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？