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The good news is that EJB 3 enables you to easily follow good object-oriented design or DDD. The entities defined by EJB 3 Java Persistence API (JPA) support OO features, such as inheritance or polymorphism. It s easy to implement a persistence object model with the EJB 3 JPA. More importantly, you can easily add business logic to your entities, so that implementing a rich domain model with EJB 3 is a trivial task. Note, though, that many people don t like adding complex business logic in the domain object itself and prefer creating a layer for procedural logic referred to as the service layer or application layer (see Patterns of Enterprise Application Architecture, by Martin Fowler [Addison-Wesley Professional, 2002]). The application layer is similar to the business logic layer of the traditional four-tier architecture, but is much thinner. Not surprisingly, you can use session beans to build the service layer. Whether you use the traditional four-tier architecture or a layered architecture with DDD, you can use entities to model domain objects, including modeling state and behavior. We ll discuss domain modeling with JPA entities in chapter 7. Despite its impressive services and vision, EJB 3 is not the only act in town. You can combine various technologies to more or less match EJB services and infrastructure. For example, you could use Spring with other open source technologies such as Hibernate and AspectJ to build your application, so why choose EJB 3 Glad you asked...

You would like a particular EJB method to have the highest possibility of success. To do this, you want to code a particular method to monitor the status of the executing code in order to detect errors or conditions that may cause a rollback to occur. If a rollback is necessary, you want your EJB to attempt to recover from the error before performing a rollback.

Specifying stateful bean business interfaces works in almost exactly the same way as it does for stateless beans with a couple of exceptions. Stateful session beans support local and remote invocation through the @Local and @Remote annotations. However, a stateful session bean cannot have a web service endpoint interface. This is because SOAP-based web services are inherently stateless in nature. Also, you should always include at least one @Remove annotated method in your stateful bean s business interface. The reason for this will become clear as we discuss the stateful bean lifecycle next.

public class FrontController extends HttpServlet { static final String ERROR_VIEW = "/error.jsp"; [...] // other URLs static Hashtable pathInfoCommandMap = new Hashtable(); public void init(javax.servlet.ServletConfig config) throws ServletException { super.init(config); pathInfoCommandMap.put("/logon","aop.j2ee.[...].LoginController"); pathInfoCommandMap.put("/subscribe","aop.j2ee. [...].SubscribeController"); [...] // other paths of application controllers } public void doGet [...] // process method invocation public void doPost[...] // process method invocation protected void process(HttpServletRequest request, HttpServletResponse resp) throws ServletException, IOException { String pathInfo = request.getPathInfo(); try { // lookup the real path by delegating to the application controller pathInfo=invokeApplicationController(pathInfo,request,resp ); } catch(Exception e) { pathInfo = ERROR_VIEW; } // forward the control to the view / command RequestDispatcher dispatcher = request.getRequestDispatcher(pathInfo); dispatcher.forward(request,resp ); }

As we mentioned in section 3.1, the lifecycle of the stateful session bean is very different from that of a stateless session bean because of passivation. In this section, we explain this concept in more depth. Let s start by looking at the lifecycle of a stateful bean, as shown in figure 3.8. The container follows these steps:

With both bean-managed and container-managed transaction EJBs, you can avoid causing a rollback by correctly handling errors.

Always creates new bean instances using the default constructor whenever a new client session is started.

2 3 4

39 40 private String invokeApplicationController( 41 String aRequestPathInfo, 42 HttpServletRequest aRequest, 43 HttpServletResponse aResponse) throws Exception { 44 ApplicationController controller = null; 45 String className = (String)pathInfoCommandMap.get(aRequestPathInfo); 46 if(className != null) { 47 Class controllerClass = Class.forName(className); 48 controller = (ApplicationController) 49 controllerClass.newInstance(); 50 if(controller != null) 51 aRequestPathInfo = (String)controller.process(aRequest,aResponse); 52 } 53 return aRequestPathInfo; 54 } 55 } The process method of line 25 is a common method for handling requests. It checks if the user is logged. If so, it forwards the request to the requested URL; otherwise, it forwards the request to the login page. When the application evolves, the front controller code becomes more complicated and handles more specific cases. A strategy to avoid this problem is to create an inheritance hierarchy to replace excessive conditional logic. For instance, for an application containing three distinct functional zones, we can factorize the commonalities within a superclass. Even though this design seems simple, it is complicated and tedious to program since the presentation layer and its associated needs are complex and often permanently redefined. The use of an aspect improves the application s modularity by separating the base logic of a front controller from a given piece of application logic. For instance, in Listing 12-16, the FrontController aspect handles the delegation and encapsulation of the requests toward the application controllers. Listing 12-16. A Simple Front Controller Aspect 01 02 03 04 05 06 07 08 09 10 11 12 13 14 package aop.j2ee.client.web.aspect; import import import import import import import java.util.Hashtable; javax.servlet.http.HttpServlet; javax.servlet.http.HttpServletRequest; javax.servlet.http.HttpServletResponse; aop.j2ee.client.web.protocol.RequestContextFactory; aop.j2ee.client.web.protocol.RequestContext; aop.j2ee.client.web.controlleur.*;

For bean-managed transaction beans Listing 5.2 contains a code section from a BMT bean business method that attempts to avoid a rollback.