java - 局域网上的点对点发现

Question

因此，正如标题所暗示的那样，我在我的 Java 项目中遇到了一些麻烦。我想做的是:

• 我有两台运行应用程序 X 的计算机
• 还有另外三台运行应用程序 Y 的计算机

我需要做的是在X和Y之间建立连接。例如，某人使用运行X的计算机，在发现过程之后，将返回一个运行Y应用程序的计算机列表，并且他们的 IP，反之亦然。

我已经使用 UDP 广播完成了此操作，但有时会失败。计算机通过 WiFi 连接，所以基本上是通过路由器。在许多情况下，任何 X 计算机都可以通过我的 UDP 发现方法看到 Y 计算机，但有时看不到，除非我手动指向 IP，有时甚至看不到。

下面是一段代码，用于发现监听特定端口的服务器:

public static ArrayList<InetAddress> searchComputer() {
    ArrayList<InetAddress> targets = new ArrayList<InetAddress>();
    try {
        c = new DatagramSocket();
        c.setBroadcast(true);

        byte[] sendData = "DISCOVER_PC_SERVER_REQUEST".getBytes();
        try {
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, InetAddress.getByName("255.255.255.255"), 2005);
            c.send(sendPacket);
        } catch (Exception e) {}
        Enumeration interfaces = NetworkInterface.getNetworkInterfaces();

        while (interfaces.hasMoreElements()) {
            NetworkInterface networkInterface = (NetworkInterface) interfaces.nextElement();

            if (networkInterface.isLoopback() || !networkInterface.isUp()) {
                continue; 
            }

            for (InterfaceAddress interfaceAddress : networkInterface.getInterfaceAddresses()) {
                InetAddress broadcast = interfaceAddress.getBroadcast();
                if (broadcast == null) {
                    continue;
                }

                try {
                    DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, broadcast, 8888);
                    c.send(sendPacket);
                } catch (Exception e) { }
            }
        }
        byte[] recvBuf = new byte[15000];
        DatagramPacket receivePacket = new DatagramPacket(recvBuf, recvBuf.length);
        if (useInstant) {
            c.setSoTimeout(500);
        }
        else {
            c.setSoTimeout(4000); //EXECUTE THE WHILE FOR 4 SECONDS, THEN RETURN WHATEVER THE RESULTS ARE.
        }
        while (true) {
            c.receive(receivePacket);
            String message = new String(receivePacket.getData()).trim();
            if (message.equals("DISCOVER_PC_SERVER_RESPONSE")) {
//              return receivePacket.getAddress();
                targets.add(receivePacket.getAddress());
            }
        }
//      c.close();
    } catch (IOException ex){}
    return targets;
}

这是我的“服务器”:

private void start_Discovery() throws Exception {
    //Keep a socket open to listen to all the UDP trafic that is destined for this port
    socket = new DatagramSocket(2005, InetAddress.getByName("0.0.0.0"));
    socket.setBroadcast(true);

    while (true) {
//      System.out.println(getClass().getName() + ">>>Ready to receive broadcast packets!");

        //Receive a packet
        byte[] recvBuf = new byte[15000];
        DatagramPacket packet = new DatagramPacket(recvBuf, recvBuf.length);
        socket.receive(packet);

        //Packet received
//      System.out.println(getClass().getName() + ">>>Discovery packet received from: " + packet.getAddress().getHostAddress());
//      System.out.println(getClass().getName() + ">>>Packet received; data: " + new String(packet.getData()));

        //See if the packet holds the right command (message)
        String message = new String(packet.getData()).trim();
        if (message.equals("DISCOVER_ANDROID_SERVER_REQUEST")) {
            byte[] sendData = "DISCOVER_ANDROID_SERVER_RESPONSE".getBytes();

            //Send a response
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, packet.getAddress(), packet.getPort());
            socket.send(sendPacket);

//          System.out.println(getClass().getName() + ">>>Sent packet to: " + sendPacket.getAddress().getHostAddress());
        }
    }
}

为什么有时他们看不到对方，即使他们连接到同一个路由器？EXTRA: 有没有特殊情况，X 台计算机通过 LAN 连接，Y 台计算机通过 WiFi 连接？

Answer 1

Why sometimes they can't see each other, even if they are connected to the same router?

因为广播是使用无连接协议(protocol)的 UDP 服务完成的。使用 UDP，您只需将数据包(数据报)发送到网络上的某个 IP 地址。您无法保证数据一定会到达。

Is there a special case, the X computers are connected via LAN, and the Y ones via WiFi?

甚至，X台计算机通过局域网连接，Y台计算机通过WiFi连接，它们都属于同一个路由器网络。因此，网络发现和网络服务将可用。不会有任何问题的。这一切都很公平，与您遇到的情况没有什么不同!