code: public static ClientTransport getReverseClientTransport(String container, AbstractChannel channel) {
          if (REVERSE_CLIENT_TRANSPORT_MAP == null) { // 初始化
              synchronized (ClientTransportFactory.class) {
                  if (REVERSE_CLIENT_TRANSPORT_MAP == null) {
                      REVERSE_CLIENT_TRANSPORT_MAP = new ConcurrentHashMap<String, ClientTransport>();
                  }
              }
          }
          String key = NetUtils.channelToString(channel.remoteAddress(), channel.localAddress());
          ClientTransport transport = REVERSE_CLIENT_TRANSPORT_MAP.get(key);
          if (transport == null) {
              synchronized (ClientTransportFactory.class) {
                  transport = REVERSE_CLIENT_TRANSPORT_MAP.get(key);
                  if (transport == null) {
                      ClientTransportConfig config = new ClientTransportConfig()
                          .setProviderInfo(new ProviderInfo().setHost(channel.remoteAddress().getHostName())
                              .setPort(channel.remoteAddress().getPort()))
                          .setContainer(container);
                      transport = ExtensionLoaderFactory.getExtensionLoader(ClientTransport.class)
                          .getExtension(config.getContainer(),
                              new Class[] { ClientTransportConfig.class },
                              new Object[] { config });
                      transport.setChannel(channel);
                      REVERSE_CLIENT_TRANSPORT_MAP.put(key, transport); // 保存唯一长连接
                  }
              }
          }
          return transport;
      }
docs: 构建反向的（服务端到客户端）虚拟长连接
  
  @param container Container of client transport
  @param channel   Exists channel from client
  @return reverse client transport of exists channel