黑马程序员---Servlet之间的跳转方式

来源：互联网发布：淘宝鹊桥app 编辑：程序博客网时间：2024/05/16 19:17

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现在的MVC框架中都使用了Servlet跳转。MVC框架把程序分成三个独立模块：业务处理系统（Model），试图模块(View)，控制模块（Control）。在Struts框架中这三部分分别为三个Servlet，程序在3个Servlet之间跳转。

转向Forward

转向是通过RequestDispatcher对象的forward(HttpServletRequest req,HttpServletResponse resp)方法实现的。The pathname specified may be relative, although it cannot extend outside the current servlet context. If the path begins with a "/" it is interpreted as relative to the current context root.（虽然它不能包含当前servlet项目以为的的资源，但是路径名可以被定义为相对路径。如果路径以‘/’开始，表示本应用程序的根目录。）代码如下：

RequestDispatcher dispatcher =req.getRequestDispacher（"/servlet/LifeCycleServlet"）;

dispatcher.forward(request,response);

ForwadServlet代码

package com.helloweenvsfei.servlet;

import java.io.IOException;
import java.util.Date;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ForwardServlet extends HttpServlet {

private static final long serialVersionUID = -291840563095094360L;

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

  String destination = request.getParameter("destination");

  // 跳转到 /WEB-INF/web.xml。通过地址栏输入网址是不能访问到该文件的，但是 forward 可以
  if("file".equals(destination)){
   RequestDispatcher dispatcher = request.getRequestDispatcher("/WEB-INF/web.xml");
   dispatcher.forward(request, response);
  }
  // 跳转到 /forward.jsp
  else if("jsp".equals(destination)){
   // 通过 setAttribute 方法传递一个 Date 对象给 JSP 页面
   Date date = new Date();
   request.setAttribute("date", date);
   RequestDispatcher dispatcher = request.getRequestDispatcher("/forward.jsp");
   dispatcher.forward(request, response);
  }
  // 跳转到另一个 Servlet
  else if("servlet".equals(destination)){
   RequestDispatcher dispatcher = request.getRequestDispatcher("/servlet/LifeCycleServlet");
   dispatcher.forward(request, response);
  }
  else{
   response.setCharacterEncoding("UTF-8");
   response.setContentType("text/html; charset=UTF-8");
   response.getWriter().println("缺少参数。用法：" + request.getRequestURL() + "?destination=jsp 或者 file 或者 servlet ");
  }

}
public void doPut(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
  this.doGet(request, response);
}

}

本程序用到一个jsp文件，forward.jsp代码如下：

<%@ page language="java" contentType="text/html; charset=UTF-8"%>
<jsp:directive.page import="java.util.Date" />
<jsp:directive.page import="java.text.SimpleDateFormat" />
<%
Date date = (Date) request.getAttribute("date");
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML>
<HEAD>
  <TITLE>Forward 跳转</TITLE>
  <link rel='stylesheet' type='text/css'
   href='<%=request.getContextPath()%>/css/style.css'>
</HEAD>
<BODY>
  <div align='center'>
   <br />
   <fieldset style=width:90%>
    <legend>
     Forward 跳转
    </legend>
    <br />
    <div style='line'>
     <div class='leftDiv' style='width:250px; '>
      从 ForwardServlet 中取到的 Date 为
     </div>
     <div align='left' class='rightDiv'>
      <%=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS")
       .format(date)%>
     </div>
    </div>
    <br />
    <div style='line'>
     <div align="center">
      <input type='button'
       onclick='location="<%=request.getContextPath()%>/servlet/ForwardServlet?destination=servlet"; '
       onmouseover="window.status='<%=request.getContextPath()%>/servlet/ForwardServlet?destination=servlet'"
       value='跳转到 Servlet' class=button>
      <input type='button'
       onclick='location="<%=request.getContextPath()%>/servlet/ForwardServlet?destination=file"; '
       onmouseover="window.status='<%=request.getContextPath()%>/servlet/ForwardServlet?destination=file'"
       value='跳转到 web.xml' class=button>
      <input type='button'
       onclick='location="<%=request.getContextPath()%>/servlet/ForwardServlet?destination=jsp"; '
       onmouseover="window.status='<%=request.getContextPath()%>/servlet/ForwardServlet?destination=jsp'"
       value='跳转到 JSP' class=button>
     </div>
    </div>
</BODY>
</HTML>
当使用forward形式跳转Servlet是，地址栏会显示跳转前的Servlet访问地址。因为该跳转是在服务器端实现的，客户端浏览器并不知道该跳转的动作。也就是说forward跳转对于客户端浏览器是透明的。

在Struts 、webWork的MVC框架中，都是用Servlet来处理用户的请求，把结果通过request.setAttribute放到request中，然后forward到JSP当中。

Redirect重定向

重定向是利用服务器返回的状态码实现的。客户端浏览器请求服务器的时候，服务器端会返回一个状态码。服务器端通过HttpServletResponse的setStatus（int status）方法设置状态码。

利用Servlet统计下载文件的下载次数。

package com.helloweenvsfei.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class RedirectServlet extends HttpServlet {

private static final long serialVersionUID = 4442189888545647793L;

Map<String, Integer> map = new HashMap<String, Integer>();

@Override
public void init() throws ServletException {
  map.put("/download/setup.exe", 0);
  map.put("/download/application.zip", 0);
  map.put("/download/01.mp3", 0);
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

  String filename = request.getParameter("filename");

  if(filename != null){
   // 取下载次数
   int hit = map.get(filename);
   // 下载次数 + 1 后保存
   map.put(filename, ++ hit);
   // 重定向到文件
   response.sendRedirect(request.getContextPath() + filename);
  }
  else{
   response.setCharacterEncoding("UTF-8");
   response.setContentType("text/html");
   PrintWriter out = response.getWriter();
   out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
   out.println("<HTML>");
   out.println(" <HEAD><TITLE>文件下载</TITLE></HEAD>");
   out.println(" <link rel='stylesheet' type='text/css' href='../css/style.css'>");
   out.println(" <BODY><br/>");

   out.println("<fieldset align=center style=width:90%><legend>文件下载</legend>");
   out.println("<table width=100%>");
   out.println(" <tr>");
   out.println("  <td><b>文件名</b></td>");
   out.println("  <td><b>下载次数</b></td>");
   out.println("  <td><b>下载</b></td>");
   out.println(" </tr>");

   for(Entry<String, Integer> entry : map.entrySet()){
    out.println("<tr>");
    out.println(" <td>" + entry.getKey() + "</td>");
    out.println(" <td>" + entry.getValue() + "</td>");
    out.println(" <td><a href='" + request.getRequestURI() + "?filename=" + entry.getKey() + "' target='_blank' onclick='location=location; '>下载</a></td>");
    out.println("</tr>");
   }

   out.println("</table>");
   out.println(" </legend>");
   out.println(" </BODY>");
   out.println("</HTML>");
   out.flush();
   out.close();
  }

}

@Override
public void destroy() {
map = null;
}

}

当使用Redirect跳转Servlet时，跳转在客户端实现。也就是说客户端浏览器实际上请求了两次服务器，第一次获取重定向状态吗与重定向的网址，第2次访问真实地址。

自动刷新Refresh

response.setHeader("Refresh","1000;URL=http://localhost:8080/servlet/exapmle.html").

某些情况下自动刷新很有用处。例如实时显示股市走势，或定时检查是否有新邮件到达。

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