<%@ 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>
<base href="<%=basePath%>">
<title>My JSP 'index.jsp' starting 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">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
<script type="text/javascript">
function reloadCode(){
var time=new Date().getTime();
document.getElementById("imagecode").src="<%=path %>/servlet/ImageServlet?d="+time;
}
</script>
</head>
<body>
验证码：<input type="text" name="checkcode" />
<img alt="验证码" id="imagecode" src="<%=path %>/servlet/ImageServlet" />
<a href="javascript:reloadCode();">看不清楚</a><br/>
</body>
</html>
package com.code;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ImageServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
BufferedImage bImage=new BufferedImage(68, 22, BufferedImage.TYPE_INT_RGB);
//画图
Graphics graphics=bImage.getGraphics();
Color color=new Color(220,150,130);
graphics.setColor(color);
//给图片画个框
graphics.fillRect(0, 0, 78, 27);
/**
* 在图片中显示的内容
* 字母和数字的组合
*/
char[] ch="ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();
//随机获取里面的内容
Random random=new Random();
int len=ch.length,index;
StringBuffer sb=new StringBuffer();
for(int i=0;i<4;i++){
index=random.nextInt(len);
graphics.setColor(new Color(random.nextInt(88),random.nextInt(188),random.nextInt(255)));
graphics.drawString(ch[index]+"",(i*15)+3, 18);
sb.append(ch[index]);
}
//将验证码保存到session为以后比较使用
request.getSession().setAttribute("piccode", sb.toString());
ImageIO.write(bImage, "JPG", response.getOutputStream());
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
}
}
ImageServlet类写完之后，需要在web.xml里面配置该servlet<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>ImageServlet</servlet-name>
<servlet-class>com.code.ImageServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ImageServlet</servlet-name>
<url-pattern>/servlet/ImageServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>
原文：http://blog.csdn.net/smilelifesky/article/details/44450237