java验证码问题

AuthImageServlet.java

package com.util.servlet;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Random;

import javax.imageio.ImageIO;
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 AuthImageServlet extends HttpServlet {

/**
  * Constructor of the object.
  */
  private static final String CONTENT_TYPE = "text/html; charset=gb2312";  
     //设置字母的大小,大小  
     private Font mFont = new Font("Times New Roman", Font.PLAIN, 17);  
     public void init() throws ServletException  
     {  
         super.init();  
     }  
     Color getRandColor(int fc,int bc)  
     {  
         Random random = new Random();  
         if(fc>255) fc=255;  
         if(bc>255) bc=255;  
         int r=fc+random.nextInt(bc-fc);  
         int g=fc+random.nextInt(bc-fc);  
         int b=fc+random.nextInt(bc-fc);  
         return new Color(r,g,b);  
     }  
  
     public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException  
     {  
         response.setHeader("Pragma","No-cache");  
         response.setHeader("Cache-Control","no-cache");  
         response.setDateHeader("Expires", 0);  
         //表明生成的响应是图片  
         response.setContentType("image/jpeg");  
           
         int width=100, height=18;  
         BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);  
           
         Graphics g = image.getGraphics();  
         Random random = new Random();  
         g.setColor(getRandColor(200,250));  
         g.fillRect(1, 1, width-1, height-1);  
         g.setColor(new Color(102,102,102));  
         g.drawRect(0, 0, width-1, height-1);  
         g.setFont(mFont);  
  
         g.setColor(getRandColor(160,200));  
  
         //画随机线  
         for (int i=0;i<155;i++)  
         {  
             int x = random.nextInt(width - 1);  
             int y = random.nextInt(height - 1);  
             int xl = random.nextInt(6) + 1;  
             int yl = random.nextInt(12) + 1;  
             g.drawLine(x,y,x + xl,y + yl);  
         }  
  
         //从另一方向画随机线  
         for (int i = 0;i < 70;i++)  
         {  
             int x = random.nextInt(width - 1);  
             int y = random.nextInt(height - 1);  
             int xl = random.nextInt(12) + 1;  
             int yl = random.nextInt(6) + 1;  
             g.drawLine(x,y,x - xl,y - yl);  
         }  
  
         //生成随机数,并将随机数字转换为字母  
         String codeList = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";
         String sRand="";    
         for (int i=0;i<6;i++){
            int a=random.nextInt(codeList.length()-1);
            String rand=codeList.substring(a,a+1);
            sRand+=rand;

g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));    
               g.drawString(rand,15*i+10,16);    
         }
                
         System.out.println("-----------------------------------"+sRand);
         HttpSession session = request.getSession(true); 
         session.setAttribute("rand",sRand);  
         g.dispose(); 
           OutputStream os =response.getOutputStream();
        ImageIO.write(image, "JPEG", os);
     }

}

CheckServlet.java

package com.util.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.mail.Session;
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 CheckServlet extends HttpServlet {

/**
  * Constructor of the object.
  */
 public CheckServlet() {
  super();
 }

/**
  * Destruction of the servlet. <br>
  */
 public void destroy() {
  super.destroy(); // Just puts "destroy" string in log
  // Put your code here
 }

public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {

this.doPost(request, response);
 }

public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {

request.setCharacterEncoding("utf-8");
  String yzm=request.getParameter("yzm");
  HttpSession session=request.getSession(true);
  String xt=session.getAttribute("rand").toString();
  if(yzm.equals(xt)){
   System.out.println("验证码相同");
  }else{
   System.out.println("验证码不同11111111111111111111");
  }
  System.out.println("yzm："+yzm);
  System.out.println("XT："+xt);
  //string account = context.Session["rand"].ToString();
  //String user = session.getAttribute("rand");
 }

/**
  * Initialization of the servlet. <br>
  *
  * @throws ServletException if an error occurs
  */
 public void init() throws ServletException {
  // Put your code here
 }

}

index.jsp

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
   
    <title>My JSP 'index.jsp' starting page</title>
 <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">
 -->
 <script type="text/javascript">
 function changeCode() {
  var imgNode = document.getElementById("vimg");
  imgNode.src="servlet/AuthImageServlet?"+Math.random();
 }
 </script>
  </head>
 
  <body>
 
   <form action="servlet/CheckServlet" method="post">
    <label>输入验证码</label><br>
    <input type="text" name="yzm"/><img alt="yzm" id="vimg" title="点击更换" onclick="changeCode()"
     src="servlet/AuthImageServlet"><br>
     <input type="submit" name="submit"/>
   </form>
    This is my JSP page. <br>
  </body>
</html>