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
<%@ page contentType="image/jpeg" 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(fc255) fc=255;
if(bc255) 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());
%
---------------使用验证码图片的文件---------a.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="http://img.jcwcn.com/attachment/portal"</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
</form
</body
</html
-----------------验证的页面----------check.jsp
（本文来源于图老师网站，更多请访问http://m.tulaoshi.com/java/）<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %
<html
<head
<title&
