<%@ page contentType="image/jpeg" pageEncoding="gb2312"
import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*"%>
<%!Color getRandColor(int fc, int bc) {//给定范围获得随机颜色
    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);
}%>
<%
//设置页面不缓存
response.setHeader("Pragma", "No-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 0);

// 在内存中创建图象
int width = 60, height = 20;
BufferedImage image = new BufferedImage(width, height,
     BufferedImage.TYPE_INT_RGB);

// 获取图形上下文
Graphics g = image.getGraphics();

//生成随机类
Random random = new Random();

// 设定背景色
g.setColor(getRandColor(200, 250));
g.fillRect(0, 0, width, height);

//设定字体
g.setFont(new Font("Times New Roman", Font.PLAIN, 18));

//画边框
//g.setColor(new Color());
//g.drawRect(0,0,width-1,height-1);

// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
g.setColor(getRandColor(160, 200));
for (int i = 0; i < 155; i++) {
    int x = random.nextInt(width);
    int y = random.nextInt(height);
    int xl = random.nextInt(12);
    int yl = random.nextInt(12);
    g.drawLine(x, y, x + xl, y + yl);
}

// 取随机产生的认证码(4位数字)
String sRand = "";
for (int i = 0; i < 4; i++) {
    String rand = String.valueOf(random.nextInt(10));
    sRand += rand;
    // 将认证码显示到图象中
    g.setColor(new Color(20 + random.nextInt(110), 20 + random
    .nextInt(110), 20 + random.nextInt(110)));//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
    g.drawString(rand, 13 * i + 6, 16);
}

// 将认证码存入SESSION
session.setAttribute("rand", sRand);

// 图象生效
g.dispose();

// 输出图象到页面
ImageIO.write(image, "JPEG", response.getOutputStream());

//加上下面两条代码,运行时才不会出现java.lang.IllegalStateException: getOutputStream() has already been called ..........等异常
out.clear();
out = pageContext.pushBody();
%>

testimage.jsp

<%@ page contentType="text/html;charset=gb2312"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
    <title>认证码输入页面</title>
    <meta http-equiv="Content-Type" content="text/html; charset=gb2312">
    <meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">
</head>
<body>
    <form method=post action="check.jsp">
     <table>
      <tr>
       <td align=left>
        系统产生的认证码：
       </td>
       <td>
        <img border=0 src="image.jsp">
       </td>
      </tr>
      <tr>
       <td align=left>
        输入上面的认证码：
       </td>
       <td>
        <input type=text name=rand maxlength=4 value="">
       </td>
      </tr>
      <tr>
       <td colspan=2 align=center>
        <input type=submit value="提交检测">
       </td>
      </tr>
     </table>
    </form>
</body>
</html>

check.jsp

<%@ page contentType="text/html; charset=gb2312" language="java"
import="java.sql.*" errorPage=""%>
<html>
<head>
    <title>认证码验证页面</title>
    <meta http-equiv="Content-Type" content="text/html; charset=gb2312">
    <meta http-equiv="Pragma" content="no-cache">
    <meta http-equiv="Cache-Control" content="no-cache">
    <meta http-equiv="Expires" content="0">
</head>
<body>
    <%
     String rand = (String) session.getAttribute("rand");
     String input = request.getParameter("rand");
    %>
    系统产生的认证码为：
    <%=rand%>
    <br>
    您输入的认证码为：
    <%=input%>
    <br>
    <br>
    <%
    if (rand.equals(input)) {
    %>
    <font color=green>输入相同，认证成功！</font>
    <%
    } else {
    %>
    <font color=red>输入不同，认证失败！</font>
    <%
    }
    %>
</body>
</html>