How to Implement a Simple Chat Application Using Sockets in Java?

Last Updated : 08 Jan, 2025

In this article, we will create a simple chat application using Java socket programming. Before we are going to discuss our topic, we must know Socket in Java. Java Socket connects two different JREs (Java Runtime Environment).

Java sockets can be connection-oriented or connection-less.
In Java, we have the java.net package.
Java Socket can be connectionless or connection-oriented.

Java Client

On the Java client side, we will pass the two most important information to the Socket class. This information connects the Java Client Socket to the Java Server Socket.

IP Address of Server and,
Port Number

In the Java client, we create a new thread when each new message is received from the Java Server Client.

Java

import java.io.*;
import java.net.*;
import java.util.Scanner;

public class Client {
    private static final String SERVER_ADDRESS = "localhost";
    private static final int SERVER_PORT = 12345;

    public static void main(String[] args) {
        try {
            Socket socket = new Socket(SERVER_ADDRESS, SERVER_PORT);
            System.out.println("Connected to the chat server!");

            // Setting up input and output streams
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            // Start a thread to handle incoming messages
            new Thread(() -> {
                try {
                    String serverResponse;
                    while ((serverResponse = in.readLine()) != null) {
                        System.out.println(serverResponse);
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }).start();

            // Read messages from the console and send to the server
            Scanner scanner = new Scanner(System.in);
            String userInput;
            while (true) {
                userInput = scanner.nextLine();
                out.println(userInput);
            }
           
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Java Server

In the server side. we will get the username of each connected client and stores each client in the CopyOnWriteArrayList after accepting the connection from the connected client. Every time create another thread for each client. Every message will be broadcast to every connected client.

Java

import java.io.*;
import java.net.*;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.Scanner;

public class Server {
    private static final int PORT = 12346;
    private static CopyOnWriteArrayList<ClientHandler> clients = new CopyOnWriteArrayList<>();

    public static void main(String[] args) {
        try {
            ServerSocket serverSocket = new ServerSocket(PORT);
            System.out.println("Server is running and waiting for connections...");

            // Thread to handle server admin input
            new Thread(() -> {
                Scanner scanner = new Scanner(System.in);
                while (true) {
                    String serverMessage = scanner.nextLine();
                    broadcast("[Server]: " + serverMessage, null);
                }
            }).start();

            // Accept incoming connections
            while (true) {
                Socket clientSocket = serverSocket.accept();
                System.out.println("New client connected: " + clientSocket);

                // Create a new client handler for the connected client
                ClientHandler clientHandler = new ClientHandler(clientSocket);
                clients.add(clientHandler);
                new Thread(clientHandler).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // Broadcast a message to all clients
    public static void broadcast(String message, ClientHandler sender) {
        for (ClientHandler client : clients) {
            if (client != sender) {
                client.sendMessage(message);
            }
        }
    }

    // Internal class to handle client connections
    private static class ClientHandler implements Runnable {
        private Socket clientSocket;
        private PrintWriter out;
        private BufferedReader in;
        private String username;

        public ClientHandler(Socket socket) {
            this.clientSocket = socket;

            try {
                out = new PrintWriter(clientSocket.getOutputStream(), true);
                in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void run() {
            try {
                // Get the username from the client
                out.println("Enter your username:");
                username = in.readLine();
                System.out.println("User " + username + " connected.");
                out.println("Welcome to the chat, " + username + "!");
                out.println("Type Your Message");

                String inputLine;
                while ((inputLine = in.readLine()) != null) {
                    System.out.println("[" + username + "]: " + inputLine);
                    broadcast("[" + username + "]: " + inputLine, this);
                }

                // Remove the client handler from the list
                clients.remove(this);
                System.out.println("User " + username + " disconnected.");
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    in.close();
                    out.close();
                    clientSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        public void sendMessage(String message) {
            out.println(message);
        }
    }
}