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JPA

The Spring JPA, available under the org.springframework.orm.jpa package, offers comprehensive support for the Java Persistence API in a manner similar to the integration with Hibernate while being aware of the underlying implementation in order to provide additional features.

Three Options for JPA Setup in a Spring Environment

The Spring JPA support offers three ways of setting up the JPA EntityManagerFactory that is used by the application to obtain an entity manager.

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Using `LocalEntityManagerFactoryBean`

You can use this option only in simple deployment environments such as stand-alone applications and integration tests.

The LocalEntityManagerFactoryBean creates an EntityManagerFactory suitable for simple deployment environments where the application uses only JPA for data access. The factory bean uses the JPA PersistenceProvider auto-detection mechanism (according to JPA’s Java SE bootstrapping) and, in most cases, requires you to specify only the persistence unit name. The following XML example configures such a bean:

<beans>
	<bean id="myEmf" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
		<property name="persistenceUnitName" value="myPersistenceUnit"/>
	</bean>
</beans>

This form of JPA deployment is the simplest and the most limited. You cannot refer to an existing JDBC DataSource bean definition, and no support for global transactions exists. Furthermore, weaving (byte-code transformation) of persistent classes is provider-specific, often requiring a specific JVM agent to specified on startup. This option is sufficient only for stand-alone applications and test environments, for which the JPA specification is designed.

Obtaining an EntityManagerFactory from JNDI

You can use this option when deploying to a Java EE server. Check your server’s documentation on how to deploy a custom JPA provider into your server, allowing for a different provider than the server’s default.

Obtaining an EntityManagerFactory from JNDI (for example in a Java EE environment), is a matter of changing the XML configuration, as the following example shows:

<beans>
	<jee:jndi-lookup id="myEmf" jndi-name="persistence/myPersistenceUnit"/>
</beans>

This action assumes standard Java EE bootstrapping. The Java EE server auto-detects persistence units (in effect, META-INF/persistence.xml files in application jars) and persistence-unit-ref entries in the Java EE deployment descriptor (for example, web.xml) and defines environment naming context locations for those persistence units.

In such a scenario, the entire persistence unit deployment, including the weaving (byte-code transformation) of persistent classes, is up to the Java EE server. The JDBC DataSource is defined through a JNDI location in the META-INF/persistence.xml file. EntityManager transactions are integrated with the server’s JTA subsystem. Spring merely uses the obtained EntityManagerFactory, passing it on to application objects through dependency injection and managing transactions for the persistence unit (typically through JtaTransactionManager).

If you use multiple persistence units in the same application, the bean names of such JNDI-retrieved persistence units should match the persistence unit names that the application uses to refer to them (for example, in @PersistenceUnit and @PersistenceContext annotations).

Using `LocalContainerEntityManagerFactoryBean`

You can use this option for full JPA capabilities in a Spring-based application environment. This includes web containers such as Tomcat, stand-alone applications, and integration tests with sophisticated persistence requirements.

If you want to specifically configure a Hibernate setup, an immediate alternative is to go with Hibernate 5.2 or 5.3 and set up a native Hibernate LocalSessionFactoryBean instead of a plain JPA LocalContainerEntityManagerFactoryBean, letting it interact with JPA access code as well as native Hibernate access code. See Native Hibernate setup for JPA interaction for details.

The LocalContainerEntityManagerFactoryBean gives full control over EntityManagerFactory configuration and is appropriate for environments where fine-grained customization is required. The LocalContainerEntityManagerFactoryBean creates a PersistenceUnitInfo instance based on the persistence.xml file, the supplied dataSourceLookup strategy, and the specified loadTimeWeaver. It is, thus, possible to work with custom data sources outside of JNDI and to control the weaving process. The following example shows a typical bean definition for a LocalContainerEntityManagerFactoryBean:

<beans>
	<bean id="myEmf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
		<property name="dataSource" ref="someDataSource"/>
		<property name="loadTimeWeaver">
			<bean class="org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver"/>
		</property>
	</bean>
</beans>

The following example shows a typical persistence.xml file:

<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">
	<persistence-unit name="myUnit" transaction-type="RESOURCE_LOCAL">
		<mapping-file>META-INF/orm.xml</mapping-file>
		<exclude-unlisted-classes/>
	</persistence-unit>
</persistence>

The <exclude-unlisted-classes/> shortcut indicates that no scanning for annotated entity classes is supposed to occur. An explicit 'true' value (<exclude-unlisted-classes>true</exclude-unlisted-classes/>) also means no scan. <exclude-unlisted-classes>false</exclude-unlisted-classes/> does trigger a scan. However, we recommend omitting the exclude-unlisted-classes element if you want entity class scanning to occur.

Using the LocalContainerEntityManagerFactoryBean is the most powerful JPA setup option, allowing for flexible local configuration within the application. It supports links to an existing JDBC DataSource, supports both local and global transactions, and so on. However, it also imposes requirements on the runtime environment, such as the availability of a weaving-capable class loader if the persistence provider demands byte-code transformation.

This option may conflict with the built-in JPA capabilities of a Java EE server. In a full Java EE environment, consider obtaining your EntityManagerFactory from JNDI. Alternatively, specify a custom persistenceXmlLocation on your LocalContainerEntityManagerFactoryBean definition (for example, META-INF/my-persistence.xml) and include only a descriptor with that name in your application jar files. Because the Java EE server looks only for default META-INF/persistence.xml files, it ignores such custom persistence units and, hence, avoids conflicts with a Spring-driven JPA setup upfront. (This applies to Resin 3.1, for example.)

When is load-time weaving required?

Not all JPA providers require a JVM agent. Hibernate is an example of one that does not. If your provider does not require an agent or you have other alternatives, such as applying enhancements at build time through a custom compiler or an Ant task, you should not use the load-time weaver.

The LoadTimeWeaver interface is a Spring-provided class that lets JPA ClassTransformer instances be plugged in a specific manner, depending on whether the environment is a web container or application server. Hooking ClassTransformers through an agent is typically not efficient. The agents work against the entire virtual machine and inspect every class that is loaded, which is usually undesirable in a production server environment.

Spring provides a number of LoadTimeWeaver implementations for various environments, letting ClassTransformer instances be applied only for each class loader and not for each VM.

See the Spring configuration in the AOP chapter for more insight regarding the LoadTimeWeaver implementations and their setup, either generic or customized to various platforms (such as Tomcat, JBoss and WebSphere).

As described in Spring configuration, you can configure a context-wide LoadTimeWeaver by using the @EnableLoadTimeWeaving annotation of the context:load-time-weaver XML element. Such a global weaver is automatically picked up by all JPA LocalContainerEntityManagerFactoryBean instances. The following example shows the preferred way of setting up a load-time weaver, delivering auto-detection of the platform (e.g. Tomcat’s weaving-capable class loader or Spring’s JVM agent) and automatic propagation of the weaver to all weaver-aware beans:

<context:load-time-weaver/>
<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
	...
</bean>

However, you can, if needed, manually specify a dedicated weaver through the loadTimeWeaver property, as the following example shows:

<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
	<property name="loadTimeWeaver">
		<bean class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/>
	</property>
</bean>

No matter how the LTW is configured, by using this technique, JPA applications relying on instrumentation can run in the target platform (for example, Tomcat) without needing an agent. This is especially important when the hosting applications rely on different JPA implementations, because the JPA transformers are applied only at the class-loader level and are, thus, isolated from each other.

Dealing with Multiple Persistence Units

For applications that rely on multiple persistence units locations (stored in various JARS in the classpath, for example), Spring offers the PersistenceUnitManager to act as a central repository and to avoid the persistence units discovery process, which can be expensive. The default implementation lets multiple locations be specified. These locations are parsed and later retrieved through the persistence unit name. (By default, the classpath is searched for META-INF/persistence.xml files.) The following example configures multiple locations:

<bean id="pum" class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager">
	<property name="persistenceXmlLocations">
		<list>
			<value>org/springframework/orm/jpa/domain/persistence-multi.xml</value>
			<value>classpath:/my/package/**/custom-persistence.xml</value>
			<value>classpath*:META-INF/persistence.xml</value>
		</list>
	</property>
	<property name="dataSources">
		<map>
			<entry key="localDataSource" value-ref="local-db"/>
			<entry key="remoteDataSource" value-ref="remote-db"/>
		</map>
	</property>
	<!-- if no datasource is specified, use this one -->
	<property name="defaultDataSource" ref="remoteDataSource"/>
</bean>

<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
	<property name="persistenceUnitManager" ref="pum"/>
	<property name="persistenceUnitName" value="myCustomUnit"/>
</bean>

The default implementation allows customization of the PersistenceUnitInfo instances (before they are fed to the JPA provider) either declaratively (through its properties, which affect all hosted units) or programmatically (through the PersistenceUnitPostProcessor, which allows persistence unit selection). If no PersistenceUnitManager is specified, one is created and used internally by LocalContainerEntityManagerFactoryBean.

Background Bootstrapping

LocalContainerEntityManagerFactoryBean supports background bootstrapping through the bootstrapExecutor property, as the following example shows:

<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
	<property name="bootstrapExecutor">
		<bean class="org.springframework.core.task.SimpleAsyncTaskExecutor"/>
	</property>
</bean>

The actual JPA provider bootstrapping is handed off to the specified executor and then, running in parallel, to the application bootstrap thread. The exposed EntityManagerFactory proxy can be injected into other application components and is even able to respond to EntityManagerFactoryInfo configuration inspection. However, once the actual JPA provider is being accessed by other components (for example, calling createEntityManager), those calls block until the background bootstrapping has completed. In particular, when you use Spring Data JPA, make sure to set up deferred bootstrapping for its repositories as well.

Implementing DAOs Based on JPA: `EntityManagerFactory` and `EntityManager`

Although EntityManagerFactory instances are thread-safe, EntityManager instances are not. The injected JPA EntityManager behaves like an EntityManager fetched from an application server’s JNDI environment, as defined by the JPA specification. It delegates all calls to the current transactional EntityManager, if any. Otherwise, it falls back to a newly created EntityManager per operation, in effect making its usage thread-safe.

It is possible to write code against the plain JPA without any Spring dependencies, by using an injected EntityManagerFactory or EntityManager. Spring can understand the @PersistenceUnit and @PersistenceContext annotations both at the field and the method level if a PersistenceAnnotationBeanPostProcessor is enabled. The following example shows a plain JPA DAO implementation that uses the @PersistenceUnit annotation:

Java

public class ProductDaoImpl implements ProductDao {

	private EntityManagerFactory emf;

	@PersistenceUnit
	public void setEntityManagerFactory(EntityManagerFactory emf) {
		this.emf = emf;
	}

	public Collection loadProductsByCategory(String category) {
		try (EntityManager em = this.emf.createEntityManager()) {
			Query query = em.createQuery("from Product as p where p.category = ?1");
			query.setParameter(1, category);
			return query.getResultList();
		}
	}
}

Kotlin

class ProductDaoImpl : ProductDao {

	private lateinit var emf: EntityManagerFactory

	@PersistenceUnit
	fun setEntityManagerFactory(emf: EntityManagerFactory) {
		this.emf = emf
	}

	fun loadProductsByCategory(category: String): Collection<*> {
		val em = this.emf.createEntityManager()
		val query = em.createQuery("from Product as p where p.category = ?1");
		query.setParameter(1, category);
		return query.resultList;
	}
}

The preceding DAO has no dependency on Spring and still fits nicely into a Spring application context. Moreover, the DAO takes advantage of annotations to require the injection of the default EntityManagerFactory, as the following example bean definition shows:

<beans>

	<!-- bean post-processor for JPA annotations -->
	<bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/>

	<bean id="myProductDao" class="product.ProductDaoImpl"/>

</beans>

As an alternative to explicitly defining a PersistenceAnnotationBeanPostProcessor, consider using the Spring context:annotation-config XML element in your application context configuration. Doing so automatically registers all Spring standard post-processors for annotation-based configuration, including CommonAnnotationBeanPostProcessor and so on.

Consider the following example:

<beans>

	<!-- post-processors for all standard config annotations -->
	<context:annotation-config/>

	<bean id="myProductDao" class="product.ProductDaoImpl"/>

</beans>

The main problem with such a DAO is that it always creates a new EntityManager through the factory. You can avoid this by requesting a transactional EntityManager (also called a “shared EntityManager” because it is a shared, thread-safe proxy for the actual transactional EntityManager) to be injected instead of the factory. The following example shows how to do so:

Java

public class ProductDaoImpl implements ProductDao {

	@PersistenceContext
	private EntityManager em;

	public Collection loadProductsByCategory(String category) {
		Query query = em.createQuery("from Product as p where p.category = :category");
		query.setParameter("category", category);
		return query.getResultList();
	}
}

Kotlin

class ProductDaoImpl : ProductDao {

	@PersistenceContext
	private lateinit var em: EntityManager

	fun loadProductsByCategory(category: String): Collection<*> {
		val query = em.createQuery("from Product as p where p.category = :category")
		query.setParameter("category", category)
		return query.resultList
	}
}

The @PersistenceContext annotation has an optional attribute called type, which defaults to PersistenceContextType.TRANSACTION. You can use this default to receive a shared EntityManager proxy. The alternative, PersistenceContextType.EXTENDED, is a completely different affair. This results in a so-called extended EntityManager, which is not thread-safe and, hence, must not be used in a concurrently accessed component, such as a Spring-managed singleton bean. Extended EntityManager instances are only supposed to be used in stateful components that, for example, reside in a session, with the lifecycle of the EntityManager not tied to a current transaction but rather being completely up to the application.

Method- and field-level Injection

You can apply annotations that indicate dependency injections (such as @PersistenceUnit and @PersistenceContext) on field or methods inside a class — hence the expressions “method-level injection” and “field-level injection”. Field-level annotations are concise and easier to use while method-level annotations allow for further processing of the injected dependency. In both cases, the member visibility (public, protected, or private) does not matter.

What about class-level annotations?

On the Java EE platform, they are used for dependency declaration and not for resource injection.

The injected EntityManager is Spring-managed (aware of the ongoing transaction). Even though the new DAO implementation uses method-level injection of an EntityManager instead of an EntityManagerFactory, no change is required in the application context XML, due to annotation usage.

The main advantage of this DAO style is that it depends only on the Java Persistence API. No import of any Spring class is required. Moreover, as the JPA annotations are understood, the injections are applied automatically by the Spring container. This is appealing from a non-invasiveness perspective and can feel more natural to JPA developers.

Spring-driven JPA transactions

We strongly encourage you to read transaction-declarative, if you have not already done so, to get more detailed coverage of Spring’s declarative transaction support.

The recommended strategy for JPA is local transactions through JPA’s native transaction support. Spring’s JpaTransactionManager provides many capabilities known from local JDBC transactions (such as transaction-specific isolation levels and resource-level read-only optimizations) against any regular JDBC connection pool (no XA requirement).

Spring JPA also lets a configured JpaTransactionManager expose a JPA transaction to JDBC access code that accesses the same DataSource, provided that the registered JpaDialect supports retrieval of the underlying JDBC Connection. Spring provides dialects for the EclipseLink and Hibernate JPA implementations. See the next section for details on the JpaDialect mechanism.

As an immediate alternative, Spring’s native HibernateTransactionManager is capable of interacting with JPA access code as of Spring Framework 5.1 and Hibernate 5.2/5.3, adapting to several Hibernate specifics and providing JDBC interaction. This makes particular sense in combination with LocalSessionFactoryBean setup. See Native Hibernate Setup for JPA Interaction for details.

Understanding `JpaDialect` and `JpaVendorAdapter`

As an advanced feature, JpaTransactionManager and subclasses of AbstractEntityManagerFactoryBean allow a custom JpaDialect to be passed into the jpaDialect bean property. A JpaDialect implementation can enable the following advanced features supported by Spring, usually in a vendor-specific manner:

Applying specific transaction semantics (such as custom isolation level or transaction timeout)
Retrieving the transactional JDBC Connection (for exposure to JDBC-based DAOs)
Advanced translation of PersistenceExceptions to Spring DataAccessExceptions

This is particularly valuable for special transaction semantics and for advanced translation of exception. The default implementation (DefaultJpaDialect) does not provide any special abilities and, if the features listed earlier are required, you have to specify the appropriate dialect.

As an even broader provider adaptation facility primarily for Spring’s full-featured LocalContainerEntityManagerFactoryBean setup, JpaVendorAdapter combines the capabilities of JpaDialect with other provider-specific defaults. Specifying a HibernateJpaVendorAdapter or EclipseLinkJpaVendorAdapter is the most convenient way of auto-configuring an EntityManagerFactory setup for Hibernate or EclipseLink, respectively. Note that those provider adapters are primarily designed for use with Spring-driven transaction management (that is, for use with JpaTransactionManager).

See the JpaDialect and JpaVendorAdapter javadoc for more details of its operations and how they are used within Spring’s JPA support.

Setting up JPA with JTA Transaction Management

As an alternative to JpaTransactionManager, Spring also allows for multi-resource transaction coordination through JTA, either in a Java EE environment or with a stand-alone transaction coordinator, such as Atomikos. Aside from choosing Spring’s JtaTransactionManager instead of JpaTransactionManager, you need to take few further steps:

The underlying JDBC connection pools need to be XA-capable and be integrated with your transaction coordinator. This is usually straightforward in a Java EE environment, exposing a different kind of DataSource through JNDI. See your application server documentation for details. Analogously, a standalone transaction coordinator usually comes with special XA-integrated DataSource implementations. Again, check its documentation.
The JPA EntityManagerFactory setup needs to be configured for JTA. This is provider-specific, typically through special properties to be specified as jpaProperties on LocalContainerEntityManagerFactoryBean. In the case of Hibernate, these properties are even version-specific. See your Hibernate documentation for details.
Spring’s HibernateJpaVendorAdapter enforces certain Spring-oriented defaults, such as the connection release mode, on-close, which matches Hibernate’s own default in Hibernate 5.0 but not any more in 5.1/5.2. For a JTA setup, either do not declare HibernateJpaVendorAdapter to begin with or turn off its prepareConnection flag. Alternatively, set Hibernate 5.2’s hibernate.connection.handling_mode property to DELAYED_ACQUISITION_AND_RELEASE_AFTER_STATEMENT to restore Hibernate’s own default. See orm-hibernate-invalid-jdbc-access-error for a related note about WebLogic.
Alternatively, consider obtaining the EntityManagerFactory from your application server itself (that is, through a JNDI lookup instead of a locally declared LocalContainerEntityManagerFactoryBean). A server-provided EntityManagerFactory might require special definitions in your server configuration (making the deployment less portable) but is set up for the server’s JTA environment.

Native Hibernate Setup and Native Hibernate Transactions for JPA Interaction

As of Spring Framework 5.1 and Hibernate 5.2/5.3, a native LocalSessionFactoryBean setup in combination with HibernateTransactionManager allows for interaction with @PersistenceContext and other JPA access code. A Hibernate SessionFactory natively implements JPA’s EntityManagerFactory interface now and a Hibernate Session handle natively is a JPA EntityManager. Spring’s JPA support facilities automatically detect native Hibernate sessions.

Such native Hibernate setup can, therefore, serve as a replacement for a standard JPA LocalContainerEntityManagerFactoryBean and JpaTransactionManager combination in many scenarios, allowing for interaction with SessionFactory.getCurrentSession() (and also HibernateTemplate) next to @PersistenceContext EntityManager within the same local transaction. Such a setup also provides stronger Hibernate integration and more configuration flexibility, because it is not constrained by JPA bootstrap contracts.

You do not need HibernateJpaVendorAdapter configuration in such a scenario, since Spring’s native Hibernate setup provides even more features (for example, custom Hibernate Integrator setup, Hibernate 5.3 bean container integration, and stronger optimizations for read-only transactions). Last but not least, you can also express native Hibernate setup through LocalSessionFactoryBuilder, seamlessly integrating with @Bean style configuration (no FactoryBean involved).

LocalSessionFactoryBean and LocalSessionFactoryBuilder support background bootstrapping, just as the JPA LocalContainerEntityManagerFactoryBean does. See Background Bootstrapping for an introduction.

On LocalSessionFactoryBean, this is available through the bootstrapExecutor property. On the programmatic LocalSessionFactoryBuilder, an overloaded buildSessionFactory method takes a bootstrap executor argument.

JPA