Concurrent Multi-Client Chat Server

A TCP-based multi-client chat server built with Core Java — demonstrating Multithreading, Java Sockets, ExecutorService, ConcurrentHashMap, and the Producer-Consumer pattern using BlockingQueue.

Features

Feature	Implementation
50+ concurrent clients	Fixed-size ExecutorService (thread pool)
Thread-safe session store	ConcurrentHashMap<clientId, ClientHandler>
Zero-loss messaging	BlockingQueue (Producer-Consumer pattern)
Broadcast messaging	Send to all connected clients
Private messaging	Direct message to a specific user
Graceful shutdown	ShutdownHook + awaitTermination
Custom exceptions	ChatServerException hierarchy
Resource management	try-with-resources on all I/O streams

Java Concepts Demonstrated

• Multithreading — ExecutorService, Thread, Runnable
• Concurrency — ConcurrentHashMap, BlockingQueue, volatile
• Java Sockets — ServerSocket, Socket, TCP communication
• Java I/O — BufferedReader, PrintWriter, InputStreamReader
• OOP — Encapsulation, Single Responsibility Principle
• Collections Framework — ConcurrentHashMap, LinkedBlockingQueue
• Design Patterns — Producer-Consumer, Thread-per-client

Project Structure

src/main/java/com/chatserver/
├── ChatServer.java          # Entry point; manages ServerSocket + thread pool
├── ClientHandler.java       # Runnable; one per connected client
├── MessageBroker.java       # Consumer thread; delivers from BlockingQueue
├── Message.java             # Immutable message model
├── MessageType.java         # Enum: BROADCAST | PRIVATE
├── ChatServerException.java # Custom exception hierarchy
└── ChatClient.java          # Terminal client for testing

Architecture

Client 1 ──┐
Client 2 ──┤──► ClientHandler (Thread Pool) ──► BlockingQueue ──► MessageBroker ──► Deliver
Client N ──┘                                    (Producer)           (Consumer)
                    ConcurrentHashMap
                  (active session store)

How to Run

Prerequisites

• Java 8 or above
• No external dependencies

Compile

javac -d out src/main/java/com/chatserver/*.java

Start Server

java -cp out com.chatserver.ChatServer

Start Client (open multiple terminals)

java -cp out com.chatserver.ChatClient

Commands

JOIN:<username>              → Set your display name
MSG:<text>                   → Broadcast to all users
PRIVATE:<username>:<text>    → Send private message
QUIT                         → Disconnect

Example Session

[SERVER] Welcome! Your ID: a3f2b1c4. Use JOIN:<name> to set your username.
JOIN:Shambhavi
[SERVER] [User-a3f2b1c4 is now known as Shambhavi]
MSG:Hello everyone!
PRIVATE:Alice:Hey, only for you!
QUIT

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Author: Shambhavi Trivedi
Tech Stack: Java 8 · TCP Sockets · ExecutorService · ConcurrentHashMap · BlockingQueue