以前，写一个页面的彩色验证码时，从网上找到几个有关于彩色验证码的代码，虽然可以用，但是总会在控制台出错，报异常：java.lang.IllegalStateException。
后来找了相关资料。更改了一小部分代码，在最后加上：
output.flush();
out.clear();
这样就能正常了。
整个image.jsp页面代码：
===============================================================================
<%@ 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%>" target="_blank" rel="external nofollow" >
<title>image page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
</head>
<body>
<%@ page contentType="image/jpeg" import="java.awt.*,
java.awt.image.*,java.util.*,javax.imageio.*,java.io.OutputStream" %>
<%!
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
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="";
String randString = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
for (int i=0;i<4;i++){
//String rand=String.valueOf(random.nextInt(10));
String rand = String.valueOf(randString.charAt(random.nextInt(61)));
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
OutputStream output=response.getOutputStream();
ImageIO.write(image, "JPEG", output);
output.flush();
out.clear();
out = pageContext.pushBody();
%>
</body>
</html>
=============================================================================
login.jsp
验证码的调用：
<div>
验证码：<input type="text" name="randCode" maxlength="4" value="" size="4">
<span><img id="imgCode" src="../image.jsp" style="cursor:pointer;" οnclick="changeImg()"/></span>
<span><a href="javascript:void(0)" target="_blank" rel="external nofollow" οnclick="changeImg()">看不清，换一张</a></span>
</div>
看不清，换一张 的JS代码
<script type="text/javascript" language="javascript">
function changeImg(){
var img =document.getElementById("imgCode");
img.src=img.src+"?";
}
</script>
==============================================================================
后台验证代码片段：
//进行验证码检验
String image=userForm.getImage();
//得到session中的验证码
String rand=(String)httpSession.getAttribute("rand");
if(rand.equals(image)){
//正确
}
else{
//不正确
}
前台实现的效果演示图：
![Java小案例——随机数猜测随机数猜测[图]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)