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

    //2. Try to get app label from OS environment variable
    m_appLabel = System.getenv(ApolloClientSystemConsts.APOLLO_LABEL_ENVIRONMENT_VARIABLES);
    if (!Utils.isBlank(m_appLabel)) {
      m_appLabel = m_appLabel.trim();
      logger.info("App Label is set to {} by APP_LABEL property from OS environment variable", m_appLabel);
      return;
    }

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

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