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：

<%@ 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：

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" 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);   
  %>