import java.awt.Color;

import java.awt.Font;

import java.awt.Graphics2D;

import java.awt.image.BufferedImage;

import java.io.IOException;

import java.util.Random;

import javax.imageio.ImageIO;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;

/**

 * Servlet implementation class VerifyCode

 */

@WebServlet("/VerifyCode")

public class VerifyCode extends HttpServlet {

    private static final long serialVersionUID = 1L;

    /**

     * @see HttpServlet#HttpServlet()

     */

    public VerifyCode() {

        super();

        // TODO Auto-generated constructor stub

    }

    /**

     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        this.doPost(request, response);

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // 首先设置浏览器不缓存

        response.setHeader("cache-control", "no-cache");

        response.setHeader("Expire", "-1"); // >0为缓存

        response.setHeader("pragma", "no-cache");

        // 存储正确的验证码

        String vali_code = "";

        String Vali_str = "abcdefghijklmnopqrstuvwsyz1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        Color[] colors =

            { Color.BLACK, Color.RED, Color.BLUE };

        // 1.创建图像

        int width = 110, height = 40;

        int x_offset = 5, y_offset = 28;

        BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_3BYTE_BGR);

        // 2.创建画布

        Graphics2D g = img.createGraphics();

        // 3.为画布添加背景颜色为亮灰色

        g.setColor(Color.LIGHT_GRAY);

        g.fillRect(0, 0, width, height);//fillRect是包括矩形框内部

        // 4.为画布添加边框

        g.setColor(Color.BLUE);

        g.drawRect(0, 0, width - 1, height - 1);//drawRect是绘制矩形边框

        // 5.绘制干扰线

        for (int i = 0; i < 10; i++)

        {

            g.setColor(new Color(randNum(0, 255), randNum(0, 255), randNum(0, 255)));

            g.drawLine(randNum(0, width), randNum(0, height), randNum(0, width), randNum(0, height));

        }

        g.setFont(new Font("黑体", Font.BOLD, 20));

        for (int i = 0; i < 4; i++)

        {

            int r = randNum(-180, 180);

            String code = Vali_str.charAt(randNum(0, Vali_str.length() - 1)) + "";

            g.rotate(r / 180 * Math.PI);

            g.setColor(colors[randNum(0, colors.length)]);

            g.drawString(code, x_offset + i * 30, y_offset);

            g.rotate(-r / 180 * Math.PI);

            vali_code += code;

        }

        //将验证码的内容存储在HttpSession中

        HttpSession session = request.getSession();

        session.setAttribute("Valicode", vali_code);

        // 通过response的输出流打给浏览器

        ImageIO.write(img, "jpg", response.getOutputStream());

    }

    private Random random = new Random();

    //随机生成begin和end之间的一个int值

    private int randNum(int begin, int end)

    {

        return random.nextInt(end - begin) + begin;

    }

}

 <input type="text" placeholder="输入验证码" name="ValiImage" id="ValiImage" /><!-- 输入验证码文本 -->

<img src="VerifyCode" onclick="changeValiImage(this)"><!-- 验证码图片 -->

<script type="text/javascript">

		function changeValiImage(img){

				img.src = "VerifyCode?time=" + new Date().getTime();//刷新

		}

</script>

//1 获得用户输入的验证码

        String verifyCode = request.getParameter("ValiImage");

        //2 获得服务器session 存放数据 ,如果没有返回null

        String sessionCacheData = (String) request.getSession().getAttribute("Valicode");

        // *将服务器缓存session数据移除

        request.getSession().removeAttribute("Valicode");

        // ** 判断服务器是否存在

        if(sessionCacheData == null){

        	request.setAttribute("message", "请不要重复提交");

            request.getRequestDispatcher("login.jsp").forward(request, response);

            return;

        }

        //3 比较

        if(! sessionCacheData.equalsIgnoreCase(verifyCode)){

            //用户输入错误

            // * 存放request作用域

        	request.setAttribute("message", "验证码输入错误");

            // * 请求转发

            request.getRequestDispatcher("login.jsp").forward(request, response);

            return;

        }

//接登录方法...