J2EE中的设计模式之四

b. 建议的解决方法 　　解决这个问题的方法是，把客户端和他们使用的EJB分割开。建议适用Session Fa?ade模式。这个模式通过一个Session Bean，为一系列的EJB提供统一的接口来实现流程。事实上，当客户端只是使用这个接口来触发流程。这样，所有关于EJB实现流程所需要的改变，都和客户端无关。 　　看下面这个例子。这段代码用来控制与客户相关的订单的处理方法。 // All imports required // Exception handling not shown in the sample code public class OrderSessionFacade implements SessionBean { // all EJB specific methods like ejbCreate defined here // Here is the business method that does the workflow // required when a customer places a new order public int placeOrder(String customerId, Details orderDetails) throws RemoteException { // Get reference to the required EJBs InitialContext ctxt = new InitialContext(); Object obj = ctxt.lookup("java:comp/env/ejb/UserAccount"); UserAccountHome acctHome = (UserAccountHome) PortableRemoteObject.narrow(obj, UserAccountHome.class); UserAccount acct = acctHome.create(); obj = ctxt.lookup("java:comp/env/ejb/CreditCheck"); CreditCheckHome creditCheckHome = (CreditCheckHome) PortableRemoteObject.narrow(obj, CreditCheckHome.class); CreditCheck credit = creditCheckHome.create(); obj = ctxt.lookup("java:comp/env/ejb/Approvals"); ApprovalsHome apprHome = (ApprovalsHome) PortableRemoteObject.narrow(obj, ApprovalsHome.class); Approvals appr = apprHome.create(); obj = ctxt.lookup("java:comp/env/ejb/CommitOrder"); CommitOrderHome orderHome = (CommitOrderHome) PortableRemoteObject.narrow(obj, CommitOrderHome.class); CommitOrder order = orderHome.create(); // Now do the required workflow to place the order int result = acct.checkStatus(customerId); if(result != OK) { // stop further steps } result = credit.checkCreditWorth(customerId, currentOrder); if(result != OK) { // stop further steps } result = appr.getApprovals(customerId, currentOrder); if(result != OK) { // stop further steps } // Everything OK; place the order int orderId = order.placeOrder(customerId, currentOrder); // Do other processing required return(orderId); } // Implement other workflows for other order related functionalities (like // updating an existing order, canceling an existing order etc.) in a // similar way } 　　在模式允许的情况下，Servlet代码将很容易实现。 // all required imports // exceptions to be caught appropriately wherever applicable public class OrderHandlingServlet extends HttpServlet { // all required declarations, definitions public void init() { // all inits required done here } public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { // other logic as required // Get reference to the session facade InitialContext ctxt = new InitialContext(); Object obj = ctxt.lookup("java:comp/env/ejb/OrderSessionFacade"); OrderSessionFacadeHome facadeHome = (OrderSessionFacadeHome) PortableRemoteObject.narrow(obj, OrderSessionFacadeHome.class); OrderSessionFacade facade = facadeHome.create(); // trigger the order workflow int orderId = facade.placeOrder(customerId, currentOrder); // do further processing as required } } 　　就象上面显示的，客户端的逻辑变得非常简单。流程中的任何改变只要修改模式中的一处地方就可以了。客户端可以仍旧使用原来的接口，而不必做任何修改。同样，这个模式可以用来响应其他处理器的流程处理。这让你能用同样的模式来处理不同客户端的不同流程。在这个例子中，模式提供了很好的伸缩性和可维护性。 J2EE中的设计模式之一 J2EE中的设计模式之二 J2EE中的设计模式之三 J2EE中的设计模式之四 J2EE中的设计模式之五