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3.4.2 Injection and stateful session beans
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For the most part, using DI is a no-brainer. There are a few nuances to keep an eye on while using DI with stateful beans, though. You can inject a stateful session into another stateful session bean instance if you need to. For example, you can inject
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Performance considerations for stateful beans
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the BidderAccountCreator stateful EJB from UserAccountRegistration EJB that is another stateful session bean as follows:
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@Stateful public class UserAccountRegistrationBean implements UserAccountRegistration { @EJB private BidderAccountCreator bidderAccountCreator; ... }
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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.
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3.5 Performance considerations for stateful beans
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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.
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3.5.1 Using stateful session beans effectively
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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
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Building business logic with session beans
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. Since the container stores session information in memory, if you have thousands of concurrent clients for your stateful session bean you may run out of memory or cause a lot of disk thrashing by the container as it passivates and activates instances to try to conserve memory. Consequently, you have to closely examine what kind of data you are storing in the conversation state and make sure the total memory footprint for the stateful bean is as small as possible. For example, it may be a lot more efficient to store just the itemId for an Item instead of storing the complete Item object in an instance variable. If you cluster stateful beans, the conversational state is replicated between different instances of the EJB container. State replication uses network bandwidth. Storing a large object in the bean state may have a significant impact on the performance of your application because the containers will spend time replicating objects to other container instances to ensure high availability. We ll discuss more about EJB clustering in chapter 13. Passivating and removing beans The rules for passivation are generally implementation specific. Improper use of passivation policies (when passivation configuration is an option) may cause performance problems. For example, the Oracle Application Server passivates bean instances when the idle time for a bean instance expires, when the maximum number of active bean instances allowed for a stateful session bean is reached, or when the threshold for JVM memory is reached. You have to check the documentation for your EJB container and appropriately set passivation rules. For example, if we set the maximum number of active instances allowed for a stateful bean
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