1、Struts an open-source MVC implementation This article introduces Struts, a Model-View-Controller implementation that uses servlets and JavaServer Pages (JSP) technology. Struts can help you control change in your Web project and promote specialization. Even if you never implement a system with Strut
2、s, you may get some ideas for your future servlets and JSP page implementation. Introduction Kids in grade school put HTML pages on the Internet. However, there is a monumental difference between a grade school page and a professionally developed Web site. The page designer (or HTML developer) must
3、understand colors, the customer, product flow, page layout, browser compatibility, 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. JavaSer
4、ver Pages (JSP) technology provides the glue between the page designer and the Java developer. 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 busine
5、ss logic and data. Struts is an MVC implementation that uses Servlets 2.2 and JSP 1.1 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. Model-
6、View-Controller (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 comes to the rescue. MVC helps resolve some of the issues with the single module approach by dividing the problem into thre
7、e categories: Model The model contains the core of the applications functionality. The model encapsulates 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 th
8、e look of the application. The view can access the model getters, but it has no knowledge of the 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
9、. MVC Model 2 The Web brought some unique challenges to software developers, most notably the stateless 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
10、modification to the state of the application. Another noticeable change is that the view uses different 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
11、programmers should develop services, not HTML. Changes to layout would require changes to code. Customers 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 W
12、eb, the classical form of MVC needed to change. Figure 1 displays the Web adaptation of MVC, also commonly known as MVC Model 2 or MVC 2. Figure 1. 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 de
13、finition implies that Struts is a framework, rather than a library, but Struts also contains an extensive tag library and utility classes that work independently of the framework. Figure 2 displays an overview of Struts. Figure 2. Struts view Struts overview Client browser An HTTP request from the c
14、lient 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 command design pattern implemented as a servlet. The struts-config
15、.xml 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 applica
16、tion. 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 bus
17、iness logic, and no model information - just tags. Tags are one of the things that make Struts unique compared to other frameworks like Velocity. Struts details Displayed in Figure 3 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).
