apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/NamespaceService.java/NamespaceService/isNamespaceUnique
class NamespaceService:  public boolean isNamespaceUnique(String appId, String cluster, String namespace) {
    Objects.requireNonNull(appId, "AppId must not be null");
    Objects.requireNonNull(cluster, "Cluster must not be null");
    Objects.requireNonNull(namespace, "Namespace must not be null");
    return Objects.isNull(
        namespaceRepository.findByAppIdAndClusterNameAndNamespaceName(appId, cluster, namespace));
  }