code: public List<LoaderListener> getListeners(String dataId) {
          if (Objects.isNull(dataId)) {
              dataId = "";
          }
          List<LoaderListener> listeners = this.listenersMap.get(dataId);
          if (Objects.isNull(listeners)) {
              synchronized (listenersMap) {
                  listeners = this.listenersMap.get(dataId);
                  if (Objects.isNull(listeners)) {
                      listeners = Collections.synchronizedList(new ArrayList<>());
                      this.listenersMap.put(dataId, listeners);
                  }
              }
          }
          return listeners;
      }
docs: Evaluate the {@link LoaderListener} instances, listening fo a dataId
  given.
  
  @param dataId The dataId, not null
  @return the according listeners