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Learning advanced EJB concepts
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This chapter covers
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EJB internals Dependency injection of the @Resource annotation Crosscutting concerns with interceptors Task scheduling with the EJB timer
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EJB internals
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In the previous two chapters we focused on developing session beans and message-driven beans (MDBs). Although we discussed a few bean type-specific features in detail, we generally avoided covering topics not closely related to introducing the basics. In this chapter we build on the material in the previous chapters and introduce advanced concepts applicable to MDBs and session beans. It is very likely that you ll find these EJB 3 features extremely helpful while using EJB in the real world. We begin by discussing the how containers provide the services behind the scenes and how to access environment information. We then move on to advanced use of dependency injection, JNDI lookups, EJB interceptors, and the EJB timer service. As you ll learn, EJB 3 largely relieves you from these system-level concerns while providing extremely robust and flexible functionality. As a foundation for the rest of the chapter, we briefly examine these EJB internals first.
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5.1 EJB internals
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Although we ve talked about the role of the container and the concept of managed services, we haven t explained how most containers go about providing managed services. The secret to understanding these and the other EJB services is knowing how the container provides them. Without going into too much detail, we ll discuss EJB objects which perform the magic of providing the service and then examine the EJB context which a bean can use to access runtime environment and use container services.
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5.1.1 EJB behind the scenes EJB centers on the idea of managed objects. As we saw in the previous chapters, EJB 3 beans are just annotated POJOs themselves. When a client invokes an EJB method using the bean interface, it doesn t work directly on the bean instance. The container makes beans special by acting as a proxy between the client and the actual bean instance. This enables the container to provide EJB services to the client on behalf of the bean instance.
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NOTE
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For each bean instance, the container automatically generates a proxy called an EJB object. The EJB object has access to all the functionality of the container, including the JNDI registry, security, transaction management, thread pools, session management, and pretty much anything else that is necessary to provide EJB services. The EJB object is aware of the bean configuration and what services the POJO is supposed to provide.
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Learning advanced EJB concepts
Since all requests to the EJB instance are passed through the EJB object proxy, the EJB object can insert container services to client requests as needed, including managing all aspects of the bean lifecycle. Figure 5.1 is a typical representation of this technique. As you ve seen in the previous chapters, the beauty of this technique is that all the service details are completely transparent to bean clients and even to bean developers. In fact, a container implementation is free to implement the services in the most effective way possible and at the same time provide vendor-specific feature and performance enhancements. This is fundamentally all there is to the magic parts of EJB. For session beans, the client interacts with the EJB object through the business interface. For MDBs, however, the EJB object or message endpoint sits between the message provider and the bean instance. Let s now take a look at how EJBs access the container environment in which the EJB object itself resides.
Figure 5.1 The magic of EJB. The container-generated EJB object receives all EJB client requests as the proxy, and reads configuration and inserts container services as required before forwarding client requests to the bean instance.
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