Java multiple multicast sockets in same group on same host and port

Question

I'm using multicast to implement a simple discovery service. Several instances on the application must be able to run on the same host, I therefore end up with several multicast sockets that are members of the same group on the same host.

On Linux, this works as expected. Every instance of the application on a host receives the messages sent to the multicast address.

On Windows however, only the first application to join the multicast group receives the messages sent.

Here is the code I'm using to create my multicast socket:

socket = new MulticastSocket(PORT);
InetAddress group = InetAddress.getByName(ADDRESS);
socket.joinGroup(group);
socket.setTimeToLive(TTL);

Is there some option I need to set? I've tried scouring the API but I can't find anything.

EDIT: According to this website, I need to set SO_REUSEADDR, but this can only be set before the socket binds. It seems the constructor for MulticastSocket seems to bind the socket upon creation.

Answer 1

you do this by creating the socket with no arguments and then call bind on that object.

socket = new MulticastSocket();
socket.setReuseAddress(true);//redundant, already set with empty constructor
SocketAddress sockAddr = new InetSocketAddress(PORT);
socket.bind(sockAddr);
InetAddress group = InetAddress.getByName(ADDRESS);
socket.joinGroup(group);
socket.setTimeToLive(TTL);