I make a Hashtable and put it into application,Hashtable is used to store online number,when user visited this page,I add him into this Hashtable,like follows:

Hashtable list = (Hashtable)application.getAttribute("list");
if(list==null)
list=new Hashtable();
list.put(user.getName(),user.getName());
application.setAttribute("list",list);

Then I can get online number through this Hashtable,like follows:

int number;
Hashtable list = (Hashtable) application.getAttribute("list");
Enumeration info= list.elements();
while(info.hasMoreElements())
  number++; //online number

When user's session is timeout or user closed IE,I remove user from this Hashtable.
My question is how to know user's status in Server?

Any idea will be appreciated!
Edward

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