<!-- web.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://java.sun.com/xml/ns/javaee"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
         id="WebApp_ID" version="3.0">

    <!--注册 Servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>ImageServlet</servlet-class>
    </servlet>
    <!--Servlet 的请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

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.io.PrintWriter;
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 5 秒自动刷新一次
        resp.setHeader("refresh", "5");

        // 在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        // 得到图片
        Graphics graphics = image.getGraphics();
        // 设置图片的背景颜色
        graphics.setColor(Color.white);
        graphics.fillRect(0, 0, 80, 20);
        graphics.setColor(Color.BLUE);
        graphics.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 {
        doGet(req, resp);
    }
}