How should I use AsynchronousServerSocketChannel for accepting connections?

Question

I would like to write an asynchronous server using Java 7 and NIO 2.

But how should I use AsynchronousServerSocketChannel?

E.g. if I start with:

final AsynchronousServerSocketChannel server = 
    AsynchronousServerSocketChannel.open().bind(
        new InetSocketAddress(port));

Then when I do server.accept(), the program terminates because that call is asynchronous. And if I put that code in an infinite loop, an AcceptPendingException is thrown.

Any suggestions on how to write a simple asynchronous server using AsynchronousServerSocketChannel?

Here is my full example (similar to the example in the JavaDoc):

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.AsynchronousServerSocketChannel;
import java.nio.channels.AsynchronousSocketChannel;
import java.nio.channels.CompletionHandler;

public class AsyncServer {

    public static void main(String[] args) {
        int port = 8060;
        try {
            final AsynchronousServerSocketChannel server = 
                    AsynchronousServerSocketChannel.open().bind(
                            new InetSocketAddress(port));

            System.out.println("Server listening on " + port);

            server.accept("Client connection", 
                    new CompletionHandler<AsynchronousSocketChannel, Object>() {
                public void completed(AsynchronousSocketChannel ch, Object att) {
                    System.out.println("Accepted a connection");

                    // accept the next connection
                    server.accept("Client connection", this);

                    // handle this connection
                    //TODO handle(ch);
                }

                public void failed(Throwable exc, Object att) {
                    System.out.println("Failed to accept connection");
                }
            });
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

You are on the right track, calling accept() from the completed callback in order to accept more connections should work.

A simple (but ugly) way to prevent the thread from terminating is simply to loop until the thread is interrupted.

// yes, sleep() is evil, but sometimes I don't care
while (true) {
    Thread.sleep(1000);
}

A cleaner way is to use AsynchronousChannelGroup. For instance:

AsynchronousChannelGroup group = AsynchronousChannelGroup.withThreadPool(Executors
            .newSingleThreadExecutor());
AsynchronousServerSocketChannel server = AsynchronousServerSocketChannel.open(group).bind(
            new InetSocketAddress(port));

// (insert server.accept() logic here)

// wait until group.shutdown()/shutdownNow(), or the thread is interrupted:
group.awaitTermination(Long.MAX_VALUE, TimeUnit.SECONDS);

You can tune how threads are handled, see the AsynchronousChannelGroup API docs for more information.

Answer 2

Using asynchronous accept is useful if you have something else to do in the same thread. In you case, you are not doing something else so I would use

while(true) {
    AsynchronousSocketChannel socket = server.accept().get();
    System.out.println("Accepted " + socket);
    socket.close();
}

Answer 3

Another alternative is to have your main method wait on a signal before returning. Then if you have some kind of external shutdown command, you just notify the signal and the main thread shuts down.

private static final Object shutdownSignal = new Object();

public static void main(String[] args) {

    ...

    synchronized (shutdownSignal) {
        try {
            shutdownSignal.wait();
        }
        catch (InterruptedException e) {
            // handle it!
        }
    }
}

Answer 4

Use count down latch like the following example

    final AsynchronousServerSocketChannel serverChannel = AsynchronousServerSocketChannel.open();
    InetSocketAddress address = new InetSocketAddress(port);
    serverChannel.bind(address);
    final CountDownLatch latch = new CountDownLatch(1);
    serverChannel.accept(null, new CompletionHandler<AsynchronousSocketChannel, Object>() {
@Override
        public void completed(final AsynchronousSocketChannel channel, Object attachment) {
            serverChannel.accept(null, this);
                        }

});
try {
        latch.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
        Thread.currentThread().interrupt();
    }