实现网站在线访问计数器功能，网站的初始值设置为1000

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（1）创建CountFilter的类，实现javax。servlet.Filter接口，是一个过滤器对象，通过过滤器实现统计网站人数功能：

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package  com.lixiyu;
import  java.io.IOException;
import  javax.servlet.Filter;
import  javax.servlet.FilterChain;
import  javax.servlet.FilterConfig;
import  javax.servlet.ServletContext;
import  javax.servlet.ServletException;
import  javax.servlet.ServletRequest;
import  javax.servlet.ServletResponse;
import  javax.servlet.http.HttpServletRequest;
/**
  * 统计过滤器
  * @author lixiyu
  */
public  class  CountFilter  implements  Filter {
     // 来访数量
     private  int  count;
                                          
     @Override
     public  void  init(FilterConfig filterConfig)  throws  ServletException {
         // 获取初始化参数
         String param = filterConfig.getInitParameter( "count" );
         // 将字符串转换为int
         count = Integer.valueOf(param);
     }
     @Override
     public  void  doFilter(ServletRequest request, ServletResponse response,
             FilterChain chain)  throws  IOException, ServletException {
         // 访问数量自增
         count ++;
         // 将ServletRequest转换成HttpServletRequest
         HttpServletRequest req =(HttpServletRequest)request;
         // 获取ServletContext
         ServletContext context =req.getSession().getServletContext();
         // 将来访数量值放入到ServletContext中
         context.setAttribute( "count" , count);
         // 向下传递过滤器
         chain.doFilter(request, response);
     }
     @Override
     public  void  destroy() {
     }
}

（2）配置已创建的CountFilter对象，设置初始值为1000，配置web.xml:

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<? xml  version = "1.0"  encoding = "UTF-8" ?>
< web-app  xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
     xmlns = "http://java.sun.com/xml/ns/javaee"  xmlns:web = "http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
     xsi:schemaLocation = "http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
     id = "WebApp_ID"  version = "2.5" >
   < filter >
     < filter-name >CountFilter</ filter-name >
     < filter-class >com.lixiyu.CountFilter</ filter-class >
     < init-param >
       < param-name >count</ param-name >
       < param-value >1000</ param-value >
     </ init-param >
   </ filter >
   < filter-mapping >
     < filter-name >CountFilter</ filter-name >
     < url-pattern >/index.jsp</ url-pattern >
   </ filter-mapping >
</ web-app >

(3)创建程序首页index.jsp，在该页面通过JSP内置对象Application获取计数器的值：

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<%@ page language="java" contentType="text/html; charset=UTF-8"
     pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
< html >
< head >
< meta  http-equiv = "Content-Type"  content = "text/html; charset=UTF-8" >
< title >人数统计</ title >
</ head >
< body >
     < h2 >
     欢迎光临，< br >
     您是本站的第【
     <%=application.getAttribute("count") %>
      】位访客！
      </ h2 >
</ body >
</ html >

本文转自lixiyu 51CTO博客，原文链接：http://blog.51cto.com/lixiyu/1351934，如需转载请自行联系原作者