Servlet写一个3秒刷新的验证码

package com.kevin.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;

//给浏览器写一个验证码登录
public class ImageServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        //如何让浏览器五秒刷新一次
        resp.setHeader("refresh","3");

        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片的背景颜色
        g.setColor(Color.WHITE);
        g.fillRect(0,0,200,50);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),10,20);  //将下面获取的随机数画在图片上

        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/png");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");//缓存控制的策略 换成不缓存
        resp.setHeader("Programe","no-cache");

        //把图片写给浏览器
        boolean write = ImageIO.write(image,"png", resp.getOutputStream());
    }
//生成随机数
    private  String makeNum (){
    
    
        Random random = new Random();
        String num = random.nextInt(99999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7 - num.length(); i++) {
    
    
            sb.append("0");
        }
        String s = sb.toString() + num; //字符串拼接，如果不足七位，就用0来填充，保证必定是7位
        return  num;
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

长宽有些不合适，挡住了（懒得琢磨怎么改好），具体看代码还有注释
这种用sevlet获取页面二维码的方式很老套，不需要学，但是得知道原理