生成有4个随机数字和杂乱背景的图片，数字和背景颜色会改变，服务器端刷新（用history.go(-1)也会变）
原型参考ALIBABA http://china.alibaba.com/member/showimage
产生验证码图片的文件-----image.jsp
1@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*"
1!
2Color getRandColor(int fc,int bc){//给定范围获得随机颜色
3Random random = new Random();
4if(fc>255) fc=255;
5if(bc>255) bc=255;
6int r=fc+random.nextInt(bc-fc);
7int g=fc+random.nextInt(bc-fc);
8int b=fc+random.nextInt(bc-fc);
9return new Color(r,g,b);
10}
1
2//设置页面不缓存
3response.setHeader("Pragma","No-cache");
4response.setHeader("Cache-Control","no-cache");
5response.setDateHeader("Expires", 0);
6
7// 在内存中创建图象
8int width=60, height=20;
9BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
10
11// 获取图形上下文
12Graphics g = image.getGraphics();
13
14//生成随机类
15Random random = new Random();
16
17// 设定背景色
18g.setColor(getRandColor(200,250));
19g.fillRect(0, 0, width, height);
20
21//设定字体
22g.setFont(new Font("Times New Roman",Font.PLAIN,18));
23
24//画边框
25//g.setColor(new Color());
26//g.drawRect(0,0,width-1,height-1);
27
28
29// 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
30g.setColor(getRandColor(160,200));
31for (int i=0;i<155;i++)
32{
33int x = random.nextInt(width);
34int y = random.nextInt(height);
35int xl = random.nextInt(12);
36int yl = random.nextInt(12);
37g.drawLine(x,y,x+xl,y+yl);
38}
39
40// 取随机产生的认证码(4位数字)
41String sRand="";
42for (int i=0;i<4;i++){
43String rand=String.valueOf(random.nextInt(10));
44sRand+=rand;
45// 将认证码显示到图象中
46g.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
47g.drawString(rand,13*i+6,16);
48}
49
50// 将认证码存入SESSION
51session.setAttribute("rand",sRand);
52
53
54// 图象生效
55g.dispose();
56
57// 输出图象到页面
58ImageIO.write(image, "JPEG", response.getOutputStream());
59
60
---------------使用验证码图片的文件---------a.jsp------------------------------------
1@ page contentType="text/html;charset=gb2312"
1<html>
2<head>
3<title>认证码输入页面</title>
4<meta content="text/html; charset=utf-8" http-equiv="Content-Type"/>
5<meta content="no-cache" http-equiv="Pragma"/>
6<meta content="no-cache" http-equiv="Cache-Control"/>
7<meta content="0" http-equiv="Expires"/>
8</head>
9<body>
10<form action="check.jsp" method="post">
11<table>
12<tr>
13<td align="left">系统产生的认证码：</td>
14<td><img border="0" src="image.jsp"/></td>
15</tr>
16<tr>
17<td align="left">输入上面的认证码：</td>
18<td><input maxlength="4" name="rand" type="text" value=""/></td>
19</tr>
20<tr>
21<td align="center" colspan="2"><input type="submit" value="提交检测"/></td>
22</tr>
23</table></form>
24</body>
25</html>
-----------------验证的页面----------check.jsp
1@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage=""
1<html>
2<head>
3<title>认证码验证页面</title>
4<meta content="text/html; charset=utf-8" http-equiv="Content-Type"/>
5<meta content="no-cache" http-equiv="Pragma"/>
6<meta content="no-cache" http-equiv="Cache-Control"/>
7<meta content="0" http-equiv="Expires"/>
8</head>
9<body>
String rand = (String)session.getAttribute("rand");
String input = request.getParameter("rand");
1系统产生的认证码为： ```
2= rand
3```<br/>
4您输入的认证码为： ```
5= input
6```<br/>
7<br/>
if (rand.equals(input)) {
1<font color="green">输入相同，认证成功！</font>
} else {
1<font color="red">输入不同，认证失败！</font>
}
1</body>
2</html>