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