Response验证码实现

ImageServlet：

package com.allen.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

public class ImageServlet extends HttpServlet {

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

        //如何让浏览器3秒自动刷新一次；
        resp.setHeader("refresh", "3");

        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(100, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics(); //笔
        //设置图片背景颜色
        g.setColor(Color.white);
        g.fillRect(0, 0, 100, 20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null, Font.BOLD, 20));
        g.drawString(makeNum(), 0, 20);

        //告诉浏览器，这个请求用图片的打开方式
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires", -1);
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Pragma", "no-cache");

        //把图片写给浏览器
        ImageIO.write(image, "jpg", resp.getOutputStream());
    }

    //生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for(int i = 0; i < 7 - num.length(); i++){
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req, resp);
    }
}

web.xml

<servlet>
    <servlet-name>ImageServlet</servlet-name>
    <servlet-class>com.allen.servlet.ImageServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>ImageServlet</servlet-name>
    <url-pattern>/img</url-pattern>
</servlet-mapping>

测试：