Networking Concepts in Java

Networking means two programs on different computers (or on the same computer) sending data to each other. Java has supported it from its first version through the package java.net, and newer code also uses java.net.http. Before writing any code, you need a handful of ideas: IP addresses, ports, protocols, and the roles of client and server. In this lesson, you will learn those ideas in plain words, see where each Java class fits, and run a first program in which a client and a server talk to each other.

The Key Terms

Ports

Only one program can listen on a given port at a time. Asking for port 0 means "give me any free port", which the examples in this chapter use so they never clash with something already running.

TCP vs UDP

Two transport protocols carry almost all traffic:

Where the Java Classes Fit

Example: A First Conversation

Let's see an example in which one program plays both roles: it starts a server on a free port, then connects to it as a client.

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
 
public class Hello {
    public static void main(String[] args) throws Exception {
        try (ServerSocket server = new ServerSocket(0)) {
            int port = server.getLocalPort();
 
            Thread serverThread = new Thread(() -> {
                try (Socket client = server.accept();
                     PrintWriter out =
                         new PrintWriter(client.getOutputStream(), true)) {
                    out.println("Hello from the server");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            });
            serverThread.start();
 
            try (Socket socket = new Socket("localhost", port);
                 BufferedReader in = new BufferedReader(
                     new InputStreamReader(socket.getInputStream()))) {
                System.out.println("Client received: " + in.readLine());
                System.out.println("Port in valid range? "
                    + (port >= 1024 && port <= 65535));
            }
            serverThread.join();
        }
    }
}

Client received: Hello from the server
Port in valid range? true

Explanation:

1. new ServerSocket(0) opens a listening socket on any free port; getLocalPort() tells us which one we got.
2. The server thread calls accept(), which waits until a client connects and then returns a Socket for talking to that client.
3. new Socket("localhost", port) is the client connecting. The name localhost is resolved to 127.0.0.1, and the port selects our server.
4. Each side has an input stream and an output stream. What the server writes with println, the client reads with readLine.
5. try-with-resources closes every socket, which also ends the connection.

Real programs put the server and the client in two separate programs, usually on two machines. Keeping both in one file lets you run it with a single command.

Common Mistakes

Confusing the IP address with the port. The address chooses the machine; the port chooses the program on it.

Expecting UDP to be reliable. If you need every byte, in order, use TCP.

Hard-coding a port that is already in use. The program fails with BindException. Make the port configurable.

Forgetting that network calls block. accept() and read() wait until something happens; don't call them on a thread that must stay responsive.

Interview Corner

Practice

1. Easy: Change Hello so the server sends two lines and the client prints both.
2. Harder: Make the client send its name first, and have the server reply "Hello, " plus that name.

Key Takeaways