apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/service/AppNamespaceServiceWithCache.java/AppNamespaceServiceWithCache/findByAppIdAndNamespaces
class AppNamespaceServiceWithCache:  public List<AppNamespace> findByAppIdAndNamespaces(String appId, Set<String> namespaceNames) {
    Preconditions.checkArgument(!Strings.isNullOrEmpty(appId), "appId must not be null");
    if (namespaceNames == null || namespaceNames.isEmpty()) {
      return Collections.emptyList();
    }
    List<AppNamespace> result = Lists.newArrayList();
    for (String namespaceName : namespaceNames) {
      AppNamespace appNamespace = appNamespaceCache.get(STRING_JOINER.join(appId, namespaceName));
      if (appNamespace != null) {
        result.add(appNamespace);
      }
    }
    return result;
  }