code: void readTreeLevel(CmsUUID parentId, String openPathFragment) {
  
          CmsObject cms = A_CmsUI.getCmsObject();
          boolean openedFragment = false;
          try {
              CmsResource parent = cms.readResource(parentId, CmsResourceFilter.IGNORE_EXPIRATION);
              String folderPath = cms.getSitePath(parent);
              List<CmsResource> folderResources = cms.readResources(folderPath, FOLDERS, false);
  
              // sets the parent to leaf mode, in case no child folders are present
              m_treeContainer.setChildrenAllowed(parentId, !folderResources.isEmpty());
  
              for (CmsResource resource : folderResources) {
                  addTreeItem(cms, resource, parentId, false, m_treeContainer);
                  openedFragment = openedFragment || resource.getName().equals(openPathFragment);
              }
              if (!openedFragment && (openPathFragment != null)) {
                  CmsResource resource = cms.readResource(
                      CmsStringUtil.joinPaths(folderPath, openPathFragment),
                      CmsResourceFilter.IGNORE_EXPIRATION);
                  addTreeItem(cms, resource, parentId, true, m_treeContainer);
              }
  
              m_fileTree.markAsDirtyRecursive();
          } catch (CmsException e) {
              CmsErrorDialog.showErrorDialog(e);
              LOG.error(e.getLocalizedMessage(), e);
          }
      }
docs: Reads the given tree level.<p>
  
  @param parentId the parent id
  @param openPathFragment the open path fragment