code: protected boolean doubleCheck(String interfaceId, ProviderInfo providerInfo, ClientTransport transport) {
          if (transport.isAvailable()) {
              try { // 睡一下下 防止被连上又被服务端踢下线
                  Thread.sleep(100);
              } catch (InterruptedException e) {
                  // ignore
              }
              if (transport.isAvailable()) { // double check
                  return true;
              } else { // 可能在黑名单里，刚连上就断开了
                  if (LOGGER.isWarnEnabled(consumerConfig.getAppName())) {
                      LOGGER.warnWithApp(consumerConfig.getAppName(),
                          "Connection has been closed after connected (in last 100ms)!" +
                              " Maybe connectionNum of provider has been reached limit," +
                              " or your host is in the blacklist of provider {}/{}",
                          interfaceId, transport.getConfig().getProviderInfo());
                  }
                  providerInfo.setDynamicAttr(ProviderInfoAttrs.ATTR_RC_PERIOD_COEFFICIENT, 5);
                  return false;
              }
          } else {
              return false;
          }
      }
docs: 两次验证检查ClientTransport是否存活
  
  @param interfaceId 接口
  @param transport   ClientTransport对象
  @return 是否存活