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This code will create an instance of BidderAccountCreatorBean which will be specifically meant for the client accessing the instance of the UserAccountRegistrationBean. If the client removes the instance of UserAccountRegistrationBean, the associated instance of BidderAccountCreatorBean will also be automatically removed. Keep in mind that you must not inject a stateful session bean into a stateless object, such as a stateless session bean or servlet that may be shared by multiple concurrent clients (you should use JNDI in such cases instead). However, injecting an instance of a stateless session bean into a stateful session bean is perfectly legal. 12 discusses in much greater detail how you can use EJB from other tiers. This concludes our brief discussion on accessing session beans. Next, we ll briefly explore potential performance issues of stateful session beans.

Whether or not they deserve it, stateful session beans have received a bad rap as performance bottlenecks. There is truth behind this perception, quite possibly due to poor initial implementations for most popular application servers. In recent years, these problems have been greatly alleviated with effective underthe-hood optimizations as well as better JVM implementations. However, you still have to keep a few things in mind in order to use session beans effectively. More or less, these techniques are essential for using any stateful technology, so pay attention even if you decide against using stateful beans. In this section you ll learn techniques to effectively use stateful session beans and other alternatives for building stateful applications.

Our example message bean attempts to extract a command string from JMS messages it receives. From this command String, it will start particular business processes in an asynchronous manner. As you can tell from this recipe, you should consult your application server s documentation for the specific way it maps message-driven beans to actual JMS topics and messages. In this example, for Weblogic we needed to create an additional XML file for the message-driven bean that specifies the actual JMS topic for the EJB. This recipe shows only a small subset of the message-driven EJB capability. When developing message-driven beans, you should consider delegating business logic to other objects rather than tying it to a particular bean. This will allow you to use particular business logic in other places and not tie it to the message-driven behavior. You can read more about this idea in chapter 6.

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There is little doubt that stateful session beans provide extremely robust business logic processing functionality if maintaining conversational state is an essential

application requirement In addition, EJB 3 adds extended persistence contexts specifically geared toward stateful session beans (discussed in chapters 9 and 13), significantly increasing their capability Most popular application servers such as WebSphere, WebLogic, Oracle, and JBoss provide high availability by clustering EJB containers running the same stateful bean A clustered EJB container replicates session state across container instances If a clustered container instance crashes for any reason, the client is routed to another container instance seamlessly without losing state Such reliability is hard to match without using stateful session beans Nonetheless, there are a few things to watch out for while using stateful session beans Choosing session data appropriately Stateful session beans can become resource hogs and cause performance problems if not used properly.

One way to improve the performance of the client layer of an EJB application is to avoid repeated JNDI lookup calls to find an EJB. One of the best ways to do this is to save the EJB reference in a persistent format once the client looks it up. Saving a reference to an EJB means creating an object that can rebuild the connection to the EJB object in the EJB container.