【正文】 (ActionServlet) to the Model (Action amp。 ActionForm) （ 1） 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. （ 2） 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 HTML form. 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. （ 3） 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. Another way of thinking about Action class is as the Adapter design pattern. The purpose of the Action is to Convert the interface of a class into another interface the clients expect. Adapter lets classes work together that couldn_t otherwise because of inpatibility interface (from Design Patterns Elements of Reusable OO Software by Gof). The client in this instance is the ActionServlet that knows nothing about our specific business class interface. Therefore, Struts provides a business interface it does understand, Action. By extending the Action, we make our business interface patible with Struts business interface. (An interesting observation is that Action is a class and not an interface. Action started as an interface and changed into a class over time. Nothing39。s perfect.) （ 4） The Error classes The UML diagram (Figure 4) also included ActionError and ActionErrors. ActionError encapsulates an individual error message. ActionErrors is a container of ActionError classes that the View can access using tags. ActionErrors is Struts way of keeping up with a list of errors. Figure 4. UML diagram of the relationship of the Command (ActionServlet) to the Model (Action) （ 5） The ActionMapping class An ining event is normally in the form of an HTTP request, which the servlet Container turns into an HttpServletRequest. The Controller looks at the ining event and dispatches the request to an Action class. The determines what Action class the Controller calls. The configuration information is translated into a set of ActionMapping, which are put into container of ActionMappings. (If you have not noticed it, classes that end with s are containers) The ActionMapping contains the knowledge of how a specific event maps to specific Actions. The ActionServlet (Command) passes the ActionMapping to the Action class via the perform() method. This allows Action to access the information to control flow. （ 6） ActionMappings ActionMappings is a collection of ActionMapping objects. a)Struts pros ? Use of JSP tag mechanism The tag feature promotes reusable code and abstracts Java code from the JSP file. This feature allows nice integration into JSPbased development tools that allow authoring with tags. ? Tag library Wh