Multi-Client Chat Application

Overview

A Java-based multi-client chat application built using socket programming, object-oriented principles (OOP), and multi-threading. The server handles multiple clients simultaneously, allowing users to communicate in a group chat environment with unique usernames.

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Key Technologies & Concepts

1. Socket Programming (TCP/IP Communication)

• ServerSocket listens for incoming client connections on a specified port (12345).
• Socket establishes a two-way communication channel between the server and clients.
• BufferedReader and PrintWriter handle text-based I/O streams for sending and receiving messages.
• Connection Lifecycle:
  • Server waits for clients (serverSocket.accept()).
  • Clients connect (new Socket("localhost", 12345)).
  • Messages are exchanged via input/output streams.
  • Clean disconnection using /quit command.

2. Object-Oriented Programming (OOP)

• Encapsulation:
  • The ClientHandler class encapsulates client-specific logic (username, I/O streams).
  • Private fields with controlled access (e.g., socket, username).
• Separation of Concerns:
  • Server class manages connections and broadcasting.
  • Client class handles user input and server responses.
• Reusability:
  • The ClientHandler thread can be reused for each new client connection.

3. Multi-Threading (Concurrent Client Handling)

• Server-Side Threading:
  • Each client runs in a separate ClientHandler thread.
  • The main thread continues listening for new connections.
• Client-Side Threading:
  • Receiving Thread: Continuously listens for incoming messages.
  • Sending Thread: Handles user input and message transmission.
• Thread Safety:
  • synchronizedSet ensures safe access to the connected clients list.
  • synchronized blocks prevent race conditions during broadcasts.

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Features

• Multi-client support (handles many users at once)
• Username-based identification (no anonymous messaging)
• Real-time group chat (messages broadcast to all)
• Graceful disconnections (/quit command)
• Join/leave notifications (automatic alerts)

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How It Works

Server Workflow

1. Starts and waits for clients (ServerSocket.accept()).
2. For each new client:
   • Creates a ClientHandler thread.
   • Prompts for a username.
   • Adds client to the active connections list.
3. Broadcasts messages to all clients (except sender).
4. Removes disconnected clients and notifies others.

Client Workflow

1. Connects to the server (Socket("localhost", 12345)).
2. Enters a username (sent to the server).
3. Runs two threads:
   • Receiver: Listens for messages from the server.
   • Sender: Reads user input and sends messages.
4. Disconnects cleanly with /quit.

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Project Structure

chat-app/  
├── Client/  
│   └── Client.java       # Manages client-side connection & messaging  
├── Server/  
│   └── Server.java       # Main server logic    
├── build.bat
├── runClient.bat
├── runServer.bat
└── README.md  

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Conclusion

This project demonstrates socket programming for network communication, OOP for clean architecture, and multi-threading for handling concurrent clients. It serves as a foundation for building more advanced chat systems with additional features.