apollo-client/src/main/java/com/ctrip/framework/apollo/internals/DefaultConfig.java/DefaultConfig/getPropertyNamesFromAdditional
class DefaultConfig:  protected Set<String> getPropertyNamesFromAdditional() {
    Properties properties = m_resourceProperties;
    if (properties == null) {
      return Collections.emptySet();
    }
    return this.stringPropertyNames(properties);
  }