请问为什么ArrayList xx=(ArrayList)session .getAttribute取得的值为空
他会直接跳转到bottom页面，请问为什么ArrayList xx=(ArrayList)session .getAttribute取得的值为空，
updatePassword的代码如下
```java
<%--
Document : updatePassword
Created on : 2012-3-20, 23:58:33
Author : Administrator
--%>
<%@page import="ch1.LoginBean"%>
<%@page import="java.util.ArrayList"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>JSP Page</title>
</head>
<body bgcolor="CCCFFF">
<hr noshade>
<div align="center">
<table border="0" cellspacing="0" cellpadding="0" width="100%" align="center">
<tr>
<td width="33%">
<a href="lookMessage.jsp">查看个人信息</a>
</td>
<td width="33%">
<a href="updateMessage.jsp">修改个人信息</a>
</td>
<td width="33%">
修改密码
</td>
</tr>
</table>
</div>
<hr noshade>
<br><br>
<form action="UpdatePasswordServlet" method="post">
<table border="2" cellspacing="0" cellpadding="0" bgcolor="CCCFFF" width="60%" align="center">
<%
ArrayList login=(ArrayList)session.getAttribute("login");
if(login==null||login.size()==0){
response.sendRedirect("bottom.jsp");
}else{
for(int i=login.size()-1;i>=0;i--){
LoginBean nn=(LoginBean)login.get(i);
%>
<tr>
<td height="30">用户密码</td>
<td><input type="password" name="password1" value="<%=nn.getPassword()%>"></td>
</tr>
<tr>
<td height="30">重复密码</td>
<td><input type="password" name="password2" value="<%=nn.getPassword()%>"></td>
</tr>
<tr>
<td colspan="2" align="center">
<input type="submit" value="确 定" size="12">
<input type="reset" value="清 除" size="12">
</td>
</tr>
<%
}
}
%>
</table>
</form>
</body>
</html>
```这是UpdatePasswordServlet的
```java
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package ch1;
import java.io.IOException;
import java.sql.*;
import java.util.ArrayList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.swing.JOptionPane;
import ch1.LoginBean;
/**
*
* @author Administrator
*/
public class UpdatePasswordServlet extends HttpServlet {
public void wrong1(){
String msg="不允许有空，修改失败！";
int type=JOptionPane.YES_NO_CANCEL_OPTION;
String title="消息提示";
JOptionPane.showMessageDialog(null, msg, title, type);
}
public void wrong2(){
String msg="两次密码不同，修改失败！";
int type=JOptionPane.YES_NO_CANCEL_OPTION;
String title="消息提示";
JOptionPane.showMessageDialog(null, msg, title, type);
}
public void right(){
String msg="填写信息合格，修改成功！";
int type=JOptionPane.YES_NO_CANCEL_OPTION;
String title="消息提示";
JOptionPane.showMessageDialog(null, msg, title, type);
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String password1=new String(request.getParameter("password1").getBytes("ISO-8859-1"),"UTF-8");
String password2=new String(request.getParameter("password2").getBytes("ISO-8859-1"),"UTF-8");
if(password1.length()==0||password2.length()==0){
wrong1();
response.sendRedirect("updatePassword.jsp");
}else if(!(password1.equals(password2))){
wrong2();
response.sendRedirect("updatePassword.jsp");
}else{
try{
Connection con=null;
Statement stmt=null;
ResultSet rs=null;
Class.forName("com.mysql.jdbc.Driver");
String url="jdbc:mysql://localhost:3306/person?useUnicode=true&characterEncoding=gbk";
con=DriverManager.getConnection(url,"root","123456");
stmt=con.createStatement();
String userName="";
HttpSession session=request.getSession();
ArrayList login=(ArrayList)session.getAttribute("login");
if(login==null||login.size()==0){
response.sendRedirect("login.jsp");
}else{
for(int i=login.size()-1;i>=0;i--){
LoginBean nn=(LoginBean)login.get(i);
userName=nn.getUserName();
}
}
String sql1="Update user set password='"+password1+"' where userName='"+userName+"'";
stmt.executeUpdate(sql1);
String sql2="select * from user where userName='"+userName+"'";
rs=stmt.executeQuery(sql2);
LoginBean nn=new LoginBean();
nn.setPassword(password1);
ArrayList wordlist=null;
wordlist=new ArrayList();
wordlist.add(nn);
session.setAttribute("login", login);
rs.close();
stmt.close();
con.close();
right();
response.sendRedirect("lookMessage.jsp");
}catch(Exception e){
e.printStackTrace();
}
}
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
这是LoginBean 的
package ch1;
public class LoginBean {
private String userName;
private String password;
public LoginBean(){
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
```
代码贴全