code: protected ClientTransport selectByProvider(SofaRequest message, ProviderInfo providerInfo) {
          ClientTransport transport = connectionHolder.getAvailableClientTransport(providerInfo);
          if (transport != null) {
              if (transport.isAvailable()) {
                  lastProviderInfo = providerInfo;
                  checkAlias(providerInfo, message); //检查分组
                  return transport;
              } else {
                  connectionHolder.setUnavailable(providerInfo, transport);
              }
          }
          return null;
      }
docs: 得到provider得到连接
  
  @param message      调用对象
  @param providerInfo 指定Provider
  @return 一个可用的transport或者null