[[orm-jpa]]
= JPA

The Spring JPA, available under the `org.springframework.orm.jpa` package, offers
comprehensive support for the
https://www.oracle.com/technetwork/articles/javaee/jpa-137156.html[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.


[[orm-jpa-setup]]
== 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.

* xref:data-access/orm/jpa.adoc#orm-jpa-setup-lemfb[Using `LocalEntityManagerFactoryBean`]
* xref:data-access/orm/jpa.adoc#orm-jpa-setup-jndi[Obtaining an EntityManagerFactory from JNDI]
* xref:data-access/orm/jpa.adoc#orm-jpa-setup-lcemfb[Using `LocalContainerEntityManagerFactoryBean`]

[[orm-jpa-setup-lemfb]]
=== 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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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 be specified on startup. This
option is sufficient only for stand-alone applications and test environments, for which
the JPA specification is designed.

[[orm-jpa-setup-jndi]]
=== Obtaining an EntityManagerFactory from JNDI

You can use this option when deploying to a Jakarta 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 Jakarta EE environment),
is a matter of changing the XML configuration, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<beans>
		<jee:jndi-lookup id="myEmf" jndi-name="persistence/myPersistenceUnit"/>
	</beans>
----

This action assumes standard Jakarta EE bootstrapping. The Jakarta EE server auto-detects
persistence units (in effect, `META-INF/persistence.xml` files in application jars) and
`persistence-unit-ref` entries in the Jakarta 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 Jakarta 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).

[[orm-jpa-setup-lcemfb]]
=== 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.

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`:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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>
----

NOTE: 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 Jakarta EE server. In a
full Jakarta 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 Jakarta 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
{java-api}/java.instrument/java/lang/instrument/package-summary.html[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 xref:core/aop/using-aspectj.adoc#aop-aj-ltw-spring[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 xref:core/aop/using-aspectj.adoc#aop-aj-ltw-spring[Spring configuration], you can configure
a context-wide `LoadTimeWeaver` by using the `@EnableLoadTimeWeaving` annotation or 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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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.

[[orm-jpa-setup-multiple]]
=== 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:

[source,xml,indent=0,subs="verbatim"]
----
	<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`.

[[orm-jpa-setup-background]]
=== Background Bootstrapping

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

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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.


[[orm-jpa-dao]]
== Implementing DAOs Based on JPA: `EntityManagerFactory` and `EntityManager`

NOTE: 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:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	public class ProductDaoImpl implements ProductDao {

		private EntityManagerFactory emf;

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

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

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	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::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	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 Jakarta 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 bean definition
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.

[[orm-jpa-dao-autowired]]
=== Implementing DAOs Based on `@Autowired` (typically with constructor-based injection)

`@PersistenceUnit` and `@PersistenceContext` can only be declared on methods and fields.
What about providing JPA resources via constructors and other `@Autowired` injection points?

`EntityManagerFactory` can easily be injected via constructors and `@Autowired` fields/methods
as long as the target is defined as a bean, e.g. via `LocalContainerEntityManagerFactoryBean`.
The injection point matches the original `EntityManagerFactory` definition by type as-is.

However, an `@PersistenceContext`-style shared `EntityManager` reference is not available for
regular dependency injection out of the box. In order to make it available for type-based
matching as required by `@Autowired`, consider defining a `SharedEntityManagerBean` as a
companion for your `EntityManagerFactory` definition:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
		...
	</bean>

	<bean id="em" class="org.springframework.orm.jpa.support.SharedEntityManagerBean">
		<property name="entityManagerFactory" ref="emf"/>
	</bean>
----

Alternatively, you may define an `@Bean` method based on `SharedEntityManagerCreator`:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Bean("em")
	public static EntityManager sharedEntityManager(EntityManagerFactory emf) {
		return SharedEntityManagerCreator.createSharedEntityManager(emf);
	}
----

In case of multiple persistence units, each `EntityManagerFactory` definition needs to be
accompanied by a corresponding `EntityManager` bean definition, ideally with qualifiers
that match with the distinct `EntityManagerFactory` definition in order to distinguish
the persistence units via `@Autowired @Qualifier("...")`.


[[orm-jpa-tx]]
== Spring-driven JPA Transactions

NOTE: We strongly encourage you to read xref:data-access/transaction/declarative.adoc[Declarative Transaction Management],
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, without requiring
a JTA transaction coordinator and XA-capable resources.

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
xref:data-access/orm/jpa.adoc#orm-jpa-dialect[next section] for details on `JpaDialect`.

For JTA-style lazy retrieval of actual resource connections, Spring provides a
corresponding `DataSource` proxy class for the target connection pool: see
{spring-framework-api}/jdbc/datasource/LazyConnectionDataSourceProxy.html[`LazyConnectionDataSourceProxy`].
This is particularly useful for JPA read-only transactions which can often
be processed from a local cache rather than hitting the database.


[[orm-jpa-dialect]]
== 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 `PersistenceException` to Spring's `DataAccessException`

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.

TIP: 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 {spring-framework-api}/orm/jpa/JpaDialect.html[`JpaDialect`] and
{spring-framework-api}/orm/jpa/JpaVendorAdapter.html[`JpaVendorAdapter`] javadoc for
more details of its operations and how they are used within Spring's JPA support.


[[orm-jpa-jta]]
== 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 Jakarta 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 Jakarta 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` variants. 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 Hibernate 5.1+. For a JTA setup, make sure to declare
your persistence unit transaction type as "JTA". 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 xref:data-access/orm/hibernate.adoc#orm-hibernate-invalid-jdbc-access-error[Spurious Application Server Warnings with Hibernate] for related notes.

* 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.


[[orm-jpa-hibernate]]
== Native Hibernate Setup and Native Hibernate Transactions for JPA Interaction

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).

[NOTE]
====
`LocalSessionFactoryBean` and `LocalSessionFactoryBuilder` support background
bootstrapping, just as the JPA `LocalContainerEntityManagerFactoryBean` does.
See xref:data-access/orm/jpa.adoc#orm-jpa-setup-background[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.
====




