image.jsp
代码如下：

%@ page language="java" import="java.util.*" pageEncoding="GBK"%
%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%
%@ 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;i155;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;i4;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());
%


a.jsp------------------------------------------------------------------------------------------------------ 
代码如下：

%@ page language="java" import="java.util.*" pageEncoding="GBK"%
%
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
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 style="text-align:center"
input type=submit value="提交检测"
/td
/tr
/table
/form
/body
/html


check.jsp----------------------------------------------------------------------------------------------------------------------
代码如下：

%@ page language="java" import="java.util.*" pageEncoding="GBK"%
%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%
!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
%@ 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