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Learning advanced EJB concepts
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It is illegal to inject a MessageDrivenContext into a session bean or a SessionContext into an MDB.
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This is about as much time as we need to spend on the EJB context right now. Rest assured that you ll see more of it in chapter 6. In the meantime, let s turn our attention back to a vital part of EJB 3 dependency injection. We provided a brief overview of DI in chapter 2 and have been seeing EJB DI in action in the last few chapters. We just saw an intriguing use case in injecting EJB contexts. In reality, EJB DI is a like a Swiss army knife: it is an all-in-one tool that can be used in unexpected ways. Let s take a look at some of these advanced uses next.
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5.2 Accessing resources using DI and JNDI
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We ve seen EJB 3 DI in its primary incarnations already the @javax.ejb.EJB and @javax.annotation.Resource annotations. EJB 3 DI comes in two more forms the @javax.persistence.PersistenceContext and @javax.persistence.PersistenceUnit annotations. We ll see these two annotations in action in part 3 of this book. We ve also witnessed only a small part of the power of the @Resource annotation. So far, we ve used the @Resource annotation to inject JDBC data sources, JMS connection factories, and JMS destinations. Unlike some lightweight containers such as Spring, EJB 3 does not permit injection of POJOs that aren t beans. However, the @Resource annotation allows for a variety of other uses, some of which we cover in the coming section. In this section we ll show you how to use the @Resource annotation and its parameters. You ll learn the difference between setter and field injection, and you ll see the @Resource annotation in action when we inject a variety of resources such as e-mail, environment entries, and the timer service. Finally, you ll learn how to look up resources using JNDI and the lookup method in EJBContext.
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5.2.1 Resource injection using @Resource
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The @Resource annotation is by far the most versatile mechanism for DI in EJB 3. As we noted, in most cases the annotation is used to inject JDBC data sources, JMS resources, and EJB contexts. However, the annotation can also be used for e-mail server resources, environment entries, ORB reference, or even EJB references. Let s take a brief look at each of these cases. For convenience, we ll use the familiar JDBC data source example to explain the basic features of the @Resource annotation before moving on to the more involved cases. The following code injects a data source into the PlaceBid bean from chapter 2:
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Accessing resources using DI and JNDI
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@Stateless public class PlaceBidBean implements PlaceBid { ... @Resource(name="jdbc/actionBazaarDB") private javax.sql.DataSource dataSource;
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In this case, the container would not have to work very hard to figure out what resource to inject because the name parameter is explicitly specified. As we know, this parameter specifies the JNDI name of the resource to be injected, which in our case is specified as jdbc/actionBazaarDB. Although we didn t mention this little detail before, the value specified by the name parameter is actually interpreted further by the container similar to a value specified in the res-ref-name in the <resource-ref> tag in the deployment descriptor, as in the following example:
<resource-ref> <res-ref-name>jdbc/actionBazaarDB</res-ref-name> <res-type>javax.sql.DataSource</res-type> </resource-ref>
The value of the name parameter in @Resource (or res-ref-name) is translated to a fully qualified JNDI mapping in the form java:comp/env/[value of the name parameter] (see the accompanying sidebar). In our example, the complete JNDI path for the resource will be java:comp/env/jdbc/actionBazaarDB. If you don t specify the name element in the @Resource annotation, the JNDI name for the resource will be of the form java:comp/env/ [bean class name including package]/ [name of the annotated field/property]. If we didn t specify the name element in the @Resource annotation, the container would use java:comp/env/actionbazaar.buslogic.PlaceBidBean/dataSource as the JNDI name.
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