import java.awt.Color;

import java.awt.Font;

import java.awt.Graphics;

import java.awt.image.BufferedImage;

import java.io.IOException;

import java.util.Date;

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 GenerateImageCodeServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    private static final char[] CH = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".toCharArray();

    private static final int IMAGE_WIDTH = 73;

    private static final int IMAGE_HEIGHT = 28;

    private static final int LINE_NUM = 30;

    private static final int RANDOM_NUM = 4;

    Random random = new Random();

    @Override

    protected void doGet(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("image/jpg");//设置相应类型,告诉浏览器输出的内容为图片

        response.setHeader("Pragma", "No-cache");//设置响应头信息，告诉浏览器不要缓存此内容

        response.setHeader("Cache-Control", "no-cache");

        response.setDateHeader("Expire", new Date().getTime());

        BufferedImage bi = new BufferedImage(IMAGE_WIDTH, IMAGE_HEIGHT, BufferedImage.TYPE_INT_BGR);

        Graphics g = bi.getGraphics();

        g.fillRect(0, 0, IMAGE_WIDTH, IMAGE_HEIGHT);

        g.setColor(getRandomColor(110, 133));

        g.setFont(new Font("Times New Roman", Font.ROMAN_BASELINE, 18));

        // 绘制干扰线

        for (int i = 1; i <= LINE_NUM; i++) {

            int x = random.nextInt(IMAGE_WIDTH);

            int y = random.nextInt(IMAGE_HEIGHT);

            int xl = random.nextInt(13);

            int yl = random.nextInt(15);

            g.drawLine(x, y, x + xl, y + yl);

        }

        // 绘制随机字符

        StringBuilder sb = new StringBuilder();

        String str = null;

        for (int i = 0; i < RANDOM_NUM; i++) {

            g.setFont(new Font("Fixedsys", Font.CENTER_BASELINE, 18));

            g.setColor(new Color(random.nextInt(101), random.nextInt(111), random.nextInt(121)));

            str = CH[random.nextInt(CH.length)] + "";

            g.drawString(str, 13 * i, 16);

            g.translate(random.nextInt(3), random.nextInt(3));

            sb.append(str);

        }

        g.dispose();

        request.getSession().setAttribute("safeCode", sb.toString());

        ImageIO.write(bi, "JPG", response.getOutputStream());

    }

    /**

     * 获得颜色

     */

    private Color getRandomColor(int fc, int bc) {

        if (fc > 255)

            fc = 255;

        if (bc > 255)

            bc = 255;

        int r = fc + random.nextInt(bc - fc - 16);

        int g = fc + random.nextInt(bc - fc - 14);

        int b = fc + random.nextInt(bc - fc - 18);

        return new Color(r, g, b);

    }

}

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class LoginServlet extends HttpServlet{

    private static final long serialVersionUID = 1L;

    @Override

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        resp.setContentType("text/html;charset=gbk");

        String safeCode = (String) req.getSession().getAttribute("safeCode");

        String checkcode = req.getParameter("checkcode");

        PrintWriter out = resp.getWriter();

        if (safeCode.equalsIgnoreCase(checkcode)) {

            out.println("验证码正确");

        }

        else {

            out.println("验证码错误");

        }

        out.flush();

        out.close();

    }

}

<?xml version="1.0" encoding="UTF-8"?>

<web-app id="WebApp_ID" version="3.0" 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_3_0.xsd">

    <servlet>

        <servlet-name>ImageCodeServlet</servlet-name>

        <servlet-class>com.lijy.servlet.GenerateImageCodeServlet</servlet-class>

    </servlet>

    <servlet>

        <servlet-name>LoginServlet</servlet-name>

        <servlet-class>com.lijy.servlet.LoginServlet</servlet-class>

    </servlet>

    <servlet-mapping>

        <servlet-name>ImageCodeServlet</servlet-name>

        <url-pattern>/safe_code</url-pattern>

    </servlet-mapping>

    <servlet-mapping>

        <servlet-name>LoginServlet</servlet-name>

        <url-pattern>/login</url-pattern>

    </servlet-mapping>

    <welcome-file-list>

        <welcome-file>index.jsp</welcome-file>

    </welcome-file-list>

</web-app>

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">

<title>checkcodes</title>

<script type="text/javascript">

    function reloadCode() {

        var time = new Date().getTime();

        document.getElementById("imagecode").src="<%=request.getContextPath()%>/safe_code?d="+time;

    }

</script>

</head>

<body>

<form action="<%=request.getContextPath()%>/login" method="get">

验证码：<input type="text" name="checkcode" />

    <img alt="验证码" id="imagecode" src="<%=request.getContextPath()%>/safe_code">

    <a href="javascript:reloadCode();">看不清楚</a><br/>

    <input type="submit" value="提交" />

<hr>

</form>

</body>

</html>