一、程式碼：

package com.servlet.study;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ResponseD extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
    	//設定refresh響應頭控制瀏覽器每隔5秒鐘重新整理一次
        response.setHeader("refresh", "5");
        
        //1.在記憶體中建立一張圖片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        
        //2.得到圖片
        //Graphics g = image.getGraphics();
        Graphics2D g = (Graphics2D)image.getGraphics();
        g.setColor(Color.WHITE);//設定圖片的背景色
        g.fillRect(0, 0, 80, 20);//填充背景色
        
        //3.向圖片上寫資料
        g.setColor(Color.BLUE);//設定圖片上字型的顏色
        g.setFont(new Font(null, Font.BOLD, 20));
        g.drawString(makeNum(), 0, 20);
        
        //4.設定響應頭控制瀏覽器瀏覽器以圖片的方式開啟
        response.setContentType("image/jpeg");//等同於response.setHeader("Content-Type", "image/jpeg");
        
        //5.設定響應頭控制瀏覽器不快取圖片資料
        response.setDateHeader("expries", -1);
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        
        //6.將圖片寫給瀏覽器
        ImageIO.write(image, "jpg", response.getOutputStream());
    }

    /**
     * 生成隨機數字
     * @return
     */
    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;
    }
    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

}
 

二、解釋：

2.1 設定refresh響應頭控制瀏覽器每隔5秒鐘重新整理一次

response.setHeader("refresh", "5");

2.2 設定http響應頭控制瀏覽器禁止快取當前文件內容

response.setDateHeader("expries", -1);
response.setHeader("Cache-Control", "no-cache");
response.setHeader("Pragma", "no-cache");

2.3 random.nextInt(n) 

public int nextInt(int n)

該方法的作用是生成一個隨機的int值，該值介於[0,n)的區間，也就是0到n之間的隨機int值，包含0而不包含n。
 

三、執行結果：

轉自：https://www.cnblogs.com/xdp-gacl/p/3791993.html