apollo-core/src/main/java/com/ctrip/framework/foundation/internals/provider/DefaultApplicationProvider.java/DefaultApplicationProvider/initAppId
class DefaultApplicationProvider:  private void initAppId() {
    // 1. Get app.id from System Property
    m_appId = System.getProperty(ApolloClientSystemConsts.APP_ID);
    if (!Utils.isBlank(m_appId)) {
      m_appId = m_appId.trim();
      logger.info("App ID is set to {} by app.id property from System Property", m_appId);
      return;
    }

    //2. Try to get app id from OS environment variable
    m_appId = System.getenv(ApolloClientSystemConsts.APP_ID_ENVIRONMENT_VARIABLES);
    if (!Utils.isBlank(m_appId)) {
      m_appId = m_appId.trim();
      logger.info("App ID is set to {} by APP_ID property from OS environment variable", m_appId);
      return;
    }

    // 3. Try to get app id from app.properties.
    m_appId = m_appProperties.getProperty(ApolloClientSystemConsts.APP_ID);
    if (!Utils.isBlank(m_appId)) {
      m_appId = m_appId.trim();
      logger.info("App ID is set to {} by app.id property from {}", m_appId,
          APP_PROPERTIES_CLASSPATH);
      return;
    }

    m_appId = null;
    logger.warn("app.id is not available from System Property and {}. It is set to null",
        APP_PROPERTIES_CLASSPATH);
  }