请问一个 Java 线程安全问题

public static void register(Class<? extends Event> eventClass, Subscriber subscriber) {
        CopyOnWriteArraySet<Subscriber> set = SUBSCRIBER_MAP.get(eventClass);
        if (set == null) {
            set = new CopyOnWriteArraySet<Subscriber>();
            // 这里有点意思,判断了两次是不是 null,这样能够线程安全吗?
            CopyOnWriteArraySet<Subscriber> old = SUBSCRIBER_MAP.putIfAbsent(eventClass, set);
            if (old != null) {
                set = old;
            }
        }
        set.add(subscriber);
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Register subscriber: {} of event: {}.", subscriber, eventClass);
        }
    }

两次判断 null,是不是从线程安全的角度考虑的