通过重写HttpSessionListener接口实现

//session操作类
public class SessionMap {
  
     private static Map<String,HttpSession> sesMap = new HashMap<String,HttpSession>();
  
     private static final Object flag = new Object();
  
     private SessionMap(){
  
     }
  
     private static class MapUtilsN{
         private static SessionMap sessionMap = new SessionMap();
     }
      
     public static SessionMap getInstance(){
         return MapUtilsN.sessionMap;
     }
      
     public void addMap(String sessionId,HttpSession session){
         synchronized (flag) {
             if(!sesMap.containsKey(sessionId)){
                 sesMap.put(sessionId, session);
             }
         }
     }
      
     public void removeMap(String sessionId){
         synchronized (flag) {
             sesMap.remove(sessionId);
         }
     }
      
     public int getSize(){
         synchronized (flag) {
             return sesMap.size();
         }
     }
      
     public HttpSession getSession(String key){
         synchronized (flag) {
             return sesMap.get(key);
         }
      }
}
 
 
//监听session创建及销毁
public class MySessionListner implements HttpSessionListener{
 
 public void sessionCreated(HttpSessionEvent arg0) {
     SessionMap.getInstance().addMap(arg0.getSession().getId(), arg0.getSession());
 }
 
 public void sessionDestroyed(HttpSessionEvent arg0) {
     SessionMap.getInstance().removeMap(arg0.getSession().getId());
 }
 
}
 

//web.xml中配置监听器
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
 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_2_5.xsd">
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
  <!-- 配置监听器 -->
  <listener>  
      <listener-class>  
        practice.listner.MySessionListner  
      </listener-class>  
  </listener>
   <!-- 设置session超时时间 -->
  <session-config>
   <session-timeout>10</session-timeout>
  </session-config>
</web-app>