NioServer.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;

// Just for demo of nio
public class NioServer {

    public static void main(String[] args) throws IOException {
        final int port = 5555;
        final String host = "localhost";
        new NioServer(host, port).run();
    }

    String host;
    int port;

    NioServer(String host, int port) {
        this.host = host;
        this.port = port;
    }

    public void run() throws IOException {
        ServerSocketChannel serverChannel = null;
        Selector selector = null;
        try {
            serverChannel = ServerSocketChannel.open();
            final ServerSocket s = serverChannel.socket();
            final InetSocketAddress address = new InetSocketAddress(port);
            s.bind(address);
            serverChannel.configureBlocking(false);
            selector = Selector.open();
            serverChannel.register(selector, SelectionKey.OP_ACCEPT);
        } catch (Exception ex) {
            ex.printStackTrace();
        }

        while (selector.select() > 0) {
            final Set<SelectionKey> readKeys = selector.selectedKeys();
            final Iterator<SelectionKey> iterator = readKeys.iterator();
            while (iterator.hasNext()) {
                final SelectionKey key = iterator.next();
                iterator.remove();
                try {
                    if (key.isAcceptable()) {
                        final ServerSocketChannel server = (ServerSocketChannel) key.channel();
                        final SocketChannel client = server.accept();
                        System.out.println("Accepted connection from :" + client);
                        client.configureBlocking(false);
                        final SelectionKey clientKey = client.register(selector, SelectionKey.OP_READ);
                        final ByteBuffer buffer = ByteBuffer.allocate(100);
                        clientKey.attach(buffer);
                    } else if (key.isReadable()) {
                        final SocketChannel client = (SocketChannel) key.channel();
                        final ByteBuffer buffer = (ByteBuffer) key.attachment();
                        if (client.read(buffer) < 0) {
                            client.close();
                            continue;
                        }
                        buffer.flip();
                        client.write(buffer);
                        buffer.compact();
                    }
                } catch (Exception ex) {
                    ex.printStackTrace();
                    key.cancel();
                    try {
                        key.channel().close();
                    } catch (Exception exx) {
                        exx.printStackTrace();
                    }
                }
            }
        }
    }
}