File Server

A multithreaded client-server file storage system written in Java, built as part of the JetBrains Hyperskill Java Backend Developer track.

Overview

The File Server allows clients to store, retrieve, and delete files on a remote server over a TCP socket connection. Files are persisted on disk and referenced by either their filename or a server-assigned integer ID. The server handles multiple simultaneous client connections using a thread pool.

Features

• TCP socket communication between client and server
• Concurrent client handling via ExecutorService thread pool
• ID-based file access — server assigns a unique integer ID on PUT, which the client can use for GET and DELETE
• Persistence across restarts — the ID-to-filename map is serialized to disk using ObjectOutputStream / ObjectInputStream
• Binary file transfer support
• Graceful shutdown — server exits cleanly on client request

Project Structure

File-Server/
├── src/
│   └── server/
│       ├── Main.java          # Server entry point
│       ├── Server.java        # Accepts connections, manages thread pool
│       ├── RequestHandler.java # Handles individual client sessions
│       └── Storage.java       # File I/O and ID map management
│   └── client/
│       ├── Main.java          # Client entry point
│       └── Client.java        # Sends requests to the server
└── README.md

How It Works

Server

The server listens on a configured port and accepts incoming client connections. Each connection is handed off to a RequestHandler running in a thread pool managed by ExecutorService.

ExecutorService pool = Executors.newFixedThreadPool(N_THREADS);
pool.submit(() -> handleClient(socket));

The server maintains a HashMap<Integer, String> idMap that maps integer file IDs to their filenames on disk. This map is serialized to a file after every write operation so it survives server restarts.

Protocol

Clients send plain-text commands over the socket:

Command	Format	Description
PUT	PUT <filename> <data>	Store a file; server returns the assigned ID
GET	GET <id|filename>	Retrieve a file by ID or name
DELETE	DELETE <id|filename>	Delete a file by ID or name
EXIT	EXIT	Shut down the server

ID Map Persistence

On every PUT, the server updates idMap and serializes the entire map to disk:

// Save
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(ID_MAP_PATH));
oos.writeObject(idMap);

// Load on startup
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(ID_MAP_PATH));
idMap = (HashMap<Integer, String>) ois.readObject();

Getting Started

Prerequisites

• Java 11+
• Any IDE (IntelliJ IDEA recommended) or javac/java from the command line

Running the Server

cd src
javac server/*.java
java server.Main

Running the Client

cd src
javac client/*.java
java client.Main

Concepts Covered

• Java Sockets (ServerSocket, Socket)
• Multithreading with ExecutorService and thread pools
• Java Serialization (ObjectOutputStream / ObjectInputStream)
• Binary file I/O with DataInputStream / DataOutputStream
• Synchronized access to shared state

Stage Breakdown

Stage	What was added
1	Basic single-client GET/PUT over sockets
2	File storage to disk
3	DELETE support; named file access
4	Multithreading, ID-based access, serialized persistence

Author

Dev-narayan01