docs: public void addNotificationHandler(String serviceName, NotificationHandler handler){
  
          if(handler == null || serviceName == null || serviceName.isEmpty()){
              throw new IllegalArgumentException();
          }
  
          synchronized(notificationHandlers){
              if(! notificationHandlers.containsKey(serviceName)){
                  notificationHandlers.put(serviceName, new ArrayList<NotificationHandler>());
              }
  
              notificationHandlers.get(serviceName).add(handler);
          }
      }