Servlet是如何实现MVC的

转：http://blog.csdn.net/laner0515/article/details/11266673

 

Servlet是一种服务器端的编程语言，是J2EE中比较关键的组成部分，Servlet技术的推出，扩展了Java语言在服务器端开发的功能，巩固了Java语言在服务器端开发中的地位，而且现在使用非常广泛的JSP技术也是基于Servlet的原理.

JSP+JavaBeans+Servlet成为实现MVC模式的一种有效的选择。

如果我们要实现一个对用户的增删改查，并且要求符合对扩展开发，对修改关闭的原则，该怎么做呢？

首先，这是我们的类图以及类与类之间调用的时序图。

根据UML图来实现代码：

TestServlet类

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1. package com.bjpowernode.servlet;  
2.   
3. import java.io.IOException;  
4. import java.util.List;  
5.   
6. import javax.servlet.ServletException;  
7. import javax.servlet.http.HttpServlet;  
8. import javax.servlet.http.HttpServletRequest;  
9. import javax.servlet.http.HttpServletResponse;  
10.   
11. public class TestServlet extends HttpServlet {  
12.   
13.       
14.     protected void doGet(HttpServletRequest request, HttpServletResponse response)  
15.             throws ServletException, IOException {  
16.         String requestURI = request.getRequestURI();  
17.         System.out.println("requestURI=" + requestURI);  
18.         String path = requestURI.substring(requestURI.indexOf("/",1),requestURI.indexOf("."));  
19.         System.out.println("path = " +path);  
20.           
21.         String username = request.getParameter("username");  
22.           
23.         Action action = null;  
24.         if("/servlet/delUser".equals(path)){  
25.             action = new DelUserAction();  
26.         }else if("/servlet/addUser".equals(path)){  
27.             action = new AddUserAction();  
28.         }else if("/servlet/modifyUser".equals(path)){  
29.             action = new ModifyAction();  
30.         }else if("/servlet/queryUser".equals(path)){  
31.             action = new QueryUserAction();  
32.         }else{  
33.             throw new RuntimeException("请求失败");  
34.         }  
35.         String forward ="";  
36.         try {  
37.             forward= action.execute(request, response);  
38.         } catch (Exception e) {  
39.             e.printStackTrace();  
40.         }  
41.         request.getRequestDispatcher(forward).forward(request, response);  
42.           
43.   
44.     }  
45.   
46.   
47.     protected void doPost(HttpServletRequest request, HttpServletResponse response)  
48.             throws ServletException, IOException {  
49.         doGet(request,response);  
50.     }  
51.   
52. }  

package com.bjpowernode.servlet;import java.io.IOException;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class TestServlet extends HttpServlet {protected void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {String requestURI = request.getRequestURI();System.out.println("requestURI=" + requestURI);String path = requestURI.substring(requestURI.indexOf("/",1),requestURI.indexOf("."));System.out.println("path = " +path);String username = request.getParameter("username");Action action = null;if("/servlet/delUser".equals(path)){action = new DelUserAction();}else if("/servlet/addUser".equals(path)){action = new AddUserAction();}else if("/servlet/modifyUser".equals(path)){action = new ModifyAction();}else if("/servlet/queryUser".equals(path)){action = new QueryUserAction();}else{throw new RuntimeException("请求失败");}String forward ="";try {forward= action.execute(request, response);} catch (Exception e) {e.printStackTrace();}request.getRequestDispatcher(forward).forward(request, response);}protected void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request,response);}}

抽象接口Action

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1. package com.bjpowernode.servlet;  
2.   
3. import javax.servlet.http.HttpServletRequest;  
4. import javax.servlet.http.HttpServletResponse;  
5.   
6. public interface Action {  
7.   
8.     public String execute(HttpServletRequest request, HttpServletResponse response)  
9.     throws Exception;  
10.       
11. }  

package com.bjpowernode.servlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public interface Action {public String execute(HttpServletRequest request, HttpServletResponse response)throws Exception;}

添加类，这里就只写添加类了，其他类一样

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1. package com.bjpowernode.servlet;  
2.   
3. import javax.servlet.http.HttpServletRequest;  
4. import javax.servlet.http.HttpServletResponse;  
5.   
6. public class AddUserAction implements Action {  
7.   
8.     public String execute(HttpServletRequest request,  
9.             HttpServletResponse response) throws Exception {  
10.         String username = request.getParameter("username");  
11.         //int age = Integer.parseInt(request.getParameter("username"));  
12.         //String sex = request.getParameter("sex");  
13.           
14.         //调用业务逻辑  
15.         UserManager userManager = new UserManager();  
16.         userManager.add(username);  
17.           
18.         return "/add_success.jsp";  
19.     }  
20.   
21. }  

package com.bjpowernode.servlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class AddUserAction implements Action {public String execute(HttpServletRequest request,HttpServletResponse response) throws Exception {String username = request.getParameter("username");//int age = Integer.parseInt(request.getParameter("username"));//String sex = request.getParameter("sex");//调用业务逻辑UserManager userManager = new UserManager();userManager.add(username);return "/add_success.jsp";}}

Web.xml配置

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1. <welcome-file-list>  
2.     <welcome-file>index.jsp</welcome-file>  
3.   </welcome-file-list>  
4.     
5.   <servlet>  
6.     <servlet-name>TestServlet</servlet-name>  
7.     <servlet-class>com.bjpowernode.servlet.TestServlet</servlet-class>  
8.   </servlet>  
9.   <servlet-mapping>  
10.      <servlet-name>TestServlet</servlet-name>  
11.      <!--  
12.      <url-pattern>/servlet/TestServlet</url-pattern> 
13.       -->  
14.       <url-pattern>*.do</url-pattern>  
15.   </servlet-mapping>  

<welcome-file-list>    <welcome-file>index.jsp</welcome-file>  </welcome-file-list>    <servlet>  <servlet-name>TestServlet</servlet-name>  <servlet-class>com.bjpowernode.servlet.TestServlet</servlet-class>  </servlet>  <servlet-mapping>     <servlet-name>TestServlet</servlet-name>     <!--      <url-pattern>/servlet/TestServlet</url-pattern>      -->      <url-pattern>*.do</url-pattern>  </servlet-mapping>

 

这里我们发现，在testAction类中，用到了大量的if和else，而当我们需要扩展一个上传类的时候，就需要修改if/else，所以，这就不符合对扩展开发，对修改关闭原则，所以，我们需要把这部分提取出来，配置到配置文件里，用反射进行配置。

 

我们需要先配置配置文件

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1. <action-config>  
2.         <action path ="/servlet/delUser" type = "com.bjpowernode.servlet.DelUserAction">  
3.             <forward name = "success">del_success.jsp</forward>  
4.             <forward name = "error">del_error.jsp</forward>  
5.         </action>   
6.         <action path ="/servlet/addUser" type = "com.bjpowernode.servlet.AddUserAction">  
7.             <forward name = "success">add_success.jsp</forward>  
8.             <forward name = "error">add_error.jsp</forward>  
9.         </action>   
10.         <action path ="/servlet/modifyUser" type = "com.bjpowernode.servlet.ModifyAction">  
11.             <forward name = "success">modify_success.jsp</forward>  
12.             <forward name = "error">modify_error.jsp</forward>  
13.         </action>   
14.         <action path ="/servlet/queryUser" type = "com.bjpowernode.servlet.QueryUserAction">  
15.             <forward name = "success">query_success.jsp</forward>  
16.             <forward name = "error">query_error.jsp</forward>  
17.         </action>   
18.   </action-config>  

<action-config> <action path ="/servlet/delUser" type = "com.bjpowernode.servlet.DelUserAction">  <forward name = "success">del_success.jsp</forward>  <forward name = "error">del_error.jsp</forward>  </action>  <action path ="/servlet/addUser" type = "com.bjpowernode.servlet.AddUserAction">  <forward name = "success">add_success.jsp</forward>  <forward name = "error">add_error.jsp</forward>  </action>  <action path ="/servlet/modifyUser" type = "com.bjpowernode.servlet.ModifyAction">  <forward name = "success">modify_success.jsp</forward>  <forward name = "error">modify_error.jsp</forward>  </action>  <action path ="/servlet/queryUser" type = "com.bjpowernode.servlet.QueryUserAction">  <forward name = "success">query_success.jsp</forward>  <forward name = "error">query_error.jsp</forward>  </action>  </action-config>

需要获取actionMapping对象

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1. ActionMapping{  
2.     private String path;  
3.     private String type;  
4.    Map forwardMap;  
5.     
6.     
7.   }  
8.   forwardMap{  
9.     key = "success";  
10.     value ="/del_success.jsp"  
11.     key ="error"  
12.     value ="del_error.jsp";  
13.    
14.    
15.    
16.   }  
17.    
18.   Map map = new HashMap();  
19.   map.put("/servlet/delUser",actionMapping);  
20.   map.put("/servlet/addUser",actionMapping);  
21.   map.put("/servlet/modifyUser",actionMapping);  
22.   map.put("/servlet/queryUser",actionMapping);  
23.    
24.  // 如果是删除ActionMapping存储如下：  
25.   actionMapping{  
26.     path ="/servlet/delUser";  
27.    type ="com.bjpowernode.servlet.DelUserAction"  
28.     forwardMap{  
29.         key ="success",value ="/del_success.jsp"  
30.         key ="error",value"/del_error.jsp"  
31.     }  
32.   }  
33.    

  ActionMapping{  private String path;private String type;   Map forwardMap;      }  forwardMap{  key = "success";  value ="/del_success.jsp"  key ="error"  value ="del_error.jsp";     }   Map map = new HashMap();  map.put("/servlet/delUser",actionMapping);  map.put("/servlet/addUser",actionMapping);  map.put("/servlet/modifyUser",actionMapping);  map.put("/servlet/queryUser",actionMapping);  // 如果是删除ActionMapping存储如下：  actionMapping{  path ="/servlet/delUser";   type ="com.bjpowernode.servlet.DelUserAction"   forwardMap{   key ="success",value ="/del_success.jsp"   key ="error",value"/del_error.jsp"   }  } 

最后根据获取的对象采用反射来动态实例化Action，达到我们的目的。

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1.  String path = "/servlet/delUser";  
2.   
3. /  根据截取的URL请求，到Map中取得本次请求对应的Action  
4.     ActionMapping actionMapping =(ActionMapping)map.get(path);  
5.       
6. /取得本次请求对应的Action类的完整路径  
7.  String type = actionMapping.getType();  //com.bjpowernode.servlet.DelUserAction  
8.   
9. // 采用反射动态实例化Action  
10.  Action action  = (Action)class.forName(type).newInstance();  
11.   
12. /动态待用Action中的execute方法  
13.  String forward = action.execute(request.response);  
14.   
15.  //根据路径完成转向        
16. request.getRequestDispatcher(forward).forward(request, response);  

  String path = "/servlet/delUser"; //  根据截取的URL请求，到Map中取得本次请求对应的Action  ActionMapping actionMapping =(ActionMapping)map.get(path);  //取得本次请求对应的Action类的完整路径  String type = actionMapping.getType();  //com.bjpowernode.servlet.DelUserAction  // 采用反射动态实例化Action  Action action  = (Action)class.forName(type).newInstance(); //动态待用Action中的execute方法  String forward = action.execute(request.response);   //根据路径完成转向  request.getRequestDispatcher(forward).forward(request, response);

 

        这样，我们基本就是实现了对扩展开发，对修改封闭的原则，但是，这仅仅是一个功能我们就需要做这么多事情，写这么多代码，对程序员来说这是一项不小的工作量，那么我们有没有其他的办法让我们的工作量小一点呢？