Java Socket Programming: TCP/UDP Fundamentals, InetAddress, URL, and Multithreaded Server‑Client Examples

Java was originally designed as a network programming language, offering strong support for socket communication that enables client‑server interaction. This tutorial introduces the fundamentals of computer networking, including IP addresses, protocols, ports, the TCP/IP five‑layer model, and how these elements combine to form a socket.

The Java platform provides several networking APIs:

InetAddress : Represents an IP address and offers methods to retrieve host names and address bytes.

URL : Uniform Resource Locator for accessing resources on the Internet.

Socket and ServerSocket : Classes for TCP communication.

DatagramSocket and DatagramPacket : Classes for UDP communication.

InetAddress example:

InetAddress address = InetAddress.getLocalHost();
address.getHostName(); // host name
address.getHostAddress(); // IP address
byte[] bytes = address.getAddress(); // byte array of IP
InetAddress address2 = InetAddress.getByName("otherHost");
InetAddress address3 = InetAddress.getByName("192.168.1.10");

URL example:

URL baidu = new URL("http://www.baidu.com");
URL url = new URL(baidu, "/index.html?username=tom#test");
url.getProtocol(); // http
url.getHost(); // www.baidu.com
url.getPort(); // -1 (default)
url.getPath(); // /index.html
url.getFile(); // /index.html?username=tom
url.getRef(); // test
url.getQuery(); // username=tom

For TCP programming, the typical workflow is:

Create a ServerSocket on the server side and bind it to a port.

Call accept() to wait for client connections.

Obtain input and output streams from the resulting Socket to read and write data.

Close streams and sockets when communication ends.

Sample server‑side TCP code:

// Server side
ServerSocket serverSocket = new ServerSocket(10086);
Socket socket = serverSocket.accept();
InputStream is = socket.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(is));
String line;
while ((line = br.readLine()) != null) {
    System.out.println("Server received: " + line);
}
PrintWriter pw = new PrintWriter(socket.getOutputStream());
pw.write("Welcome!");
pw.flush();
// Close resources
pw.close();
br.close();
socket.close();
serverSocket.close();

Client‑side TCP code mirrors the server steps, creating a Socket with the server’s address and port, sending data via an output stream, and reading the response via an input stream.

UDP communication uses DatagramSocket and DatagramPacket. The server creates a socket, receives a packet, processes the data, and sends a response packet back to the client’s address and port.

// UDP server example
DatagramSocket socket = new DatagramSocket(10010);
byte[] buf = new byte[1024];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
socket.receive(packet);
String info = new String(buf, 0, packet.getLength());
System.out.println("Server received: " + info);
InetAddress clientAddr = packet.getAddress();
int clientPort = packet.getPort();
byte[] reply = "Welcome!".getBytes();
DatagramPacket replyPacket = new DatagramPacket(reply, reply.length, clientAddr, clientPort);
socket.send(replyPacket);
socket.close();

To handle multiple clients concurrently, the server can spawn a new thread for each accepted Socket, allowing simultaneous communication while the main thread continues listening for new connections.

Important notes:

Adjust thread priorities appropriately to avoid performance degradation.

Closing the output stream of a socket also closes the socket; therefore, close the socket directly instead of individual streams.

When transmitting objects over TCP, consider Java serialization mechanisms.

File transfer over sockets requires careful handling of I/O streams.