apollo-client/src/main/java/com/ctrip/framework/apollo/util/ConfigUtil.java/ConfigUtil/getAppId
class ConfigUtil:  public String getAppId() {
    String appId = Foundation.app().getAppId();
    if (Strings.isNullOrEmpty(appId)) {
      appId = ConfigConsts.NO_APPID_PLACEHOLDER;
      if (warnLogRateLimiter.tryAcquire()) {
        logger.warn(
            "app.id is not set, please make sure it is set in classpath:/META-INF/app.properties, now apollo will only load public namespace configurations!");
      }
    }
    return appId;
  }