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：
<%@ 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" rel="nofollow noopener noreferrer" 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：
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：
<%@ 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" rel="nofollow noopener noreferrer" 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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