[[orm-hibernate]]
= Hibernate

We start with a coverage of https://hibernate.org/[Hibernate 5] in a Spring environment,
using it to demonstrate the approach that Spring takes towards integrating OR mappers.
This section covers many issues in detail and shows different variations of DAO
implementations and transaction demarcation. Most of these patterns can be directly
translated to all other supported ORM tools. The later sections in this chapter then
cover the other ORM technologies and show brief examples.

[NOTE]
====
As of Spring Framework 6.0, Spring requires Hibernate ORM 5.5+ for Spring's
`HibernateJpaVendorAdapter` as well as for a native Hibernate `SessionFactory` setup.
We recommend Hibernate ORM 5.6 as the last feature branch in that Hibernate generation.

Hibernate ORM 6.x is only supported as a JPA provider (`HibernateJpaVendorAdapter`).
Plain `SessionFactory` setup with the `orm.hibernate5` package is not supported anymore.
We recommend Hibernate ORM 6.1/6.2 with JPA-style setup for new development projects.
====


[[orm-session-factory-setup]]
== `SessionFactory` Setup in a Spring Container

To avoid tying application objects to hard-coded resource lookups, you can define
resources (such as a JDBC `DataSource` or a Hibernate `SessionFactory`) as beans in the
Spring container. Application objects that need to access resources receive references
to such predefined instances through bean references, as illustrated in the DAO
definition in the xref:data-access/orm/hibernate.adoc#orm-hibernate-straight[next section].

The following excerpt from an XML application context definition shows how to set up a
JDBC `DataSource` and a Hibernate `SessionFactory` on top of it:

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

		<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
			<property name="driverClassName" value="org.hsqldb.jdbcDriver"/>
			<property name="url" value="jdbc:hsqldb:hsql://localhost:9001"/>
			<property name="username" value="sa"/>
			<property name="password" value=""/>
		</bean>

		<bean id="mySessionFactory" class="org.springframework.orm.hibernate5.LocalSessionFactoryBean">
			<property name="dataSource" ref="myDataSource"/>
			<property name="mappingResources">
				<list>
					<value>product.hbm.xml</value>
				</list>
			</property>
			<property name="hibernateProperties">
				<value>
					hibernate.dialect=org.hibernate.dialect.HSQLDialect
				</value>
			</property>
		</bean>

	</beans>
----

Switching from a local Jakarta Commons DBCP `BasicDataSource` to a JNDI-located
`DataSource` (usually managed by an application server) is only a matter of
configuration, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<beans>
		<jee:jndi-lookup id="myDataSource" jndi-name="java:comp/env/jdbc/myds"/>
	</beans>
----

You can also access a JNDI-located `SessionFactory`, using Spring's
`JndiObjectFactoryBean` / `<jee:jndi-lookup>` to retrieve and expose it.
However, that is typically not common outside of an EJB context.

[NOTE]
====
Spring also provides a `LocalSessionFactoryBuilder` variant, seamlessly integrating
with `@Bean` style configuration and programmatic setup (no `FactoryBean` involved).

Both `LocalSessionFactoryBean` and `LocalSessionFactoryBuilder` support background
bootstrapping, with Hibernate initialization running in parallel to the application
bootstrap thread on a given bootstrap executor (such as a `SimpleAsyncTaskExecutor`).
On `LocalSessionFactoryBean`, this is available through the `bootstrapExecutor`
property. On the programmatic `LocalSessionFactoryBuilder`, there is an overloaded
`buildSessionFactory` method that takes a bootstrap executor argument.

As of Spring Framework 5.1, such a native Hibernate setup can also expose a JPA
`EntityManagerFactory` for standard JPA interaction next to native Hibernate access.
See xref:data-access/orm/jpa.adoc#orm-jpa-hibernate[Native Hibernate Setup for JPA] for details.
====


[[orm-hibernate-straight]]
== Implementing DAOs Based on the Plain Hibernate API

Hibernate has a feature called contextual sessions, wherein Hibernate itself manages
one current `Session` per transaction. This is roughly equivalent to Spring's
synchronization of one Hibernate `Session` per transaction. A corresponding DAO
implementation resembles the following example, based on the plain Hibernate API:

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

		private SessionFactory sessionFactory;

		public void setSessionFactory(SessionFactory sessionFactory) {
			this.sessionFactory = sessionFactory;
		}

		public Collection loadProductsByCategory(String category) {
			return this.sessionFactory.getCurrentSession()
					.createQuery("from test.Product product where product.category=?")
					.setParameter(0, category)
					.list();
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	class ProductDaoImpl(private val sessionFactory: SessionFactory) : ProductDao {

		fun loadProductsByCategory(category: String): Collection<*> {
			return sessionFactory.currentSession
					.createQuery("from test.Product product where product.category=?")
					.setParameter(0, category)
					.list()
		}
	}
----
======

This style is similar to that of the Hibernate reference documentation and examples,
except for holding the `SessionFactory` in an instance variable. We strongly recommend
such an instance-based setup over the old-school `static` `HibernateUtil` class from
Hibernate's CaveatEmptor sample application. (In general, do not keep any resources in
`static` variables unless absolutely necessary.)

The preceding DAO example follows the dependency injection pattern. It fits nicely into a Spring IoC
container, as it would if coded against Spring's `HibernateTemplate`.
You can also set up such a DAO in plain Java (for example, in unit tests). To do so,
instantiate it and call `setSessionFactory(..)` with the desired factory reference. As a
Spring bean definition, the DAO would resemble the following:

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

		<bean id="myProductDao" class="product.ProductDaoImpl">
			<property name="sessionFactory" ref="mySessionFactory"/>
		</bean>

	</beans>
----

The main advantage of this DAO style is that it depends on Hibernate API only. No import
of any Spring class is required. This is appealing from a non-invasiveness
perspective and may feel more natural to Hibernate developers.

However, the DAO throws plain `HibernateException` (which is unchecked, so it does not have
to be declared or caught), which means that callers can treat exceptions only as being
generally fatal -- unless they want to depend on Hibernate's own exception hierarchy.
Catching specific causes (such as an optimistic locking failure) is not possible without
tying the caller to the implementation strategy. This trade off might be acceptable to
applications that are strongly Hibernate-based, do not need any special exception
treatment, or both.

Fortunately, Spring's `LocalSessionFactoryBean` supports Hibernate's
`SessionFactory.getCurrentSession()` method for any Spring transaction strategy,
returning the current Spring-managed transactional `Session`, even with
`HibernateTransactionManager`. The standard behavior of that method remains
to return the current `Session` associated with the ongoing JTA transaction, if any.
This behavior applies regardless of whether you use Spring's
`JtaTransactionManager`, EJB container managed transactions (CMTs), or JTA.

In summary, you can implement DAOs based on the plain Hibernate API, while still being
able to participate in Spring-managed transactions.


[[orm-hibernate-tx-declarative]]
== Declarative Transaction Demarcation

We recommend that you use Spring's declarative transaction support, which lets you
replace explicit transaction demarcation API calls in your Java code with an AOP
transaction interceptor. You can configure this transaction interceptor in a Spring
container by using either Java annotations or XML. This declarative transaction capability
lets you keep business services free of repetitive transaction demarcation code and
focus on adding business logic, which is the real value of your application.

NOTE: Before you continue, we are strongly encourage you to read xref:data-access/transaction/declarative.adoc[Declarative Transaction Management]
if you have not already done so.

You can annotate the service layer with `@Transactional` annotations and instruct the
Spring container to find these annotations and provide transactional semantics for
these annotated methods. The following example shows how to do so:

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

		private ProductDao productDao;

		public void setProductDao(ProductDao productDao) {
			this.productDao = productDao;
		}

		@Transactional
		public void increasePriceOfAllProductsInCategory(final String category) {
			List productsToChange = this.productDao.loadProductsByCategory(category);
			// ...
		}

		@Transactional(readOnly = true)
		public List<Product> findAllProducts() {
			return this.productDao.findAllProducts();
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	class ProductServiceImpl(private val productDao: ProductDao) : ProductService {

		@Transactional
		fun increasePriceOfAllProductsInCategory(category: String) {
			val productsToChange = productDao.loadProductsByCategory(category)
			// ...
		}

		@Transactional(readOnly = true)
		fun findAllProducts() = productDao.findAllProducts()
	}
----
======

In the container, you need to set up the `PlatformTransactionManager` implementation
(as a bean) and a `<tx:annotation-driven/>` entry, opting into `@Transactional`
processing at runtime. The following example shows how to do so:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:aop="http://www.springframework.org/schema/aop"
		xmlns:tx="http://www.springframework.org/schema/tx"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			https://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/tx
			https://www.springframework.org/schema/tx/spring-tx.xsd
			http://www.springframework.org/schema/aop
			https://www.springframework.org/schema/aop/spring-aop.xsd">

		<!-- SessionFactory, DataSource, etc. omitted -->

		<bean id="transactionManager"
				class="org.springframework.orm.hibernate5.HibernateTransactionManager">
			<property name="sessionFactory" ref="sessionFactory"/>
		</bean>

		<tx:annotation-driven/>

		<bean id="myProductService" class="product.SimpleProductService">
			<property name="productDao" ref="myProductDao"/>
		</bean>

	</beans>
----


[[orm-hibernate-tx-programmatic]]
== Programmatic Transaction Demarcation

You can demarcate transactions in a higher level of the application, on top of
lower-level data access services that span any number of operations. Nor do restrictions
exist on the implementation of the surrounding business service. It needs only a Spring
`PlatformTransactionManager`. Again, the latter can come from anywhere, but preferably
as a bean reference through a `setTransactionManager(..)` method. Also, the
`productDAO` should be set by a `setProductDao(..)` method. The following pair of snippets show
a transaction manager and a business service definition in a Spring application context
and an example for a business method implementation:

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

		<bean id="myTxManager" class="org.springframework.orm.hibernate5.HibernateTransactionManager">
			<property name="sessionFactory" ref="mySessionFactory"/>
		</bean>

		<bean id="myProductService" class="product.ProductServiceImpl">
			<property name="transactionManager" ref="myTxManager"/>
			<property name="productDao" ref="myProductDao"/>
		</bean>

	</beans>
----

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

		private TransactionTemplate transactionTemplate;
		private ProductDao productDao;

		public void setTransactionManager(PlatformTransactionManager transactionManager) {
			this.transactionTemplate = new TransactionTemplate(transactionManager);
		}

		public void setProductDao(ProductDao productDao) {
			this.productDao = productDao;
		}

		public void increasePriceOfAllProductsInCategory(final String category) {
			this.transactionTemplate.execute(new TransactionCallbackWithoutResult() {
				public void doInTransactionWithoutResult(TransactionStatus status) {
					List productsToChange = this.productDao.loadProductsByCategory(category);
					// do the price increase...
				}
			});
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	class ProductServiceImpl(transactionManager: PlatformTransactionManager,
							private val productDao: ProductDao) : ProductService {

		private val transactionTemplate = TransactionTemplate(transactionManager)

		fun increasePriceOfAllProductsInCategory(category: String) {
			transactionTemplate.execute {
				val productsToChange = productDao.loadProductsByCategory(category)
				// do the price increase...
			}
		}
	}
----
======

Spring's `TransactionInterceptor` lets any checked application exception be thrown
with the callback code, while `TransactionTemplate` is restricted to unchecked
exceptions within the callback. `TransactionTemplate` triggers a rollback in case of
an unchecked application exception or if the transaction is marked rollback-only by
the application (by setting `TransactionStatus`). By default, `TransactionInterceptor`
behaves the same way but allows configurable rollback policies per method.


[[orm-hibernate-tx-strategies]]
== Transaction Management Strategies

Both `TransactionTemplate` and `TransactionInterceptor` delegate the actual transaction
handling to a `PlatformTransactionManager` instance (which can be a
`HibernateTransactionManager` (for a single Hibernate `SessionFactory`) by using a
`ThreadLocal` `Session` under the hood) or a `JtaTransactionManager` (delegating to the
JTA subsystem of the container) for Hibernate applications. You can even use a custom
`PlatformTransactionManager` implementation. Switching from native Hibernate transaction
management to JTA (such as when facing distributed transaction requirements for certain
deployments of your application) is only a matter of configuration. You can replace
the Hibernate transaction manager with Spring's JTA transaction implementation. Both
transaction demarcation and data access code work without changes, because they
use the generic transaction management APIs.

For distributed transactions across multiple Hibernate session factories, you can combine
`JtaTransactionManager` as a transaction strategy with multiple
`LocalSessionFactoryBean` definitions. Each DAO then gets one specific `SessionFactory`
reference passed into its corresponding bean property. If all underlying JDBC data
sources are transactional container ones, a business service can demarcate transactions
across any number of DAOs and any number of session factories without special regard, as
long as it uses `JtaTransactionManager` as the strategy.

Both `HibernateTransactionManager` and `JtaTransactionManager` allow for proper
JVM-level cache handling with Hibernate, without container-specific transaction manager
lookup or a JCA connector (if you do not use EJB to initiate transactions).

`HibernateTransactionManager` can export the Hibernate JDBC `Connection` to plain JDBC
access code for a specific `DataSource`. This ability allows for high-level
transaction demarcation with mixed Hibernate and JDBC data access completely without
JTA, provided you access only one database. `HibernateTransactionManager` automatically
exposes the Hibernate transaction as a JDBC transaction if you have set up the passed-in
`SessionFactory` with a `DataSource` through the `dataSource` property of the
`LocalSessionFactoryBean` class. Alternatively, you can specify explicitly the
`DataSource` for which the transactions are supposed to be exposed through the
`dataSource` property of the `HibernateTransactionManager` class.


[[orm-hibernate-resources]]
== Comparing Container-managed and Locally Defined Resources

You can switch between a container-managed JNDI `SessionFactory` and a locally defined
one without having to change a single line of application code. Whether to keep
resource definitions in the container or locally within the application is mainly a
matter of the transaction strategy that you use. Compared to a Spring-defined local
`SessionFactory`, a manually registered JNDI `SessionFactory` does not provide any
benefits. Deploying a `SessionFactory` through Hibernate's JCA connector provides the
added value of participating in the Jakarta EE server's management infrastructure, but does
not add actual value beyond that.

Spring's transaction support is not bound to a container. When configured with any strategy
other than JTA, transaction support also works in a stand-alone or test environment.
Especially in the typical case of single-database transactions, Spring's single-resource
local transaction support is a lightweight and powerful alternative to JTA. When you use
local EJB stateless session beans to drive transactions, you depend both on an EJB
container and on JTA, even if you access only a single database and use only stateless
session beans to provide declarative transactions through container-managed
transactions. Direct use of JTA programmatically also requires a Jakarta EE environment.

Spring-driven transactions can work as well with a locally defined Hibernate
`SessionFactory` as they do with a local JDBC `DataSource`, provided they access a
single database. Thus, you need only use Spring's JTA transaction strategy when you
have distributed transaction requirements. A JCA connector requires container-specific
deployment steps, and (obviously) JCA support in the first place. This configuration
requires more work than deploying a simple web application with local resource
definitions and Spring-driven transactions.

All things considered, if you do not use EJBs, stick with local `SessionFactory` setup
and Spring's `HibernateTransactionManager` or `JtaTransactionManager`. You get all of
the benefits, including proper transactional JVM-level caching and distributed
transactions, without the inconvenience of container deployment. JNDI registration of a
Hibernate `SessionFactory` through the JCA connector adds value only when used in
conjunction with EJBs.


[[orm-hibernate-invalid-jdbc-access-error]]
== Spurious Application Server Warnings with Hibernate

In some JTA environments with very strict `XADataSource` implementations (currently
some WebLogic Server and WebSphere versions), when Hibernate is configured without
regard to the JTA transaction manager for that environment, spurious warnings or
exceptions can show up in the application server log. These warnings or exceptions
indicate that the connection being accessed is no longer valid or JDBC access is no
longer valid, possibly because the transaction is no longer active. As an example,
here is an actual exception from WebLogic:

[literal]
[subs="verbatim,quotes"]
----
java.sql.SQLException: The transaction is no longer active - status: 'Committed'. No
further JDBC access is allowed within this transaction.
----

Another common problem is a connection leak after JTA transactions, with Hibernate
sessions (and potentially underlying JDBC connections) not getting closed properly.

You can resolve such issues by making Hibernate aware of the JTA transaction manager,
to which it synchronizes (along with Spring). You have two options for doing this:

* Pass your Spring `JtaTransactionManager` bean to your Hibernate setup. The easiest
  way is a bean reference into the `jtaTransactionManager` property for your
  `LocalSessionFactoryBean` bean (see xref:data-access/transaction/strategies.adoc#transaction-strategies-hibernate[Hibernate Transaction Setup]).
  Spring then makes the corresponding JTA strategies available to Hibernate.
* You may also configure Hibernate's JTA-related properties explicitly, in particular
  "hibernate.transaction.coordinator_class", "hibernate.connection.handling_mode"
  and potentially "hibernate.transaction.jta.platform" in your "hibernateProperties"
  on `LocalSessionFactoryBean` (see Hibernate's manual for details on those properties).

The remainder of this section describes the sequence of events that occur with and
without Hibernate's awareness of the JTA `PlatformTransactionManager`.

When Hibernate is not configured with any awareness of the JTA transaction manager,
the following events occur when a JTA transaction commits:

* The JTA transaction commits.
* Spring's `JtaTransactionManager` is synchronized to the JTA transaction, so it is
  called back through an `afterCompletion` callback by the JTA transaction manager.
* Among other activities, this synchronization can trigger a callback by Spring to
  Hibernate, through Hibernate's `afterTransactionCompletion` callback (used to clear
  the Hibernate cache), followed by an explicit `close()` call on the Hibernate session,
  which causes Hibernate to attempt to `close()` the JDBC Connection.
* In some environments, this `Connection.close()` call then triggers the warning or
  error, as the application server no longer considers the `Connection` to be usable,
  because the transaction has already been committed.

When Hibernate is configured with awareness of the JTA transaction manager,
the following events occur when a JTA transaction commits:

* The JTA transaction is ready to commit.
* Spring's `JtaTransactionManager` is synchronized to the JTA transaction, so the
  transaction is called back through a `beforeCompletion` callback by the JTA
  transaction manager.
* Spring is aware that Hibernate itself is synchronized to the JTA transaction and
  behaves differently than in the previous scenario. In particular, it aligns with
  Hibernate's transactional resource management.
* The JTA transaction commits.
* Hibernate is synchronized to the JTA transaction, so the transaction is called back
  through an `afterCompletion` callback by the JTA transaction manager and can
  properly clear its cache.



