apollo-client/src/main/java/com/ctrip/framework/apollo/internals/LocalFileConfigRepository.java/LocalFileConfigRepository/setUpstreamRepository
class LocalFileConfigRepository:  @Override
  public void setUpstreamRepository(ConfigRepository upstreamConfigRepository) {
    if (upstreamConfigRepository == null) {
      return;
    }
    //clear previous listener
    if (m_upstream != null) {
      m_upstream.removeChangeListener(this);
    }
    m_upstream = upstreamConfigRepository;
    upstreamConfigRepository.addChangeListener(this);
  }