK, mode = JOIN)
    Publisher publisher;

    @ManyToMany
    @FetchProfileOverride(profile = Book_.PROFILE_EAGER_BOOK, mode = JOIN)
    Set<Author> authors;

    ...
}
@Entity
class Author {
    ...

    @OneToOne
    @FetchProfileOverride(profile = Book_.PROFILE_EAGER_BOOK, mode = JOIN)
    Person person;

    ...
}

Here, once again, Book_.PROFILE_EAGER_BOOK is generated by the Metamodel Generator, and is just a constant with the value "EagerBook".

For collections, we may even request subselect fetching:

@FetchProfile(name = "EagerBook")
@FetchProfile(name = "BookWithAuthorsBySubselect")
@Entity
class Book {
    ...

    @OneToOne
    @FetchProfileOverride(profile = Book_.PROFILE_EAGER_BOOK, mode = JOIN)
    Person person;

    @ManyToMany
    @FetchProfileOverride(profile = Book_.PROFILE_EAGER_BOOK, mode = JOIN)
    @FetchProfileOverride(profile = Book_.BOOK_WITH_AUTHORS_BY_SUBSELECT,
                          mode = SUBSELECT)
    Set<Author> authors;

    ...
}

We may define as many different fetch profiles as we like.

Table 70. Annotations for defining fetch profiles
Annotation Purpose

@FetchProfile

Declares a named fetch profile, optionally including a list of @FetchOverrides

@FetchProfile.FetchOverride

Declares a fetch strategy override as part of the @FetchProfile declaration

@FetchProfileOverride

Specifies the fetch strategy for the annotated association, in a given fetch profile

A fetch profile must be explicitly enabled for a given session by calling enableFetchProfile():

session.enableFetchProfile(Book_.PROFILE_EAGER_BOOK);
Book eagerBook = session.find(Book.class, bookId);

So why or when might we prefer named fetch profiles to entity graphs? Well, it’s really hard to say. It’s nice that this feature exists, and if you love it, that’s great. But Hibernate offers alternatives that we think are more compelling most of the time.

The one and only advantage unique to fetch profiles is that they let us very selectively request subselect fetching. We can’t do that with entity graphs, and we can’t do it with HQL.

There’s a special built-in fetch profile named org.hibernate.defaultProfile which is defined as the profile with @FetchProfileOverride(mode=JOIN) applied to every eager @ManyToOne or @OneToOne association. If you enable this profile:

session.enableFetchProfile("org.hibernate.defaultProfile");

Then outer joins for such associations will automatically be added to every HQL or criteria query. This is nice if you can’t be bothered typing out those join fetches explicitly. And in principle it even helps partially mitigate the problem of JPA having specified the wrong default for the fetch member of @ManyToOne.

9. Credits

The full list of contributors to Hibernate ORM can be found on the GitHub repository.

The following contributors were involved in this documentation:

  • Gavin King