Skip to main content

Using EJB Annotations Outside of EJB

Posted by edburns on August 29, 2005 at 1:05 PM PDT

The Common
Annotations
and href="http://www.jcp.org/en/jsr/detail?id=220">EJB 3 JSRs both
provide a number of annotations that are useful to enterprise Java
developers. This blog entry examines the use of two from EJB3 that I
feel really should be in Common Annotations: @PostConstruct
and @PreDestroy.

Let's review the purpose of these annotations.

@PostConstruct
 * Designates a method to receive a callback during creation of a
* session bean or message driven bean instance.

@PreDestroy
 * Designates a method to receive a callback before a session bean or 
* message driven instance is destroyed.

These annotations are useful for any container managed instance, not
just session beans or message driven beans. This blog shows how I
leveraged the new ManagedBeanFactory SPI in Sun's
implementation of JSF technology to create a prototype that allows JSF
authors to use these annotations in their managed beans. I'm doing this
to make the case to move these two annotations out of EJB and into
Common Annotations. If this is done, it's easy to see that using this
feature will find its way into a future version of the JSF spec.

The running code for this example is checked into the "components"
jsf-demo in the JSF
project on java.net
. You have to get a nightly build more recent
than 30 August 2005 to get a build with this code in it, though.

Details

The com.sun.faces.spi.ManagedBeanFactoryWrapper class in
the Sun JSF implementation allows the user to decorate the internal
implementation class used to instantiate managed beans such that the
creation process can be intercepted and methods annotated with
@PostConstruct can be called. The listeners for request,
session, and ServletContext classes in the Servlet API can
be used to intercept destruction events and call managed bean methods
annotated with @PreDestroy.

The following two classes leverage these hook points to implement the
feature. The classes are presented in the form of a JSF project href="https://javaserverfaces.dev.java.net/faq.html#change_bundle">change-bundle
To see the real code, please href="https://javaserverfaces.dev.java.net/faq.html#Code_checkout">get
the source from java.net.

A
components/src/java/components/model/LifecycleManagedBeanFactory.java



This class leverages the decorator pattern provided by the
ManagedBeanFactory SPI to decorate the
newInstance() method and inject calling the
@PostConstruct method on the ManagedBean instance being
created, if present. It also establishes a contract with the
LifecycleManagedBeanListener to call the
@PreDestroy annotated methods, if present, when the time
comes.

A components/src/java/components/model/LifecycleManagedBeanListener.java

This class implements

ServletRequestListener,
HttpSessionListener
, and ServletContextListener and
uses the *Destroyed methods to look into the list
established by the LifecycleManagedBeanFactory for this
scope and call any appropriately annotated methods.

M components/src/java/demo/model/RepeaterBean.java

annotate some methods to be called.

M components/web/WEB-INF/web.xml

Declare the above LifecycleManagedBeanListener.

M components/build.xml

Declare a compile time dependency on jsf-impl.jar.

Summary

It could be said that using annotations in this way is just making up
for Java's lack of a proper destructor, as one has in Objective C and
C++. However, using annotations in this way is far more flexible than
constructors and destructors, while not adding any conceptual burden for
the new developer. New developers can simply avoid using annotations
until they feel ready to do so. Another factor to consider is that the
servlet API could be enhanced to automatically call methods annotated
with these or similar annotations when objects are added to the request,
session, or application scope. I hope I've demonstrated the usefulness
of these two annotations outside of EJB and made a good case for moving
them out to Common Annotations.

Technorati Tags:

Related Topics >>