Servlet是如何实现MVC的？

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

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

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

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

  <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

<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

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。

 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？