Networking

Java provides a complete set of networking APIs through the java.net package. Socket and ServerSocket for TCP client-server communication. URL and HttpURLConnection for HTTP requests. Java 11+ adds a modern HttpClient that supports HTTP/2, WebSockets, and asynchronous requests.

A Socket is a two-way communication endpoint between a client and a server. A ServerSocket listens on a specific port and accepts connections from clients. Each connection produces a new Socket that is handled in a separate thread.

HttpClient (Java 11+) is the modern replacement for HttpURLConnection — cleaner, more powerful, and supports async. It uses a builder pattern for configuration and returns a structured HttpResponse.

Java Networking APIs:

Code Example

import java.net.*;
import java.net.http.*;
import java.io.*;
import java.time.Duration;

public class NetworkingJava {
    public static void main(String[] args) throws Exception {
        // HTTP GET (Java 11+ HttpClient)
        HttpClient client = HttpClient.newBuilder()
            .connectTimeout(Duration.ofSeconds(10))
            .build();

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://jsonplaceholder.typicode.com/posts/1"))
            .header("Accept", "application/json")
            .GET()
            .build();

        HttpResponse<String> response = client.send(request,
            HttpResponse.BodyHandlers.ofString());

        System.out.println("Status: " + response.statusCode());
        System.out.println("Body: " + response.body().substring(0, 200));

        // Simple URL fetch
        URL url = new URL("https://api.ipify.org");
        try (BufferedReader br = new BufferedReader(
                new InputStreamReader(url.openStream()))) {
            System.out.println("IP Publik: " + br.readLine());
        }

        // DNS lookup
        InetAddress[] addresses = InetAddress.getAllByName("google.com");
        for (InetAddress addr : addresses) {
            System.out.println("IP: " + addr.getHostAddress());
        }
    }
}

Code Explanation

1. HttpClient: a builder pattern for configuring timeouts, proxies, and more. send() for synchronous requests, sendAsync() for asynchronous.
2. HttpRequest: the .uri(), .header(), .GET()/.POST() methods build the request. The body handler determines the type of the response body.
3. HttpResponse: statusCode() for the HTTP code (200, 404, etc.), body() for the response contents.
4. URL.openStream(): the old way to fetch a URL's content — enough for simple needs.
5. InetAddress: DNS lookup — converts a hostname to an IP address.

Everyday Example

A simple client-server chat using Sockets:

// === SERVER ===
import java.net.*;
import java.io.*;

public class ChatServer {
    public static void main(String[] args) throws IOException {
        try (ServerSocket server = new ServerSocket(5000)) {
            System.out.println("Server berstreet di port 5000...");
            try (Socket client = server.accept();
                 BufferedReader in = new BufferedReader(
                     new InputStreamReader(client.getInputStream()));
                 PrintWriter out = new PrintWriter(client.getOutputStream(), true)) {

                String message;
                while ((message = in.readLine()) != null) {
                    System.out.println("Client: " + message);
                    out.println("Server: " + message.toUpperCase());
                }
            }
        }
    }
}

// === CLIENT ===
import java.net.*;
import java.io.*;
import java.util.Scanner;

public class ChatClient {
    public static void main(String[] args) throws IOException {
        try (Socket socket = new Socket("localhost", 5000);
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
             BufferedReader in = new BufferedReader(
                 new InputStreamReader(socket.getInputStream()));
             Scanner sc = new Scanner(System.in)) {

            System.out.println("Terhubung to server. Ketik message:");
            while (sc.hasNextLine()) {
                out.println(sc.nextLine());
                System.out.println(in.readLine());
            }
        }
    }
}

Summary

Java's networking APIs are very complete — from low-level Socket TCP/UDP to the modern HttpClient. For web services, use HttpClient (Java 11+) instead of the more verbose HttpURLConnection. For chat/real-time apps, Socket and ServerSocket are still relevant and effective.