本篇文章由北京北大青鸟通州校区ACCP学术部提供：
1. random.jsp (产生四位的随机字符,由0-9,a-z,A-Z构成.并把最终字符串放到session中保存以让后续页面验证真伪)
代码如下
<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*;" pageEncoding="UTF-8" %>
<%!
private 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（北京北大青鸟）
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 = 100, height = 30;
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);
// 随机产生5条干扰线，使图象中的认证码不易被其它程序探测到（北京北大青鸟）
for (int i = 0; i < 5; i++) {
g.setColor(new Color(random.nextInt(50),random.nextInt(50),random.nextInt(50)));
//设定字体
g.setFont(new Font("Times New Roman", Font.PLAIN, random.nextInt(10)+20));
int x = random.nextInt(30);
int y = random.nextInt(30);
int xl = random.nextInt(30)+80;
int yl = random.nextInt(30);
g.drawLine(x, y, xl, yl);
}
// 取随机产生的认证码(4位随机符号)
String sRand = "";
//种子,可以放中文
String rand = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
for (int i = 0; i < 4; i++) {
String temp = String.valueOf(rand.charAt(random.nextInt(rand.length())));
sRand += temp;
// 将认证码显示到图象中,调用函数出来的颜色相同，可能是因为种子太接近，所以直接生成（北京北大青鸟）
g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
g.drawString(temp, 20 * i + 6, 20);
}
// 将认证码存入SESSION
session.setAttribute("numRand", sRand);
// 图象生效
g.dispose();
// 输出图象到页面
ImageIO.write(image, "JPEG", response.getOutputStream());
out.clear();
out = pageContext.pushBody();
%>
2. index.jsp (显示验证图片)
<%@ 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>验证码测试</title>
</head>
<body>
<img src="random.jsp" alt="不清楚，单击更换图片" onclick="this.src='',this.src='random.jsp'" />
</body>
</html>
（北京北大青鸟）
