Servlet是如何實現MVC的
Servlet是一種服務器端的編程語言，是J2EE中比較關鍵的組成部分，Servlet技術的推出，擴展了Java語言在服務器端開發的功能，鞏固了Java語言在服務器端開發中的地位，而且現在使用非常廣泛的JSP技術也是基于Servlet的原理.
JSP+JavaBeans+Servlet成為實現MVC模式的一種有效的選擇。
如果我們要實現一個對用戶的增刪改查，并且要求符合對擴展開發，對修改關閉的原則，該怎么做呢？
首先，這是我們的類圖以及類與類之間調用的時序圖。
根據UML圖來實現代碼：
TestServlet類
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("請求失敗"); } 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); }}</pre>
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;}</pre>
添加類，這里就只寫添加類了，其他類一樣
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"); //調用業務邏輯 UserManager userManager = new UserManager(); userManager.add(username); return "/add_success.jsp"; }}</pre>
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>這里我們發現，在testAction類中，用到了大量的if和else，而當我們需要擴展一個上傳類的時候，就需要修改if/else，所以，這就不符合對擴展開發，對修改關閉原則，所以，我們需要把這部分提取出來，配置到配置文件里，用反射進行配置。
我們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>需要獲取actionMapping對象
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); // 如果是刪除ActionMapping存儲如下： actionMapping{ path ="/servlet/delUser"; type ="com.bjpowernode.servlet.DelUserAction" forwardMap{ key ="success",value ="/del_success.jsp" key ="error",value"/del_error.jsp" } }最后根據獲取的對象采用反射來動態實例化Action，達到我們的目的。
String path = "/servlet/delUser"; // 根據截取的URL請求，到Map中取得本次請求對應的Action ActionMapping actionMapping =(ActionMapping)map.get(path); //取得本次請求對應的Action類的完整路徑 String type = actionMapping.getType(); //com.bjpowernode.servlet.DelUserAction // 采用反射動態實例化Action Action action = (Action)class.forName(type).newInstance(); //動態待用Action中的execute方法 String forward = action.execute(request.response); //根據路徑完成轉向 request.getRequestDispatcher(forward).forward(request, response);這樣，我們基本就是實現了對擴展開發，對修改封閉的原則，但是，這僅僅是一個功能我們就需要做這么多事情，寫這么多代碼，對程序員來說這是一項不小的工作量，那么我們有沒有其他的辦法讓我們的工作量小一點呢？