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New features: simplifying EJB
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EJB 3 solves this problem by allowing you to override annotations with XML
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deployment descriptors where appropriate. Know your deployment descriptor A deployment descriptor is simply an XML file that contains application configuration information. Every deployment unit in Java EE can have a deployment descriptor that describes its contents and environment. Some typical examples of deployment units are the Enterprise Archive (EAR), Web Application Archive (WAR), and the EJB (ejb-jar) module. If you have ever used EJB 2, you know how verbose the XML (ejb-jar.xml) descriptor was. Most elements were required even if they were trivial. This added to the complexity of using EJB. For example, you could have had the following deployment descriptor for the HelloUserBean that we saw in chapter 1:
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<enterprise-beans> <session> <ejb-name>HelloUserBean</ejb-name> <local>ejb3inaction.example.Hello</local> <ejb-class>ejb3inaction.example.HelloUserBean</ejb-class> <session-type>Stateless</session-type> <transaction-type>Container</transaction-type> </session> </enterprise-beans>
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We ll discuss deployment descriptors in greater detail when we talk about EJB packaging in chapter 11. The good news is that EJB 3 makes deployment descriptors completely optional. You can now use metadata annotations instead of descriptor entries, thus making the development experience much simpler. Note that we ll primarily use annotations throughout this book. This is not because we think deployment descriptors are unimportant or outdated, but because concepts are more easily explained using annotations. As a matter of fact, although deployment descriptors involve dealing with often confusing and verbose XML, we think they can be an excellent mechanism for separating coding concerns from deployment and configuration concerns. With this fact in mind, we present the deployment descriptor counterparts for each of the annotations described in the chapter (and more) in appendix D. You can use deployment descriptor entries only for corner cases where you need them. (A corner case is a problem or situation that occurs only outside normal operating parameters.)
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A first taste of EJB
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Mixing annotations and deployment descriptors Annotations and descriptors are not mutually exclusive. In fact, in EJB 3 they re designed for harmonious coexistence. Deployment descriptor entries override configuration values hard-coded into EJB components. As an example, we could override the @Author annotation we just introduced with the following imaginary deployment descriptor:
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<ManningBooks> <ManningBook> <BookClass>EJB3InAction</BookClass> <Author>Larry, Moe and Curly</Author> </ManningBook> </ManningBooks>
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At runtime, the Manning website engine would detect that the authors of the EJB3InAction book really are Larry, Moe, and Curly, and not Debu Panda, Reza Rahman, and Derek Lane. This is an invaluable feature if you develop enterprise applications that can be deployed to a variety of environments. In the simplest case, the differing environments could be a test and a production server. In the most complex case, you could be selling shrink-wrapped enterprise applications deployed to an unknown customer environment. The most obvious way of mixing and matching annotation and XML metadata is to use XML for deployment environment specific configurations while using annotations for everything else. If you really don t like annotations, that s fine too. You can avoid using them completely in favor of XML deployment descriptors. We ll primarily focus on annotations rather than deployment descriptors in this book simply because they are so much more intuitive to look at and explain. Common metadata annotations Obviously, EJB defines its own set of standard annotations. We ll be discussing these annotations throughout this book. During the course of developing Java EE 5.0, it became apparent that the Java EE container as a whole could use some of the annotations geared toward EJB 3. In particular, these annotations are extremely useful in integrating EJB with the web/servlet tier. Some of these annotations were separated out of the EJB 3 spec and christened common metadata annotations. These annotations are a core part of what makes EJB 3 development, including dependency injection, easy. Table 2.1 lists some of the major common metadata annotations. We ll discuss them throughout this part of the book, starting with some of the most fundamental ones in this chapter.
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