apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/service/AppNamespaceServiceWithCache.java/AppNamespaceServiceWithCache/findByAppIdAndNamespace
class AppNamespaceServiceWithCache:  public AppNamespace findByAppIdAndNamespace(String appId, String namespaceName) {
    Preconditions.checkArgument(!StringUtils.isContainEmpty(appId, namespaceName), "appId and namespaceName must not be empty");
    return appNamespaceCache.get(STRING_JOINER.join(appId, namespaceName));
  }