apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/service/ItemService.java/ItemService/getNamespaceId
class ItemService:  private long getNamespaceId(NamespaceIdentifier namespaceIdentifier) {
    String appId = namespaceIdentifier.getAppId();
    String clusterName = namespaceIdentifier.getClusterName();
    String namespaceName = namespaceIdentifier.getNamespaceName();
    Env env = namespaceIdentifier.getEnv();
    NamespaceDTO namespaceDTO;
    try {
      namespaceDTO = namespaceAPI.loadNamespace(appId, env, clusterName, namespaceName);
    } catch (HttpClientErrorException e) {
      if (e.getStatusCode() == HttpStatus.NOT_FOUND) {
        throw new BadRequestException(String.format(
            "namespace not exist. appId:%s, env:%s, clusterName:%s, namespaceName:%s", appId, env, clusterName,
            namespaceName));
      }
      throw e;
    }
    return namespaceDTO.getId();
  }