Java Socket Server Implementations

This repository demonstrates three variations of a simple TCP server in Java, each highlighting different concurrency models for handling multiple clients:

1. Multithreaded Server (per-client Thread)
2. Thread Pool Server (using ExecutorService)
3. Single-threaded Server (sequential handling)

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🔧 Requirements

• Java 8 or above
• Basic understanding of Java networking (java.net.Socket, java.net.ServerSocket)

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📁 Project Structure

Server.java // One of the 3 server implementations at a time Client.java // Simple client to test the servers

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🚀 How to Run

1. Compile the Java files

javac Server.java Client.java
2. Run the Server

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java Server

Make sure the server is up and listening before running the client.

#3. Run the Client

java Client You can run the client multiple times (even in a loop) to test concurrency.

🧠 Key Concepts Explained

✅ 1. Multithreaded Server (Per Client Thread)

Thread thread = new Thread(() -> server.getConsumer().accept(clientSocket)); thread.start(); A new thread is created for each client connection.

Suitable for a small number of concurrent clients.

Simple to implement but not scalable due to thread overhead.

✅ 2. Thread Pool Server (Using ExecutorService)

ExecutorService pool = Executors.newFixedThreadPool(100); pool.execute(() -> handleClient(clientSocket)); A fixed number of threads are reused to handle multiple clients.

Efficient and scalable under high load.

Prevents creation of too many threads.

Uses ExecutorService to manage thread lifecycle.

✅ 3. Single-threaded Server

Socket acceptedConnection = socket.accept(); handleClient(acceptedConnection); // directly Handles one client at a time, sequentially.

Simple but not suitable for production or multiple clients.

Useful for debugging or learning socket basics.

📡 Communication Flow

Client:

Connects to server via TCP socket.

Sends an initial message (optional).

Reads a welcome message from server.

Server:

Accepts client socket.

Sends a message like "Hello from server ".

Closes the connection.

🧪 Testing with Multiple Clients

You can simulate load by modifying the client:

for (int i = 0; i < 100; i++) {
    new Thread(client.getRunnable()).start();
}

Try this with:

Multithreaded server (works fine for small N)

Thread pool server (more controlled concurrency)

Single-threaded server (only one handled at a time)

⚠️ Common Issues Connection Refused: Server not started before client.

SocketTimeoutException: Server times out before a client connects. (Set a higher timeout or remove it)

Too many open files: Too many simultaneous threads/sockets, especially in multithreaded version.

✅ Conclusion

This repo shows how Java handles socket communication with varying degrees of concurrency and control. The thread pool model is best for most real-world use cases due to its efficiency and scalability.

📬 Author

Created by Amit Kumar

Feel free to fork, modify, and learn!