java验证码学习一

package com.sys.web.sesoion;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.awt.Graphics;import java.awt.image.BufferedImage;import java.util.Random;import javax.imageio.ImageIO;import javax.servlet.http.HttpSession;public class CheckCodeServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        BufferedImage image = new BufferedImage(50,25,BufferedImage.TYPE_INT_RGB);        Graphics g = image.getGraphics();        g.drawString(Token.getNum(),15,15);        ImageIO.write(image,"JPG",response.getOutputStream());        response.setHeader("expires","-1");        response.setHeader("cache-control","no-cache");  //没有缓成        response.setHeader("pragma","no-cache");    }}//用于产生随机数class Token{    public static String getNum(){        Random r = new Random();        int num = r.nextInt(10000);        return num + "";    }}