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3.1.5 Bean lifecycle callbacks
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A session bean has a lifecycle. This mean that beans go through a predefined set of state transitions. If you ve used Spring or EJB 2, this should come as no surprise. If you haven t, the concept can be a little tricky to grasp.
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Building business logic with session beans
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To understand the bean lifecycle, it is important to revisit the concept of managed resources. Recall that the container manages almost every aspect of session beans. This means that neither the client nor the bean is responsible for determining when bean instances are created, when dependencies are injected, when bean instances are destroyed, or when to take optimization measures. Managing these actions enables the container to provide the abstractions that constitute some of the real value of using EJBs, including DI, automated transaction management, AOP, transparent security management, and so on. The lifecycle events The lifecycle of a session bean may be categorized into several phases or events. The most obvious two events of a bean lifecycle are creation and destruction. All EJBs go through these two phases. In addition, stateful session beans go through the passivation/activation cycle, which we discuss in depth in section 3.3.5. Here, we take a close look at the phases shared by all session beans: creation and destruction. The lifecycle for a session bean starts when a bean instance is created. This typically happens when a client receives a reference to the bean either by doing a JNDI lookup or by using dependency injection. The following steps occur when a bean is initialized:
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The container invokes the newInstance method on the bean object. This essentially translates to a constructor invocation on the bean implementation class. If the bean uses DI, all dependencies on resources, other beans, and environment components are injected into the newly created bean instance.
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Figure 3.2 depicts this series of events. After the container determines that an instance is no longer needed, the instance is destroyed. This sounds just fine until you realize that the bean might need to know when some of its lifecycle transitions happen. For example, suppose that the resource being injected into a bean is a JDBC data source. That means that it would be nice to be able to know when it is injected so you can open the JDBC database connection to be used in the next business method invocation. In a similar way, the bean would also need to be notified before it is destroyed so that the open database connection can be properly closed. This is where callbacks come in.
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Getting to know session beans
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Figure 3.2 The lifecycle of an EJB starts when a method is invoked. The container creates a bean instance and then dependencies on resources are injected. The instance is then ready for method invocation.
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Understanding lifecycle callbacks Lifecycle callbacks are bean methods (not exposed by a business interface) that the container calls to notify the bean about a lifecycle transition, or event. When the event occurs, the container invokes the corresponding callback method, and you can use these methods to perform business logic or operations such as initialization and cleanup of resources. Callback methods are bean methods that are marked with metadata annotations such as @PostContruct and @PreDestroy. They can be public, private, protected, or package-protected. As you might have already guessed, a PostConstruct callback is invoked just after a bean instance is created and dependencies are injected. A PreDestroy callback is invoked just before the bean is destroyed and is helpful for cleaning up resources used by the bean. While all session beans have PostConstruct and PreDestroy lifecycle events, stateful session beans have two additional ones: PrePassivate and PostActivate. Since stateful session beans maintain state, there is a stateful session bean instance for each client, and there could be many instances of a stateful session bean in the container. If this happens, the container may decide to deactivate a stateful bean instance temporarily when not in use; this process is called passivation. The
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