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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

container activates the bean instance again when the client needs it; this process is called activation. The @PrePassivate and @PostActivate annotations apply to the passivation and activation lifecycle events.

Figure 7-28. The blue indicator is informing you that Visual Studio would like to help you write some code.

You can define a lifecycle callback method either in the bean class or in a separate interceptor class.

Figure 7-10. This view has both a page view and a block view that share default values. Please note that if you make any changes to settings within a page, block, or feed display, you must click the Override button if you want changes to be reflected for that specific display, as shown in Figure 7-11. If you do not click the Override button, then any changes made will be changed to the default display. This actually happens quite a bit for both beginners and experienced users of this module; I can t tell you how many times I ve thrown my hands in the air and exclaimed Gosh! when I realized I completely changed a view because I forgot to use the Override function. One workaround to this problem you may want to try is to export the view before you start making any changes. Exporting a view gives you a complete backup of the view; if you make any mistakes, such as forgetting to override one or more fields, you can simply delete the view and import the backup.

Table 3.1 lists the lifecycle callback method annotations, where they are applied, and what the callback methods are typically used for. In sections 3.2.4 and 3.3.4, you ll learn how to define lifecycle callback methods in the bean class for stateless and stateful beans. We ll defer our discussion of lifecycle callback methods in the interceptor classes to chapter 5.

Figure 7-29. Press Control+. (period) to have Visual Studio implement the abstract class for you. Now you can see that Visual Studio went ahead and created a method for you, called Invoke (see Figure 7-30).

Table 3.1 Lifecycle callbacks are created to handle lifecycle events for an EJB. You can create these callback methods either in the bean class or in an external interceptor class. Callback Annotation javax.annotation. PostConstruct Type of EJB Stateless, stateful, MDB Typically Used For... This annotated method is invoked after a bean instance is created and dependency injection is complete. Generally this callback is used to initialize resources (for example, opening database connections). This annotated method is invoked prior to a bean instance being destroyed. Generally this callback is used to clean up resources (for example, closing database connections). This annotated method is invoked prior to a bean instance being passivated. Generally this callback is used to clean up resources, such as database connections, TCP/IP sockets, or any resources that cannot be serialized during passivation. This annotated method is invoked after a bean instance is activated. Generally this callback is used to restore resources, such as database connections that you cleaned up in the PrePassivate method.