2хсторонняя передача файла NIO.2

Можно каналы использовать и в многопоточной среде, только разносить по отдельным потокам надо не приём и передачу, а обработку каждого нового подключения:

public class ExecutorNioServer {
    private static final int PORT = 1234;
    private static final String FILE_NAME = "received_%d.jpg";
    public static void main(String args[]) throws IOException {
        ExecutorService executor = null;
        try {
            executor = Executors.newFixedThreadPool(10);
            try (ServerSocketChannel serverSocketChannel = ServerSocketChannel.open()) {
                serverSocketChannel.bind(new InetSocketAddress(PORT));
                while (true) {
                    SocketChannel socketChannel = serverSocketChannel.accept();
                    executor.execute(() -> {
                        String fileName = String.format(FILE_NAME, System.currentTimeMillis());
                        try (FileChannel fileChannel = FileChannel.open(Paths.get(fileName),
                                                                        StandardOpenOption.CREATE,
                                                                        StandardOpenOption.WRITE);
                             socketChannel) {
                            fileChannel.transferFrom(socketChannel, 0, Long.MAX_VALUE);
                        }
                        catch (IOException exc) {
                            exc.printStackTrace();
                        }
                    });
                }
            }
        }
        finally {
            if (executor != null)
                executor.shutdown();
        }
    }
}

Но более интересный и естественный для каналов вариант - это не потоки, а асинхронность:

public class SelectorNioServer {
    private static final int PORT = 1234;
    private static final String FILE_NAME = "received_%d.jpg";
    private static class Storage {
        private final AsynchronousFileChannel fileChannel;
        private Future<Integer> writeOperaion;
        private long position;
        public Storage() throws IOException {
            String fileName = String.format(FILE_NAME, System.currentTimeMillis());
            fileChannel = AsynchronousFileChannel.open(Paths.get(fileName),
                                                       StandardOpenOption.CREATE,
                                                       StandardOpenOption.WRITE);
        }
        public void write(ByteBuffer buffer) throws InterruptedException, ExecutionException {
            try {
                if (writeOperaion != null)
                    position += writeOperaion.get(5, TimeUnit.SECONDS);
                writeOperaion = fileChannel.write(buffer, position);
            }
            catch (TimeoutException exc) {
                close();
                System.err.println("Write timeout");
            }
        }
        public void close() {
            try {
                fileChannel.close();
            }
            catch (IOException exc) {
                exc.printStackTrace();
            }
        }
    }
    public static void main(String args[]) throws Exception {
        Selector selector = Selector.open();
        try (ServerSocketChannel serverSocketChannel = ServerSocketChannel.open()) {
            serverSocketChannel.bind(new InetSocketAddress(PORT));
            serverSocketChannel.configureBlocking(false);
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
            while (true) {
                selector.select();
                Set<SelectionKey> keys = selector.selectedKeys();
                Iterator<SelectionKey> iterator = keys.iterator();
                while (iterator.hasNext()) {
                    SelectionKey key = iterator.next();
                    if (key.isAcceptable()) {
                        SocketChannel socketChannel = serverSocketChannel.accept();
                        socketChannel.configureBlocking(false);
                        socketChannel.register(selector, SelectionKey.OP_READ, new Storage());
                    }
                    if (key.isReadable()) {
                        SocketChannel socketChannel = (SocketChannel) key.channel();
                        Storage storage = (Storage) key.attachment();
                        ByteBuffer buffer = ByteBuffer.allocate(256);
                        int read = socketChannel.read(buffer);
                        if (read != -1) {
                            buffer.flip();
                            storage.write(buffer);
                        }
                        else {
                            storage.close();
                            socketChannel.close();
                        }
                    }
                    iterator.remove();
                }
            }
        }
    }
}

Я здесь использовал и мультиплексирование сетевых каналов, и асинхронные файловые каналы, чтобы показать обе возможности реализации асинхронности в NIO.2