public static void unavailableSession(ClientSession session, Presence presence) {
        if (!listeners.isEmpty()) {
            for (PresenceEventListener listener : listeners) {
                try {
                    listener.unavailableSession(session, presence);
                } catch (Exception e) {
                    Log.warn("An exception occurred while dispatching a 'unavailableSession' event!", e);
                }
            }
        }
    }
Notification message indicating that a session that was available is no longer
available. A session becomes unavailable when an unavailable presence is received.
The entity may still be connected to the server and may send an available presence
later to indicate that communication can proceed.

@param session the session that is no longer available.
@param presence the received unavailable presence.
xmppserver/src/main/java/org/jivesoftware/openfire/user/PresenceEventDispatcher.java