image.jsp------------------------------生成随机验证码图片的Jsp页面
代码如下： 
代码如下：

%@ page contentType="image/jpeg" import="java.awt.*,
java.awt.image.*,java.util.*,javax.imageio.*" %
%!
Color getRandColor(int fc,int bc)
{
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);
}
%
%
out.clear();//这句针对resin服务器，如果是tomacat可以不要这句
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);
int width=60, height=20;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();
Random random = new Random();
g.setColor(getRandColor(200,250));
g.fillRect(0, 0, width, height);
g.setFont(new Font("Times New Roman",Font.PLAIN,18));
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);
}
String sRand="";
for (int i=0;i4;i++){
String rand=String.valueOf(random.nextInt(10));
sRand+=rand;
g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));
g.drawString(rand,13*i+6,16);
}
// 将认证码存入SESSION
session.setAttribute("rand",sRand);
g.dispose();
ImageIO.write(image, "JPEG", response.getOutputStream());
%

logic.jsp--------------------------------登陆页面，在这里提供输入验证码然后提交并验证
代码如下： 
代码如下：

%@ page contentType="text/html;charset=gbk" %
%@ page language="java" import="java.sql.*" errorPage="" %
html
head
meta http-equiv="Content-Type" content="text/html; charset=gb2312"
title用户登录/title
script language="javascript"
function loadimage(){
document.getElementById("randImage").src = "image.jsp?"+Math.random();
}
/script
/head
body
table width="256" cellpadding="0" cellspacing="0"
!--DWLayoutTable--
form action="validate.jsp" method="post" name="loginForm"
tr
td width="118" height="22" valign="middle" style="text-align:center"input type="text" name="rand" size="15"/td
td width="138" valign="middle" style="text-align:center"img alt="" name="randImage" id="randImage" src="image.jsp" width="60" height="20" align="absmiddle"/td
/tr
tr
td height="36" colspan="2" style="text-align:center" valign="middle"a href="javascript:loadimage();"font class=pt95看不清点我/font/a/td
/tr
tr
td height="36" colspan="2" style="text-align:center" valign="middle"input type="submit" name="login" value="提交"/td
/tr
/form
/table
/body
/html

validate.jsp--------------------------用来验证输入的验证码是否正确
代码如下：
代码如下：

%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %
%
String rand = (String)session.getAttribute("rand");
String input = request.getParameter("rand");
if(rand.equals(input)){
out.print("scriptalert('验证通过！');/script");
} else{
out.print("scriptalert('请输入正确的验证码！');location.href='login.jsp';/script");
}
%

以上三个JSP页面放在和WEB-INF同级目录下，如果你放在不同的文件夹下，那么里面相关页面的路径自己稍作更改就可以了。
友情提示：Tomcat5以下版本会出现异常