Setting timeout on blocking I/O ops with ServerSocketChannel doesn't work as expected

Question

I'm working on a small group conversation server in Java and I'm currently hacking network code, but it seems like I cannot set right timeout on blocking I/O ops: chances are I've been bitten by some Java weirdness (or, simply, I misinterpret javadoc).

So, this is the pertinent code from ConversationServer class (with all security checks and logging stripped for simplicity):

class ConversationServer {

    // ...

    public int setup() throws IOException {
        ServerSocketChannel server = ServerSocketChannel.open();
        server.bind(new InetSocketAddress(port), Settings.MAX_NUMBER_OF_PLAYERS + 1);

        server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);

        int numberOfPlayers;

        for (numberOfPlayers = 0; numberOfPlayers < Settings.MAX_NUMBER_OF_PLAYERS; ++numberOfPlayers) {
            SocketChannel clientSocket;

            try {
                clientSocket = server.accept();
            } catch (SocketTimeoutException timeout) {
                break;
            }

            clients.add(messageStreamFactory.create(clientSocket));
        }

        return numberOfPlayers;
    }

    // ...

}

The expected behaviour is to let connect Settings.MAX_NUMBER_OF_PLAYERS clients at most, or terminate setup anyway after Settings.AWAIT_PLAYER_MS milliseconds (currently, 30000L).

What happens, is that if I connect Settings.MAX_NUMBER_OF_PLAYERS clients, everything is fine (exit because of for condition), but if I don't, the SocketTimeoutException I'd expect is never thrown and the server hangs forever.

If I understand right, server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS); should be sufficient, but it doesn't give the expected behaviour.

So, can anyone spot the error here?

Answer 1

It looks like the timeout works if you change from

server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);
server.accept();

to

server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);
server.socket().accept();

I.e. call accept() on the same object on which you set the SO timeout. I don't know enough about NIO sockets to say exactly what results from doing this. Maybe somebody else can shed some light.