{
            // Do not use interators as this method tends to be called recursively 
            // by the invalidate calls.
            while (_sessions.containsKey(id))
            {
                Session session=(Session)_sessions.getValue(id,0);
                if (session.isValid())
                    session.invalidate();
                else
                    _sessions.removeValue(id,session);
            }
        }
    }