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?

I can confirm it doesn't time out for me when run with IDEA on 64-bit JDK 1.7.0_21 or a JDK8 build from a few weeks ago in Win 7. — Ryan Stewart, Jun 16 '13 at 13:34
...and the same in an Ubuntu server with OpenJDK 1.7.0_21 directly from the command line. — Ryan Stewart, Jun 16 '13 at 13:39

score 3 · Accepted Answer · answered Jun 16 '13 at 13:49

3

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.

answered Jun 16 '13 at 13:49

Ryan Stewart

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2

In this case, though, the result will be a `Socket`; you'll have to `.getChannel()` on it to get a `SocketChannel`. Strangely enough, since 1.7, you have a `.setOption()` on a `SocketChannel` but no way of setting the timeout:/ – fge Jun 16 '13 at 13:57
The `.socket().accept().getChannel()` workaround works well. Then it was a Java-weirdness (sadly)! It's a bit upsetting that people report this happening on JDK8 too. I mean, is the use of NIO so rare? – Stefano Sanfilippo Jun 16 '13 at 14:35

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

1 Answers1