Typical business applications are often cluttered with repetitive resource management code. Many projects try to invent their own solutions for this issue, sometimes sacrificing proper handling of failures for programming convenience. Spring advocates strikingly simple solutions for proper resource handling, namely IoC via templating; for example infrastructure classes with callback interfaces, or applying AOP interceptors. The infrastructure cares for proper resource handling, and for 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 cares for connection handling, and for proper conversion of SQLException to the DataAccessException hierarchy, including translation of database-specific SQL error codes to meaningful exception classes. It supports both JTA and JDBC transactions, via respective Spring transaction managers.

Spring also offers Hibernate and JDO support, consisting of a HibernateTemplate / JdoTemplate analogous to JdbcTemplate, a HibernateInterceptor / JdoInterceptor, and a Hibernate / JDO transaction manager. The major goal is to allow for clear application layering, with any data access and transaction technology, and for loose coupling of application objects. No more business service dependencies on the data access or transaction strategy, no more hard-coded resource lookups, no more hard-to-replace singletons, no more custom service registries. One simple and consistent approach to wiring up application objects, keeping them as reusable and free from container dependencies as possible. All the individual data access features are usable on their own but integrate nicely with Spring's application context concept, providing XML-based configuration and cross-referencing of plain JavaBean instances that don't need to be Spring-aware. In a typical Spring app, many important objects are JavaBeans: data access templates, data access objects (that use the templates), transaction managers, business services (that use the data access objects and transaction managers), web view resolvers, web controllers (that use the business services),and so on.

To avoid tying application objects to hard-coded resource lookups, Spring allows you to define resources like a JDBC DataSource or a Hibernate SessionFactory as beans in an application context. Application objects that need to access resources just receive references to such pre-defined instances via bean references (the DAO definition in the next section illustrates this). 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>

Note that 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>

  <bean id="myDataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
    <property name="jndiName" value="java:comp/env/jdbc/myds"/>
  </bean>

</beans>

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

The basic programming model for templating looks as follows, for methods that can be part of any custom data access object or business service. There are no restrictions on the implementation of the surrounding object at all, it just needs to provide a Hibernate SessionFactory. It can get the latter from anywhere, but preferably as bean reference from a Spring IoC container - via a simple setSessionFactory(..) bean property setter. The following snippets show a DAO definition in a Spring container, referencing the above defined SessionFactory, and an example for a DAO method implementation.

<beans>

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

</beans>

public class ProductDaoImpl implements ProductDao {

    private HibernateTemplate hibernateTemplate;

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

    public Collection loadProductsByCategory(String category) throws DataAccessException {
    	return this.hibernateTemplate.find("from test.Product product where product.category=?", category);
    }
}

The HibernateTemplate class provides many methods that mirror the methods exposed on the Hibernate Session interface, in addition to a number of convenience methods such as the one shown above. If you need access to the Session to invoke methods that are not exposed on the HibernateTemplate, you can always drop down to a callback-based approach like so.

public class ProductDaoImpl implements ProductDao {

    private HibernateTemplate hibernateTemplate;

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

    public Collection loadProductsByCategory(final String category) throws DataAccessException {
        return this.hibernateTemplate.execute(new HibernateCallback() {
            public Object doInHibernate(Session session) {
                Criteria criteria = session.createCriteria(Product.class);
                criteria.add(Expression.eq("category", category));
                criteria.setMaxResults(6);
                return criteria.list();
            }
        };
    }
}

A callback implementation effectively can be used for any Hibernate data access. HibernateTemplate will ensure that Session instances are properly opened and closed, and automatically participate in transactions. The template instances are thread-safe and reusable, they can thus be kept as instance variables of the surrounding class. For simple single step actions like a single find, load, saveOrUpdate, or delete call, HibernateTemplate offers alternative convenience methods that can replace such one line callback implementations. Furthermore, Spring provides a convenient HibernateDaoSupport base class that provides a setSessionFactory(..) method for receiving a SessionFactory, and getSessionFactory() and getHibernateTemplate()for use by subclasses. In combination, this allows for very simple DAO implementations for typical requirements:

public class ProductDaoImpl extends HibernateDaoSupport implements ProductDao {

    public Collection loadProductsByCategory(String category) throws DataAccessException {
        return this.getHibernateTemplate().find(
            "from test.Product product where product.category=?", category);
    }
}

As alternative to using Spring's HibernateTemplate to implement DAOs, data access code can also be written in a more traditional fashion, without wrapping the Hibernate access code in a callback, while still complying to Spring's generic DataAccessException hierarchy. Spring's HibernateDaoSupport base class offers methods to access the current transactional Session and to convert exceptions in such a scenario; similar methods are also available as static helpers on the SessionFactoryUtils class. Note that such code will usually pass "false" into the getSession(..) method's "allowCreate" argument, to enforce running within a transaction (which avoids the need to close the returned Session, as it's lifecycle is managed by the transaction).

public class ProductDaoImpl extends HibernateDaoSupport implements ProductDao {

    public Collection loadProductsByCategory(String category)
            throws DataAccessException, MyException {

        Session session = getSession(getSessionFactory(), false);
        try {
            List result = session.find(
                "from test.Product product where product.category=?",
                category, Hibernate.STRING);
            if (result == null) {
                throw new MyException("invalid search result");
            }
            return result;
        }
        catch (HibernateException ex) {
            throw convertHibernateAccessException(ex);
        }
    }
}

The major advantage of such direct Hibernate access code is that it allows any checked application exception to be thrown within the data access code, while HibernateTemplate is restricted to unchecked exceptions within the callback. Note that one can often defer the corresponding checks and the throwing of application exceptions to after the callback, which still allows working with HibernateTemplate. In general, the HibernateTemplate class' convenience methods are simpler and more convenient for many scenarios.

Hibernate 3.0.1 introduced a feature called "contextual Sessions", where 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 looks like as follows, 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 very similar to what you will find in 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 like 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, it would look as follows:

<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 tieing the caller to the implementation strategy. This tradeoff 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: returning the current Session associated with the ongoing JTA transaction, if any (no matter whether driven by Spring's JtaTransactionManager, by EJB CMT, or by JTA).

In summary: DAOs can be implemented based on the plain Hibernate3 API, while still being able to participate in Spring-managed transactions.

Transactions can be demarcated in a higher level of the application, on top of such lower-level data access services spanning any number of operations. There are no restrictions on the implementation of the surrounding business service here as well, it just needs a Spring PlatformTransactionManager. Again, the latter can come from anywhere, but preferably as bean reference via a setTransactionManager(..) method - just like the productDAO should be set via 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...
                }
            }
        );
    }
}

Alternatively, one can use Spring's declarative transaction support, which essentially enables you to replace explicit transaction demarcation API calls in your Java code with an AOP transaction interceptor configured in a Spring container. This allows you to keep business services free of repetitive transaction demarcation code, and allows you to focus on adding business logic which is where the real value of your application lies. Furthermore, transaction semantics like propagation behavior and isolation level can be changed in a configuration file and do not affect the business service implementations.

<beans>

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

  <bean id="myProductService" class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="proxyInterfaces" value="product.ProductService"/>
    <property name="target">
        <bean class="product.DefaultProductService">
            <property name="productDao" ref="myProductDao"/>
        </bean>
    </property>
    <property name="interceptorNames">
      <list>
        <value>myTxInterceptor</value> <!-- the transaction interceptor (configured elsewhere) -->
      </list>
    </property>
  </bean>

</beans>

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);
        // ...
    }
}

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 will trigger a rollback in case of an unchecked application exception, or if the transaction has been marked rollback-only by the application (via TransactionStatus). TransactionInterceptor behaves the same way by default but allows configurable rollback policies per method.

The following higher level approach to declarative transactions doesn't use the ProxyFactoryBean, and as such may be easier to use if you have a large number of service objects that you wish to make transactional.

	Note
	You are strongly encouraged to read the section entitled Section 9.5, “Declarative transaction management” if you have not done so already prior to continuing.

<?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-2.0.xsd
       http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
       http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">

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

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

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 could even use a custom PlatformTransactionManager implementation. So 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, as 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 of your DAOs then gets one specific SessionFactory reference passed into it's respective 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>

  <bean id="myDataSource1" class="org.springframework.jndi.JndiObjectFactoryBean">
    <property name="jndiName value="java:comp/env/jdbc/myds1"/>
  </bean>

  <bean id="myDataSource2" class="org.springframework.jndi.JndiObjectFactoryBean">
    <property name="jndiName" value="java:comp/env/jdbc/myds2"/>
  </bean>

  <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>

  <!-- this shows the Spring 1.x style of declarative transaction configuration -->
  <!-- it is totally supported, 100% legal in Spring 2.x, but see also above for the sleeker, Spring 2.0 style -->
  <bean id="myProductService"
      class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
    <property name="transactionManager" ref="myTxManager"/>
    <property name="target">
      <bean class="product.ProductServiceImpl">
        <property name="productDao" ref="myProductDao"/>
        <property name="inventoryDao" ref="myInventoryDao"/>
      </bean>
    </property>
    <property name="transactionAttributes">
      <props>
        <prop key="increasePrice*">PROPAGATION_REQUIRED</prop>
        <prop key="someOtherBusinessMethod">PROPAGATION_REQUIRES_NEW</prop>
        <prop key="*">PROPAGATION_SUPPORTS,readOnly</prop>
      </props>
    </property>
  </bean>

</beans>

Both HibernateTransactionManager and JtaTransactionManager allow for proper JVM-level cache handling with Hibernate - without container-specific transaction manager lookup or JCA connector (as long as not using EJB to initiate transactions).

HibernateTransactionManager can export the JDBC Connection used by Hibernate to plain JDBC access code, for a specific DataSource. This allows for high-level transaction demarcation with mixed Hibernate/JDBC data access completely without JTA, as long as you are just accessing one database! HibernateTransactionManager will automatically expose the Hibernate transaction as JDBC transaction if the passed-in SessionFactory has been set up with a DataSource (through the "dataSource" property of the LocalSessionFactoryBean class). Alternatively, the DataSource that the transactions are supposed to be exposed for can also be specified explicitly, through the "dataSource" property of the HibernateTransactionManager class.

Spring's resource management allows for simple switching between a JNDI SessionFactory and a local one, without having to change a single line of application code. The decision as to whether to keep the resource definitions in the container or locally within the application, is mainly a matter of the transaction strategy being used. 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 J2EE server's management infrastructure, but does not add actual value beyond that.

An important benefit of Spring's transaction support is that it isn't bound to a container at all. Configured to any other strategy than JTA, it will work in a standalone or test environment too. Especially for the typical case of single-database transactions, this is a very lightweight and powerful alternative to JTA. When using local EJB Stateless Session Beans to drive transactions, you depend both on an EJB container and JTA - even if you just access a single database anyway, and just use SLSBs for declarative transactions via CMT. The alternative of using JTA programmatically requires a J2EE environment as well. JTA does not just involve 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 being configured for proper JVM-level caching.

Spring-driven transactions can work with a locally defined Hibernate SessionFactory nicely, just like with a local JDBC DataSource - if accessing a single database, of course. Therefore you just have to fall back to Spring's JTA transaction strategy when actually facing distributed transaction requirements. Note that a JCA connector needs container-specific deployment steps, and obviously JCA support in the first place. This is far more hassle than deploying a simple web app with local resource definitions and Spring-driven transactions. And you often need the Enterprise Edition of your container, as for example WebLogic Express does not provide JCA. A Spring application with local resources and transactions spanning one single database will work in any J2EE web container (without JTA, JCA, or EJB) - like Tomcat, Resin, or even plain Jetty. Additionally, such a middle tier can be reused in desktop applications or test suites easily.

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

In some JTA environments with very strict XADataSource implementations -- currently only some WebLogic and WebSphere versions -- when using Hibernate configured without any awareness of 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 will say something to the effect 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.

This warning is easy to resolve by simply making Hibernate aware of the JTA PlatformTransactionManager instance, to which it will also synchronize (along with Spring). This may be done in two ways:

It is not necessary to read any more for proper usage, but the full sequence of events with and without Hibernate being aware of the JTA PlatformTransactionManager will now be described.

When Hibernate is not configured with any awareness of the JTA PlatformTransactionManager, the sequence of events when a JTA transaction commits is as follows:

When Hibernate is configured with awareness of the JTA PlatformTransactionManager, the sequence of events when a JTA transaction commits is instead as follows:

Spring provides a LocalPersistenceManagerFactoryBean class that allows for defining 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, a PersistenceManagerFactory can also be set up through direct instantiation of a PersistenceManagerFactory implementation class. A JDO PersistenceManagerFactory implementation class is supposed to follow the JavaBeans pattern, just like a JDBC DataSource implementation class, which is a natural fit for a Spring bean definition. This setup style usually supports a Spring-defined JDBC DataSource, passed into the "connectionFactory" property. For example, for the open source JDO implementation JPOX (http://www.jpox.org):

<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.jpox.PersistenceManagerFactoryImpl" destroy-method="close">
   <property name="connectionFactory" ref="dataSource"/>
   <property name="nontransactionalRead" value="true"/>
 </bean>

</beans>

A JDO PersistenceManagerFactory can also be set up in the JNDI environment of a J2EE application server, usually through the JCA connector provided by the particular JDO implementation. Spring's standard JndiObjectFactoryBean can be used to retrieve and expose such a PersistenceManagerFactory. However, outside an EJB context, there is often no compelling benefit in holding the PersistenceManagerFactory in JNDI: only choose such setup for a good reason. See "container resources versus local resources" in the Hibernate section for a discussion; the arguments there apply to JDO as well.

Each JDO-based DAO will then receive the PersistenceManagerFactory through dependency injection. Such a DAO could be coded against plain JDO API, working with the given PersistenceManagerFactory, but will usually rather be used with the Spring Framework's JdoTemplate:

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

public class ProductDaoImpl implements ProductDao {
  
    private JdoTemplate jdoTemplate;

    public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) {
        this.jdoTemplate = new JdoTemplate(pmf);
    }

    public Collection loadProductsByCategory(final String category) throws DataAccessException {
        return (Collection) this.jdoTemplate.execute(new JdoCallback() {
            public Object doInJdo(PersistenceManager pm) throws JDOException {
                Query query = pm.newQuery(Product.class, "category = pCategory");
                query.declareParameters("String pCategory"); 
                List result = query.execute(category);
                // do some further stuff with the result list
                return result;
            }
        });
    }
}

A callback implementation can effectively be used for any JDO data access. JdoTemplate will ensure that PersistenceManagers are properly opened and closed, and automatically participate in transactions. The template instances are thread-safe and reusable, they can thus be kept as instance variables of the surrounding class. For simple single-step actions such as a single find, load, makePersistent, or delete call, JdoTemplate offers alternative convenience methods that can replace such one line callback implementations. Furthermore, Spring provides a convenient JdoDaoSupport base class that provides a setPersistenceManagerFactory(..) method for receiving a PersistenceManagerFactory, and getPersistenceManagerFactory() and getJdoTemplate() for use by subclasses. In combination, this allows for very simple DAO implementations for typical requirements:

public class ProductDaoImpl extends JdoDaoSupport implements ProductDao {
  
    public Collection loadProductsByCategory(String category) throws DataAccessException {
        return getJdoTemplate().find(
            Product.class, "category = pCategory", "String category", new Object[] {category});
    }
}

As alternative to working with Spring's JdoTemplate, you can also code Spring-based DAOs at the JDO API level, explicitly opening and closing a PersistenceManager. As elaborated in the corresponding Hibernate section, the main advantage of this approach is that your data access code is able to throw checked exceptions. JdoDaoSupport offers a variety of support methods for this scenario, for fetching and releasing a transactional PersistenceManager as well as for converting exceptions.

DAOs can also be written against plain JDO API, without any Spring dependencies, directly using an injected PersistenceManagerFactory. A corresponding DAO implementation looks like as follows:

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();
        }
    }
}

As the above DAO still follows the Dependency Injection pattern, it still fits nicely into a Spring container, just like it would if coded against Spring's JdoTemplate:

<beans>

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

</beans>

The main issue with such DAOs is that they always get a new PersistenceManager from the factory. To still access a Spring-managed transactional PersistenceManager, consider defining a TransactionAwarePersistenceManagerFactoryProxy (as included in Spring) in front of your target PersistenceManagerFactory, passing the proxy into your DAOs.

<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 then receive a transactional PersistenceManager (if any) from the PersistenceManagerFactory.getPersistenceManager() method that it calls. The latter method call goes through the proxy, which will first check for a current transactional PersistenceManager before getting a new one from the factory. close() calls on the PersistenceManager will be ignored in case of a transactional PersistenceManager.

If your data access code will always run 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 prefer 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 to enforce active transactions through turning TransactionAwarePersistenceManagerFactoryProxy's "allowCreate" flag off:

<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 generally fatal - unless they 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 tradeoff might be acceptable to applications that are strongly JDO-based and/or do not need any special exception treatment.

In summary: DAOs can be implemented based on plain JDO API, while still being able to participate in Spring-managed transactions. This might in particular appeal to people already familiar with JDO, feeling more natural to them. However, such DAOs will throw plain JDOException; conversion to Spring's DataAccessException would have to happen explicitly (if desired).

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-2.0.xsd
   http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
   http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.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>

Note that JDO requires an active transaction when modifying a persistent object. There is no concept like a non-transactional flush in JDO, in contrast to Hibernate. For this reason, the chosen JDO implementation needs to be set up for a specific environment: in particular, it needs to be explicitly set up 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 for participating in JTA transactions (whether driven by Spring's JtaTransactionManager or by EJB CMT / 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 by default the case for JDBC-based JDO 2.0 implementations; for JDO 1.0 implementations, a custom JdoDialect needs to be used. See next section for details on the JdoDialect mechanism.

As an advanced feature, both JdoTemplate and interfacename support a custom JdoDialect, to be passed into the "jdoDialect" bean property. In such a scenario, the DAOs won't receive a PersistenceManagerFactory reference but rather a full JdoTemplate instance instead (for example, passed into JdoDaoSupport's "jdoTemplate" property). A JdoDialect implementation can enable some advanced features supported by Spring, usually in a vendor-specific manner:

This is particularly valuable for JDO 1.0 implementations, where none of those features are covered by the standard API. On JDO 2.0, most of those features are supported in a standard manner: Hence, Spring's DefaultJdoDialect uses the corresponding JDO 2.0 API methods by default (as of Spring 1.2). For special transaction semantics and for advanced translation of exception, it is still valuable to derive vendor-specific JdoDialect subclasses.

See the JdoDialect Javadoc for more details on its operations and how they are used within Spring's JDO support.

TopLink itself does not ship with a SessionFactory abstraction. Instead, multi-threaded access is based on the concept of a central ServerSession, which in turn is able to spawn ClientSession instances for single-threaded usage. For flexible setup options, Spring defines a SessionFactory abstraction for TopLink, enabling to switch between different Session creation strategies.

As a one-stop shop, Spring provides a LocalSessionFactoryBean class that allows for defining a TopLink SessionFactory with bean-style configuration. It needs to be configured with the location of the TopLink session configuration file, and usually also receives a Spring-managed JDBC DataSource to use.

<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="mySessionFactory" class="org.springframework.orm.toplink.LocalSessionFactoryBean">
    <property name="configLocation" value="toplink-sessions.xml"/>
    <property name="dataSource" ref="dataSource"/>
  </bean>
    
</beans>

<toplink-configuration>

  <session>
    <name>Session</name>
    <project-xml>toplink-mappings.xml</project-xml>
    <session-type>
      <server-session/>
    </session-type>
    <enable-logging>true</enable-logging>
    <logging-options/>
  </session>

</toplink-configuration>

Usually, LocalSessionFactoryBean will hold a multi-threaded TopLink ServerSession underneath and create appropriate client Sessions for it: either a plain Session (typical), a managed ClientSession, or a transaction-aware Session (the latter are mainly used internally by Spring's TopLink support). It might also hold a single-threaded TopLink DatabaseSession; this is rather unusual, though.

Each TopLink-based DAO will then receive the SessionFactory through dependency injection, i.e. through a bean property setter or through a constructor argument. Such a DAO could be coded against plain TopLink API, fetching a Session from the given SessionFactory, but will usually rather be used with Spring's TopLinkTemplate:

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

public class TopLinkProductDao implements ProductDao {
  
    private TopLinkTemplate tlTemplate;

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

    public Collection loadProductsByCategory(final String category) throws DataAccessException {
        return (Collection) this.tlTemplate.execute(new TopLinkCallback() {
            public Object doInTopLink(Session session) throws TopLinkException {
                ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class);
                findOwnersQuery.addArgument("Category");
                ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder();
                findOwnersQuery.setSelectionCriteria(
                    builder.get("category").like(builder.getParameter("Category")));

                Vector args = new Vector();
                args.add(category);
                List result = session.executeQuery(findOwnersQuery, args);
                // do some further stuff with the result list
                return result;
            }
        });
    }
}

A callback implementation can effectively be used for any TopLink data access. TopLinkTemplate will ensure that Sessions are properly opened and closed, and automatically participate in transactions. The template instances are thread-safe and reusable, they can thus be kept as instance variables of the surrounding class. For simple single-step actions such as a single executeQuery, readAll, readById, or merge call, JdoTemplate offers alternative convenience methods that can replace such one line callback implementations. Furthermore, Spring provides a convenient TopLinkDaoSupport base class that provides a setSessionFactory(..) method for receiving a SessionFactory, and getSessionFactory() and getTopLinkTemplate() for use by subclasses. In combination, this allows for simple DAO implementations for typical requirements:

public class ProductDaoImpl extends TopLinkDaoSupport implements ProductDao {
  
    public Collection loadProductsByCategory(String category) throws DataAccessException {
        ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class);
        findOwnersQuery.addArgument("Category");
        ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder();
        findOwnersQuery.setSelectionCriteria(
            builder.get("category").like(builder.getParameter("Category")));

        return getTopLinkTemplate().executeQuery(findOwnersQuery, new Object[] {category});
    }
}

Side note: TopLink query objects are thread-safe and can be cached within the DAO, i.e. created on startup and kept in instance variables.

As alternative to working with Spring's TopLinkTemplate, you can also code your TopLink data access based on the raw TopLink API, explicitly opening and closing a Session. As elaborated in the corresponding Hibernate section, the main advantage of this approach is that your data access code is able to throw checked exceptions. TopLinkDaoSupport offers a variety of support methods for this scenario, for fetching and releasing a transactional Session as well as for converting exceptions.

DAOs can also be written against plain TopLink API, without any Spring dependencies, directly using an injected TopLink Session. The latter will usually be based on a SessionFactory defined by a LocalSessionFactoryBean, exposed for bean references of type Session through Spring's TransactionAwareSessionAdapter.

The getActiveSession() method defined on TopLink's Session interface will return the current transactional Session in such a scenario. If there is no active transaction, it will return the shared TopLink ServerSession as-is, which is only supposed to be used directly for read-only access. There is also an analogous getActiveUnitOfWork() method, returning the TopLink UnitOfWork associated with the current transaction, if any (returning null else).

A corresponding DAO implementation looks like as follows:

public class ProductDaoImpl implements ProductDao {

    private Session session;

    public void setSession(Session session) {
        this.session = session;
    }

    public Collection loadProductsByCategory(String category) {
        ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class);
        findOwnersQuery.addArgument("Category");
        ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder();
        findOwnersQuery.setSelectionCriteria(
            builder.get("category").like(builder.getParameter("Category")));

        Vector args = new Vector();
        args.add(category);
        return session.getActiveSession().executeQuery(findOwnersQuery, args);
    }
}

As the above DAO still follows the Dependency Injection pattern, it still fits nicely into a Spring application context, analogous to like it would if coded against Spring's TopLinkTemplate. Spring's TransactionAwareSessionAdapter is used to expose a bean reference of type Session, to be passed into the DAO:

<beans>

  <bean id="mySessionAdapter"
      class="org.springframework.orm.toplink.support.TransactionAwareSessionAdapter">
    <property name="sessionFactory" ref="mySessionFactory"/>
  </bean>

  <bean id="myProductDao" class="product.ProductDaoImpl">
    <property name="session" ref="mySessionAdapter"/>
  </bean>

</beans>

The main advantage of this DAO style is that it depends on TopLink 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 TopLink developers.

However, the DAO throws plain TopLinkException (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 TopLink'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 tradeoff might be acceptable to applications that are strongly TopLink-based and/or do not need any special exception treatment.

A further disadvantage of that DAO style is that TopLink's standard getActiveSession() feature just works within JTA transactions. It does not work with any other transaction strategy out-of-the-box, in particular not with local TopLink transactions.

Fortunately, Spring's TransactionAwareSessionAdapter exposes a corresponding proxy for the TopLink ServerSession which supports TopLink's Session.getActiveSession() and Session.getActiveUnitOfWork() methods for any Spring transaction strategy, returning the current Spring-managed transactional Session even with TopLinkTransactionManager. Of course, the standard behavior of that method remains: returning the current Session associated with the ongoing JTA transaction, if any (no matter whether driven by Spring's JtaTransactionManager, by EJB CMT, or by plain JTA).

In summary: DAOs can be implemented based on plain TopLink API, while still being able to participate in Spring-managed transactions. This might in particular appeal to people already familiar with TopLink, feeling more natural to them. However, such DAOs will throw plain TopLinkException; conversion to Spring's DataAccessException would have to happen explicitly (if desired).

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-2.0.xsd
   http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
   http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">

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

Note that TopLink requires an active UnitOfWork for modifying a persistent object. (You should never modify objects returned by a plain TopLink Session - those are usually read-only objects, directly taken from the second-level cache!) There is no concept like a non-transactional flush in TopLink, in contrast to Hibernate. For this reason, TopLink needs to be set up for a specific environment: in particular, it needs to be explicitly set up for JTA synchronization, to detect an active JTA transaction itself and expose a corresponding active Session and UnitOfWork. This is not necessary for local transactions as performed by Spring's TopLinkTransactionManager, but it is necessary for participating in JTA transactions (whether driven by Spring's JtaTransactionManager or by EJB CMT / plain JTA).

Within your TopLink-based DAO code, use the Session.getActiveUnitOfWork() method to access the current UnitOfWork and perform write operations through it. This will only work within an active transaction (both within Spring-managed transactions and plain JTA transactions). For special needs, you can also acquire separate UnitOfWork instances that won't participate in the current transaction; this is hardly needed, though.

TopLinkTransactionManager is capable of exposing a TopLink transaction to JDBC access code that accesses the same JDBC DataSource, provided that TopLink works with JDBC in the backend and is thus able to expose the underlying JDBC Connection. The DataSource to expose the transactions for needs to be specified explicitly; it won't be autodetected.

If we want to map the previous Account class with iBATIS 2.x we need to create the following SQL map 'Account.xml':

<sqlMap namespace="Account">

  <resultMap id="result" class="examples.Account">
    <result property="name" column="NAME" columnIndex="1"/>
    <result property="email" column="EMAIL" columnIndex="2"/>
  </resultMap>

  <select id="getAccountByEmail" resultMap="result">
    select ACCOUNT.NAME, ACCOUNT.EMAIL
    from ACCOUNT
    where ACCOUNT.EMAIL = #value#
  </select>

  <insert id="insertAccount">
    insert into ACCOUNT (NAME, EMAIL) values (#name#, #email#)
  </insert>

</sqlMap>

The configuration file for iBATIS 2 looks like this:

<sqlMapConfig>

  <sqlMap resource="example/Account.xml"/>

</sqlMapConfig>

Remember that iBATIS loads resources from the class path, so be sure to add the 'Account.xml' file to the class path.

We can use the SqlMapClientFactoryBean in the Spring container. Note that with iBATIS SQL Maps 2.x, the JDBC DataSource is usually specified on the SqlMapClientFactoryBean, which enables lazy loading.

<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="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
    <property name="configLocation" value="WEB-INF/sqlmap-config.xml"/>
    <property name="dataSource" ref="dataSource"/>
  </bean>

</beans>

The SqlMapClientDaoSupport class offers a supporting class similar to the SqlMapDaoSupport. We extend it to implement our DAO:

public class SqlMapAccountDao extends SqlMapClientDaoSupport implements AccountDao {

    public Account getAccount(String email) throws DataAccessException {
        return (Account) getSqlMapClientTemplate().queryForObject("getAccountByEmail", email);
    }

    public void insertAccount(Account account) throws DataAccessException {
        getSqlMapClientTemplate().update("insertAccount", account);
    }
}

In the DAO, we use the pre-configured SqlMapClientTemplate to execute the queries, after setting up the SqlMapAccountDao in the application context and wiring it with our SqlMapClient instance:

<beans>

  <bean id="accountDao" class="example.SqlMapAccountDao">
    <property name="sqlMapClient" ref="sqlMapClient"/>
  </bean>

</beans>

Note that a SqlMapTemplate instance could also be created manually, passing in the SqlMapClient as constructor argument. The SqlMapClientDaoSupport base class simply pre-initializes a SqlMapClientTemplate instance for us.

The SqlMapClientTemplate also offers a generic execute method, taking a custom SqlMapClientCallback implementation as argument. This can, for example, be used for batching:

public class SqlMapAccountDao extends SqlMapClientDaoSupport implements AccountDao {

    public void insertAccount(Account account) throws DataAccessException {
        getSqlMapClientTemplate().execute(new SqlMapClientCallback() {
            public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
                executor.startBatch();
                executor.update("insertAccount", account);
                executor.update("insertAddress", account.getAddress());
                executor.executeBatch();
            }
        });
    }
}

In general, any combination of operations offered by the native SqlMapExecutor API can be used in such a callback. Any SQLException thrown will automatically get converted to Spring's generic DataAccessException hierarchy.

DAOs can also be written against plain iBATIS API, without any Spring dependencies, directly using an injected SqlMapClient. A corresponding DAO implementation looks like as follows:

public class SqlMapAccountDao implements AccountDao {
        
    private SqlMapClient sqlMapClient;
    
    public void setSqlMapClient(SqlMapClient sqlMapClient) {
        this.sqlMapClient = sqlMapClient;
    }

    public Account getAccount(String email) {
        try {
            return (Account) this.sqlMapClient.queryForObject("getAccountByEmail", email);
        }
        catch (SQLException ex) {
            throw new MyDaoException(ex);
        }
    }

    public void insertAccount(Account account) throws DataAccessException {
        try {
            this.sqlMapClient.update("insertAccount", account);
        }
        catch (SQLException ex) {
            throw new MyDaoException(ex);
        }
    }
}

In such a scenario, the SQLException thrown by the iBATIS API needs to be handled in a custom fashion: usually, wrapping it in your own application-specific DAO exception. Wiring in the application context would still look like before, due to the fact that the plain iBATIS-based DAO still follows the Dependency Injection pattern:

<beans>

  <bean id="accountDao" class="example.SqlMapAccountDao">
    <property name="sqlMapClient" ref="sqlMapClient"/>
  </bean>

</beans>

Spring JPA offers two ways of setting up JPA EntityManagerFactory:

<beans>

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

</beans>

<beans>

    <jndi:lookup id="entityManagerFactory" jndi-name="jpa/myPersistenceUnit"/>

</beans>

12.6.1.2. LocalContainerEntityManagerFactoryBean

The LocalContainerEntityManagerFactoryBean gives full control over the JPA EntityManagerFactory and is appropriate for environments where fine customizations are required. The LocalContainerEntityManagerFactoryBean will create a PersistenceUnitInfo based on the 'persistence.xml' file, the supplied dataSourceLookup strategy and the loadTimeWeaver. It is thus possible to work with custom datasources outside of JNDI and control the weaving process.

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

When is load time weaving required?

Not all JPA providers impose the need of a JDK agent (Hibernate being an example). If your provider does not require an agent or you have other alternatives (for example 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 on the environment (web container/application server). Hooking ClassTransformers through a JDK 5.0 agent is typically not efficient - the agents work against the entire virtual machine and inspect every class that is loaded - something that is typically undesirable in a production server enviroment.

Spring provides a number of LoadTimeWeaver implementations for various environments, allowing ClassTransformer instances to be applied only per classloader and not per VM.

12.6.1.2.1. Tomcat setup

Jakarta Tomcat's default classloader does not support class transformation but allows custom classloaders to be used. Spring offers the TomcatInstrumentableClassLoader (inside the org.springframework.instrument.classloading.tomcat package) which extends the Tomcat classloader (WebappClassLoader) and allows JPA ClassTransformer instances to 'enhance' all classes loaded by it. In short, JPA transformers will be applied only inside a specific web application (which uses the TomcatInstrumentableClassLoader).

In order to use the custom classloader on:

1. Copy spring-tomcat-weaver.jar into $CATALINA_HOME/server/lib (where $CATALINA_HOME represents the root of the Tomcat installation).

2. Instruct Tomcat to use the custom classloader (instead of the default one) by editing the web application context file:

   <Context path="/myWebApp" docBase="/my/webApp/location">
      <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" />
   </Context>

   Tomcat 5.0.x and 5.5.x series support several context locations: server configuration file ($CATALINA_HOME/conf/server.xml), the default context configuration ($CATALINA_HOME/conf/context.xml) that affects all deployed web applications and per-webapp configurations, deployed on the server ($CATALINA_HOME/conf/[enginename]/[hostname]/my-webapp-context.xml) side or along with the webapp (your-webapp.war/META-INF/context.xml). For efficiency, inside the web-app configuration style is recommended since only applications which use JPA will use the custom classloader. See the Tomcat 5.x documentation for more details about available context locations.

   Note that versions prior to 5.5.20 contained a bug in the XML configuration parsing preventing usage of Loader tag inside server.xml (no matter if a classloader is specified or not (be it the official or a custom one). See Tomcat's bugzilla for more details.

   If you are using Tomcat 5.5.20+ you can set useSystemClassLoaderAsParent to false to fix the problem:

   <Context path="/myWebApp" docBase="/my/webApp/location">
      <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" 
                             useSystemClassLoaderAsParent="false"/>
   </Context>

   In Tomcat 4.x, one can use the same contex.xml and place it under $CATALINA_HOME/webapps or modify $CATALINA_HOME/conf/server.xml to use the custom classloader by default. See the Tomcat 4.x documentation for more information.

1. Copy spring-tomcat-weaver.jar into $CATALINA_HOME/lib (where $CATALINA_HOME represents the root of the Tomcat installation).

2. Instruct Tomcat to use the custom classloader (instead of the default one) by editing the web application context file:

   <Context path="/myWebApp" docBase="/my/webApp/location">
      <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/>
   </Context>

   Tomcat 6.0.x (similar to 5.0.x/5.5.x) series support several context locations: server configuration file ($CATALINA_HOME/conf/server.xml), the default context configuration ($CATALINA_HOME/conf/context.xml) that affects all deployed web applications and per-webapp configurations, deployed on the server ($CATALINA_HOME/conf/[enginename]/[hostname]/my-webapp-context.xml) side or along with the webapp (your-webapp.war/META-INF/context.xml). For efficiency, inside the web-app configuration style is recommended since only applications which use JPA will use the custom classloader. See the Tomcat 5.x documentation for more details about available context locations.

• Tomcat 4.x/5.x/5.5.x

• Tomcat 6.0.x

The last step required on all Tomcat versions, is to use the appropriate the LoadTimeWeaver when configuring LocalContainerEntityManagerFactoryBean:

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

Using this technique, JPA applications relying on instrumentation, can run in Tomcat without the need of an agent. This is important especially when hosting applications which rely on different JPA implementations since the JPA transformers are applied only at classloader level and thus, are isolated from each other.

	Note
	If TopLink is being used a JPA provider under Tomcat, please place the toplink-essentials jar under $CATALINA_HOME/shared/lib folder instead of your war.

12.6.1.2.2. OC4J setup (10.1.3.1+)

As Oracle's OC4J classloader has native bytecode transformation support, switching from an JDK agent to a LoadTimeWeaver can be done just through the application Spring configuration:

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
  <property name="loadTimeWeaver">
    <bean class="org.springframework.instrument.classloading.oc4j.OC4JLoadTimeWeaver"/>
  </property>
</bean>

12.6.1.2.3. GlassFish

GlassFish application server provides out of the box, an instrumentation cable classloader. Spring supports it through GlassFishLoadTimeWeaver:

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
  <property name="loadTimeWeaver">
    <bean class="org.springframework.instrument.classloading.glassfish.GlassFishLoadTimeWeaver"/>
  </property>
</bean>

12.6.1.2.4. Resin setup (3.1.0+)

Caucho Resin 3.1 sesies classloader provides native bytecode capabilities which can be used by Spring through ReflectiveLoadTimeWeaver:

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

12.6.1.2.5. General LoadTimeWeaver

For environments where class instrumentation is required but are not supported by the existing LoadTimeWeaver implementations, a JDK agent can be the only solution. For such cases, Spring provides InstrumentationLoadTimeWeaver which requires a Spring-specific (but very general) VM agent (spring-agent.jar):

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

Note that the virtual machine has to be started with the Spring agent, by supplying the following JVM options:

-javaagent:/path/to/spring-agent.jar

<bean id="persistenceUnitManager" class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager">
  <property name="persistenceXmlLocation">
    <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>
   <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="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
 <property name="persistenceUnitManager" ref="persistenceUnitManager"/>
</bean>

Each JPA-based DAO will then receive a EntityManagerFactory via dependency injection. Such a DAO can be coded against plain JPA and work with the given EntityManagerFactory or through Spring's JpaTemplate:

<beans>

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

</beans>

public class JpaProductDao implements ProductDao {
  
    private JpaTemplate jpaTemplate;

    public void setEntityManagerFactory(EntityManagerFactory emf) {
        this.jpaTemplate = new JpaTemplate(emf);
    }

    public Collection loadProductsByCategory(final String category) throws DataAccessException {
        return (Collection) this.jpaTemplate.execute(new JpaCallback() {
            public Object doInJpa(EntityManager em) throws PersistenceException {
                Query query = em.createQuery("from Product as p where p.category = :category");
                query.setParameter("category", category);
                List result = query.getResultList(); 
                // do some further processing with the result list
                return result;
            }
        });
    }
}

The JpaCallback implementation allows any type of JPA data access. The JpaTemplate will ensure that EntityManagers are properly opened and closed and automatically participate in transactions. Moreover, the JpaTemplate properly handles exceptions, making sure resources are cleaned up and the appropriate transactions rolled back. The template instances are thread-safe and reusable and they can be kept as instance variable of the enclosing class. Note that JpaTemplate offers single-step actions such as find, load, merge, etc along with alternative convenience methods that can replace one line callback implementations.

Furthermore, Spring provides a convenient JpaDaoSupport base class that provides the get/setEntityManagerFactory and getJpaTemplate() to be used by subclasses:

public class ProductDaoImpl extends JpaDaoSupport implements ProductDao {
  
    public Collection loadProductsByCategory(String category) throws DataAccessException {
        Map<String, String> params = new HashMap<String, String>();
        params.put("category", category);
        return getJpaTemplate().findByNamedParams("from Product as p where p.category = :category", params);
    }
}

Besides working with Spring's JpaTemplate, one can also code Spring-based DAOs against the JPA, doing one's own explicit EntityManager handling. As elaborated in the corresponding Hibernate section, the main advantage of this approach is that your data access code is able to throw checked exceptions. JpaDaoSupport offers a variety of support methods for this scenario, for retrieving and releasing a transaction EntityManager, as well as for converting exceptions.

Note

While EntityManagerFactory instances are thread safe, EntityManager instances are not. The injected JPA EntityManager behave just like an EntityManager fetched from an application server's JNDI environment, as defined by the JPA specification. It will delegate all calls to the current transactional EntityManager, if any; else, it will fall back to a newly created EntityManager per operation, making it thread safe.

It is possible to write code against the plain JPA without using any Spring dependencies, using an injected EntityManagerFactory or EntityManager. Note that Spring can understand @PersistenceUnit and @PersistenceContext annotations both at field and method level if a PersistenceAnnotationBeanPostProcessor is enabled. A corresponding DAO implementation 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, just like it would if coded against Spring's JpaTemplate. Moreover, the DAO takes advantage of annotations to require the injection of the default EntityManagerFactory:

<beans>

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

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

</beans>

The main issue with such a DAO is that it always creates a new EntityManager via the factory. This can be easily overcome by requesting a transactional EntityManager (also called "shared EntityManager", since 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(); 
    }
}

Note that 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 implementation prefers method level injection (of an EntityManager instead of an EntityManagerFactory), no change is required in the application context XML due to annotation usage.

The main advantage of this DAO style is that it depends 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 of course appealing from a non-invasiveness perspective, and might feel more natural to JPA developers.

However, the DAO throws the plain PersistenceException exception class (which is unchecked, and so does not have to be declared or caught) but also IllegalArgumentException and IllegalStateException, which means that callers can only treat exceptions as generally fatal - unless they want to depend on JPA'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 tradeoff might be acceptable to applications that are strongly JPA-based and/or do not need any special exception treatment. However, Spring offers a solution allowing 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 will automatically look for all exception translators (implementations of the PersistenceExceptionTranslator interface) and advice all beans maked with the @Repository annotation so that the discovered translators can intercept and apply the appropriate translation on the thrown exceptions.

In summary: DAOs can be implemented based on the plain Java Persistence API and annotations, while still being able to benefit from Spring-managed transactions, dependency injection, and transparent exception conversion (if desired) to Spring's custom exception hierarchies.