软件毕设外文翻译.docx

1、软件毕设外文翻译Strutsan open-source MVC implementationBy Christian Kirkegaard and Anders Moller,BRICS, University of Aarhus, DenmarkThis 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

2、 project and promote specialization. Even if you never implement a system with Struts, you may get some ideas for your future servlets and JSP page implementation.IntroductionKids in grade school put HTML pages on the Internet. However, there is a monumental difference between a grade school page an

3、d a professionally developed Web site. The page designer (or HTML developer) must 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 inte

4、rested 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 large-scale Web application, you understand the term change. Model-View-Controller (MVC) is a desig

5、n 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, from the J2EE specifications, as part of the implementation. You may never implement a system with Struts, but looking at Stru

6、ts 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 validation, flow control, and updating the state of the application. This is where MVC comes to the rescue. MVC helps resolve some

7、 of the issues with the single module approach by dividing the problem into three categories: ModelThe 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 vie

8、w or controller. ViewThe 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 setters. In addition, it knows nothing about the controller. The view should be notified when changes to the model occur. Con

9、trollerThe 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 stateless connection between the client and the server. This stateless behavior made it difficult for the model to notify the view

10、 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 different technology for implementation than the model or controller. Of course, we could use Java (or PERL, C/C+ or what ever) cod

11、e 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. Customers of the service should be able to create pages to meet their specific needs. The page designer isnt able to have direct

12、involvement in page development. HTML embedded into code is ugly. For the Web, 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 2Struts, an MVC 2 implementationStruts is a set of cooperating clas

13、ses, 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 extensive tag library and utility classes that work independently of the framework. Figure 2 displays an overview of Struts. Figure

14、 2. Struts viewStruts overview Client browser An HTTP request from the client browser creates an event. The Web container will respond with an HTTP response. ControllerThe Controller receives the request from the browser, and makes the decision where to send the request. With Struts, the Controller

15、is a command design pattern implemented as a servlet. The struts-config.xml the Controller. Business logicThe 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. Mod

16、el stateThe 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 information from the ActionForm bean using JSP tags. ViewThe view is simply a

17、 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 other frameworks like Velocity. Struts detailsDisplayed in Figure 3 is a stripped-down UML diagram of the org.apache.struts.action package. Figur

18、e 6 shows the minimal relationships among ActionServlet (Controller), ActionForm (Form State), and Action (Model Wrapper). Figure 3:the relationship between ActionServlet (Controller)、 ActionForm (Form State) and Action (Model Wrapper)The ActionServlet classDo you remember the days of function mappi

19、ngs? You would map some input event to a pointer to a function. If you where slick, you would place the configuration information into a load the 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 a

20、ll 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 you don_t have to hard-code the values. Life changes, but stays the same. ActionServlet is the Command part of the MVC implementation

21、and is the core of the Framework. ActionServlet (Command) creates and uses Action, an ActionForm, and ActionForward. As mentioned earlier, the struts-config.xml the Command. During the creation of the Web project, Action and ActionForm are extended to solve the specific problem space. The instructs

22、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

23、 Java code to understand the flow of the application. The Java developer does not need to 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 abstr

24、act class that is sub-classed 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

25、 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 request.getParameter() calls. For instance, the Struts framework will take fname from request stream and

26、 call UserActionForm.setFname(). 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

27、 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 struts-config.xml whic

28、h 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 classThe Action class is a wrapper around the business logic. The purpose of Action class is to translate the HttpServ

29、letRequest 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 request.getParameter() calls. By the time the event gets here, the input form d

30、ata (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

31、allow flexibility and reuse.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 incompatibility i

32、nterface (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 compatible 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. Nothings perfect.)