public static void addListener(ExternalComponentManagerListener listener) {
        if (listener == null) {
            throw new NullPointerException();
        }
        listeners.add(listener);
    }
Registers a listener to receive events when a configuration change happens. Listeners
have the chance to deny the operation from happening.

@param listener the listener.
xmppserver/src/main/java/org/jivesoftware/openfire/component/ExternalComponentManager.java