java验证码及其刷新

1.servlet产生验证码：
package com.servlet;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
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 Img extends HttpServlet {
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  response.setContentType("image/jpeg");
  response.setHeader("Pragma","No-cache");
  response.setHeader("Cache-Control","no-cache");
  response.setDateHeader("Expires",0);
  
  OutputStream out=response.getOutputStream();
  int w=80,h=20;
  BufferedImage img=new BufferedImage(w,h,BufferedImage.TYPE_INT_RGB);
  Graphics g=img.getGraphics();
  Random ran=new Random();
  g.fillRect(0, 0, w, h);
  g.setFont(new Font("Times New Roman",Font.ITALIC,18));
  String sRan="";
  for(int i=0;i<4;i++){
   String rand=String.valueOf(ran.nextInt(10));
   sRan+=rand;
   g.setColor(new Color(20+ran.nextInt(110),20+ran.nextInt(110),20+ran.nextInt(110)));
   g.drawString(rand,20*i+6,16);
  }
  request.getSession().setAttribute("ch", sRan);
  g.dispose();
  ImageIO.write(img, "jpeg", out);
  
 }
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }
}
2.嵌入html中
<img src="Img" >
3.验证码的刷新
1）点击验证码刷新
<img