🚀 Multithreaded Chat Server in Java

A concurrent real-time chat server built in Java using sockets and multithreading for efficient client communication. This project demonstrates advanced Java networking, thread synchronization, and object-oriented programming principles.

🎯 Project Overview

This chat server can handle multiple client connections simultaneously using dedicated threads for each client. It features thread-safe operations and broadcast messaging where all messages are sent to all connected users.

🧠 Key Learning Objectives

Core Java Networking: ServerSocket, Socket, I/O streams
Multithreading: Thread creation, Runnable interface, thread lifecycle
Synchronization: synchronized blocks, thread-safe collections (ConcurrentHashMap)
Socket Programming: Client-server communication, connection management
Clean Architecture: Separation of concerns, OOP design patterns

🏗️ Architecture

The project is organized into server and client folders:

📁 Multithreaded-Chat-Server/
├── 📁 server/
│   ├── 📄 ChatServer.java      → Main server class, manages client threads
│   ├── 📄 ClientHandler.java   → Handles individual client communication  
│   ├── 📄 UserManager.java     → Manages user connections and history
│   └── 📄 Message.java         → Message data structure (POJO)
├── 📁 client/
│   ├── 📄 ChatClient.java      → Client application
│   └── 📄 Message.java         → Message data structure (POJO)
└── 📄 README.md               → This documentation

🔧 Core Components

Server Side:

ChatServer.java: Main entry point, accepts connections, spawns client threads
ClientHandler.java: Manages individual client sessions in separate threads
UserManager.java: Handles user registration, tracking, and re-entry logic
Message.java: Data structure for message types (broadcast and system)

Client Side:

ChatClient.java: Client application for connecting to the server and sending messages
Message.java: Shared message data structure

✨ Features

🌐 Server Features

✅ Concurrent client handling with dedicated threads
✅ Thread-safe client management using synchronized blocks
✅ Broadcast messaging to all connected users
✅ User re-entry support - existing users can reconnect seamlessly
✅ Connection history tracking with personalized welcome messages
✅ Graceful client disconnection handling
✅ Server statistics and logging
✅ Shutdown hooks for clean server termination

💬 Chat Mode

All messages are broadcast to everyone. Simply type your message and press Enter - no commands needed!

Messages appear to all connected users except the sender
System notifications for user join/leave events
Simple and straightforward chat experience

🔒 Thread Safety

Synchronized blocks for critical operations
Thread-safe user management via UserManager
Atomic operations for client add/remove
Safe message broadcasting without race conditions

🔄 User Re-entry Support

Seamless Reconnection: Users can disconnect and reconnect with the same username
Connection History: Server tracks user connection statistics and timestamps
Personalized Welcome: Different messages for new vs. returning users
Connection Tracking: Server maintains history of all user connections

🚀 Quick Start

Prerequisites

Java JDK 8 or higher
Command line interface (Terminal, PowerShell, CMD)

1. Compile the Project

# Navigate to project directory
cd Multithreaded-Chat-Server

# Compile server files
cd server
javac *.java

# Compile client files
cd ../client
javac *.java
cd ..

2. Start the Server

# Navigate to server directory
cd server

# Start server on default port 12345
java ChatServer

# Or specify a custom port
java ChatServer 8080

Expected Output:

==================================================
Chat Server Started Successfully!
Listening on port: 12345
Server started at: Fri Jan 17 10:30:45 EST 2026
==================================================
Waiting for client connections...

3. Connect Clients

Using the Chat Client

# In a new terminal window, navigate to client directory
cd client

# Run the client
java ChatClient

# Follow the prompts to enter your username and start chatting

Note: Update the SERVER_IP constant in ChatClient.java to match your server's IP address (default is 192.168.56.1).

Alternative: Using Telnet (Windows)

telnet localhost 12345

Alternative: Using Netcat (Linux/Mac)

nc localhost 12345

📋 Usage Examples

Example 1: Basic Chat Session

Start the server:
```
cd server
java ChatServer
```

Connect first client:

cd client
java ChatClient
# Enter username: Alice

Connect second client (in another terminal):

cd client
java ChatClient
# Enter username: Bob

Chat conversation:

[Alice]: Hello everyone!
# Output on Bob's screen: Alice: Hello everyone!

[Bob]: Hi Alice! How are you?
# Output on Alice's screen: Bob: Hi Alice! How are you?

[Alice]: Great to chat with you!
# Output on Bob's screen: Alice: Great to chat with you!

Example 2: Multiple Clients Broadcasting

# Terminal 1 - Server
cd server
java ChatServer

# Terminal 2 - Client 1 (Alice)
cd client
java ChatClient
# Username: Alice
[Alice]: Hello chat room!

# Terminal 3 - Client 2 (Bob)
cd client
java ChatClient
# Username: Bob
# Sees: [SYSTEM] Alice has joined the chat for the first time!
[Bob]: Hi everyone!

# Terminal 4 - Client 3 (Charlie)
cd client
java ChatClient
# Username: Charlie
# Sees: [SYSTEM] Alice has joined...
# Sees: [SYSTEM] Bob has joined...
[Charlie]: Hey folks!

# All messages are broadcast to everyone except the sender

🔍 Testing the Server

Manual Testing Checklist

Multiple clients can connect simultaneously
Messages are broadcasted to all users except sender
System messages appear for user join/leave events
Returning users get personalized welcome messages
Client disconnection is handled gracefully
Server handles malformed input without crashing
Thread-safe operations (no race conditions)

Load Testing

# Test with multiple simultaneous connections
cd client
for i in {1..10}; do
    java ChatClient &
done

🛠️ Advanced Configuration

Custom Port Configuration

Server:

cd server
java ChatServer 9999

Client:

Edit ChatClient.java and change the SERVER_PORT constant:

private static final int SERVER_PORT = 9999;

Then recompile and run:

cd client
javac ChatClient.java
java ChatClient

Server Monitoring

The server provides real-time statistics:

Active connections count
Total connections served
Connected usernames
Message broadcasting logs
Connection/disconnection events

🐛 Troubleshooting

Common Issues

Issue	Cause	Solution
`Address already in use`	Port is busy	Use different port or kill existing process
`Connection refused`	Server not running	Start server first
`Cannot resolve hostname`	Invalid server address	Check server IP/hostname
Compilation errors	Missing Java files	Ensure all .java files are present

Debug Mode

Add debug output by modifying the server:

// Add to ChatServer.java main method
System.setProperty("java.util.logging.ConsoleHandler.level", "ALL");

🔒 Security Considerations

Input Validation: All user inputs are sanitized
Resource Management: Proper cleanup of sockets and streams
Thread Safety: No race conditions or deadlocks
Graceful Shutdown: Clean disconnection handling

🎓 Learning Outcomes

After completing this project, you'll understand:

Socket Programming: Client-server communication patterns
Multithreading: Concurrent programming and thread management
Synchronization: Thread-safe operations and data protection
Network Protocols: TCP/IP communication fundamentals
Resource Management: Proper cleanup and memory management
Error Handling: Robust exception handling and recovery

🚀 Future Enhancements

Potential improvements for advanced learning:

📄 License

This project is created for educational purposes. Feel free to use and modify for learning Java networking and multithreading concepts.

Built with ❤️ using Java | Demonstrating Multithreading & Socket Programming

Name		Name	Last commit message	Last commit date
Latest commit History 10 Commits
client		client
server		server
.gitignore		.gitignore
README.md		README.md

ashokkumar272/Multithreaded-Chat-Server

Folders and files

Latest commit

History

Repository files navigation