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1、A.1 英文原文an open-source MVC implementationThis 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.IntroductionKids 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 un
3、derstand 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. JavaServe
4、r 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 business
5、 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-Vie
6、w-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 three ca
7、tegories: 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 view or controller.ViewThe view provides the presentation of the model. It is the look
8、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. ControllerThe controller reacts to the user input. It creates and sets the model. MVC Mo
9、del 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 of changes. On the Web, the browser has to re-query the server to discover modificat
10、ion 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 programmer
11、s 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 cl
12、assical 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 classes, servlets, and JSP tags that make up a reusable MVC 2 design. This definition imp
13、lies 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 viewStruts overview Client browser An HTTP request from the client browser
14、 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 is a command design pattern implemented as a servlet. The struts-config.xml file conf
15、igures 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 busine
16、ss 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. ViewThe view is simply a JSP file. There is no flow logic, no business logic, and
17、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. Figure 6 shows the minimal relationships among ActionServlet (C
18、ontroller), 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 mappings? You would map some input event to a pointer to a func
19、tion. 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 in C. Life is better now that we have Java technology, XML, J2EE, and all that. The Struts Controller is a servl
20、et 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, but stays the same. ActionServlet is the Command part of the MVC implementation and is the core of the Framework.
21、 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 project, Action and ActionForm are extended to solve the specific problem space. The file struts-config.xml inst
22、ructs 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 t
23、hrough 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
24、 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. For instance, you may have a UserActionForm that is set by an HTML Form. The Struts framework will: Check to
25、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 more dreadful request.getParameter() calls. For instance, the Struts framework will take fname from request stre
26、am 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 class, Struts will also conduct form state validation by calling the validation() method on UserActionForm.Note: T
27、his 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 better in the UserAction class. The UserActionForm can be maintained at a session level. Notes: The struts-config.xm
28、l 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 wizards. The Action classThe Action class is a wrapper around the business logic. The purpose of Action class is to tr
29、anslate the HttpServletRequest 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 he
30、re, the input form data (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
31、 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 of a class into another interface the clients expect. Adapter lets classes work together that couldn_t otherwise because
32、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
33、 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.) The Error classesThe UML diagram (Figure 4) also includ
34、ed ActionError and ActionErrors. ActionError encapsulates an individual error message. ActionErrors is a container of ActionError classes that the View can access using tags. ActionErrors is Struts way of keeping up with a list of errors.Figure 4:the relationship between Command (ActionServlet) and
35、Model (Action)The ActionMapping classAn incoming event is normally in the form of an HTTP request, which the servlet Container turns into an HttpServletRequest. The Controller looks at the incoming event and dispatches the request to an Action class. The struts-config.xml determines what Action clas
36、s the Controller calls. The struts-config.xml configuration information is translated into a set of ActionMapping, which are put into container of ActionMappings. (If you have not noticed it, classes that end with s are containers)The ActionMapping contains the knowledge of how a specific event maps
37、 to specific Actions. The ActionServlet (Command) passes the ActionMapping to the Action class via the perform() method. This allows Action to access the information to control flow. A.1 中文翻译MVC 的一种开放源码实现本文介绍Struts，它是使用 servlet 和 JavaServer Pages 技术的一种 Model-View-Controller 实现。Struts 可帮助您控制 Web 项目中的
38、变化并提高专业化水平。尽管您可能永远不会用 Struts 实现一个系统，但您可以将其中的一些思想用于您以后的 servlet 和 JSP 网页的实现中。简介小学生也可以在因特网上发布 HTML 网页。但是，小学生的网页和专业开发的网站有质的区别。网页设计人员（或者 HTML 开发人员）必须理解颜色、用户、生产流程、网页布局、浏览器兼容性、图像创建和 JavaScript 等等。设计漂亮的网站需要做大量的工作，大多数 Java 开发人员更注重创建优美的对象接口，而不是用户界面。JavaServer Pages (JSP) 技术为网页设计人员和 Java 开发人员提供了一种联系钮带。如果您开发过大
39、型 Web 应用程序，您就理解 变化 这个词的含义。“模型-视图-控制器”(MVC) 就是用来帮助您控制变化的一种设计模式。MVC 减弱了业务逻辑接口和数据接口之间的耦合。Struts 是一种 MVC 实现，它将 Servlet 2.2 和 JSP 1.1 标记（属于 J2EE 规范）用作实现的一部分。尽管您可能永远不会用 Struts 实现一个系统，但了解一下 Struts 或许使您能将其中的一些思想用于您以后的 Servlet 的 JSP 实现中。模型-视图-控制器 (MVC)JSP 标记只解决了部分问题。我们还得处理验证、流程控制和更新应用程序的状态等问题。这正是 MVC 发挥作用的地方
40、。MVC 通过将问题分为三个类别来帮助解决单一模块方法所遇到的某些问题：Model（模型）模型包含应用程序的核心功能。模型封装了应用程序的状态。有时它包含的唯一功能就是状态。它对视图或控制器一无所知。 View（视图）视图提供模型的表示。它是应用程序的 外观。视图可以访问模型的读方法，但不能访问写方法。此外，它对控制器一无所知。当更改模型时，视图应得到通知。 Controller（控制器）控制器对用户的输入作出反应。它创建并设置模型。 MVC Model 2Web 向软件开发人员提出了一些特有的挑战，最明显的就是客户机和服务器的无状态连接。这种无状态行为使得模型很难将更改通知视图。在 Web
41、上，为了发现对应用程序状态的修改，浏览器必须重新查询服务器。另一个重大变化是实现视图所用的技术与实现模型或控制器的技术不同。当然，我们可以使用 Java（或者 PERL、C/C+ 或别的语言）代码生成 HTML。这种方法有几个缺点：Java 程序员应该开发服务，而不是 HTML。 更改布局时需要更改代码。 服务的用户应该能够创建网页来满足它们的特定需要。 网页设计人员不能直接参与网页开发。 嵌在代码中的 HTML 很难看。 对于 Web，需要修改标准的 MVC 形式。图1显示了 MVC 的 Web 改写版，通常也称为 MVC Model 2 或 MVC 2。图1. MVC Model 2Str
42、uts，MVC 2 的一种实现Struts 是一组相互协作的类、servlet 和 JSP 标记，它们组成一个可重用的 MVC 2 设计。这个定义表示 Struts 是一个框架，而不是一个库，但 Struts 也包含了丰富的标记库和独立于该框架工作的实用程序类。图2显示了 Struts 的一个概览。图2. Struts 概览Struts 概览Client browser（客户浏览器）来自客户浏览器的每个 HTTP 请求创建一个事件。Web 容器将用一个 HTTP 响应作出响应。Controller（控制器）控制器接收来自浏览器的请求，并决定将这个请求发往何处。就 Struts 而言，控制器是以
43、 servlet 实现的一个命令设计模式。 struts-config.xml 文件配置控制器。业务逻辑业务逻辑更新模型的状态，并帮助控制应用程序的流程。就 Struts 而言，这是通过作为实际业务逻辑“瘦”包装的 Action 类完成的。Model（模型）的状态模型表示应用程序的状态。业务对象更新应用程序的状态。ActionForm bean 在会话级或请求级表示模型的状态，而不是在持久级。JSP 文件使用 JSP 标记读取来自 ActionForm bean 的信息。View（视图）视图就是一个 JSP 文件。其中没有流程逻辑，没有业务逻辑，也没有模型信息 - 只有标记。标记是使 Stru
44、ts 有别于其他框架（如 Velocity）的因素之一。详细分析 Struts图3示的是 org.apache.struts.action 包的一个最简 UML 图。图3显示了 ActionServlet (Controller)、 ActionForm (Form State) 和 Action (Model Wrapper) 之间的最简关系。图3. Command (ActionServlet) 与 Model (Action & ActionForm) 之间的关系的 UML 图ActionServlet 类 您还记得函数映射的日子吗？在那时，您会将某些输入事件映射到一个函数指针上。如果您
45、对此比较熟悉，您会将配置信息放入一个文件，并在运行时加载这个文件。函数指针数组曾经是用 C 语言进行结构化编程的很好方法。现在好多了，我们有了 Java 技术、XML、J2EE，等等。Struts 的控制器是将事件（事件通常是 HTTP post）映射到类的一个 servlet。正如您所料 - 控制器使用配置文件以使您不必对这些值进行硬编码。时代变了，但方法依旧。ActionServlet 是该 MVC 实现的 Command 部分，它是这一框架的核心。 ActionServlet (Command) 创建并使用 Action 、 ActionForm 和 ActionForward 。如前所
46、述， struts-config.xml 文件配置该 Command。在创建 Web 项目时，您将扩展 Action 和 ActionForm 来解决特定的问题。文件 struts-config.xml 指示 ActionServlet 如何使用这些扩展的类。这种方法有几个优点： 应用程序的整个逻辑流程都存储在一个分层的文本文件中。这使得人们更容易查看和理解它，尤其是对于大型应用程序而言。 网页设计人员不必费力地阅读 Java 代码来理解应用程序的流程。 Java 开发人员也不必在更改流程以后重新编译代码。 可以通过扩展 ActionServlet 来添加 Command 功能。 Action
47、Form 类 ActionForm 维护 Web 应用程序的会话状态。 ActionForm 是一个抽象类，必须为每个输入表单模型创建该类的子类。当我说 输入表单模型 时,是指 ActionForm 表示的是由 HTML 表单设置或更新的一般意义上的数据。例如，您可能有一个由 HTML 表单设置的 UserActionForm 。Struts 框架将执行以下操作： 检查 UserActionForm 是否存在；如果不存在，它将创建该类的一个实例。 Struts 将使用 HttpServletRequest 中相应的域设置 UserActionForm 的状态。没有太多讨厌的 request.g
48、etParameter() 调用。例如，Struts 框架将从请求流中提取 fname ，并调用 UserActionForm.setFname() 。 Struts 框架在将 UserActionForm 传递给业务包装 UserAction 之前将更新它的状态。 在将它传递给 Action 类之前，Struts 还会对 UserActionForm 调用 validation() 方法进行表单状态验证。 注： 这并不总是明智之举。别的网页或业务可能使用 UserActionForm ，在这些地方，验证可能有所不同。在 UserAction 类中进行状态验证可能更好。 可在会话级维护 UserActionForm 。 struts-config.xml 文件控制 HTML 表单请求与 ActionForm 之间的映射关系。 可将多个请求映射到 UserActionForm 。 UserActionForm 可跨多页进行映射，以执行诸如向导之类的操作。 Action 类 Action 类是业务逻辑的一个包装。 Action 类的用途是将 HttpServletRequest 转换为业务逻辑。要使用 Action ，请创建它的子类并覆盖 process() 方法。 ActionServlet (Command) 使用 perform(