計算機外文翻譯--javabean 的發(fā)展

1、外文原文The Enterprise JavaBeansIN a J2EE application Enterprise JavaBeans (EJB) tier hosts application-specific business logic and provides system-level services such as transaction management, concurrency control, and secu

2、rity. Enterprise JavaBeans technology provides a distributed component model that enables developers to focus on solving business problems while relying on the J2EE platform to handle complex system level issues This sep

3、aration of concerns allows rapid development of scalable accessible, robust, and highly secure applications. In the J2EE programming model EJB components are a fundamental link between presentation components hosted by t

4、he Web tier and business-critical data and systems maintained in the enterprise information system tier This chapter describes the concepts central to the Enterprise JavaBeans architecture and provides guidelines and rec

5、ommendations to best use EJB components. The chapter:．Examines the nature of business logic and describes the problems a developer needs to resolve when implementing business logic．Describes the component model that Ente

6、rprise JavaBeans architecture provides to address these problems·Describes remote and local client views·Provides details on the three types of enterprise beans：entity beans, session beans, and message-driven b

7、eans·Recommends design guidelines for developing EJB components and applications·Presents recommendations and practices to best utilize the EJB tier services provided by the J2EE platform·Provides guidelin

8、es to facilitate application portability5.1 Business Logic and Business Objects Business logic, in a very broad sense, is the set of procedures or methods used to manage a specific business function. Taking the object-

9、oriented approach enables the developer to decompose a business function into a set of components or element called business objects. Like other objects, business objects have both state(or data) and behavior. For exampl

10、e, an employee object has data such as a name address, social security number, and so on. It has methods for assigning it to a new department or for changing its salary by a certain percentage. To manage a business probl

11、em you and updated account information should be written to persistent storage.No other client should be able to access the particular account concurrently. Similar analysis and requirement definitions could be performed

12、 for other objects. For example, an order object has a set of general conditions on its behave for that have a significant correlation to the behavior of an account object. That is client needs to be authorized before u

13、pdating or reading the status of an order, details need to be written to a persistent storage, and so on. If you examine business objects in similar applications you will see that even though the actual structure and beh

14、avior of the object is tied closely to the business problem it is going to solve, many services that these business objects provide follow specific patterns that are quite generic in nature．5.1．1 Common Requirements of

15、Business 06jectsThis section describes common requirements of business objects.5.1.1．1 Maintain StateA business object often needs to maintain state between method invocations. This state can be either conversational o

16、r persistent. Conversational state is state main tamed in an object during the conversation between a client and the application. Persistent state is state that is stored in a database or other persistent store, outlivin

17、g the conversation between a client and the application.Consider a shopping cart object. The state of the shopping cart object represents the items and quantities of the items purchased by the client. The cart is initial

18、ly empty and gains meaningful state when a user adds an item to the cart When a user adds a second item to the cart, the cart should have both items in it.Similarly, when a user deletes an item from the cart, the cart sh

19、ould reflect the change in its state. When a user exits the application, the cart object is reclaimed and the conversational state no longer exists．When the object gains, maintains, and loses its state as a result of rep

20、eated interactions with the same client, we say the object maintains conversational state.To understand persistent state, consider an account object. When a user creates an account, the account information needs to be st

21、ored permanently so that when the user exits the application and re-enters the application, the account information can be presented to the user again. The state of an account object needs to be maintained in persistent