Typical business applications are cluttered with repetitive resource management code. Many projects try to invent their own solutions, sometimes sacrificing proper handling of failures for programming convenience. Spring advocates simple solutions for proper resource handling, namely IoC through templating in the case of JDBC and applying AOP interceptors for the ORM technologies.

The infrastructure provides proper resource handling and appropriate conversion of specific API exceptions to an unchecked infrastructure exception hierarchy. Spring introduces a DAO exception hierarchy, applicable to any data access strategy. For direct JDBC, the JdbcTemplate class mentioned in a previous section provides connection handling and proper conversion of SQLException to the DataAccessException hierarchy, including translation of database-specific SQL error codes to meaningful exception classes. For ORM technologies, see the next section for how to get the same exception translation benefits.

When it comes to transaction management, the JdbcTemplate class hooks in to the Spring transaction support and supports both JTA and JDBC transactions, through respective Spring transaction managers. For the supported ORM technologies Spring offers Hibernate, JPA and JDO support through the Hibernate, JPA, and JDO transaction managers as well as JTA support. For details on transaction support, see the Chapter 11, Transaction Management chapter.

When you use Hibernate, JPA, or JDO in a DAO, you must decide how to handle the persistence technology’s native exception classes. The DAO throws a subclass of a HibernateException, PersistenceException or JDOException depending on the technology. These exceptions are all run-time exceptions and do not have to be declared or caught. You may also have to deal with IllegalArgumentException and IllegalStateException. This means that callers can only treat exceptions as generally fatal, unless they want to depend on the persistence technology’s own exception structure. 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 ORM-based and/or do not need any special exception treatment. However, Spring enables exception translation to be applied transparently through the @Repository annotation:

@Repository
public class ProductDaoImpl implements ProductDao {

    // class body here...

}

<beans>

    <!-- Exception translation bean post processor -->
    <bean class="org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor"/>

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

</beans>

The postprocessor automatically looks for all exception translators (implementations of the PersistenceExceptionTranslator interface) and advises all beans marked with the @Repository annotation so that the discovered translators can intercept and apply the appropriate translation on the thrown exceptions.

In summary: you can implement DAOs based on the plain persistence technology’s API and annotations, while still benefiting from Spring-managed transactions, dependency injection, and transparent exception conversion (if desired) to Spring’s custom exception hierarchies.

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 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:

<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.hibernate3.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 just a matter of configuration:

<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.

Hibernate 3 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:

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();
    }
}

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 above DAO follows the dependency injection pattern: it fits nicely into a Spring IoC container, just as it would if coded against Spring’s HibernateTemplate. Of course, such a DAO can also be set up in plain Java (for example, in unit tests). Simply instantiate it and call setSessionFactory(..) with the desired factory reference. As a Spring bean definition, the DAO would resemble the following:

<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 of course appealing from a non-invasiveness perspective, and will no doubt feel more natural to Hibernate developers.

However, the DAO throws plain HibernateException (which is unchecked, so does not have to be declared or caught), which means that callers can only treat exceptions as 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 and/or do not need any special exception treatment.

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. Of course, the standard behavior of that method remains the return of the current Session associated with the ongoing JTA transaction, if any. This behavior applies regardless of whether you are using Spring’s JtaTransactionManager, EJB container managed transactions (CMTs), or JTA.

In summary: you can implement DAOs based on the plain Hibernate 3 API, while still being able to participate in Spring-managed transactions.

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

	Note
	Prior to continuing, you are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so.

Furthermore, transaction semantics like propagation behavior and isolation level can be changed in a configuration file and do not affect the business service implementations.

The following example shows how you can configure an AOP transaction interceptor, using XML, for a simple service class:

<?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
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx.xsd
        http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop.xsd">

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

    <bean id="transactionManager"
            class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory"/>
    </bean>

    <aop:config>
        <aop:pointcut id="productServiceMethods"
                expression="execution(* product.ProductService.(..))"/>
        <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/>
    </aop:config>

    <tx:advice id="txAdvice" transaction-manager="myTxManager">
        <tx:attributes>
            <tx:method name="increasePrice" propagation="REQUIRED"/>
            <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/>
            <tx:method name="*" propagation="SUPPORTS" read-only="true"/>
        </tx:attributes>
    </tx:advice>

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

</beans>

This is the service class that is advised:

public class ProductServiceImpl implements ProductService {

    private ProductDao productDao;

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

    // notice the absence of transaction demarcation code in this method
    // Spring's declarative transaction infrastructure will be demarcating
    // transactions on your behalf
    public void increasePriceOfAllProductsInCategory(final String category) {
        List productsToChange = this.productDao.loadProductsByCategory(category);
        // ...
    }
}

We also show an attribute-support based configuration, in the following example. You annotate the service layer with @Transactional annotations and instruct the Spring container to find these annotations and provide transactional semantics for these annotated methods.

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();
    }

}

As you can see from the following configuration example, the configuration is much simplified, compared to the XML example above, while still providing the same functionality driven by the annotations in the service layer code. All you need to provide is the TransactionManager implementation and a "<tx:annotation-driven/>" entry.

<?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
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx.xsd
        http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop.xsd">

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

    <bean id="transactionManager"
            class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory"/>
    </bean>

    <tx:annotation-driven/>

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

</beans>

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

<beans>

    <bean id="myTxManager" class="org.springframework.orm.hibernate3.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>

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...
            }
        });
    }
}

Spring’s TransactionInterceptor allows any checked application exception to 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 (via TransactionStatus). TransactionInterceptor behaves the same way by default but allows configurable rollback policies per method.

Both TransactionTemplate and TransactionInterceptor delegate the actual transaction handling to a PlatformTransactionManager instance, which can be a HibernateTransactionManager (for a single Hibernate SessionFactory, 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 just a matter of configuration. Simply replace the Hibernate transaction manager with Spring’s JTA transaction implementation. Both transaction demarcation and data access code will work without changes, because they just use the generic transaction management APIs.

For distributed transactions across multiple Hibernate session factories, simply 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 is using JtaTransactionManager as the strategy.

<beans>

    <jee:jndi-lookup id="dataSource1" jndi-name="java:comp/env/jdbc/myds1"/>

    <jee:jndi-lookup id="dataSource2" jndi-name="java:comp/env/jdbc/myds2"/>

    <bean id="mySessionFactory1"
            class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
        <property name="dataSource" ref="myDataSource1"/>
        <property name="mappingResources">
            <list>
                <value>product.hbm.xml</value>
            </list>
        </property>
        <property name="hibernateProperties">
            <value>
                hibernate.dialect=org.hibernate.dialect.MySQLDialect
                hibernate.show_sql=true
            </value>
        </property>
    </bean>

    <bean id="mySessionFactory2"
            class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
        <property name="dataSource" ref="myDataSource2"/>
        <property name="mappingResources">
            <list>
                <value>inventory.hbm.xml</value>
            </list>
        </property>
        <property name="hibernateProperties">
            <value>
                hibernate.dialect=org.hibernate.dialect.OracleDialect
            </value>
        </property>
    </bean>

    <bean id="myTxManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>

    <bean id="myProductDao" class="product.ProductDaoImpl">
        <property name="sessionFactory" ref="mySessionFactory1"/>
    </bean>

    <bean id="myInventoryDao" class="product.InventoryDaoImpl">
        <property name="sessionFactory" ref="mySessionFactory2"/>
    </bean>

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

    <aop:config>
        <aop:pointcut id="productServiceMethods"
                expression="execution(* product.ProductService.(..))"/>
        <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/>
    </aop:config>

    <tx:advice id="txAdvice" transaction-manager="myTxManager">
        <tx:attributes>
            <tx:method name="increasePrice" propagation="REQUIRED"/>
            <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/>
            <tx:method name="*" propagation="SUPPORTS" read-only="true"/>
        </tx:attributes>
    </tx:advice>

</beans>

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 are not using EJB to initiate transactions).

HibernateTransactionManager can export the Hibernate JDBC Connection to plain JDBC access code, for a specific DataSource. This capability allows for high-level transaction demarcation with mixed Hibernate and JDBC data access completely without JTA, if you are accessing 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.

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 Java EE server’s management infrastructure, but does not add actual value beyond that.

Spring’s transaction support is not bound to a container. 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 JTA, even if you access only a single database, and only use stateless session beans to provide declarative transactions through container-managed transactions. Also, direct use of JTA programmatically requires a Java EE environment as well. JTA does not involve only container dependencies in terms of JTA itself and of JNDI DataSource instances. For non-Spring, JTA-driven Hibernate transactions, you have to use the Hibernate JCA connector, or extra Hibernate transaction code with the TransactionManagerLookup configured for proper JVM-level caching.

Spring-driven transactions can work as well with a locally defined Hibernate SessionFactory as they do with a local JDBC DataSource if they are accessing a single database. Thus you only have to 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. Also, you often need the Enterprise Edition of your container if you are using, for example, WebLogic Express, which does not provide JCA. A Spring application with local resources and transactions spanning one single database works in any Java EE web container (without JTA, JCA, or EJB) such as Tomcat, Resin, or even plain Jetty. Additionally, you can easily reuse such a middle tier in desktop applications or test suites.

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 only adds value when used in conjunction with EJBs.

In some JTA environments with very strict XADataSource implementations — currently only some WebLogic Server and WebSphere versions — when Hibernate is configured without regard to the JTA PlatformTransactionManager object for that environment, it is possible for spurious warning or exceptions to 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:

java.sql.SQLException: The transaction is no longer active - status: Committed. No
further JDBC access is allowed within this transaction.

You resolve this warning by simply making Hibernate aware of the JTA PlatformTransactionManager instance, to which it will synchronize (along with Spring). You have two options for doing this:

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 PlatformTransactionManager, the following events occur when a JTA transaction commits:

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

Spring provides a LocalPersistenceManagerFactoryBean class that allows you to define a local JDO PersistenceManagerFactory within a Spring application context:

<beans>

    <bean id="myPmf" class="org.springframework.orm.jdo.LocalPersistenceManagerFactoryBean">
        <property name="configLocation" value="classpath:kodo.properties"/>
    </bean>

</beans>

Alternatively, you can set up a PersistenceManagerFactory through direct instantiation of a PersistenceManagerFactory implementation class. A JDO PersistenceManagerFactory implementation class follows the JavaBeans pattern, just like a JDBC DataSource implementation class, which is a natural fit for a configuration that uses Spring. This setup style usually supports a Spring-defined JDBC DataSource, passed into the connectionFactory property. For example, for the open source JDO implementation DataNucleus (formerly JPOX) ( http://www.datanucleus.org/), this is the XML configuration of the PersistenceManagerFactory implementation:

<beans>

 <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
    <property name="driverClassName" value="${jdbc.driverClassName}"/>
    <property name="url" value="${jdbc.url}"/>
    <property name="username" value="${jdbc.username}"/>
    <property name="password" value="${jdbc.password}"/>
 </bean>

 <bean id="myPmf" class="org.datanucleus.jdo.JDOPersistenceManagerFactory" destroy-method="close">
    <property name="connectionFactory" ref="dataSource"/>
    <property name="nontransactionalRead" value="true"/>
 </bean>

</beans>

You can also set up JDO PersistenceManagerFactory in the JNDI environment of a Java EE application server, usually through the JCA connector provided by the particular JDO implementation. Spring’s standard JndiObjectFactoryBean or <jee:jndi-lookup> can be used to retrieve and expose such a PersistenceManagerFactory. However, outside an EJB context, no real benefit exists in holding the PersistenceManagerFactory in JNDI: only choose such a setup for a good reason. See Section 14.3.6, “Comparing container-managed and locally defined resources” for a discussion; the arguments there apply to JDO as well.

DAOs can also be written directly against plain JDO API, without any Spring dependencies, by using an injected PersistenceManagerFactory. The following is an example of a corresponding DAO implementation:

public class ProductDaoImpl implements ProductDao {

    private PersistenceManagerFactory persistenceManagerFactory;

    public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) {
        this.persistenceManagerFactory = pmf;
    }

    public Collection loadProductsByCategory(String category) {
        PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager();
        try {
            Query query = pm.newQuery(Product.class, "category = pCategory");
            query.declareParameters("String pCategory");
            return query.execute(category);
        }
        finally {
            pm.close();
        }
    }
}

Because the above DAO follows the dependency injection pattern, it fits nicely into a Spring container, just as it would if coded against Spring’s JdoTemplate:

<beans>

    <bean id="myProductDao" class="product.ProductDaoImpl">
        <property name="persistenceManagerFactory" ref="myPmf"/>
    </bean>

</beans>

The main problem with such DAOs is that they always get a new PersistenceManager from the factory. To access a Spring-managed transactional PersistenceManager, define a TransactionAwarePersistenceManagerFactoryProxy (as included in Spring) in front of your target PersistenceManagerFactory, then passing a reference to that proxy into your DAOs as in the following example:

<beans>

    <bean id="myPmfProxy"
            class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy">
        <property name="targetPersistenceManagerFactory" ref="myPmf"/>
    </bean>

    <bean id="myProductDao" class="product.ProductDaoImpl">
        <property name="persistenceManagerFactory" ref="myPmfProxy"/>
    </bean>

</beans>

Your data access code will receive a transactional PersistenceManager (if any) from the PersistenceManagerFactory.getPersistenceManager() method that it calls. The latter method call goes through the proxy, which first checks for a current transactional PersistenceManager before getting a new one from the factory. Any close() calls on the PersistenceManager are ignored in case of a transactional PersistenceManager.

If your data access code always runs within an active transaction (or at least within active transaction synchronization), it is safe to omit the PersistenceManager.close() call and thus the entire finally block, which you might do to keep your DAO implementations concise:

public class ProductDaoImpl implements ProductDao {

    private PersistenceManagerFactory persistenceManagerFactory;

    public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) {
        this.persistenceManagerFactory = pmf;
    }

    public Collection loadProductsByCategory(String category) {
        PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager();
        Query query = pm.newQuery(Product.class, "category = pCategory");
        query.declareParameters("String pCategory");
        return query.execute(category);
    }
}

With such DAOs that rely on active transactions, it is recommended that you enforce active transactions through turning off TransactionAwarePersistenceManagerFactoryProxy's allowCreate flag:

<beans>

    <bean id="myPmfProxy"
            class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy">
        <property name="targetPersistenceManagerFactory" ref="myPmf"/>
        <property name="allowCreate" value="false"/>
    </bean>

    <bean id="myProductDao" class="product.ProductDaoImpl">
        <property name="persistenceManagerFactory" ref="myPmfProxy"/>
    </bean>

</beans>

The main advantage of this DAO style is that it depends on JDO API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and might feel more natural to JDO developers.

However, the DAO throws plain JDOException (which is unchecked, so does not have to be declared or caught), which means that callers can only treat exceptions as fatal, unless you want to depend on JDO’s own exception structure. 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 JDO-based and/or do not need any special exception treatment.

In summary, you can DAOs based on the plain JDO API, and they can still participate in Spring-managed transactions. This strategy might appeal to you if you are already familiar with JDO. However, such DAOs throw plain JDOException, and you would have to convert explicitly to Spring’s DataAccessException (if desired).

	Note
	You are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so, to get a more detailed coverage of Spring’s declarative transaction support.

To execute service operations within transactions, you can use Spring’s common declarative transaction facilities. For example:

<?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
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx.xsd
        http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop.xsd">

    <bean id="myTxManager" class="org.springframework.orm.jdo.JdoTransactionManager">
        <property name="persistenceManagerFactory" ref="myPmf"/>
    </bean>

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

    <tx:advice id="txAdvice" transaction-manager="txManager">
        <tx:attributes>
            <tx:method name="increasePrice*" propagation="REQUIRED"/>
            <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/>
            <tx:method name="" propagation="SUPPORTS" read-only="true"/>
        </tx:attributes>
    </tx:advice>

    <aop:config>
        <aop:pointcut id="productServiceMethods"
                expression="execution( product.ProductService.*(..))"/>
        <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/>
    </aop:config>

</beans>

JDO requires an active transaction to modify a persistent object. The non-transactional flush concept does not exist in JDO, in contrast to Hibernate. For this reason, you need to set up the chosen JDO implementation for a specific environment. Specifically, you need to set it up explicitly for JTA synchronization, to detect an active JTA transaction itself. This is not necessary for local transactions as performed by Spring’s JdoTransactionManager, but it is necessary to participate in JTA transactions, whether driven by Spring’s JtaTransactionManager or by EJB CMT and plain JTA.

JdoTransactionManager is capable of exposing a JDO transaction to JDBC access code that accesses the same JDBC DataSource, provided that the registered JdoDialect supports retrieval of the underlying JDBC Connection. This is the case for JDBC-based JDO 2.0 implementations by default.

As an advanced feature, both JdoTemplate and JdoTransactionManager support a custom JdoDialect that can be passed into the jdoDialect bean property. In this scenario, the DAOs will not receive a PersistenceManagerFactory reference but rather a full JdoTemplate instance (for example, passed into the jdoTemplate property of JdoDaoSupport). Using a JdoDialect implementation, you can enable advanced features supported by Spring, usually in a vendor-specific manner:

See the JdoDialect javadocs for more details on its operations and how to use them within Spring’s JDO support.

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

LocalEntityManagerFactoryBean

	Note
	Only use this option 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 autodetection mechanism (according to JPA’s Java SE bootstrapping) and, in most cases, requires you to specify only the persistence unit name:

<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

	Note
	Use this option when deploying to a Java EE 5 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 5 environment), is simply a matter of changing the XML configuration:

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

This action assumes standard Java EE 5 bootstrapping: the Java EE server autodetects 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 multiple persistence units are used 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.

LocalContainerEntityManagerFactoryBean

	Note
	Use this option for full JPA capabilities in a Spring-based application environment. This includes web containers such as Tomcat as well as 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:

<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>

Note

The <exclude-unlisted-classes/> shortcut indicates that no scanning for annotated entity classes is supposed to occur. An explicit true value specified - <exclude-unlisted-classes>true</exclude-unlisted-classes/> - also means no scan. <exclude-unlisted-classes>false</exclude-unlisted-classes/> does trigger a scan; however, it is recommended to simply omit 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 5 server. In a full Java EE 5 environment, consider obtaining your EntityManagerFactory from JNDI. Alternatively, specify a custom persistenceXmlLocation on your LocalContainerEntityManagerFactoryBean definition, for example, META-INF/my-persistence.xml, and only include a descriptor with that name in your application jar files. Because the Java EE 5 server only looks for default META-INF/persistence.xml files, it ignores such custom persistence units and hence avoid 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, the load-time weaver should not be used.

The LoadTimeWeaver interface is a Spring-provided class that allows JPA ClassTransformer instances to be plugged in a specific manner, depending whether the environment is a web container or application server. Hooking ClassTransformers through an agent typically is 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, allowing ClassTransformer instances to be applied only per class loader and not per VM.

Refer to the section called “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, WebLogic, GlassFish, Resin and JBoss).

As described in the aforementioned section, you can configure a context-wide LoadTimeWeaver using the @EnableLoadTimeWeaving annotation of context:load-time-weaver XML element. Such a global weaver is picked up by all JPA LocalContainerEntityManagerFactoryBeans automatically. This is the preferred way of setting up a load-time weaver, delivering autodetection of the platform (WebLogic, GlassFish, Tomcat, Resin, JBoss or VM 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, if needed, one can manually specify a dedicated weaver through the loadTimeWeaver property:

<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, using this technique, JPA applications relying on instrumentation can run in the target platform (ex: Tomcat) without needing an agent. This is important especially when the hosting applications rely on different JPA implementations because the JPA transformers are applied only at class loader level and thus are isolated from each other.

<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>

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 @PersistenceUnit and @PersistenceContext annotations both at field and method level if a PersistenceAnnotationBeanPostProcessor is enabled. A plain JPA DAO implementation using the @PersistenceUnit annotation might look like this:

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();
            }
        }
    }
}

The DAO above 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:

<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 defining a PersistenceAnnotationBeanPostProcessor explicitly, 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.

<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 "shared EntityManager" because it is a shared, thread-safe proxy for the actual transactional EntityManager) to be injected instead of the factory:

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();
    }
}

The @PersistenceContext annotation has an optional attribute type, which defaults to PersistenceContextType.TRANSACTION. This default is what you need 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 EntityManagers 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.

The injected EntityManager is Spring-managed (aware of the ongoing transaction). It is important to note that 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 only depends on 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 might feel more natural to JPA developers.

	Note
	You are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so, to get a more detailed coverage of Spring’s declarative transaction support.

To execute service operations within transactions, you can use Spring’s common declarative transaction facilities. For example:

<?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
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx.xsd
        http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop.xsd">

    <bean id="myTxManager" class="org.springframework.orm.jpa.JpaTransactionManager">
        <property name="entityManagerFactory" ref="myEmf"/>
    </bean>

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

    <aop:config>
        <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.(..))"/>
        <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/>
    </aop:config>

    <tx:advice id="txAdvice" transaction-manager="myTxManager">
        <tx:attributes>
            <tx:method name="increasePrice" propagation="REQUIRED"/>
            <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/>
            <tx:method name="*" propagation="SUPPORTS" read-only="true"/>
        </tx:attributes>
    </tx:advice>

</beans>

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

As an advanced feature JpaTemplate, JpaTransactionManager and subclasses of AbstractEntityManagerFactoryBean support a custom JpaDialect, to be passed into the jpaDialect bean property. In such a scenario, the DAOs do not receive an EntityManagerFactory reference but rather a full JpaTemplate instance (for example, passed into the jpaTemplate property of JpaDaoSupport). A JpaDialect implementation can enable some advanced features supported by Spring, usually in a vendor-specific manner:

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

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