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Java Multi-Client Chat Application with File Transfer

A console-based multi-client chat application built using Java Socket Programming that supports:

  • Real-time chat
  • Private messaging
  • Online user tracking
  • Targeted file transfer between specific users
  • Communication across different computers on the same network

This project follows real-world client–server architecture and uses a protocol-enforced design to ensure correctness and reliability.

Features

Chat Features

  • Multiple clients can connect simultaneously
  • Broadcast messaging (send message to all users)
  • Private messaging using @username
  • Real-time online/offline user updates
  • Error handling

File Transfer Features

  • Send files to a specific user only
  • Receiver approval before file transfer (commented down in code for now)
  • File transfer runs in parallel threads
  • Chat does not freeze during file transfer
  • Works across different PCs on the same LAN
  • Prevents empty or corrupted files

🏗 Architecture Overview

Client ──(CHAT: 7777)──▶ Server ◀──(CHAT: 7777)── Client
Client ──(FILE: 8888)──▶ Server ◀──(FILE: 8888)── Client

Ports Used

Port Purpose
7777 Chat & control messages
8888 File transfer (binary data)

Project Structure

├── Server.java
├── ClientHandler.java
├── Client.java
└── README.md

File Description

  • Server.java
    Starts the chat server and file relay server.

  • ClientHandler.java
    Handles one connected client (chat + file protocol).

  • Client.java
    Console-based client application.

Communication Protocol

Chat Commands/Protocols

Command Description
ENTER_USERNAME Username handshake
MESSAGE|user|text Private message
BROADCAST|text Broadcast message
ONLINE_USERS|list Online users
ERROR Error from client
LOGOUT Disconnect

File Transfer Protocol (Protocol-Enforced Pairing)

  1. Sender requests file transfer
    FILE_REQUEST|receiver|filename|size

  2. Receiver receives offer
    FILE_OFFER|sender|filename|size

  3. Receiver accepts
    FILE_ACCEPT|sender|filename

  4. Server authorizes transfer
    FILE_START|sender|receiver|filename|size

  5. Both clients connect to FILE_PORT (8888)

  6. Server relays file bytes File transfer starts only after FILE_START
    Prevents wrong pairing and empty files