Java多播发送数据，而不是接收数据

Question

我正在用Java编写一个类，用于大大简化多播过程。但是，我遇到了两个大问题：

1. 该类发送数据（我可以使用我的网络监视器 Wireshark 验证这一点），但同一组中的任何其他人都不会收到该数据。
2. 在某些机器上，发送数据包在传输过程中会超出 TTL（同样，根据 Wireshark 的说法）。

有人可以帮我吗？我已经尝试和寻找答案几个小时了，看来我的代码遵循连接、加入、发送和接收来自多播主机的数据的所有基本过程。

以下是该类相关部分的片段：

Multicaster 类：

public class Multicaster {
  public int port = 5540;

  protected String IPAddress;

  private MulticastSocket msConn;
  private InetAddress netAddr;

  public Multicaster(String IPAddress) {
    this.IPAddress = IPAddress;
  }

  public String recieveData() {
    byte[] buf = new byte[1000];
    DatagramPacket pack = new DatagramPacket(buf, buf.length);

    try {
      this.msConn.receive(pack);
      new Message(pack);
      String out = new String(pack.getData());
      return out.substring(0, pack.getLength());
    } catch (IOException e) {
      return new String("");
    }
  }

  public void joinGroup() { 
    try {
      this.msConn.joinGroup(this.netAddr);
    } catch (IOException e) {
    //This error shouldn't occur since it would caught by the connect() method during initial connection to the host
    }
  }

  public void connect() throws MulticasterInitException {
  //Try to create a multicast connection on the given IP address and port
    try {
      try {
      //Create a multicast connection on a given port, throws UnknownHostException
        this.msConn = new MulticastSocket(this.port);

      //If all goes well, then create a connection to a given IP address using the above port number, throws IOException and SecurityException
        this.netAddr = InetAddress.getByName(this.IPAddress);
      }
    }

  /**
   * Here all of the possible exceptions that are thrown above
   * are caught and handled here. This works just fine.
   */
  }

  public void sendData(String data) throws MulticasterSendException {
    DatagramPacket packet = new DatagramPacket(data.getBytes(), data.length(), this.netAddr, this.port);

    try {
      this.msConn.send(packet);
    } catch (IOException e) {
      throw new MulticasterSendException("Java could not communicate with the server. Please check your network connections.", e);
    }
  }
}

发送数据的示例用法：

Multicaster multicast = new Multicaster("239.0.0.0");

try {
  multicast.connect();
} catch (MulticasterInitException e) {
  //Handle exception...
}

multicast.joinGroup();

try {
  multicast.sendData("Hi");
} catch (MulticasterSendException e) {
  //Handle exception... 
}

接收数据的示例用法：

Multicaster multicast = new Multicaster("239.0.0.0");

try {
  multicast.connect();
} catch (MulticasterInitException e) {
  //Handle exception...
}

multicast.joinGroup();
System.out.print(multicast.recieveData());

Answer 1

我之前遇到过类似的问题，并且必须确保在接收端指定了 NetworkInterface。

SocketAddress socketAddress = new InetSocketAddress(groupIp, groupPort);
NetworkInterface networkInterface = NetworkInterface.getByName(interfaceName);

socket.joinGroup(socketAddress, networkInterface);

其中 interfaceName 是在 Linux 或 Mac 上运行 ifconfig 时显示的接口名称之一。