apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/AppService.java/AppService/isAppIdUnique
class AppService:  public boolean isAppIdUnique(String appId) {
    Objects.requireNonNull(appId, "AppId must not be null");
    return Objects.isNull(appRepository.findByAppId(appId));
  }