docs: public void addNotificationHandler(String serviceName, NotificationHandler handler){
  
          ServiceInstanceUtils.validateServiceName(serviceName);
          if (handler == null) {
              throw new ServiceException(ErrorCode.SERVICE_DIRECTORY_NULL_ARGUMENT_ERROR,
                      ErrorCode.SERVICE_DIRECTORY_NULL_ARGUMENT_ERROR.getMessageTemplate(),
                      "NotificationHandler");
          }
    
          synchronized(notificationHandlers){
              if(! notificationHandlers.containsKey(serviceName)){
                  notificationHandlers.put(serviceName, new ArrayList<NotificationHandler>());
              }
              notificationHandlers.get(serviceName).add(handler);
          }
      }