class ServerSessionEventDispatcher {

    private static final Logger Log = LoggerFactory.getLogger(ServerSessionEventDispatcher.class);

    private static List<ServerSessionEventListener> listeners =
            new CopyOnWriteArrayList<>();

    private ServerSessionEventDispatcher() {
        // Not instantiable.
    }

    /**
     * Registers a listener to receive events.
     *
     * @param listener the listener.
     */
    public static void addListener(ServerSessionEventListener listener) {<FILL_FUNCTION_BODY>}

    /**
     * Unregisters a listener to receive events.
     *
     * @param listener the listener.
     */
    public static void removeListener(ServerSessionEventListener listener) {
        listeners.remove(listener);
    }

    /**
     * Dispatches an event to all listeners.
     *
     * @param session the session.
     * @param eventType the event type.
     */
    public static void dispatchEvent(Session session, EventType eventType) {
        for (ServerSessionEventListener listener : listeners) {
            try {
                switch (eventType) {
                    case session_created: {
                        listener.sessionCreated(session);
                        break;
                    }
                    case session_destroyed: {
                        listener.sessionDestroyed(session);
                        break;
                    }
                    default:
                        break;
                }
            }
            catch (Exception e) {
                Log.error(e.getMessage(), e);
            }
        }
    }

    /**
     * Represents valid event types.
     */
    public enum EventType {

        /**
         * A session was created.
         */
        session_created,

        /**
         * A session was destroyed
         */
        session_destroyed,
    }
}

        if (listener == null) {
            throw new NullPointerException();
        }
        listeners.add(listener);
    
<no_super_class>