JSP中验证码问题

以下代码随机产生四位数字的验证码：
- 1）生成验证码的jsp代码：

<%@ page language="java" import="java.awt.*" contentType="image/jpeg"
    import="java.awt.image.BufferedImage" import="java.util.*"
    import="javax.imageio.*" pageEncoding="UTF-8"%>
<%
    response.setHeader("Cache-Control", "no-cache");
    // 在内存中创建图象 
    int width = 60, height = 20;
    BufferedImage image = new BufferedImage(width, height,
            BufferedImage.TYPE_INT_RGB);
    //获取画笔
    Graphics g = image.getGraphics();
    //设定背景色 
    g.setColor(new Color(200, 200, 200));
    g.fillRect(0, 0, width, height);
    //取随机产生的验证码(4位数字) 
    Random rnd = new Random();
    int randNum = rnd.nextInt(8999) + 1000;
    String randStr = String.valueOf(randNum);
    //将验证码存入session
    session.setAttribute("randStr", randStr);
    //将验证码显示到图象中 
    g.setColor(Color.black);
    g.setFont(new Font("", Font.PLAIN, 20));
    g.drawString(randStr, 10, 17);
    // 随机产生100个干扰点，使图象中的验证码不易被其他程序探测到 
    for (int i = 0; i < 100; i++) {
        int x = rnd.nextInt(width);
        int y = rnd.nextInt(height);
        g.drawOval(x, y, 1, 1);
    }
    // 输出图象到页面 
    ImageIO.write(image, "JPEG",response.getOutputStream());

    out.clear();
    out = pageContext.pushBody(); 
%>

登录界面显示验证码，点击看不清，换一个更换验证码

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>登录</title>
</head>
<body>
    <script language="javascript" type="text/javascript">
        function refresh(imgUrl){
            var getImgCode=document.getElementById("codeImg");
            getImgCode.src=imgUrl+"?id="+Math.random();
        }
    </script>
    欢迎登陆本系统！<br>
    <form name="loginForm" action="login.jsp" method="post">
        账号：<input type="text" name="account"/><br>
        密码：<input type="password" name="password" /><br>
        验证码：<input type="text" name="code" size="10" >
        <img id="codeImg" name="codeImg" alt="验证码" src="validate.jsp" >
        <a href="javascript:refresh('validate.jsp')">看不清，换一个</a><br>
        <input type="submit" value="登录" >
    </form>
</body>
</html>

效果图:
这里写图片描述