TCP/IP Server Sockets and Datagram (UDP) Programming

TCP/IP Server Sockets

A ServerSocket listens on a port and accepts incoming client connections. Each accepted connection returns a regular Socket used to communicate with that specific client.

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

public class TCPServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(5000);
        System.out.println("Server listening on port 5000...");

        while (true) {
            Socket clientSocket = serverSocket.accept(); // blocks until a client connects
            System.out.println("Client connected: " + clientSocket.getInetAddress());

            BufferedReader in = new BufferedReader(
                new InputStreamReader(clientSocket.getInputStream()));
            PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);

            String message = in.readLine();
            System.out.println("Received: " + message);
            out.println("Echo: " + message);

            clientSocket.close();
        }
    }
}

Key ServerSocket Methods

Concurrent Servers with Threads

A single-threaded server handles one client at a time. Spawning a thread per client allows concurrent connections:

while (true) {
    Socket clientSocket = serverSocket.accept();
    new Thread(() -> handleClient(clientSocket)).start();
}

Datagram (UDP) Programming

UDP (User Datagram Protocol) is connectionless and unreliable but fast — no handshake, no guaranteed delivery or order. Java represents UDP communication with DatagramSocket and DatagramPacket.

UDP Client

import java.net.*;

public class UDPClient {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket();
        byte[] message = "Hello UDP Server".getBytes();

        InetAddress serverAddr = InetAddress.getByName("localhost");
        DatagramPacket packet = new DatagramPacket(message, message.length, serverAddr, 6000);
        socket.send(packet);

        socket.close();
    }
}

UDP Server

import java.net.*;

public class UDPServer {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(6000);
        byte[] buffer = new byte[1024];

        while (true) {
            DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
            socket.receive(packet); // blocks until a packet arrives

            String received = new String(packet.getData(), 0, packet.getLength());
            System.out.println("Received: " + received + " from " + packet.getAddress());
        }
    }
}

TCP vs UDP

Key Takeaway: ServerSocket.accept() blocks until a TCP client connects, and threading lets a server handle many clients concurrently. UDP trades reliability for speed — DatagramSocket sends/receives self-contained DatagramPackets with no connection setup.