先创建ServletContextListener

在全局对象application中设置count属性

package com.yyy.listener;

import java.util.ArrayList;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

/**
 * Application Lifecycle Listener implementation class TestServletContextListener
 *
 */
@WebListener
public class TestServletContextListener implements ServletContextListener {

    /**
     * Default constructor. 
     */
    public TestServletContextListener() {
        // TODO Auto-generated constructor stub
    }

	/**
     * @see ServletContextListener#contextDestroyed(ServletContextEvent)
     */
    public void contextDestroyed(ServletContextEvent arg0)  { 
         // TODO Auto-generated method stub
    	System.out.println("服务器关闭了");
    }

	/**
     * @see ServletContextListener#contextInitialized(ServletContextEvent)
     */
    public void contextInitialized(ServletContextEvent arg0)  { 
         // TODO Auto-generated method stub
    	// TODO Auto-generated method stub
    			//初始化数据 变量  数据库连接信息  连接池的信息
    			ServletContext application=  arg0.getServletContext();
    			
    			application.setAttribute("count", 0);
    			
    			
    			System.out.println("服务器启动了");
    }
	
}

创建SessionListener修改count的值

所有的session共用一个application对象

package com.yyy.listener;

import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;

/**
 * Application Lifecycle Listener implementation class HttpSessionListener
 *
 */
@WebListener
public class HttpSessionListener implements javax.servlet.http.HttpSessionListener {

    /**
     * Default constructor. 
     */
    public HttpSessionListener() {
        // TODO Auto-generated constructor stub
    }

	/**
     * @see HttpSessionListener#sessionCreated(HttpSessionEvent)
     */
    public void sessionCreated(HttpSessionEvent arg0)  { 
         // TODO Auto-generated method stub
    	ServletContext application=  arg0.getSession().getServletContext();
		
		int count=(int)application.getAttribute("count");
		
		count++;
		
		application.setAttribute("count", count);
		
		System.out.println("新用户上线,当前总人数为:"+count);
    }

	/**
     * @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
     */
    public void sessionDestroyed(HttpSessionEvent arg0)  { 
         // TODO Auto-generated method stub
    	 ServletContext application=  arg0.getSession().getServletContext();
 		
 		int count=(int)application.getAttribute("count");
 		
 		count--;
 		
 		application.setAttribute("count", count);
 		
 		System.out.println("用户下线,当前总人数为:"+count);
    }
	
}

web.xml

<?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" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>web5</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <listener>
    <listener-class>com.yyy.listener.TestServletContextListener</listener-class>
  </listener>
  <session-config>
    <session-timeout>1</session-timeout>
  </session-config>
</web-app>

创建servlet运行测试

package com.yyy.servlet;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class TestServlet
 */
@WebServlet("/TestServlet")
public class TestServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public TestServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		HttpSession session=request.getSession();
		System.out.println(session.getId()+":上线了");
		
//		ServletContext application=  session.getServletContext();
//		
//		int count=(int)application.getAttribute("count");
//		
//		System.out.println("在线人数为:"+count);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}