1、If you have worked on a large-scale Web application, you understand the term change. Model-View-Controller (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 2.2 and JSP 1.1 tags, fro
2、m 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.Model-View-Controller (MVC)JSP tags solved only part of our problem. We still have issues with validat
3、ion, flow control, and updating the state of the application. This is where MVC comes 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 applications functionality. The model enca
4、psulates the state of the application. Sometimes the only functionality it contains is state. It knows nothing about the view or controller. View The view provides the presentation of the model. It is the look of the application. The view can access the model getters, but it has no knowledge of the
5、setters. In addition, it knows nothing about the controller. The view should be notified when changes to the model occur. Controller The controller reacts to the user input. It creates and sets the model. MVC Model 2The Web brought some unique challenges to software developers, most notably the stat
6、eless connection between the client and the server. This stateless behavior made it difficult for the model to notify the view of changes. On the Web, the browser has to re-query the server to discover modification to the state of the application.Another noticeable change is that the view uses diffe
7、rent technology for implementation than the model or controller. Of course, we could use Java (or PERL, C/C+ or what ever) code to generate HTML. There are several disadvantages to that approach: Java programmers should develop services, not HTML. Changes to layout would require changes to code. Cus
8、tomers of the service should be able to create pages to meet their specific needs. The page designer isnt able to have direct involvement in page development. HTML embedded into code is ugly. For the Web, the classical form of MVC needed to change. Figure 4 displays the Web adaptation of MVC, also c
9、ommonly known as MVC Model 2 or MVC 2. Struts detailsDisplayed in Figure 6 is a stripped-down UML diagram of the org.apache.struts.action package. Figure 6 shows the minimal relationships among ActionServlet (Controller), ActionForm (Form State), and Action (Model Wrapper). Figure 6. UML diagram of
10、the relationship of the Command (ActionServlet) to the Model (Action & ActionForm)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 t
11、he 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 C
12、ontroller uses a configuration file so you don_t have to hard-code 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 mention
13、ed earlier, the struts-config.xml file configures the Command. During the creation of the Web project, Action and ActionForm are extended to solve the specific problem space. The file struts-config.xml instructs ActionServlet on how to use the extended classes. There are several advantages to this a
14、pproach: 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 t
15、o recompile 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 sub-classed for each input form model. When I say input form model, I
16、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 s
17、et the state of the UserActionForm using corresponding fields from the HttpServletRequest. No more dreadful request.getParameter() calls. For instance, the Struts framework will take fname from request stream and call UserActionForm.setFname(). The Struts framework updates the state of the UserActio
18、nForm 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 b
19、usiness 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 struts-config.xml file controls which HTML form request maps to which ActionForm. Multiple requests can be ma
20、pped 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
21、the process() method. The ActionServlet (Command) passes the parameterized classes to ActionForm using the perform() method. Again, no more dreadful request.getParameter() calls. By the time the event gets here, the input form data (or HTML form data) has already been translated out of the request s
22、tream and into an ActionForm class. Figure 4. MVC Model 2Struts, an MVC 2 implementationStruts 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 also contains an extensiv
23、e tag library and utility classes that work independently of the framework. Figure 5 displays an overview of Struts. Figure 5. Struts overviewStruts overview Client browser An HTTP request from the client browser creates an event. The Web container will respond with an HTTP response. The Controller
24、receives the request from the browser, and makes the decision where to send the request. With Struts, the Controller is a command design pattern implemented as a servlet. The struts-config.xml file configures the Controller. Business logic The business logic updates the state of the model and helps
25、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
26、 or request level, and not at a persistent level. The JSP file reads information from the ActionForm bean using JSP tags. 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 compared to oth
27、er frameworks like Velocity. 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 th
28、e 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 incompatibility interface (from Design Patterns - Elements of Reusable OO Software by Gof). The
29、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 compatible with Struts business interface. (An interesting observation is that Action is a class and not an interface.
copyright@ 2008-2023 冰点文库 网站版权所有
经营许可证编号:鄂ICP备19020893号-2