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Lưu ý ghi nhãn một kiến trúc cụ thể như ba tầng, năm tầng, và như vậy là gần như đảm bảo để thúc đẩy một số cuộc tranh luận học thuật. Một số nhấn mạnh rằng lớp được định nghĩa bởi các phân vùng vật lý, do đó, nếu các thành phần ứng dụng cư trú trên các máy trạm của khách hàng, máy chủ ứng dụng | CHAPTER 1 JAVA EE ESSENTIALS 19 As you ll see in Chapter 9 building EJBs follows the same basic steps as creating an RMI object. You create an interface that exposes the operations or services provided by the EJB. You then create a class that implements the interface. When you deploy an EJB to an application server the EJB is associated with a name in a registry. Clients can look up the EJB in the registry and then remotely call the methods of the EJB. Since the EJB container provides all of the enterprise plumbing you get to spend more time building your application and less time messing around with trying to shoehorn in services like security and transaction support. EJBs come in a few different flavors session beans entity beans and message beans. Session beans as the name implies live only as long as the conversation or session between the client application and the bean lasts. The session bean s primary reason for being is to provide application services defined and designed by the application developer to client applications. Depending on the design a session bean may maintain state during the session or may be stateless. With a stateful EJB when a subsequent request comes from a client the values of the internal member variables have the same values they had when the previous request ended so that the EJB can maintain a conversation with the client. A stateless EJB provides business rules through its exposed operations but doesn t provide any sense of state that responsibility is delegated to the client. Entity beans represent business objects such as customers invoices and products in the application domain. These business objects are persisted so they can be stored and retrieved at will. The Java EE architecture provides a lot of flexibility for the persistence model. You can defer all of the work of storing and retrieving the bean s state information to the container as shown in Figure 1-11. This is known as container-managed persistence. Figure 1-11 . In
