networking3UdpDatagram

1package pkg10networking;2 3import java.net.DatagramPacket;4import java.net.DatagramSocket;5import java.net.InetAddress;6 7/*8 * networking3UdpDatagram.java9 * ---------------------------10 * UDP datagrams: connectionless, unordered, best-effort messages.11 *12 * DEFINITION:13 *   UDP sends independent packets (datagrams) with no connection, no ordering,14 *   and no delivery guarantee — but with very low overhead. Good for DNS, video,15 *   games, metrics. This demo sends a packet to itself over loopback.16 *17 * KEY POINTS:18 *   - DatagramSocket sends/receives; DatagramPacket holds bytes + address + port.19 *   - receive() blocks until a packet arrives (or timeout via setSoTimeout).20 *   - No accept()/connect() handshake — just fire packets.21 *   - Packets can be lost, duplicated, or reordered: the app must cope.22 */23public class networking3UdpDatagram {24 25    public static void main(String[] args) throws Exception {26        InetAddress loop = InetAddress.getLoopbackAddress();27 28        // Receiver socket on an OS-chosen port29        DatagramSocket receiver = new DatagramSocket(0);30        int port = receiver.getLocalPort();31 32        Thread receiverThread = new Thread(() -> {33            try {34                byte[] buf = new byte[1024];35                DatagramPacket packet = new DatagramPacket(buf, buf.length);36                receiver.receive(packet);             // blocks until a datagram arrives37                String msg = new String(packet.getData(), 0, packet.getLength());38                System.out.println("[receiver] got: \"" + msg + "\" from port " + packet.getPort());39            } catch (Exception e) {40                System.out.println("[receiver] error: " + e.getMessage());41            }42        });43        receiverThread.start();44 45        // Sender fires one datagram at the receiver46        try (DatagramSocket sender = new DatagramSocket()) {47            byte[] data = "hello over UDP".getBytes();48            DatagramPacket packet = new DatagramPacket(data, data.length, loop, port);49            sender.send(packet);50            System.out.println("[sender] sent " + data.length + " bytes to port " + port);51        }52 53        receiverThread.join();54        receiver.close();55        System.out.println("Done.");56    }57}