Client <-> Server - jak to wykonać?

uint8 - typ pakietu
uint16 - rozmiar pakietu

Jeżeli typ pakietu to `LOG_IN` (`1`), wtedy dalsza część to:
   uint16 - przydzielone ID gracza

Jeżeli typ pakietu to `ENTITY_MOVE` (`2`), wtedy dalsza część to:
   uint16 - ID jednostki w grze
   int32 - nowa pozycja X
   int32 - nowa pozycja Y

Jeżeli typ pakietu to `ENTITY_DESTROYED` (`3`), wtedy dalsza część to:
   uint16 - ID jednostki w grze

(i tak dalej)

package core;

import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketAddress;
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.*;
import java.util.concurrent.*;
import device.*;
import configuration.*;

public class YourServer {
    
    public final int MAX_DEVICES = 100000;
    
    private InetAddress addr;
    private int port;
    private Selector selector;
    public Map<SocketChannel,List<byte[]>> dataMap;
    private Map<SocketChannel, Device> deviceMap;
    private DeviceManager deviceManager;
    private ExecutorService executorService;
    public Configuration configuration;
    
    public int clientsCounter = 0;

    public YourServer(InetAddress addr, int port) throws IOException {
        this.addr = addr;
        this.port = port;
        dataMap = new HashMap<SocketChannel,List<byte[]>>();
        deviceMap = new HashMap<SocketChannel, Device>();
        this.deviceManager = new DeviceManager(this);
        this.configuration = new Configuration();
        try{
            this.configuration.loadConfiguration();
        }
        catch( Exception ignored ) {
            
        }
        
        //ustawiamy pule 32 wątków
        executorService = Executors.newFixedThreadPool(32);
        startServer();
        executorService.shutdown();
    }

    private void startServer() throws IOException {
        this.selector = Selector.open();
        ServerSocketChannel serverChannel = ServerSocketChannel.open();
        serverChannel.configureBlocking(false);

        InetSocketAddress listenAddr = new InetSocketAddress(this.addr, this.port);
        serverChannel.socket().bind(listenAddr);
        serverChannel.register(this.selector, SelectionKey.OP_ACCEPT);

        while (true) {
            this.selector.select();

            Iterator keys = this.selector.selectedKeys().iterator();
            while (keys.hasNext()) {
                SelectionKey key = (SelectionKey) keys.next();

                keys.remove();

                if (! key.isValid()) {
                    continue;
                }
                
                if (key.isAcceptable()) {
                    this.accept(key);
                }
                else if (key.isReadable()) {
                    this.read(key);
                }
                else if (key.isWritable()) {
                    this.write(key);
                }
            }
        }
    }

    private void accept(SelectionKey key) throws IOException {
        ServerSocketChannel serverChannel = (ServerSocketChannel) key.channel();
        SocketChannel channel = serverChannel.accept();
        channel.configureBlocking(false);

        Socket socket = channel.socket();
        SocketAddress remoteAddr = socket.getRemoteSocketAddress();
        log(clientsCounter++ +" Connected to: " + remoteAddr);

        dataMap.put(channel, new ArrayList<byte[]>());
        channel.register(this.selector, SelectionKey.OP_READ);
        
        deviceMap.put(channel, new Device(channel));
        //jesli operujemy na metodach klasy Device, powinnismy to robic jako nowe zadanie egzekutora  
        deviceManager.sendWelcome(executorService, deviceMap.get(channel));
    }

    private void read(SelectionKey key) throws IOException {
        SocketChannel channel = (SocketChannel) key.channel();

        ByteBuffer buffer = ByteBuffer.allocate(8192);
        int numRead = -1;
        try {
            numRead = channel.read(buffer);
        }
        catch (IOException e) {
            e.printStackTrace();
        }

        if (numRead == -1) {
            this.dataMap.remove(channel);
            Socket socket = channel.socket();
            SocketAddress remoteAddr = socket.getRemoteSocketAddress();
            log("Connection closed by client: " + remoteAddr);
            channel.close();
            key.cancel();
            return;
        }

        byte[] data = new byte[numRead];
        System.arraycopy(buffer.array(), 0, data, 0, numRead);
        deviceManager.task(executorService, deviceMap.get(key.channel()), data);
    }

    private void write(SelectionKey key) throws IOException {
        SocketChannel channel = (SocketChannel) key.channel();
        List<byte[]> pendingData = this.dataMap.get(channel);
        Iterator<byte[]> items = pendingData.iterator();
        while (items.hasNext()) {
            byte[] item = items.next();
            items.remove();
            channel.write(ByteBuffer.wrap(item));
        }
        key.interestOps(SelectionKey.OP_READ);
    }
    
    private void addDevice(SelectionKey key) {
	
    }

    private void doEcho(SelectionKey key, byte[] data) {
        SocketChannel channel = (SocketChannel) key.channel();
        List<byte[]> pendingData = this.dataMap.get(channel);
        pendingData.add(data);
        key.interestOps(SelectionKey.OP_WRITE);
    }

    private static void log(String s) {
        System.out.println(s);
    }

    public static void main(String[] args) throws Exception {
	new YourServer(null, 1234);
    }

}