spring-framework/spring-framework-reference/src/ejb.xml

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<chapter id="ejb">
	<title>Enterprise JavaBeans (EJB) integration</title>

	<section id="ejb-introduction">
    <title>Introduction</title>
    <para>
      As a lightweight container, Spring is often considered an EJB
      replacement. We do believe that for many if not most applications and use
      cases, Spring as a container, combined with its rich supporting
      functionality in the area of transactions, ORM and JDBC access, is a better
      choice than implementing equivalent functionality via an EJB container and
      EJBs.
      </para>
    <para>
      However, it is important to note that using Spring does not prevent
      you from using EJBs. In fact, Spring makes it much easier to access EJBs and
      implement EJBs and functionality within them. Additionally, using Spring to
      access services provided by EJBs allows the implementation of those services
      to later transparently be switched between local EJB, remote EJB, or POJO
      (plain old Java object) variants, without the client code having to
      be changed.
      </para>
    <para>
      In this chapter, we look at how Spring can help you access and
      implement EJBs. Spring provides particular value when accessing stateless
      session beans (SLSBs), so we'll begin by discussing this.
    </para>
	</section>

	<section id="ejb-access">
		<title>Accessing EJBs</title>

		<section id="ejb-access-concepts">
			<title>Concepts</title>
			<para>
				To invoke a method on a local or remote stateless session bean,
				client code must normally perform a JNDI lookup to obtain the (local or
				remote) EJB Home object, then use a 'create' method call on that object
				to obtain the actual (local or remote) EJB object. One or more methods
				are then invoked on the EJB.
      </para>
			<para>
				To avoid repeated low-level code, many EJB applications use the
				Service Locator and Business Delegate patterns. These are better than
				spraying JNDI lookups throughout client code, but their usual
				implementations have significant disadvantages. For example:
            </para>
			<itemizedlist>
				<listitem>
					<para>
						Typically code using EJBs depends on Service Locator or
						Business Delegate singletons, making it hard to test.
					</para>
				</listitem>
				<listitem>
					<para>
						In the case of the Service Locator pattern used without a
						Business Delegate, application code still ends up having to invoke
						the create() method on an EJB home, and deal with the resulting
						exceptions. Thus it remains tied to the EJB API and the complexity
						of the EJB programming model.
					</para>
				</listitem>
				<listitem>
					<para>
						Implementing the Business Delegate pattern typically results
						in significant code duplication, where we have to write numerous
						methods that simply call the same method on the EJB.
					</para>
				</listitem>
			</itemizedlist>
			<para>
				The Spring approach is to allow the creation and use of proxy objects,
				normally configured inside a Spring container, which act as codeless
				business delegates. You do not need	to write another Service Locator, another
				JNDI lookup, or duplicate methods in a hand-coded Business Delegate unless
				you are actually adding real value in such code.
			</para>
		</section>

		<section id="ejb-access-local">
			<title>Accessing local SLSBs</title>
			<para>
				Assume that we have a web controller that needs to use a local
				EJB. We’ll follow best practice and use the EJB Business Methods
				Interface pattern, so that the EJB’s local interface extends a non
				EJB-specific business methods interface. Let’s call this business
				methods interface <classname>MyComponent</classname>.
			</para>
			<programlisting language="java"><![CDATA[public interface MyComponent {
    ...
}]]></programlisting>
			<para>
				One of the main reasons to use the Business Methods Interface pattern
				is to ensure that synchronization between method signatures in local
				interface and bean implementation class is automatic. Another reason is
				that it later makes it much easier for us to switch to a POJO (plain old
				Java object) implementation of the service if it makes sense to do so.
				Of course we’ll also need to implement the local home interface and
				provide an implementation class that implements <classname>SessionBean</classname>
				and the <classname>MyComponent</classname> business methods interface. Now the
				only Java coding we’ll  need to do to hook up our web tier controller to the
				EJB implementation is to expose a setter method of type <classname>MyComponent</classname>
				on the controller. This will save the reference as an instance variable in the
				controller:
			</para>
			<programlisting language="java"><![CDATA[private MyComponent myComponent;

public void setMyComponent(MyComponent myComponent) {
    this.myComponent = myComponent;
}]]></programlisting>
			<para>
				We can subsequently use this instance variable in any business
				method in the controller. Now assuming we are obtaining our controller
				object out of a Spring container, we can (in the same context) configure a
				<classname>LocalStatelessSessionProxyFactoryBean</classname> instance, which
				will be the EJB proxy object. The configuration of the proxy, and setting of
				the <literal>myComponent</literal> property of the controller is done
				with a configuration entry such as:
			</para>
			<programlisting language="xml"><![CDATA[<bean id="myComponent"
      class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean">
  <property name="jndiName" value="ejb/myBean"/>
  <property name="businessInterface" value="com.mycom.MyComponent"/>
</bean>

<bean id="myController" class="com.mycom.myController">
  <property name="myComponent" ref="myComponent"/>
</bean>]]></programlisting>
			<para>
				There’s a lot of work happening behind the scenes, courtesy of
				the Spring AOP framework, although you aren’t forced to work with AOP
				concepts to enjoy the results. The <literal>myComponent</literal> bean
				definition creates a proxy for the EJB, which implements the business
				method interface. The EJB local home is cached on startup, so there’s
				only a single JNDI lookup. Each time the EJB is invoked, the proxy
				invokes the <literal>classname</literal> method on the local EJB and
				invokes the	corresponding business method on the EJB.
			</para>
			<para>
				The <literal>myController</literal> bean definition sets the
				<literal>myComponent</literal> property of the controller class to the
				EJB proxy.
			</para>
      <para>
        Alternatively (and preferably in case of many such proxy definitions),
        consider using the <literal>&lt;jee:local-slsb&gt;</literal>
        configuration element in Spring's "jee" namespace:
      </para>
      <programlisting language="xml"><![CDATA[<jee:local-slsb id="myComponent" jndi-name="ejb/myBean"
      business-interface="com.mycom.MyComponent"/>

<bean id="myController" class="com.mycom.myController">
  <property name="myComponent" ref="myComponent"/>
</bean>]]></programlisting>
			<para>
				This EJB access mechanism delivers huge simplification of
				application code: the web tier code (or other EJB client code) has no
				dependence on the use of EJB. If we want to replace this EJB reference
				with a POJO or a mock object or other test stub, we could simply change
				the <literal>myComponent</literal> bean definition without changing a
				line of Java code. Additionally, we haven’t had to write a single line of
				JNDI lookup or other EJB plumbing code as part of our application.
			</para>
			<para>
				Benchmarks and experience in real applications indicate that the
				performance overhead of this approach (which involves reflective
				invocation of the target EJB) is minimal, and is typically undetectable
				in typical use. Remember that we don’t want to make fine-grained calls
				to EJBs anyway, as there’s a cost associated with the EJB infrastructure
				in the application server.
			</para>
			<para>
				There is one caveat with regards to the JNDI lookup. In a bean
				container, this class is normally best used as a singleton (there simply
				is no reason to make it a prototype). However, if that bean container
				pre-instantiates singletons (as do the various XML
				<classname>ApplicationContext</classname> variants)
				you may have a problem if the bean container is loaded before the EJB
				container loads the target EJB. That is because the JNDI lookup will be
				performed in the <literal>init()</literal> method of this class and then
				cached, but the EJB will not have been bound at the target location yet.
				The solution is to not pre-instantiate this factory object, but allow it
				to be created on first use. In the XML containers, this is controlled via
				the <literal>lazy-init</literal> attribute.
			</para>
			<para>
				Although this will not be of interest to the majority of Spring
				users, those doing programmatic AOP work with EJBs may want to look at
				<classname>LocalSlsbInvokerInterceptor</classname>.
			</para>
		</section>

		<section id="ejb-access-remote">
			<title>Accessing remote SLSBs</title>
			<para>
				Accessing remote EJBs is essentially identical to accessing local
				EJBs, except that the
				<classname>SimpleRemoteStatelessSessionProxyFactoryBean</classname> or
        <literal>&lt;jee:remote-slsb&gt;</literal> configuration element is used.
				Of course, with or without Spring, remote invocation semantics apply; a
				call to a method on an object in another VM in another computer does
				sometimes have to be treated differently in terms of usage scenarios and
				failure handling.
			</para>
			<para>
				Spring's EJB client support adds one more advantage over the
				non-Spring approach. Normally it is problematic for EJB client code to
				be easily switched back and forth between calling EJBs locally or
				remotely. This is because the remote interface methods must declare that
				they throw <classname>RemoteException</classname>, and client code must deal
				with this, while the local interface methods don't. Client code
				written for local EJBs which needs to be moved to remote EJBs
				typically has to be modified to add handling for the remote exceptions,
				and client code written for remote EJBs which needs to be moved to local
				EJBs, can either stay the same but do a lot of unnecessary handling of
				remote exceptions, or needs to be modified to remove that code. With the
				Spring remote EJB proxy, you can instead not declare any thrown
				<classname>RemoteException</classname> in your Business Method Interface and
				implementing EJB code, have a remote interface which is identical except
				that it does throw <classname>RemoteException</classname>, and rely on the
				proxy to dynamically treat the two interfaces as if they were the same.
				That is, client code does not have to deal with the checked
				<classname>RemoteException</classname> class. Any actual
				<classname>RemoteException</classname> that is thrown during the EJB
				invocation will be re-thrown as the non-checked
				<classname>RemoteAccessException</classname> class, which is a subclass of
				<classname>RuntimeException</classname>. The target service can then be
				switched at will between a local EJB or remote EJB (or even plain Java
				object) implementation, without the client code knowing or caring. Of
				course, this is optional; there is nothing stopping you from declaring
				<classname>RemoteExceptions</classname> in your business interface.
			</para>
		</section>

    <section id="ejb-access-ejb2-ejb3">
      <title>Accessing EJB 2.x SLSBs versus EJB 3 SLSBs</title>
      <para>
        Accessing EJB 2.x Session Beans and EJB 3 Session Beans via Spring
        is largely transparent. Spring's EJB accessors, including the
        <literal>&lt;jee:local-slsb&gt;</literal> and <literal>&lt;jee:remote-slsb&gt;</literal>
        facilities, transparently adapt to the actual component at runtime.
        They handle a home interface if found (EJB 2.x style), or perform straight
        component invocations if no home interface is available (EJB 3 style).
      </para>
      <para>
        Note: For EJB 3 Session Beans, you could effectively use a
        <classname>JndiObjectFactoryBean</classname> / <literal>&lt;jee:jndi-lookup&gt;</literal>
        as well, since fully usable component references are exposed for plain
        JNDI lookups there. Defining explicit <literal>&lt;jee:local-slsb&gt;</literal>
        / <literal>&lt;jee:remote-slsb&gt;</literal> lookups simply provides
        consistent and more explicit EJB access configuration.
      </para>
    </section>
	</section>

	<section id="ejb-implementation">
		<title>Using Spring's EJB implementation support classes</title>

    <section id="ejb-implementation-ejb2">
      <title>EJB 2.x base classes</title>
      <para>
        Spring provides convenience classes to help you implement EJBs.
        These are designed to encourage the good practice of putting business
        logic behind EJBs in POJOs, leaving EJBs responsible for transaction
        demarcation and (optionally) remoting.
      </para>
      <para>
        To implement a Stateless or Stateful session bean, or a Message Driven
        bean, you need only derive your implementation class from
        <classname>AbstractStatelessSessionBean</classname>,
        <classname>AbstractStatefulSessionBean</classname>, and
        <classname>AbstractMessageDrivenBean</classname>/<classname>AbstractJmsMessageDrivenBean</classname>,
        respectively.
      </para>
      <para>
        Consider an example Stateless Session bean which actually delegates
        the implementation to a plain java service object. We have the business
        interface:
      </para>
      <programlisting language="java"><![CDATA[public interface MyComponent {
    public void myMethod(...);
    ...
}]]></programlisting>
      <para>We also have the plain Java implementation object:</para>
      <programlisting language="java"><![CDATA[public class MyComponentImpl implements MyComponent {
    public String myMethod(...) {
        ...
    }
    ...
}]]></programlisting>
      <para>And finally the Stateless Session Bean itself:</para>
      <programlisting language="java"><![CDATA[public class MyFacadeEJB extends AbstractStatelessSessionBean
        implements MyFacadeLocal {

    private MyComponent myComp;

    /**
     * Obtain our POJO service object from the BeanFactory/ApplicationContext
     * @see org.springframework.ejb.support.AbstractStatelessSessionBean#onEjbCreate()
     */
    protected void onEjbCreate() throws CreateException {
        myComp = (MyComponent) getBeanFactory().getBean(
            ServicesConstants.CONTEXT_MYCOMP_ID);
    }

    // for business method, delegate to POJO service impl.
    public String myFacadeMethod(...) {
        return myComp.myMethod(...);
    }
    ...
}]]></programlisting>
      <para>
        The Spring EJB support base classes will by default create and load
        a Spring IoC container as part of their lifecycle, which is then available
        to the EJB (for example, as used in the code above to obtain the POJO
        service object). The loading is	done via a strategy object which is a subclass of
        <classname>BeanFactoryLocator</classname>. The actual implementation of
        <classname>BeanFactoryLocator</classname> used by default is
        <classname>ContextJndiBeanFactoryLocator</classname>, which creates the
        ApplicationContext from a resource locations specified as a JNDI
        environment variable (in the case of the EJB classes, at
        <literal>java:comp/env/ejb/BeanFactoryPath</literal>). If there is a need
        to change the BeanFactory/ApplicationContext loading strategy, the default
        <classname>BeanFactoryLocator</classname> implementation used may be overridden
        by calling the <literal>setBeanFactoryLocator()</literal> method, either
        in <literal>setSessionContext()</literal>, or in the actual constructor of
        the EJB. Please see the Javadocs for more details.
      </para>
      <para>
        As described in the Javadocs, Stateful Session beans expecting to be
        passivated and reactivated as part of their lifecycle, and which use a
        non-serializable container instance (which is the normal case) will have
        to manually call <literal>unloadBeanFactory()</literal> and
        <literal>loadBeanFactory()</literal> from <literal>ejbPassivate()</literal>
        and <literal>ejbActivate()</literal>, respectively, to unload and reload the
        BeanFactory on passivation and activation, since it can not be saved by
        the EJB container.
      </para>
      <para>
        The default behavior of the
        <classname>ContextJndiBeanFactoryLocator</classname> class is to load an
        <classname>ApplicationContext</classname> for use by an EJB, and is
        adequate for some situations. However, it is problematic when the
        <classname>ApplicationContext</classname> is loading a number of beans,
        or the initialization of those beans is time consuming or memory
        intensive (such as a Hibernate <classname>SessionFactory</classname>
        initialization, for example), since every EJB will have their own copy.
        In this case, the user may want to override the default
        <classname>ContextJndiBeanFactoryLocator</classname> usage and use
        another <classname>BeanFactoryLocator</classname> variant, such as the
        <classname>ContextSingletonBeanFactoryLocator</classname> which can load
        and use a shared container to be used by multiple EJBs or other clients.
        Doing this is relatively simple, by adding code similar to this to the
        EJB:
      </para>
      <programlisting language="java"><![CDATA[   /**
    * Override default BeanFactoryLocator implementation
    * @see javax.ejb.SessionBean#setSessionContext(javax.ejb.SessionContext)
    */
   public void setSessionContext(SessionContext sessionContext) {
       super.setSessionContext(sessionContext);
       setBeanFactoryLocator(ContextSingletonBeanFactoryLocator.getInstance());
       setBeanFactoryLocatorKey(ServicesConstants.PRIMARY_CONTEXT_ID);
   }]]></programlisting>
      <para>
        You would then need to create a bean definition file named <literal>beanRefContext.xml</literal>.
        This file defines all bean factories (usually in the form of application contexts) that may be used
        in the EJB. In many cases, this file will only contain a single bean definition such as this (where
        <literal>businessApplicationContext.xml</literal> contains the bean	definitions for all business
        service POJOs):
      </para>
      <programlisting language="xml"><![CDATA[<beans>
    <bean id="businessBeanFactory" class="org.springframework.context.support.ClassPathXmlApplicationContext">
        <constructor-arg value="businessApplicationContext.xml" />
    </bean>
</beans>]]></programlisting>
      <para>
        In the above example, the <literal>ServicesConstants.PRIMARY_CONTEXT_ID</literal> constant
        would be defined as follows:
        </para>
        <programlisting language="java"><![CDATA[public static final String ServicesConstants.PRIMARY_CONTEXT_ID = "businessBeanFactory";]]></programlisting>
        <para>
        Please see the respective Javadocs for the <classname>BeanFactoryLocator</classname> and
        <classname>ContextSingletonBeanFactoryLocator</classname> classes for more information on
        their usage.
      </para>
    </section>

    <section id="ejb-implementation-ejb3">
      <title>EJB 3 injection interceptor</title>
      <para>
        For EJB 3 Session Beans and Message-Driven Beans, Spring provides a convenient
        interceptor that resolves Spring 2.5's <literal>@Autowired</literal> annotation
        in the EJB component class:
        <classname>org.springframework.ejb.interceptor.SpringBeanAutowiringInterceptor</classname>.
        This interceptor can be applied through an <code>@Interceptors</code> annotation
        in the EJB component class, or through an <literal>interceptor-binding</literal>
        XML element in the EJB deployment descriptor.
      </para>
      <programlisting language="java"><![CDATA[@Stateless
@Interceptors(SpringBeanAutowiringInterceptor.class)
public class MyFacadeEJB implements MyFacadeLocal {

    // automatically injected with a matching Spring bean
    @Autowired
    private MyComponent myComp;

    // for business method, delegate to POJO service impl.
    public String myFacadeMethod(...) {
        return myComp.myMethod(...);
    }
    ...
}]]></programlisting>
      <para>
        <classname>SpringBeanAutowiringInterceptor</classname> by default obtains target
        beans from a <classname>ContextSingletonBeanFactoryLocator</classname>, with the
        context defined in a bean definition file named <literal>beanRefContext.xml</literal>.
        By default, a single context definition is expected, which is obtained by type rather
        than by name. However, if you need to choose between multiple context definitions,
        a specific locator key is required. The locator key (i.e. the name of the context
        definition in <literal>beanRefContext.xml</literal>) can be explicitly specified
        either through overriding the <literal>getBeanFactoryLocatorKey</literal> method
        in a custom <classname>SpringBeanAutowiringInterceptor</classname> subclass.
      </para>
      <para>
        Alternatively, consider overriding <classname>SpringBeanAutowiringInterceptor</classname>'s
        <literal>getBeanFactory</literal> method, e.g. obtaining a shared
        <interfacename>ApplicationContext</interfacename> from a custom holder class.
      </para>
    </section>

	</section>

</chapter>