code: public void registerListener(ConfigurationListener<?> listener) {
      if(!listeners.contains(listener)) {
        listeners.add(listener);
        Set<ConfigurationListener<?>> newListener = new HashSet<ConfigurationListener<?>>();
        newListener.add(listener);
        notifyListeners(newListener, liveConfigParser, log);
      }
    }
docs: Registers a {@link ConfigurationListener} instance with the current instance of ConfigurationManager.
  @param listener