1.request.getRequestDispatcher()是請求轉發，前后頁面共享一個request ;
response.sendRedirect()是重新定向，前后頁面不是一個request。

2.RequestDispatcher.forward()是在服務器端運行;
HttpServletResponse.sendRedirect()是通過向客戶瀏覽器發送命令來完成.
所以RequestDispatcher.forward()對于瀏覽器來說是“透明的”；
而HttpServletResponse.sendRedirect()則不是。

3.ServletContext.getRequestDispatcher(String url)中的url只能使用絕對路徑； 而ServletRequest.getRequestDispatcher(String url)中的url可以使用相對路徑。因為ServletRequest具有相對路徑的概念；而ServletContext對象無次概念。

RequestDispatcher對象從客戶端獲取請求request，并把它們傳遞給服務器上的servlet,html或jsp。它有兩個方法:

1.void forward(ServletRequest request,ServletResponse response)

用來傳遞request的，可以一個Servlet接收request請求，另一個Servlet用這個request請 求來產生response。request傳遞的請求，response是客戶端返回的信息。forward要在response到達客戶端之前調用，也 就是 before response body output has been flushed。如果不是的話，它會報出異常。

2.void include(ServletRequest request,ServletResponse response)

用來記錄保留request和response，以后不能再修改response里表示狀態的信息。

如果需要把請求轉移到另外一個Web App中的某個地址，可以按下面的做法：
1. 獲得另外一個Web App的ServletConext對象(currentServletContext.getContext(uripath)).

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

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

代碼實例：
index.jsp：
復制代碼 代碼如下:
<%@ 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">
  用戶名:<input type="text" name="username" />

  密碼:<input type="password" name="password" />

  <input type="submit" />
  </form>

session.Java：
復制代碼 代碼如下:
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");
        */
                //這個路徑必須是這樣寫，而不能像上面的request.getRequestDispatcher那樣使用相對路徑
                //  而且要是使用response.sendRedirect的話在下面的session.jsp中不能通過request.getAttribute來獲取request對象
                //因為前后使用的不是同一個request，但是session可以，因為session會一直存在直到用戶關閉瀏覽器
    }

    /**
     * Initialization of the servlet.

     *
     * @throws ServletException if an error occurs
     */
    public void init() throws ServletException {
        // Put your code here
    }

}

getsession.jsp：

復制代碼 代碼如下:
<%@ 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的話就不能獲取到name和pwd  
 out.println("name " + name + "pwd "+ pwd);   
  %>

jsp技術：jsp跳轉getRequestDispatcher()和sendRedirect()的區別，轉載需保留來源！

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