重写URL机制：为了保证一个url的地址唯一，可每次向服务器传递的参数不一样即可。
由数据请求的抱头信息可分析到：抱头信息包括http协议，IP地址，端口号，工程名，请求参数列表，要想访问的资源不发生变化，只能变化参数连表。
此处在实现验证码的换一张的功能时，就是利用了改变参数列表的值进行刷新。
详细代码实现：
<%@page import="java.awt.Font"%>
<%@page import="java.awt.Color"%>
<%@page import="java.awt.Graphics"%>
<%@page import="java.awt.image.BufferedImage"%>
<%@ page contentType="image/jpeg" language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
int w=100;
int h=30;
BufferedImage bi=new BufferedImage(w,h,BufferedImage.TYPE_INT_RGB);
Graphics g=bi.getGraphics();
Color c=g.getColor();
Font f=g.getFont();
Random r=new Random();
Color bg=new Color(150+r.nextInt(100),150+r.nextInt(100),150+r.nextInt(100));
g.setColor(bg);
g.fillRect(0,w,h);
String code="";
for(int i=1;i<=4;i++){
int num=r.nextInt(10);
code=code+num;
Color num_c=new Color(r.nextInt(150),r.nextInt(150),r.nextInt(150));
g.setColor(num_c);
g.drawString(String.valueOf(num),20*i,h/2);
}
request.getSession().setAttribute("code",code);
//清空缓存
response.setHeader("pragma","bo-cache");
response.setHeader("cache-control","bo-cache");
response.addDateHeader("expires",0);
ImageIO.write(bi,"jpeg",response.getOutputStream());
out.close();
%>
<%String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
My JSP 'login.jsp' starting page
<Meta http-equiv="pragma" content="no-cache">
<Meta http-equiv="cache-control" content="no-cache">
<Meta http-equiv="expires" content="0">
<Meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<Meta http-equiv="description" content="This is my page">