【Jsp精品源码栏目提醒】:网学会员鉴于大家对Jsp精品源码十分关注,论文会员在此为大家搜集整理了“(精品)计算机 JSP_软件 毕业论文 外文翻译:Struts——MVC 的一种开放源码实现 - 毕业设计”一文,供大家参考学习
Struts——an open-source MVC implementation By: Malcolm Davis. Source: Struts--an open-source MVC implementationJ.IBM Systems Journal This article introduces Struts a Model-View-Controller implementation that usesservlets and JavaServer Pages JSP technology. Struts can help you control change in yourWeb project and promote specialization. Even if you never implement a system with Strutsyou 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 monumentaldifference between a grade school page and a professionally developed Web site. The pagedesigner or HTML developer must understand colors the customer product flow pagelayout browser compatibility image creation JavaScript and more. Putting a great lookingsite together takes a lot of work and most Java developers are more interested in creating agreat looking object interface than a user interface. Java Server Pages JSP technologyprovides 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. MVCdecouples interface from business logic and data. Struts is an MVC implementation that usesServlets 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 someideas 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 flowcontrol 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 theproblem into three categories: ModelThe model contains the core of the applications functionality. The model encapsulates thestate of the application. Sometimes the only functionality it contains is state. It knows nothingabout the view or controller. View The view provides the presentation of the model. It is the look of the application. Theview can access the model getters but it has no knowledge of the setters. In addition it knowsnothing about the controller. The view should be notified when changes to the model occur.ControllerThe controller reacts to the user input. It creates and sets the model.MVC Model 2 The Web brought some unique challenges to software developers most notably thestateless connection between the client and the server. This stateless behavior made it difficultfor the model to notify the view of changes. On the Web the browser has to re-query theserver to discover modification to the state of the application. Another noticeable change is that the view uses different technology for implementationthan the model or controller. Of course we could use Java or PERL C/C or what evercode 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 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 Webadaptation of MVC also commonly known as MVC Model 2 or MVC 2. Figure 4. MVC Model 2Struts an MVC 2 implementation Struts is a set of cooperating classes servlets and JSP tags that make up a reusable MVC2 design. This definition implies that Struts is a framework rather than a library but Strutsalso contains an extensive tag library and utility classes that work independently of theframework. Figure 5 displays an overview of Struts. Figure 5. Struts overview Struts overview Client browserAn HTTP request from the client browser creates an event. The Web container will respondwith an HTTP response. ControllerThe Controller receives the request from the browser and makes the decision where to sendthe request. With Struts the Controller is a command design pattern implemented as a servlet.The struts-config.xml file configures 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. Model stateThe model represents the state of the application. The business objects update the applicationstate. ActionForm bean represents the Model state at a session or request level and not at apersistent level. The JSP file reads information from the ActionForm bean using JSP tags. ViewThe view is simply a JSP file. There is no flow logic no business logic and no modelinformation -- just tags. Tags are one of the things that make Struts unique compared to otherframeworks like Velocity.Struts details Displayed in Figure 6 is a stripped-down UML diagram of the org.apache.struts.actionpackage. Figure 6 shows the minimal relationships among ActionServlet ControllerActionForm Form State and Action Model Wrapper. Figure 6. UML diagram of the relationship of the Command ActionServlet to theModel Action amp ActionForm The ActionServlet class Do you remember the days of function mappings You would map some input event to apointer to a function. If you where slick you would place the configuration information into afile and load the file at run time. Function pointer arrays were the good old days of structuredprogramming in C. Life is better now that we have Java technology XML J2EE and all that. The StrutsController 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 thevalues. Life changes but stays the same. ActionServlet is the Command part of the MVC implementation and is the core of theFramework. ActionServlet Command creates and uses Action an ActionForm andActionForward. As mentioned earlier the struts-config.xml file configures the Command.During the creation of the Web project Action and ActionForm are extended to solve thespecific problem space. The file struts-config.xml instructs ActionServlet on how to use theextended classes. There are several advantages to this approach: The entire logical flow of the application is in a hierarchical text file. This makes iteasier to view and understand especially with large applications. The page designer does not have to wade through Java code to understand the flow ofthe 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 anabstract class that is sub-classed for each input form model. When I say input form model Iam saying ActionForm represents a general concept of data that is set or updated by a HTMLform. For instance you may have a UserActionForm that is set by an HTML Form. TheStruts 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 theHttpServletRequest. No more dreadful request.getParameter calls. For instance the Strutsframework will take fname from request stream and call UserActionForm.setFname. The Struts framework updates the state of the UserActionForm before passing it to thebusiness wrapper UserAction. Before passing it to the Action class Struts will also conduct form state validation bycalling 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 thevalidation 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 whichActionForm. Multiple requests can be mapped 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 isto translate the HttpServletRequest to the business logic. To use Action subclass andoverwrite the process method. The ActionServlet Command passes the parameterized classes to ActionForm using theperform method. Again no more dreadful request.getParameter calls. By the time theevent gets here the input form data or HTML form data has already been translated out ofthe request stream and into an ActionForm class. Note: quotThink thinquot when extending the Action class. The Action class should control theflow and not the logic of the application. By placing the business logic in a separate packageor EJB we allow flexibility and reuse. Another way of thinking about Action class is as the Adapter design pattern. Thepurpose of the Action is to quotConvert the interface of a class into another interface the clientsexpect. Adapter lets classes work together that couldn_t otherwise because of incompatibilityinterfacequot from Design Patterns - Elements of Reusable OO Software by Gof. The client inthis instance is the ActionServlet that knows nothing about our specific business classinterface. Therefore Struts provides a business interface it does understand Action. Byextending the Action we make our business interface compatible with Struts businessinterface. An interesting observation is that Action is a class and not an interface. Actionstarted as an interface and changed into a class over time. Nothings perfect. The Error classes The UML diagram Figure 6 also included ActionError and ActionErrors. ActionErrorencapsulates an individual error message. ActionErrors is a container of ActionError classesthat the View can access using tags. ActionErrors is Struts way of keeping up with a list oferrors. Figure 7. UML diagram of the relationship of the Command ActionServlet to theModel Action The ActionMapping class An incoming event is normally in the form of an HTTP request which the servletContainer turns into an HttpServletRequest. The Controller looks at the incoming event anddispatches the request to an Action class. The struts-config.xml determines what Action classthe Controller calls. The struts-config.xml configuration information is translated into a set ofActionMapping which are put into container of ActionMappings. If you have not noticed itclasses that end with s are containers The ActionMapping contains the knowledge of how a specific event maps to specificActions. The ActionServlet Command passes the ActionMapping to the Action class via theperform method. This allows Action to access the information to control flow. ActionMappings ActionMappings is a collection of ActionMapping objects.Struts pros Use of JSP tag mechanism The tag feature promotes reusable code and abstracts Java code from the JSP file. Thisfeature allows nice integration into JSP-based development tools that allow authoring withtags. Tag libraryWhy re-invent the wheel or a tag library If you cannot find something you need in thelibrary contribute. In addition Struts provides a starting point if you are learning JSP tagtechnology. Open sourceYou have all the advantages of open source such as being able to see the code and havingeveryone else using the library reviewing the code. Many eyes make for great code review. Sample MVC implementationStruts offers some insight if you want to create your own MVC implementation. Manage the problem spaceDivide and conquer is a nice way of solving the problem and making the problem manageable.Of course the sword cuts both ways. The problem is more complex and needs moremanagement.Struts cons YouthStruts development is still in preliminary form. They are working toward releasing a version1.0 but as with any 1.0 version it does not provide all the bells and whistles. ChangeThe framework is undergoing a rapid amount of change. A great deal of change has occurredbetween Struts 0.5 and 1.0. You may want to download the most current Struts nightlydistributions to avoid deprecated methods. In the last 6 months I have seen the Struts librarygrow from 90K to over 270K. I had to modify my examples several times because of changesin Struts and I am not going to guarantee my examples will work with the version of Strutsyou download. Correct level of abstractionDoes Struts provide the correct level of abstraction What is the proper level of abstraction forthe page designer That is the 64K question. Should we allow a page designer access to Javacode in page development Some frameworks like Velocity say no and provide yet anotherlanguage to learn for Web development. There is some validity to limiting Java code access inUI development. Most importantly give a page designer a little bit of Java and he will use alot of Java. I saw this happen all the time in Microsoft ASP development. In ASPdevelopment you were supposed to create COM objects and then write a little ASP script toglue it all together. Instead the ASP developers would go crazy with ASP script. I wouldhearquotWhy wait for a COM developer to create it when I can program it directly withVBScriptquot Struts helps limit the amount of Java code required in a JSP file via tag libraries.One such library is the Logic Tag which manages conditional generation of output but thisdoes not prevent the UI developer from going nuts with Java code. Whatever type offramework you decide to use you should understand the environment in which you aredeploying and maintaining the framework. Of course this task is easier said than done. Limited scopeStruts is a Web-based MVC solution that is meant be implemented with HTML JSP files andservlets. J2EE application supportStruts requires a servlet container that supports JSP 1.1 and Servlet 2.2 specifications. Thisalone will not solve all your install issues unless you are using Tomcat 3.2. I have had a greatdeal of problems installing the library with Netscape iPlanet 6.0 which is supposedly the firstJ2EE-compliant application server. I recommend visiting the Struts User Mailing List archivesee Resources when you run into problems. ComplexitySeparating the problem into parts introduces complexity. There is no question that someeducation will have to go on to understand Struts. With the constant changes occurring thiscan be frustrating at times. Welcome to the Web. Where is...I could point out other issues for instance where are the client side validations adaptableworkflow and dynamic strategy pattern for the controller However at this point it is tooeasy to be a critic and some of the issues are insignificant or are reasonable for a 1.0 release.The way the Struts team goes at it Struts might have these features by the time you read thisarticle or soon after. Future of StrutsThings change rapidly in this new age of software development. In less than 5 years I haveseen things go from cgi/perl to ISAPI/NSAPI to ASP with VB and now Java and J2EE. Sunis working hard to adapt changes to the JSP/servlet architecture just as they have in the pastwith the Java language and API. You can obtain drafts of the new JSP 1.2 and Servlet 2.3specifications from the Sun Web site. Additionally a standard tag library for JSP files isappearing. Struts——MVC 的一种开放源码实现 作者: Malcolm Davis 来源: Struts--an open-source MVC implementationJ. IBM Syste.