apollo-client/src/main/java/com/ctrip/framework/apollo/internals/PropertiesConfigFile.java/PropertiesConfigFile/getContent
class PropertiesConfigFile:  @Override
  public String getContent() {
    if (m_contentCache.get() == null) {
      m_contentCache.set(doGetContent());
    }
    return m_contentCache.get();
  }