Networking Basics: Socket, ServerSocket, and URL

Java was built for the internet era, and the java.net package has provided TCP/IP networking support since the very first release. Two classes handle raw socket-based communication — Socket for the client side and ServerSocket for the server side — while URL and URLConnection provide a higher-level way to work with web resources without managing sockets directly.

Understanding both layers matters: sockets are the foundation that protocols like HTTP are built on top of, and most real applications use the higher-level URL classes (or a proper HTTP client library) rather than raw sockets. Advanced Java developers should be comfortable with both.

Why it matters

Every call your back end makes, to a database, a payment gateway or another service, travels over a socket to a host and a port. Frameworks hide the plumbing, but the vocabulary of connections, ports, blocking reads and timeouts is what you need to read a stack trace or a firewall rule when something stops responding.

Socket and ServerSocket

A ServerSocket binds to a port and listens for incoming TCP connections. Calling accept() blocks the current thread until a client connects, then returns a Socket object representing that specific connection. A client creates its own Socket by specifying the server's host and port; once connected, both sides read and write through the socket's input and output streams, exactly like reading and writing a file.

• ServerSocket(port) — opens a listening socket on the given port.
• serverSocket.accept() — blocks until a client connects, then returns a connected Socket.
• new Socket(host, port) — the client-side call that initiates a TCP connection.
• Both sides use getInputStream() / getOutputStream() to exchange data.

URL and URLConnection

The URL class parses and represents a Uniform Resource Locator — it can break a web address down into its protocol, host, port, path, and query string without making any network call at all. URLConnection (obtained via url.openConnection()) is what actually performs network I/O: it opens a connection to the resource and lets you read its content as a stream, inspect response headers, or send data. This is the mechanism behind simple HTTP downloads in older and simpler Java code, though modern code typically prefers the more capable java.net.http.HttpClient introduced in Java 11.

Blocking Behavior and Real Applications

Both accept() and stream reads are blocking calls by default — they pause the calling thread until data or a connection arrives. That is why simple socket servers usually accept connections in a loop, often handing each accepted socket off to its own thread (or a thread pool) so one slow client cannot stall every other client. Production servers rarely write raw socket loops like this by hand; they use frameworks (Netty, Spring's embedded servers, and so on) that solve these concurrency and buffering problems for you — but the concepts underneath are exactly Socket, ServerSocket, and the read/write stream model shown here.

Examples

import java.io.*;
import java.net.*;

public class SocketDemo {
    public static void main(String[] args) throws Exception {
        // Server: accept one connection on its own thread
        Thread server = new Thread(() -> {
            try (ServerSocket serverSocket = new ServerSocket(5000)) {
                Socket client = serverSocket.accept(); // blocks until a client connects
                BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
                PrintWriter out = new PrintWriter(client.getOutputStream(), true);
                String received = in.readLine();
                out.println("Server received: " + received);
                client.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        });
        server.start();

        Thread.sleep(200); // give the server a moment to start listening

        // Client: connect, send a line, read the reply
        try (Socket socket = new Socket("localhost", 5000)) {
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out.println("Hello from client");
            System.out.println(in.readLine());
        }

        server.join();
    }
}

Server received: Hello from client

import java.net.URL;

public class UrlDemo {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://api.example.com:8443/v1/data?id=42");
        System.out.println("Protocol: " + url.getProtocol());
        System.out.println("Host: " + url.getHost());
        System.out.println("Port: " + url.getPort());
        System.out.println("Path: " + url.getPath());
        System.out.println("Query: " + url.getQuery());
    }
}

Protocol: https
Host: api.example.com
Port: 8443
Path: /v1/data
Query: id=42

Exercise

Parse two addresses with java.net.URI, which needs no network access. For the first, print the scheme, the path and the fragment. For the second, which names no port, print what getPort() returns.

Expected output: https, /products/42, reviews, -1 (one per line)

import java.net.URI;

public class ParseAddress {
    public static void main(String[] args) {
        URI full = URI.create("https://shop.example.com:8443/products/42?ref=home#reviews");
        URI plain = URI.create("http://example.com/about");

        // TODO: print the scheme, the path and the fragment of "full"
        // TODO: print the port of "plain"
    }
}

import java.net.URI;

public class ParseAddress {
    public static void main(String[] args) {
        URI full = URI.create("https://shop.example.com:8443/products/42?ref=home#reviews");
        URI plain = URI.create("http://example.com/about");

        System.out.println(full.getScheme());   // https
        System.out.println(full.getPath());     // /products/42
        System.out.println(full.getFragment()); // reviews
        System.out.println(plain.getPort());    // -1
    }
}

Quick quiz

Interview questions