在http中，控制缓存开关的字段有两个：Pragma 和 Cache-Control。
Pragma有两个字段Pragma和Expires。Pragma的值为no-cache时，表示禁用缓存，Expires的值是一个GMT时间，表示该缓存的有效时间。 Pragma是旧产物，已经逐步抛弃，有些网站为了向下兼容还保留了这两个字段。如果一个报文中同时出现Pragma和Cache-Control时，以Pragma为准。同时出现Cache-Control和Expires时，以Cache-Control为准。即优先级从高到低是 Pragma -> Cache-Control -> Expires
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( "/CheckCode") public class CheckCode extends HttpServlet { protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //让浏览器每隔一段时间自动刷新 response.setHeader("refresh","3"); //在内存中创建一个图片 BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB); //得到笔 Graphics2D graphics = (Graphics2D)image.getGraphics(); //设置图片的背景颜色 graphics.setColor(Color.white); //填充图形 graphics.fillRect(0,0,80,20); //给图片写数据 graphics.setColor(Color.BLACK); graphics.setFont(new Font(null,Font.BOLD,20)); graphics.drawString(makeNum(),0,20); //告诉浏览器这个请求用图片的格式打开 response.setContentType("image/jpeg"); //不让浏览器缓存 response.setDateHeader("expires",-1); response.setHeader("Cache-Control","no-Cache"); response.setHeader("pragma","no-Cache"); //把图片写给浏览器 ImageIO.write(image,"jpeg",response.getOutputStream()); } private String makeNum() { Random random = new Random(); String s = random.nextInt(99999999) + ""; StringBuffer buffer = new StringBuffer(); for(int i=0;i<8-s.length();i++) buffer.append("0"); s=buffer.toString()+s; return s; } }
