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Learning advanced EJB concepts
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5.2.2 The @Resource annotation in action
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In the previous sections we discussed the various parameters of the @Resource annotation, and you learned how to use field or setter injection with @Resource to inject JDBC data sources. Next you ll see how to use the @Resource annotation to inject resources such as JMS objects, mail resources, EJBContext, environment entries, and the timer service. Injecting JMS resources Recall the discussion on messaging and MDBs in chapter 4. If your application has anything to do with messaging, it is going to need to use JMS resources such as javax.jms.Queue, javax.jms.Topic, javax.jms.QueueConnectionFactory, or javax. jms.TopicConnectionFactory. Just like JDBC data sources, these resources are stored in the application server s JNDI context and can be injected through the @Resource annotation. As an example, the following code injects a Queue bound to the name jms/actionBazaarQueue to the queue field:
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@Resource(name="jms/actionBazaarQueue") private Queue queue;
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EJBContext Earlier (section 5.2) we discussed the EJBContext, SessionContext, and MessageDrivenContext interfaces. One of the most common uses of injection is to gain access to EJB contexts. The following code, used in the PlaceBid session bean, injects the EJB type specific context into the context instance variable:
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@Resource SessionContext context;
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Note that the injected session context is not stored in JNDI. In fact, it would be incorrect to try to specify the name parameters in this case at all and servers will probably ignore the element if specified. Instead, when the container detects the @Resource annotation on the context variable, it figures out that the EJB context specific to the current bean instance must be injected by looking at the variable data type, javax.ejb.SessionContext. Since PlaceBid is a session bean, the result of the injection would be the same if the variable were specified to be the parent class, EJBContext. In the following code, an underlying instance of javax.ejb. SessionContext is still injected into the context variable, even if the variable data type is javax.ejb.EJBContext:
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@Resource EJBContext context;
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Accessing resources using DI and JNDI
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Using this code in a session bean would make a lot of sense if you did not plan to use any of the bean-type specific methods available through the SessionContext interface anyway. Accessing environment entries If you have been working with enterprise applications for any length of time, it is likely you have encountered situations where some parameters of your application change from one deployment to another (customer site information, product version, and so on). It is overkill to save this kind of semi-static information in the database. This is exactly the situation environment entry values are designed to solve. For example, in the ActionBazaar application, suppose we want to set the censorship flag for certain countries. If this flag is on, the ActionBazaar application checks items posted against a censorship list specific to the country the application deployment instance is geared toward. We can inject an instance of an environment entry as follows:
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@Resource private boolean censorship;
Environment entries are specified in the deployment descriptor and are accessible via JNDI. The ActionBazaar censorship flag could be specified like this:
<env-entry> <env-entry-name>censorship</env-entry-name> <env-entry-type>java.lang.Boolean</env-entry-type> <env-entry-value>true</env-entry-value> </env-entry>
Environment entries are essentially meant to be robust application constants and support a relatively small range of data types. Specifically, the values of the <env-entry-type> tag are limited to these Java types: String, Character, Byte, Short, Integer, Long, Boolean, Double, and Float. Because environment entries are accessible via JNDI they can be injected by name. We could inject the censorship flag environment entry into any EJB by explicitly specifying the JNDI name as follows:
@Resource(name="censorship") private boolean censorship;
As you might gather, the data types of the environment entry and the injected variable must be compatible. Otherwise, the container throws a runtime exception while attempting DI.
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