apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/NamespaceService.java/NamespaceService/findByItem
class NamespaceService:  public Page<Namespace> findByItem(String itemKey, Pageable pageable) {
    Page<Item> items = itemService.findItemsByKey(itemKey, pageable);

    if (!items.hasContent()) {
      return Page.empty();
    }

    Set<Long> namespaceIds = BeanUtils.toPropertySet("namespaceId", items.getContent());

    return new PageImpl<>(namespaceRepository.findByIdIn(namespaceIds));
  }