package org.ait;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.ServerSocket;
    import java.net.Socket;
    
    public class Server {
    	ServerSocket providerSocket;
    	Socket connection = null;
    	ObjectOutputStream out;
    	ObjectInputStream in;
    	String message;
    
    	Server() {
    	}
    
    	void run() {
    		try {
    			// 1. create a socket server listening to port 8080
    			providerSocket = new ServerSocket(8080);
    			// 2. waiting for the connection (here we are waiting until next connection)
    			connection = providerSocket.accept();
    			// 3. create Input and Output streams
    			out = new ObjectOutputStream(connection.getOutputStream());
    			in = new ObjectInputStream(connection.getInputStream());
    			// 4. socket communication
    			do {
    				try {
    					message = (String) in.readObject();
    					System.out.println("client>" + message);
    					if (message.equals("bye")) {
    					    sendMessage("bye");
    					} 
    				} catch (ClassNotFoundException classnot) {
    					System.err.println("Data received in unknown format");
    				}
    			} while (!message.equals("bye"));
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		} finally {
    			// 4: close connection
    			try {
    				in.close();
    				out.close();
    				providerSocket.close();
    			} catch (IOException ioException) {
    				ioException.printStackTrace();
    			}
    		}
    	}
    
    	void sendMessage(String msg) {
    		try {
    			out.writeObject(msg);
    			out.flush();
    			System.out.println("server>" + msg);
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		}
    	}
    
    	public static void main(String args[]) {
    		Server server = new Server();
    		while (true) {
    			server.run();
    		}
    	}
    }

    package org.ait;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.Socket;
    import java.net.UnknownHostException;
    
    public class Client {
    	Socket requestSocket;
    	ObjectOutputStream out;
    	ObjectInputStream in;
    	String message;
    
    	Client() {
    	}
    
    	void run() {
    		try {
    			// 1. try to connect to the socket: localhost:8080
    			requestSocket = new Socket("localhost", 8080);
    			// 2. Input and Output streams
    			out = new ObjectOutputStream(requestSocket.getOutputStream());
    			in = new ObjectInputStream(requestSocket.getInputStream());
    			// 3: communications
    			do {
    				try {
    					sendMessage("Hello server");
    					sendMessage("bye");
    					message = (String) in.readObject();
    				} catch (Exception e) {
    					System.err.println("data received in unknown format");
    				}
    			} while (!message.equals("bye"));
    		} catch (UnknownHostException unknownHost) {
    			System.err.println("You are trying to connect to an unknown host!");
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		} finally {
    			// 4: close connection
    			try {
    				in.close();
    				out.close();
    				requestSocket.close();
    			} catch (IOException ioException) {
    				ioException.printStackTrace();
    			}
    		}
    	}
    
    	void sendMessage(String msg) {
    		try {
    			out.writeObject(msg);
    			out.flush();
    			System.out.println("client>" + msg);
    		} catch (IOException ioException) {
    			ioException.printStackTrace();
    		}
    	}
    
    	public static void main(String args[]) {
    		Client client = new Client();
    		client.run();
    	}
    }

The following Agent sends a message and waits for a response on port 8080, also with UDP. Older versions of the Eclipse IDE, the text you type on the console can be sent by pressing ctrl+z

package org.ait;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPClient {
    public static void main(String args[]) throws Exception {
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        DatagramSocket clientSocket = new DatagramSocket();
        InetAddress IPAddress = InetAddress.getByName("localhost");

        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];

        String sentence = inFromUser.readLine();
        sendData = sentence.getBytes();

        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 8080);
        clientSocket.send(sendPacket);

        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        clientSocket.receive(receivePacket);
        String modifiedSentence = new String(receivePacket.getData());

        System.out.println("converted:" + modifiedSentence);
        clientSocket.close();
    }
}

The UDP server waits for the agents messages on port 8080 and converts them to uppercase letters and sends them back to the client UDP socket.

package org.ait;

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPServer {
    public static void main(String args[]) throws Exception {

        DatagramSocket serverSocket = new DatagramSocket(8080);

        byte[] bytesReceived = new byte[1024];
        byte[] bytesSent = new byte[1024];

        DatagramPacket receivePacket = new DatagramPacket(bytesReceived, bytesReceived.length);
        // here we are waiting for the packets
        serverSocket.receive(receivePacket);

        String textMessage = new String(receivePacket.getData());

        System.out.println("I got: " + textMessage);

        InetAddress IPAddress = receivePacket.getAddress();
        int port = receivePacket.getPort();

        String upperCaseText = textMessage.toUpperCase();
        bytesSent = upperCaseText.getBytes();

        // send back
        DatagramPacket sendPacket = new DatagramPacket(bytesSent, bytesSent.length, IPAddress, port);
        serverSocket.send(sendPacket);
        serverSocket.close();

    }
}

Reading:

• http://tutorials.jenkov.com/java-nio/nio-vs-io.html
• http://www.javaworld.com/article/2073344/core-java/use-select-for-high-speed-networking.html

   ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
   
   serverSocketChannel.socket().bind(new InetSocketAddress(9999));
   serverSocketChannel.configureBlocking(false);  // ez a sor jelzi a blokkolásmentes működést
   
   while(true){
    SocketChannel socketChannel = serverSocketChannel.accept();
   
    if(socketChannel != null){
        // the connection is accepted
    }
   }

  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.net.Socket;
  import java.util.Random;
  
  public class Client implements Runnable {
	private String host;
	private int port;
  
	// Bounds on how much we write per cycle
	private static final int minWriteSize = 1024;
	private static final int maxWriteSize = 65536;
  
	// Bounds on how long we wait between cycles
	private static final int minPause = (int) (0.05 * 1000);
	private static final int maxPause = (int) (0.5 * 1000);
  
	// Random number generator
	Random rand = new Random();
  
	public Client(String host, int port, int numThreads) {
		this.host = host;
		this.port = port;
  
		for (int i = 0; i < numThreads; ++i) {
			new Thread(this).start();
		}
	}
  
	public void run() {
		byte buffer[] = new byte[maxWriteSize];
		try {
			Socket s = new Socket(host, port);
			InputStream in = s.getInputStream();
			OutputStream out = s.getOutputStream();
  
			while (true) {
				int numToWrite = minWriteSize
						+ (int) (rand.nextDouble() * (maxWriteSize - minWriteSize));
				for (int i = 0; i < numToWrite; ++i) {
					buffer[i] = (byte) rand.nextInt(256);
				}
				out.write(buffer, 0, numToWrite);
				int sofar = 0;
				while (sofar < numToWrite) {
					sofar += in.read(buffer, sofar, numToWrite - sofar);
				}
				System.out.println(Thread.currentThread() + " wrote " + numToWrite);
  
				int pause = minPause + (int) (rand.nextDouble() * (maxPause - minPause));
				try {
					Thread.sleep(pause);
				} catch (InterruptedException ie) {
				}
			}
		} catch (IOException ie) {
			ie.printStackTrace();
		}
	}
  
	static public void main(String args[]) throws Exception {
		new Client("localhost", 4444, 3);
	}
  }

  public class Server implements Runnable {
	// The port we will listen on
	private int port;
	// A pre-allocated buffer for encrypting data
	private final ByteBuffer buffer = ByteBuffer.allocate(16384);
	public Server(int port) {
		this.port = port;
		new Thread(this).start();
	}
	public void run() {
		try {
			// Instead of creating a ServerSocket,
			// create a ServerSocketChannel
			ServerSocketChannel ssc = ServerSocketChannel.open();
			// Set it to non-blocking, so we can use select
			ssc.configureBlocking(false);
			// Get the Socket connected to this channel, and bind it
			// to the listening port
			ServerSocket ss = ssc.socket();
			InetSocketAddress isa = new InetSocketAddress(port);
			ss.bind(isa);
			// Create a new Selector for selecting
			Selector selector = Selector.open();
			// Register the ServerSocketChannel, so we can
			// listen for incoming connections
			ssc.register(selector, SelectionKey.OP_ACCEPT);
			System.out.println("Listening on port " + port);
			while (true) {
				// See if we've had any activity -- either
				// an incoming connection, or incoming data on an
				// existing connection
				int num = selector.select();
				// If we don't have any activity, loop around and wait
				// again
				if (num == 0) {
					continue;
				}
				// Get the keys corresponding to the activity
				// that has been detected, and process them
				// one by one
				Set keys = selector.selectedKeys();
				Iterator it = keys.iterator();
				while (it.hasNext()) {
					// Get a key representing one of bits of I/O
					// activity
					SelectionKey key = (SelectionKey) it.next();
					// What kind of activity is it?
					if ((key.readyOps() & SelectionKey.OP_ACCEPT) == SelectionKey.OP_ACCEPT) {
						System.out.println("acc");
						// It's an incoming connection.
						// Register this socket with the Selector
						// so we can listen for input on it
						Socket s = ss.accept();
						System.out.println("Got connection from " + s);
						// Make sure to make it non-blocking, so we can
						// use a selector on it.
						SocketChannel sc = s.getChannel();
						sc.configureBlocking(false);
						// Register it with the selector, for reading
						sc.register(selector, SelectionKey.OP_READ);
					} else if ((key.readyOps() & SelectionKey.OP_READ) == SelectionKey.OP_READ) {
						SocketChannel sc = null;
						try {
							// It's incoming data on a connection, so
							// process it
							sc = (SocketChannel) key.channel();
							boolean ok = processInput(sc);
							// If the connection is dead, then remove it
							// from the selector and close it
							if (!ok) {
								key.cancel();
								Socket s = null;
								try {
									s = sc.socket();
									s.close();
								} catch (IOException ie) {
									System.err.println("Error closing socket "
											+ s + ": " + ie);
								}
							}
						} catch (IOException ie) {
							// On exception, remove this channel from the
							// selector
							key.cancel();
							try {
								sc.close();
							} catch (IOException ie2) {
								System.out.println(ie2);
							}
							System.out.println("Closed " + sc);
						}
					}
				}
				// We remove the selected keys, because we've dealt
				// with them.
				keys.clear();
			}
		} catch (IOException ie) {
			System.err.println(ie);
		}
	}
	// Do some cheesy encryption on the incoming data,
	// and send it back out
	private boolean processInput(SocketChannel sc) throws IOException {
		buffer.clear();
		sc.read(buffer);
		buffer.flip();
		// If no data, close the connection
		if (buffer.limit() == 0) {
			return false;
		}
		// Simple rot-13 encryption
		for (int i = 0; i < buffer.limit(); ++i) {
			byte b = buffer.get(i);
			if ((b >= 'a' && b <= 'm') || (b >= 'A' && b <= 'M')) {
				b += 13;
			} else if ((b >= 'n' && b <= 'z') || (b >= 'N' && b <= 'Z')) {
				b -= 13;
			}
			buffer.put(i, b);
		}
		sc.write(buffer);
		System.out.println("Processed " + buffer.limit() + " from " + sc);
		return true;
	}
	static public void main(String args[]) throws Exception {
		new Server(4444);
	}
  }