思路

1. 让浏览器每三秒刷新一次，通过refresh实现
2. 创建一个验证码图片
   1. 设置图片背景
   2. 将数据渲染进图片
3. 提供验证码中的数据
   1. 写一个生成随机数方法
4. 关闭网站缓存
5. 用imageIO将图片写进浏览器

实现

package com.yang.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 {@Overrideprotected 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(80,20,BufferedImage.TYPE_INT_RGB);//得到图片Graphics2D g = (Graphics2D) image.getGraphics(); //得到一支笔//设置图片的背景颜色g.setColor(Color.white);g.fillRect(0,0,80,20);//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");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(99999999)+"";StringBuffer sb = new StringBuffer();for (int i = 0; i < 7-num.length(); i++) {sb.append("0");}num = sb.toString()+num;return num;}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req, resp);}
}