网络工程专业毕业外文翻译.docx

1、网络工程专业毕业外文翻译XX大学毕业设计（论文）说明书英文翻译专 业网络工程班 级网X姓 名X学 号X指导教师X二O一五年六月十七日外文原文出自IBM System Journal.2014 作者： Malcolm Davis StrutsAn Open-source MVC ImplementationThis article introduces Struts, a Model-View-Controller implementation that uses servlets and JavaServer Pages (JSP) technology. Struts can help yo

2、u control change in your Web 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 be

3、tween a grade school page and 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 J

4、ava developers are more interested 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

5、-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, from the J2EE specifications, as part of the implementation. You may never implement a system with

6、 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 validation, flow control, and updating the state of the application. This is where MVC comes to the re

7、scue. 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 encapsulates the state of the application. Sometimes the only functionality it contains is state. I

8、t 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 setters. In addition, it knows nothing about the controller. The view should be notified when c

9、hanges 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 stateless connection between the client and the server. This stateless behavior made it difficult f

10、or 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 different technology for implementation than the model or controller. Of course, we could use Java (

11、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. Customers of the service should be able to create pages to meet their specific needs. The page des

12、igner 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 commonly known as MVC Model 2 or MVC 2. Struts detailsDisplayed in Figure 6 is a stripped-down U

13、ML 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 the relationship of the Command (ActionServlet) to the Model (Action & ActionForm)The ActionSer

14、vlet 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 i

15、n 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 hard-code the values. Life changes, bu

16、t 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 struts-config.xml file configures the Command. During the creation of the Web p

17、roject, 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 approach: The entire logical flow of the application is in a hierarchical text file. This makes

18、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 recompile code when making flow changes. Command functionality can be added by extending Acti

19、onServlet.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 am saying ActionForm represents a general concept of data that is set or updated by a HTML form

20、. 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 m

21、ore 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 UserActionForm before passing it to the business wrapper UserAction. Before passing it to the Action cla

22、ss, 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 bett

23、er 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 mapped UserActionForm. UserActionForm can be mapped over multiple pages for things such as wizard

24、s. 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 ActionFor

25、m 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 stream and into an ActionForm class.Figure 4. MVC Model 2Struts, an MVC 2 implementationStruts i

26、s 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 extensive tag library and utility classes that work independently of the framework. Figure 5 displays an

27、 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. Controller The Controller receives the request from the browser, and makes the decision where to send the reque

28、st. 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 control the flow of the application. With Struts this is done with an Action class as

29、 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 or request level, and not at a persistent level. The JSP file reads information from

30、 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 compared to other frameworks like Velocity. Note: Think thin when extending the Action class. T

31、he 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 Action is to Convert the interface

32、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 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. (