Java DatagramChannel组播默认为IPv6

Question

I'm trying to write an abstract class that will be inherited by three classes:

• UDPServer
• MulticastServer
• TCPServer

MulticastServer is the only one with which I am having issues. As of now, it is UDP only. When a connection is received, I would like it to hand the socket to a predefined Consumer<*SocketType*> stored in the member m_handler . I'm using a Selector to do this.

The main issue I'm running into at this time is that it seems to default to IPv6. I'm getting the message:

java.lang.IllegalArgumentException: IPv6 socket cannot join IPv4 multicast group

// m_chan is a member instance of the channel type. DatagramChannel for multicast.

if(tcpmode) // true if the generic socket type passed is ServerSocket
    ((ServerSocketChannel)m_chan).socket().bind(sa);
else{
    if(multicast) // true if the generic socket type passed is MulticastSocket
        ((DatagramChannel)m_chan).join(Inet4Address.getByName(m_host), getIPAddr());
    else
        ((DatagramChannel)m_chan).socket().bind(sa);
}

And here is the code for getIPAddr() :

static NetworkInterface getIPAddr() throws SocketException, UnknownHostException{   
    InetAddress iaddr = InetAddress.getLocalHost();     
    NetworkInterface iface = NetworkInterface.getByInetAddress(iaddr);
    return iface;
}

I've tried adding this:

System.setProperty("java.net.preferIPv4Stack" , "true");

to no avail.

I'm definitely using IPv4.

Answer 1

The following works fine for me. I only get the same exception you describe, when I change the StandardProtocolFamily.INET to StandardProtocolFamily.INET6, so I assume you somehow used the wrong option when creating the DatagramChannel?

public class Test {
    public static void main(String[] args) throws IOException {
        NetworkInterface ni = NetworkInterface.getNetworkInterfaces().nextElement();
        DatagramChannel server = DatagramChannel.open(StandardProtocolFamily.INET)
            .setOption(StandardSocketOptions.SO_REUSEADDR, true)
            .bind(new InetSocketAddress(5000))
            .setOption(StandardSocketOptions.IP_MULTICAST_IF, ni);
        server.join(Inet4Address.getByName("225.4.5.6"), ni);
    }
}