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3.3.3 Business interfaces for stateful beans
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Specifying stateful bean business interfaces works in almost exactly the same way as it does for stateless beans with a couple of exceptions. Stateful session beans support local and remote invocation through the @Local and @Remote annotations. However, a stateful session bean cannot have a web service endpoint interface. This is because SOAP-based web services are inherently stateless in nature. Also, you should always include at least one @Remove annotated method in your stateful bean s business interface. The reason for this will become clear as we discuss the stateful bean lifecycle next.
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3.3.4 Stateful bean lifecycle callbacks
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As we mentioned in section 3.1, the lifecycle of the stateful session bean is very different from that of a stateless session bean because of passivation. In this section, we explain this concept in more depth. Let s start by looking at the lifecycle of a stateful bean, as shown in figure 3.8. The container follows these steps:
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Always creates new bean instances using the default constructor whenever a new client session is started.
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Stateful session beans
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Injects resources. Stores the instance in memory. Executes the requested business method invoked through the business interface by the client. Waits for and executes subsequent client requests. If the client remains idle for a period of time, the container passivates the bean instance. Essentially, passivation means that the bean is moved out of active memory, serialized, and stored in temporary storage. If the client invokes a passivated bean, it is activated (brought back into memory from temporary storage). If the client does not invoke a passivated bean instance for a period of time, it is destroyed. If the client requests the removal of a bean instance, it is first activated if necessary and then destroyed.
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Figure 3.8 The lifecycle of a stateful session bean. A stateful bean maintains client state and cannot be pooled. It may be passivated when the client is not using it and must be activated when the client needs it again.
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Building business logic with session beans
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Like a stateless session bean, the stateful session bean has lifecycle callback methods, or callbacks, that are invoked when the PostConstruct event occurs (as an instance is created) and when the PreDestroy event occurs (before the instance is destroyed). But now we have two new callback events for which we can have callbacks: PrePassivate and PostActivate, which are part of the passivation process. We ll discuss them next. Just as in listing 3.1, we use a PostConstruct callback in listing 3.2 to open a database connection from the injected data source so that it can be used by business methods. Also as in listing 3.1, we close the cached connection in preparation for bean destruction in a PreDestroy callback. However, you should note that we invoke the very same method for both the PreDestroy and PrePassivate callbacks:
@PrePassivate @PreDestroy public void cleanup() { ... }
Similarly, the exact same action is taken for both the PostConstruct and PostActivate callbacks:
@PostConstruct @PostActivate public void openConnection() { ... }
To see why this is the case, let s discuss activation and passivation in a little more detail. Passivation and activation If clients don t invoke a bean for a long enough time, it is not a good idea to continue keeping it in memory. For a large number of beans, this could easily make the machine run out of memory. The container employs the technique of passivation to save memory when possible.
NOTE
Passivation essentially means saving a bean instance into disk instead of holding it in memory. The container accomplishes this task by serializing the entire bean instance and moving it into permanent storage like a file or the database. Activation is the opposite of passivation and is done when the bean instance is needed again. The container activates a bean instance by retrieving it from permanent storage, deserializing it, and moving it back into memory. This means that all bean instance
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