尝试接收 UDP 多播时出现空指针异常

问题描述

在尝试让一个简单的 UDP 多播接收器工作后，我感到很困惑.在我自己的代码没有按预期工作后，我尝试了 vertx 文档中发布的确切示例:

i'm puzzeled after a few attempts to get a simple UDP multicast receiver working.After my own code did not work as expected i tried out the exact example posted in the vertx documentation:

DatagramSocket socket = vertx.createDatagramSocket(new DatagramSocketOptions());
socket.listen(1234, "0.0.0.0", asyncResult -> {
  if (asyncResult.succeeded()) {
    socket.handler(packet -> {
      // Do something with the packet
    });

    // join the multicast group
    socket.listenMulticastGroup("230.0.0.1", asyncResult2 -> {
        System.out.println("Listen succeeded? " + asyncResult2.succeeded());
    });
  } else {
    System.out.println("Listen failed" + asyncResult.cause());
  }
});

执行后，我得到以下异常:

After executing, i got the following exception:

java.lang.NullPointerException: networkInterface
    at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:409)
    at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:368)
    at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:362)
    at io.vertx.core.datagram.impl.DatagramSocketImpl.listenMulticastGroup(DatagramSocketImpl.java:90)

当提供正确的网络接口 IP 地址(例如 192.168.178.52)而不是 0.0.0.0 时，我可以让它工作.然而，这意味着遍历所有网络接口并为每个接口添加一个套接字.

I can get it to work when providing the right IP address of the network interface (e.g. 192.168.178.52) instead of 0.0.0.0. However, this means iterating over all network interfaces and adding a socket per interface.

有什么想法吗?谢谢！

推荐答案

UDP 多播监听极其容易出错，尤其是在虚拟机中.

UDP multicast listening is incredibly error prone, especially in virtual machines.

这是来自我的生产代码.首先，要获得一个有效的接口:

This is from my production code. First, to get a valid interface:

static NetworkInterface mainInterface() throws SocketException {
    final ArrayList<NetworkInterface> interfaces = Collections.list(NetworkInterface.getNetworkInterfaces());
    final NetworkInterface networkInterface = interfaces.stream().filter(ni -> {
        boolean isLoopback = false;
        boolean supportsMulticast = false;
        boolean isVirtualbox = false;
        try {
            isLoopback = ni.isLoopback();
            supportsMulticast = ni.supportsMulticast();
            isVirtualbox = ni.getDisplayName().contains("VirtualBox") || ni.getDisplayName().contains("Host-Only");
        } catch (IOException loopbackTestFailure) {
        }
        final boolean hasIPv4 = ni.getInterfaceAddresses().stream().anyMatch(ia -> ia.getAddress() instanceof Inet4Address);
        return supportsMulticast && !isLoopback && !ni.isVirtual() && hasIPv4 && !isVirtualbox;
    }).sorted(Comparator.comparing(NetworkInterface::getName)).findFirst().orElse(null);
    return networkInterface;
}

然后，最防弹的数据报套接字创建:

Then, the most bullet-proof datagram socket creation:

String multicastAddress = "230.0.0.1";
NetworkInterface networkInterface = mainInterface();
Future isListening = Future.future();
vertx.createDatagramSocket(new DatagramSocketOptions()
            .setReuseAddress(true)
            .setReusePort(true))
            .listen(6112, "0.0.0.0", next -> {
                next.result().listenMulticastGroup(multicastAddress, networkInterface.getName(), null, listener -> {
                    final DatagramSocket socket = listener.result();
                    socket.handler(packet -> {
                        // Your handler here
                    });

                    isListening.complete();
                });
            });

请注意我为 VirtualBox 选择的高于 1024 的端口、重用标志和特殊外壳.

Observe my choice of a higher-than-1024 port, the reuse flags, and the special casing for VirtualBox.

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