apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/ItemService.java/ItemService/findOne
class ItemService:  public Item findOne(String appId, String clusterName, String namespaceName, String key) {
    Namespace namespace = namespaceService.findOne(appId, clusterName, namespaceName);
    if (namespace == null) {
      throw new NotFoundException(
          String.format("namespace not found for %s %s %s", appId, clusterName, namespaceName));
    }
    return itemRepository.findByNamespaceIdAndKey(namespace.getId(), key);
  }