package cn.hncu.listeners;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class MyHttpSessionListener implements HttpSessionListener {
    //有 游客/用户 来访问了
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        List<HttpSession> guests = (List<HttpSession>) se.getSession().getServletContext().getAttribute("guests");//在线人的集合
        if(guests==null){//第一个访问网站的人--沙发
            guests = new ArrayList<HttpSession>();
            se.getSession().getServletContext().setAttribute("guests", guests);//设置guests属性
        }
        guests.add(se.getSession());//将第一个用户的session添加到在线人集合
    }
    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        List<HttpSession> guests = (List<HttpSession>) se.getSession().getServletContext().getAttribute("fuses");
        if(guests.contains(se.getSession())){
            guests.remove(se.getSession());
        }
    }
}

package cn.hncu.listeners;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
public class MyHttpSessionAttributeListener implements HttpSessionAttributeListener{
    //session添加属性了，就会调用下面的attributeAdded方法
    @Override
    public void attributeAdded(HttpSessionBindingEvent se) {
        if(se.getName().equals("name")){//代表添加了name属性
            System.out.println("用户"+se.getValue()+"登录了");
            List<HttpSession> logins = (List<HttpSession>) se.getSession().getServletContext().getAttribute("logins");
            if(logins==null){
                logins = new ArrayList<HttpSession>();
                se.getSession().getServletContext().setAttribute("logins", logins);
            }
            logins.add(se.getSession());
        }
    }
    @Override
    public void attributeRemoved(HttpSessionBindingEvent se) {
    }
    @Override
    public void attributeReplaced(HttpSessionBindingEvent se) {
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" 
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
  <display-name></display-name>
  <filter>
    <filter-name>charset</filter-name>
    <filter-class>cn.hncu.filter.CharsetFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>charset</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  <listener>
    <listener-class>cn.hncu.listeners.MyServletContextListener</listener-class>
  </listener>
  <listener>
    <listener-class>cn.hncu.listeners.MyServletRequestListenr</listener-class>
  </listener>
  <listener>
    <listener-class>cn.hncu.listeners.MyHttpSessionAttributeListener</listener-class>
  </listener>
  <listener>
    <listener-class>cn.hncu.listeners.MyHttpSessionListener</listener-class>
  </listener>
  <servlet>
    <servlet-name>LoginServlet</servlet-name>
    <servlet-class>cn.hncu.servlets.LoginServlet</servlet-class>
  </servlet>
  <servlet>
    <servlet-name>ShowServlet</servlet-name>
    <servlet-class>cn.hncu.servlets.ShowServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>LoginServlet</servlet-name>
    <url-pattern>/servlet/LoginServlet</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>ShowServlet</servlet-name>
    <url-pattern>/servlet/ShowServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>