用java生成验证码

package com.starcpdk.servlet;

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 java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

@WebServlet("/bb")
public class ImageServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        // 如何让浏览器5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(80, 20 , BufferedImage.TYPE_INT_RGB);

        // 创建图片
        Graphics2D graphics2D = (Graphics2D)image.getGraphics();

        // 设置背景色
        graphics2D.setColor(Color.white);
        graphics2D.fillRect(0,0,80,20);

        // 给图片写数据
        graphics2D.setColor(Color.BLUE);
        graphics2D.setFont(new Font(null , Font.BOLD , 20));
        graphics2D.drawString(makeNum() , 0 , 20);

        // 告诉浏览器 ， 这个请求用图片的方式打开
        resp.setContentType("image/jpeg");

        // 网站存在缓冲 ， 不让浏览器缓冲
        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());


    }

    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;
        return num;
    }

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