Getting currently Logged in members

I want to know how many members are currently logged into the given system .

Or in other words i would like to know how many sessions are presently running into my server . how is this possible .

I am trying to do like this .

HttpSession session = req.getSession();

HttpSessionContext context = session.getSessionContext();

Enumeration en = context.getIds();

while(en.hasMoreElements())

{

String s = (String)en.nextElement();

out.println(s);

}

But i dont get any output .

Please somebody guide me .

[585 byte] By [mohitagarwal] at [2007-9-26 1:22:44]