jsp跳转getRequestDispatcher()和sendRedirect()的区别

1.request.getRequestDispatcher()是请求转发，前后页面共享一个request ;
response.sendRedirect()是重新定向，前后页面不是一个request。

2.RequestDispatcher.forward()是在服务器端运行;
HttpServletResponse.sendRedirect()是通过向客户浏览器发送命令来完成.
所以RequestDispatcher.forward()对于浏览器来说是“透明的”；
而HttpServletResponse.sendRedirect()则不是。

3.ServletContext.getRequestDispatcher(String url)中的url只能使用绝对路径； 而ServletRequest.getRequestDispatcher(String url)中的url可以使用相对路径。因为ServletRequest具有相对路径的概念；而ServletContext对象无次概念。

RequestDispatcher对象从客户端获取请求request，并把它们传递给服务器上的servlet,html或jsp。它有两个方法:

1.void forward(ServletRequest request,ServletResponse response)

用来传递request的，可以一个Servlet接收request请求，另一个Servlet用这个request请 求来产生response。request传递的请求，response是客户端返回的信息。forward要在response到达客户端之前调用，也 就是 before response body output has been flushed。如果不是的话，它会报出异常。

2.void include(ServletRequest request,ServletResponse response)

用来记录保留request和response，以后不能再修改response里表示状态的信息。

如果需要把请求转移到另外一个Web App中的某个地址，可以按下面的做法：
1. 获得另外一个Web App的ServletConext对象(currentServletContext.getContext(uripath)).

2. 调用ServletContext.getRequestDispatcher(String url)方法。

eg:ServletContext.getRequestDispatcher(“smserror.jsp”).forward(request,response);

代码实例：
index.jsp：
代码如下：

<%@ page language=”java” import=”java.util.*” pageEncoding=”GBK”%>

<% String path = request.getContextPath(); String basePath = request.getScheme()+”://”+request.getServerName()+”:”+request.getServerPort()+path+”/”; %>

My JSP ‘index.jsp’ starting page

<meta http-equiv=”pragma” content=”no-cache” />

<meta http-equiv=”cache-control” content=”no-cache” />

<meta http-equiv=”expires” content=”0″ />

<meta http-equiv=”keywords” content=”keyword1,keyword2,keyword3″ />

<meta http-equiv=”description” content=”This is my page” />

    <!–

        <link rel=”stylesheet” type=”text/css” href=”styles.css”>

    –>

<form action=”servlet/session” method=”post”>

  用户名:<input type=”text” name=”username” />

  密码:<input type=”password” name=”password” />

  <input type=”submit” />

  </form>

session.java：
代码如下：

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.RequestDispatcher;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;

public class session extends HttpServlet {

    /**

     * Constructor of the object.

     */

    public session() {

        super();

    }

    /**

     * Destruction of the servlet.

     */

    public void destroy() {

        super.destroy(); // Just puts “destroy” string in log

        // Put your code here

    }

    /**

     * The doGet method of the servlet.

     *

     * 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 {

        doPost(request, response);

    }

    /**

     * The doPost method of the servlet.

     *

     * 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 {

        String username = “”;

        String password = “”;

        username = request.getParameter(“username”);

        password = request.getParameter(“password”);

        HttpSession session = request.getSession();

        session.setAttribute(“username”, username);

        session.setAttribute(“password”, password);

        request.setAttribute(“name”, username);

        request.setAttribute(“pwd”, password);

        RequestDispatcher dis = request.getRequestDispatcher(“/getsession.jsp”);

        dis.forward(request, response);

        /*

        response.sendRedirect(“http://localhost:8080/sessiontest/getsession.jsp”);

        */

                //这个路径必须是这样写，而不能像上面的request.getRequestDispatcher那样使用相对路径

                //  而且要是使用response.sendRedirect的话在下面的session.jsp中不能通过request.getAttribute来获取request对象

                //因为前后使用的不是同一个request，但是session可以，因为session会一直存在直到用户关闭浏览器

    }

    /**

     * Initialization of the servlet.

     *

     * @throws ServletException if an error occurs

     */

    public void init() throws ServletException {

        // Put your code here

    }

}

getsession.jsp：

代码如下：

<%@ page language=”java” import=”java.util.*” pageEncoding=”GBK”%>

<% String path = request.getContextPath(); String basePath = request.getScheme()+”://”+request.getServerName()+”:”+request.getServerPort()+path+”/”; %>

My JSP ‘getsession.jsp’ starting page

<meta http-equiv=”pragma” content=”no-cache” />

<meta http-equiv=”cache-control” content=”no-cache” />

<meta http-equiv=”expires” content=”0″ />

<meta http-equiv=”keywords” content=”keyword1,keyword2,keyword3″ />

<meta http-equiv=”description” content=”This is my page” />

    <!–

        <link rel=”stylesheet” type=”text/css” href=”styles.css”>

    –>

  <%   out.print(“”);   String username = (String)session.getAttribute(“username”);  

String password = (String)session.getAttribute(“password”);  

String name = (String)request.getAttribute(“name”);  

String pwd = (String)request.getAttribute(“pwd”); 

 out.println(“username ” + username + ” password ” +password);

 //如果上面是使用response.sendRedirect的话就不能获取到name和pwd  

 out.println(“name ” + name + “pwd “+ pwd);   

  %>

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