/**
   * 更新缓存
   *
   * @param parent
   */
  private void refreshCache(Object parent) {

    if (parent != currentParent) {
      currentParent = (FileEntry) parent;
      FileEntry[] tmpChilds = fileListingService.getChildren(currentParent, false, null);
      List<FileEntry> childs = Arrays.asList(tmpChilds);
      currentChilds =
          childs
              .stream()
              .filter(fileEntry -> fileEntry.isDirectory())
              .collect(Collectors.toList())
              .toArray(new FileEntry[] {});
    }
  }
 @Override
 public Object getRoot() {
   if (fileListingService == null) return null;
   return fileListingService.getRoot();
 }