请求重定向与请求转发

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用sendRedirect方法实现请求重定向

    .sendRedirect方法用于生成302响应码和Location响应头，从而通知客户端去重新访问Location响应头中指定的URL，其完整的定义语法如下：
public void sendRedirect(java.lang.String location)
  throws java.io.IOException
    .使用下面两条语句也能完成response.sendRedirect(url)语句所完成的功能：
response.setStatus(response.SC_MOVED_TEMPORARILY );
response.setHeader ("Location", url);
    .sendRedirect 方法不仅可以重定向到当前应用程序中的其他资源，它还可以重定向到同一个站点上的其他应用程序中的资源，甚至是使用绝对URL重定向到其他站点的资源。
   .如果传递给sendRedirect 方法的相对URL以“/”开头，则是相对于整个WEB站点的根目录，而不是相对于当前WEB应用程序的根目录。

请求重定向与请求转发的比较

.RequestDispatcher.forward方法只能将请求转发给同一个WEB应用中的组件；而HttpServletResponse.sendRedirect 方法还可以重定向到同一个站点上的其他应用程序中的资源，甚至是使用绝对URL重定向到其他站点的资源。

.如果传递给HttpServletResponse.sendRedirect 方法的相对URL以“/”开头，它是相对于整个WEB站点的根目录；如果创建RequestDispatcher对象时指定的相对URL以“/”开头，它是相对于当前WEB应用程序的根目录。

.调用HttpServletResponse.sendRedirect方法重定向的访问过程结束后，浏览器地址栏中显示的URL会发生改变，由初始的URL地址变成重定向的目标URL；调用RequestDispatcher.forward 方法的请求转发过程结束后，浏览器地址栏保持初始的URL地址不变。
.HttpServletResponse.sendRedirect方法对浏览器的请求直接作出响应，响应的结果就是告诉浏览器去重新发出对另外一个URL的访问请求；

.RequestDispatcher.forward方法在服务器端内部将请求转发给另外一个资源，浏览器只知道发出了请求并得到了响应结果，并不知道在服务器程序内部发生了转发行为。

.RequestDispatcher.forward方法的调用者与被调用者之间共享相同的request对象和response对象，它们属于同一个访问请求和响应过程；而HttpServletResponse.sendRedirect方法调用者与被调用者使用各自的request对象和response对象，它们属于两个独立的访问请求和响应过程。

.无论是RequestDispatcher.forward方法，还是HttpServletResponse.sendRedirect方法，在调用它们之前，都不能有内容已经被实际输出到了客户端。如果缓冲区中已经有了一些内容，这些内容将被从缓冲区中清除。

缺省Servlet缓存问题的模拟程序（1）

public class ReportServlet extends HttpServlet
{
private int count = 0;
public void service(HttpServletRequest request,
  HttpServletResponse response) throws ServletException, IOException
{
  response.setContentType("text/html;charset=UTF-8");

  //禁止浏览器缓存
  response.setDateHeader("Expires",0);
  response.setHeader("Cache-Control","no-cache");
  response.setHeader("Pragma","no-cache");

  String department = request.getParameter("dept");
  synchronized(this)
  {
   System.out.println(++count + ":" + department);
   String path = getServletContext().getRealPath("/WEB-INF/report.html");
   ReportTool rt = new ReportTool();
   rt.setCount(count);
   rt.setDepartment(department);
   rt.createReportHTML(path);
   RequestDispatcher rd = getServletContext().getRequestDispatcher(
      "/WEB-INF/report.html");
   rd.forward(request,response);
  }
}
}

缺省Servlet缓存问题的模拟程序（2）

public class ReportTool
{
private int count = 0;
private String department = "";
public void setCount(int count)
{
  this.count = count;
}
public void setDepartment(String department)
{
  this.department = department;
}
public void createReportHTML(String path) throws IOException
{
  FileOutputStream fos = new FileOutputStream(path);
  OutputStreamWriter ops = new OutputStreamWriter(fos,"UTF-8");
  ops.write(String.valueOf(count) + ":" + department + "的统计信息");
  ops.close();
}
}

例子：
SubmitTest.java

package com.hbsi.csdn;

import java.io.IOException;
import java.io.PrintWriter;

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

public class SubmitTest extends HttpServlet {

/**
* Constructor of the object.
*/
public SubmitTest() {
super();
}

/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}

/**
* The doGet method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to get.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

this.doPost(request, response);
}

/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  response.setDateHeader("Expires", 0);
  response.setHeader("Cache-Control", "no-Cache");
  response.setHeader("Pragma", "no-Cache");

  response.setContentType("text/html;charset=utf-8");
  PrintWriter out=response.getWriter();

  String username=request.getParameter("username");
  String pass=request.getParameter("password");
  String sex=request.getParameter("sex");

  String lover[]=request.getParameterValues("lover");
  String temp="";
  for(int i=0;i<lover.length;i++){
   temp=temp+lover[i];
  }


  out.println("用户名："+new String(username.getBytes("ISO8859-1"),"utf-8")+"<br>");
  out.println("密码："+pass+"<br>");
  out.println("性别："+sex+"<br>");
  out.println("爱好是:"+temp+"<br>");
  out.println("出生年月："+request.getParameter("year")+"年"+request.getParameter("month")+"月");
}

/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}

}

Submit.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Submit.html</title>

<meta http-equiv="Expires" content="0">
  <meta http-equiv="Cache-Control" content="no-cache">
  <meta http-equiv="Pragma" content="no-cache">
</head>