【正文】 odel 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. ? 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. Struts details Displayed in Figure 6 is a strippeddown UML diagram of the package. Figure 6 shows the minimal relationships among ActionServlet (Controller), ActionForm (Form State), and Action (Model Wrapper). Figure 6. UML diagram of the relationship of the Command (ActionServlet) to the Model (Action amp。 ActionForm) 本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 4 The ActionServlet class Do you remember the days of function mappings? You would map some input event to a pointer to a function. If you where slick, you would place the configuration information into a file and load the file at run time. Function pointer arrays were the good old days of structured programming in C. Life is better now that we have Java technology, XML, J2EE, and all that. The Struts Controller is a servlet that maps events (an event generally being an HTTP post) to classes. And guess what the Controller uses a configuration file so you don_t have to hardcode the values. Life changes, but stays the same. ActionServlet is the Command part of the MVC implementation and is the core of the Framework. ActionServlet (Command) creates and uses Action, an ActionForm, and ActionForward. As mentioned earlier, the file configures the Command. During the creation of the Web project, Action and ActionForm are extended to solve the specific problem space. The file instructs ActionServlet on how to use the extended classes. There are several advantages to this approach: ? The entire logical flow of the application is in a hierarchical text file. This makes it easier to view and understand, especially with large applications. ? The page designer does not have to wade through Java code to understand the flow of the application. ? The Java developer does not need to repile code when making flow changes. Command functionality can be added by extending ActionServlet. The ActionForm class ActionForm maintains the session state for the Web application. ActionForm is an abstract class that is subclassed for each input form model. When I say input form model, I am saying ActionForm represents a general concept of data that is set or updated by a 本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 5 HTMLform. For instance, you may have a UserActionForm that is set by an HTML Form. The Struts framework will: ? Check to see if a UserActionForm exists。 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. ? 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. Note: Think thin when extending the Action class. The Action class should control the flow and not the logic of the application. By placing the business logic in a separate package or EJB, we allow flexibility and reuse. Another way of thinking about Action class is as the Adapter design pattern. The purpose of the