@Override
    public void subscribe(Node node, NotifyListener listener) {
        if (node == null) {
            throw new IllegalArgumentException("subscribe node == null");
        }
        if (listener == null) {
            throw new IllegalArgumentException("subscribe listener == null");
        }
        if (LOGGER.isInfoEnabled()) {
            LOGGER.info("Subscribe: " + node);
        }
        Set<NotifyListener> listeners = subscribed.get(node);
        if (listeners == null) {
            subscribed.putIfAbsent(node, new ConcurrentHashSet<NotifyListener>());
            listeners = subscribed.get(node);
        }
        listeners.add(listener);

    }
@author Robert HG (254963746@qq.com) on 5/17/15.
lts-core/src/main/java/com/github/ltsopensource/core/registry/AbstractRegistry.java