缓存控制
在http中，控制缓存开关的字段有两个：Pragma 和 Cache-Control。
Pragma
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;}
}