使用监听器实现在线人数统计的功能
我们学习了三个作用域（application、session、request）对应的监听器，例如ServletContextListener、ServletContextAttributeListener、ServletRequestListener、ServletRequestAttributeListener、HttpSessionListener、HttpSessionAttributeListener…
使用监听器的步骤：
编写实现类 在部署描述符文件中进行配置部署 编写测试页面
index.jsp页面代码如下：
<%@ 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>在线人数</title>
</head>
<body>
<h2>网站当前的在线用户人数：<%=application.getAttribute("usercount") %></h2>
</body>
</html>
HttpSessionListener监听器实现类
package com.online;
import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
@WebListener
public class HttpSessionListenerImpl implements HttpSessionListener {
/**
* Default constructor.
*/
public HttpSessionListenerImpl() {
// TODO Auto-generated constructor stub
}
/**
* @see HttpSessionListener#sessionCreated(HttpSessionEvent)
*/
public void sessionCreated(HttpSessionEvent se) {
HttpSession session = se.getSession();
ServletContext context = session.getServletContext();
int usercount = (int) context.getAttribute("usercount"); //从context中获取在线用户人数
context.setAttribute("usercount", usercount+1); //将在线用户人数+1
}
/**
* @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
*/
public void sessionDestroyed(HttpSessionEvent se) {
HttpSession session = se.getSession();
ServletContext context = session.getServletContext();//获取全局作用域对象
int usercount = (int) context.getAttribute("usercount");//获取请求参数
context.setAttribute("usercount", usercount-1);
}
}
ServletContextListener监听器实现类：
package com.online;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
@WebListener
public class ServletContextListenerImpl implements ServletContextListener {
/**
* Default constructor.
*/
public ServletContextListenerImpl() {
// TODO Auto-generated constructor stub
}
/**
* @see ServletContextListener#contextDestroyed(ServletContextEvent)
*/
public void contextDestroyed(ServletContextEvent sce) {
// TODO Auto-generated method stub
}
/**
* @see ServletContextListener#contextInitialized(ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent sce) {
ServletContext context = sce.getServletContext();
context.setAttribute("usercount", 0); //创建在线用户人数参数到context中，用于在整个应用程序进行统计
}
}