用java实现tcp穿越nat

我实现的过程描述一下，有A.B两个客户端分别挂在NA,NB两个nat下面，通过固定IP的服务器S，交换A,B客户端外的nat的公网IP和端口，然后由B向A发起连接，进行打洞(这次绝大多数应该是失败的，我也确实失败了，超时)，然后由A再连接B，理论上B已经打过通往A的洞了，这次连接应该能够通过NB跟B连接上，但是我这边失败了，一直连接不上。（以上A,B跟S通讯交换IP已经A，B互相连接和建立监听我都用的同一个端口）。网上有说java的API不支持tcp包组装，所以无法打洞的，我很疑惑，希望做过类似的大家一起过来讨论一下

手机不方便 大致说下b收到服务器返回的a的公网ip和端口 b调用connect方法 大致是这样 

         final int LOCAL_PORT = 8880;         SocketAddress localAddr = new InetSocketAddress(LOCAL_PORT);         ServerSocket ss = new ServerSocket();         ss.setReuseAddress(true);         ss.bind(localAddr);         Socket socket = new Socket();         socket.setReuseAddress(true);         socket.bind(localAddr);         final String SERVER_IP = "";         final int SERVER_PORT = 60000;         SocketAddress serverAddr = new InetSocketAddress(SERVER_IP,SERVER_PORT);         socket.connect(serverAddr);         sendAndRegist(socket);         List<InetSocketAddress> addresses = receiveAndGetAddresses(socket);         createThreadToDoSomeThing(socket,addresses);         while(true){             Socket s = ss.accept();             createThreadToAccept(s);         }

public static void serverDemo() throws IOException{     final String SERVER_IP = "";     final int SERVER_PORT = 60000;     SocketAddress localAddr = new InetSocketAddress(SERVER_IP,SERVER_PORT);     ServerSocket ss = new ServerSocket();     ss.bind(localAddr);     while(true){         Socket s = ss.accept();         createThreadToAccept(s);     } } private static void createThreadToAccept(Socket s) {     ClientInfo info = readRegistInfo(s);     info.setSocketAdress(s.getRemoteSocketAddress());     doRegist(info);     List<SocketAddress> online = findOnlineAddresses();     writeOnlineAddresses(online); }