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2022-12-05 09:10:02 本頁面
【正文】 nd JSP page implementation Introduction Kids in grade school put HTML pages on the Inter. However, there is a monumental difference between a grade school page and a professionally developed Web site. The page designer (or HTML developer) must understand colors, the customer, product flow, page layout, browser patibility, image creation, JavaScript, and more. Putting a great looking site together takes a lot of work, and most Java developers are more interested in creating a great looking object interface than a user interface. JavaServer Pages (JSP) technology provides the glue between the page designer and the Java developer. If you have worked on a largescale Web application, you understand the term change. ModelViewController (MVC) is a design pattern put together to help control change. MVC decouples interface from business logic and data. Struts is an MVC implementation that uses Servlets and JSP tags, from the J2EE specifications, as part of the implementation. You may never implement a system with Struts, but looking at Struts may give you some ideas on your future Servlets and JSP implementations. ModelViewController (MVC) JSP tags solved only part of our problem. We still have issues with validation, flow control, and updating the state of the application. This is where MVC es to the rescue. MVC helps resolve some of the issues with the single module approach by dividing the problem into three categories: ? Model The model contains the core of the application39。 if not, it will create an instance of the class. ? Struts will set the state of the UserActionForm using corresponding fields from the HttpServletRequest. No more dreadful () calls. For instance, the Struts framework will take fname from request stream and call (). ? The Struts framework updates the state of the UserActionForm before passing it to the business wrapper UserAction. ? Before passing it to the Action class, Struts will also conduct form state validation by calling the validation() method on UserActionForm. Note: This is not always wise to do. There might be ways of using UserActionForm in other pages or business objects, where the validation might be different. Validation of the state might be better in the UserAction class. 第 5頁 共 20頁 ? The UserActionForm can be maintained at a session level. ? Notes: ? The file controls which HTML form request maps to which ActionForm. ? Multiple requests can be mapped UserActionForm. ? UserActionForm can be mapped over multiple pages for things such as wizards. The Action class The Action class is a wrapper around the business logic. The purpose of Action class is to translate the HttpServletRequest to the business logic. To use Action, subclass and overwrite the process() method. The ActionServlet (Command) passes the parameterized classes to ActionForm using the perform() method. Again, no more dreadful () calls. By the time the event gets here, the input form data (or HTML form data) has already been translated out of the request stream and into an ActionForm class. Figure 4. MVC Model 2 Struts, an MVC 2 implementation Struts is a set of cooperating classes, servlets, and JSP tags that make up a reusable MVC 2 design. This definition implies that Struts is a framework, rather than a library, but Struts 第 6頁 共 20頁 also contains an extensive tag library and utility classes that work independently of the framework. Figure 5 displays an overview of Struts. Figure 5. Struts overview Struts overview ? Client browser An HTTP request from the client browser creates an event. The Web container will respond with an HTTP response. ? Controller The Controller receives the request from the browser, and makes the decision where to send the request. With Struts, the Controller is a mand design pattern implemented as a servlet. The file configures the Controller. ? Business logic The business logic updates the state of the model and helps control the flow of the application. With Struts this is done with an Action class as a thin wrapper to the actual business logic. ? Model state The model represents the state of the application. The business objects update the application state. ActionForm bean represents the Model state at a session or request level, and not at a persistent level. The JSP file reads information from the ActionForm bean using JSP tags. 第 7頁 共 20頁 ? View The view is simply a JSP file. There is no flow logic, no business logic, and no model information just tags. Tags are one of the things that make Struts unique pared to other frameworks like Velocity. Note: Think thin when extending the Action class. The Action class should control the flow and not the logic
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