Add unit test to the modules: 
dubbo-registry/dubbo-registry-multicast
   dubbo-registry/dubbo-registry-redis
  dubbo-registry/dubbo-registry-zookeeper 
}
    @Override
    public void register(URL url) {
        super.register(url);
        registered(url);
    }
    @Override
    public void unregister(URL url) {
        super.unregister(url);
        unregistered(url);
    }
    @Override
    public void subscribe(URL url, NotifyListener listener) {
        super.subscribe(url, listener);
        subscribed(url, listener);
    }
    @Override
    public void unsubscribe(URL url, NotifyListener listener) {
        super.unsubscribe(url, listener);
        received.remove(url);
    }
    @Override
    public List<URL> lookup(URL url) {
        List<URL> urls = new ArrayList<URL>();
        Map<String, List<URL>> notifiedUrls = getNotified().get(url);
        if (notifiedUrls != null && notifiedUrls.size() > 0) {
            for (List<URL> values : notifiedUrls.values()) {
                urls.addAll(values);
            }
        }