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Be sure you evaluate such alternatives before deciding which one best meets your needs.
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Accessing session beans from the web tier
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12.2.3 Dealing with transactions
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Remember from our discussion in chapter 6 that in your EJB applications you can use either container-managed transactions (CMT) or bean-managed transactions, in which you programmatically manage transactions using the UserTransaction API. While CMT is not available in the web container, if your application uses session beans they allow you to use CMTs and avoid UserTransaction. We highly recommend you take advantage of CMT. For example, if you want to make multiple EJB method calls from the same transaction, then you may be tempted to do the following:
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public class ActionBazaarRegistrationControllerServlet extends HttpServlet { ... //--- Do NOT do this!!! This is NOT recommended!!! @EJB ItemManager itemManager; @EJB categoryManager categoryManager; @Resource private UserTransaction ut; ... public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... ut.begin(); ... categoryManager.addCategory(category); itemManager.addItem(); itemManager.setCategory(category); ... ut.commit(); } ... }
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In this example we are injecting the instances ItemManager and CategoryManager and then invoking several methods on injected session beans. The first issue here is that you have to write error-prone code to demarcate the transaction. Second, because your EJBs are fine-grained, the business logic gets scattered between the EJBs and the web module. Finally, if the EJBs are remote, these translate to three RMI calls, which can be expensive from a performance perspective. We suggest you avoid this practice. If your application includes such a requirement, we recommend you create a session fa ade and use that to perform all operations, and then invoke that EJB from the web tier. (We covered the Session Fa ade design pattern earlier in this chapter.)
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12.2.4 Working with stateful session beans If you are building an application that requires maintaining client state end-toend, you ll most likely use stateful session beans. In section 12.2.1 we listed some reasons why you must avoid injecting instances of stateful session beans into multithreaded classes and use JNDI instead. In this section we ll look at how you can successfully use JNDI and stateful session beans from your web application. In order to look up a session bean, you have to establish a reference or dependency using the @EJB annotation at the class level and then look up the bean from the environment naming context (discussed in chapter 5) using the reference name specified. If you have worked with EJB 2, using @EJB at the class level is the same as using ejb-ref. The session beans get bound to the environment naming context, or ENC (see chapter 5) as java:comp/env/<name specified with @EJB>. To begin, you can use the @EJB annotation at the class level to establish an EJB reference or dependency on EJB as follows:
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@EJB(name = "UserRegistrationBean", beanInterface = UserRegistration.class) public class ActionBazaarRegistrationControllerServlet extends HttpServlet { ... }
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You must make a JNDI lookup and store the EJB object in the HttpSession object. This will ensure that the same user session gets back the desired bean instance as follows:
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try { InitialContext ctx = new InitialContext(); userReg = (UserRegistration)ctx.lookup( "java:comp/env/UserRegistrationBean"); session.setAttribute("user_reg", userReg); } catch (Exception e) { handleException(e); }
Now the reference to the session bean can invoke any number of methods. The container ensures that there is a bean instance reserved for a user session and that the bean instance is not shared between multiple user sessions. Be sure to remove the EJB object from the HttpSession when you are invoking a method that is annotated with @Remove. Here s the code that removes the bean instance from the session:
userReg.createAccount(); session.removeAttribute("user_reg");
Using JPA from the web tier
Note that the createAccount method has been annotated with @Remove (see chapter 3), and we must remove the reference to the stateful EJB when the bean instance is destroyed. Otherwise, if we try to use it in the future all we ll get will be the aforementioned javax.ejb.NoSuchEJBException. And we wouldn t want that now, would we Understanding how clients can best access session beans is great, but what about times when we need to access the persistence mechanism directly
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