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12.2 Accessing session beans from the web tier
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You may remember from our discussion in chapter 1 that EJB helps build applications with tiered architecture. You can use session beans to build the business logic of your applications and access the session beans from the presentation or web tier of your enterprise application. Figure 12.4 illustrates where EJBs fit into the architecture of ActionBazaar and how EJBs are accessed from the ActionBazaar web module. As we discussed in chapter 3, session beans can be accessed using either dependency injection or JNDI lookup. Dependency injection is supported only with managed classes classes such as servlets, whose lifecycle is managed by the container. While both managed and nonmanaged classes can use JNDI lookup, you ll probably use it only with classes that are not managed. Table 12.1 provides some insight as to types which of classes can access EJBs using dependency injection. Regardless of which method you use to access your EJBs, you ll interact with them in the same manner after you have obtained an EJB reference. Your web application may contain a mixture of managed and nonmanaged application components, and in the next sections we ll discuss how you can access session beans from both types of application components. We ll show you how to invoke an EJB 3 session bean from managed classes, such as a servlet, and nonmanaged classes, such as a helper class. We ll also explore what you have to do to demarcate a transaction and the steps required to build a stateful application using stateful session beans.
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Table 12.1 An EJB can be accessed from different types of classes in the web tier. Only managed classes can use dependency injection; nonmanaged application components have to use JNDI lookup to obtain a reference to a session bean. Class Type Servlets, filters, event listeners JSP tag handlers, library event listeners JSF managed beans Helper classes, JSPs Yes Yes Yes No Injection
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Figure 12.4 ActionBazaar application architecture. The ActionBazaar web module accesses the EJBs that implement the business logic and manipulate entities.
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Accessing session beans from the web tier
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12.2.1 Accessing session beans using dependency injection
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Dependency injection is by far the easiest way to access an EJB 3 session bean. In this section you ll see how to use dependency injection from the web module to access a session bean. You learned about the @EJB annotation in chapter 3, and you probably want to jump into using dependency injection for all your session beans. However, we ll explain why you would want to avoid using dependency injection when accessing a stateful session bean from a multithreaded client. You can obtain a reference to an EJB in your managed class by using injection in one of two ways:
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By using the @EJB annotation By using the ejb-ref or ejb-local-ref element in the web.xml deployment descriptor
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Assume that ActionBazaar uses a controller servlet. You can use @EJB to inject a reference to an EJB as follows:
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public class ActionBazaarBidControllerServlet extends HttpServlet { @EJB private PlaceBid placebid; ... placeBid.addBid(bidderId, itemId, ... }
bidPrice);
This example shows the PlaceBid EJB being injected into the ActionBazaarBidControllerServlet. The @EJB annotation here is clean and straightforward. If instead we use the ejb-ref or ejb-local-ref element in the deployment descriptor, we would have a web.xml like this:
<ejb-ref> <ejb-ref-name>PlaceBid</ejb-ref-name> <ejb-ref-type>Session</ejb-ref-type> <remote>actionbazaar.buslogic.PlaceBid</remote> <injection-target> <injection-target-name>placeBid</injection-target-name> <injection-target-class> actionbazaar.web.ActionBazaarBidControllerServlet </injection-target-class> </injection-target> </ejb-ref>
Compare these two examples. Which one makes more sense to you
Effectively integrating EJB 3 across your application tiers
Avoid injecting stateful session beans If you are using stateful session beans, then you must avoid injecting them into classes that are multithreaded in nature, such as servlets and JSF managed beans. Let s look at an example. Suppose you inject the BidderAccountCreator bean into a servlet that is multithreaded, as shown here:
@Stateful(name = "BidderAccountCreator") public class BidderAccountCreatorBean { private LoginInfo loginInfo; private BiographicalInfo biographicalInfo; private BillingInfo billingInfo; @Remove public void cancelAccountCreation() { ... } }
In this case, the single BidderAccountCreator instance will be shared by all users of the servlet. This is not the desired behavior since loginInfo, biographicalInfo, and billingInfo are designed to be specific to an individual client. This becomes trickier when we have a method annotated with the @Remove annotation. When a particular user invokes the cancelAccountCreation method, the EJB instance will be destroyed. The next time a user tries to access the BidderAccountCreator EJB, the container will generate a javax.ejb.NoSuchEJBException. If you want to use stateful session beans in your web application, then you need to perform a JNDI lookup and store the returned EJB instance in the HttpSession object. This way, the same user reaches the same bean instance for future activity. This also scopes the EJB instance to the user s session, avoiding the possibility of other sessions acquiring or deleting it. We ll explain more about working with stateful session beans in section 12.2.4.
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