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## Network File-Sharing chat System (Java)
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### Overview
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This project is a multi-client network chat system with file sharing, implemented using **Java sockets**, **MUltithreading** and **Object Serialization**.
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Clients connect to a central server and can:
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* send public chat message
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* send private messages
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* request the list of online users
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* transfer files to other users
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communication between client and server is implemented using Java Object Streams(`ObjectInputStream` / `ObjectOutputStream`) so that structured objects can be transferred directly over the network.
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---
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### Architecture
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1. **ChatServer** opens a `ServerSocket` and waits for connections.
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2. Each connecting client gets its own **ClientSession** thread.
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3. A shared **UserManager** tracks who's online and routes messages.
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4. Everything is exchanged as serialized objects (`ChatMessage` / `FileMessage`).
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```
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Client → Socket → Server → ClientSession (thread) → UserManager → Broadcast/Route → Clients
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```
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---
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### Message Protocol
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Two serializable classes carry everything over the wire:
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* **ChatMessage** – text-based: logins, public/private messages, user-list requests, server replies.
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* **FileMessage** – file transfers, carrying the file as a `byte[]`.
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`MessageType` is an enum (`LOGIN`, `LOGIN_SUCCESS`, `LOGIN_FAILED`, `PUBLIC_MESSAGE`, `PRIVATE_MESSAGE`, `USER_LIST`, …) so both sides agree on what each message means.
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---
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### Package Structure
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```
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com.university.chat
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├── common → shared message classes (ChatMessage, FileMessage, MessageType)
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├── server → server-side logic
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└── client → client application
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```
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#### `common`
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Already complete — these define the protocol and don't need any changes.
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#### `server`
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* **ChatServer** – entry point. Opens the server socket and creates a `ClientSession` per client.
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* **ClientSession** – one thread per client. Handles login, reads incoming messages, and dispatches them (broadcast, private message, user list, file transfer).
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* **UserManager** – thread-safe registry of online users (`ConcurrentHashMap<String, ClientSession>`). Already complete.
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* **FileManager** – manages per-user folders on disk (`server_data/<username>/sent` and `received`). Already complete.
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#### `client`
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* **chatClient** – entry point. Connects to the server, logs in, starts a listener thread, and runs the input loop for user commands.
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* **ServerListener** – background thread that continuously reads and displays messages/files coming from the server.
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* **TransferProgress** – small helper that prints a progress bar during file uploads. Already complete.
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---
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### Client Commands
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```
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/msg <username> <message> → private message
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/users → list online users
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/sendfile <username> <filepath> → send a file
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<anything else> → public message
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```
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---
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### File Transfer Flow
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1. Client reads the file and converts it to `byte[]`.
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2. It's wrapped in a `FileMessage` and sent to the server.
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3. The server saves a copy (in `sent/` for the sender, `received/` for the receiver) and forwards it to the recipient.
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---
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### Threading Model
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* **Server**: one `ClientSession` thread per connected client.
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* **Client**: main thread handles user input, a `ServerListener` thread handles incoming messages — so you can chat and receive **at the same time**.
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---
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## Your Task
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Several core pieces are left as **TODOs** for you to implement. Follow the comments in each file alongside the structure mentioned above to complete them.
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| File | What to implement |
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|------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| `Server/ChatServer.java` | Pick a port, create a shared `UserManager`, open the server socket, accept clients in a loop, and create a `ClientSession` thread per client. |
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| `Server/ClientSession.java` | Set up object streams in the constructor; handle login (success/failure); receive messages in a loop and dispatch `ChatMessage`/`FileMessage`; broadcast, private message, and user-list logic; remove user on disconnect; forward files. |
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| `Client/chatClient.java` | Connect to the server, set up streams, log in, start `ServerListener`, and implement the command loop (`/msg`, `/users`, `/sendfile`, plain messages). |
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| `Client/ServerListener.java` | Continuously read objects from the server and print chat messages / file-received notifications. |
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**Tips:**
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* Always create the `ObjectOutputStream` *before* the `ObjectInputStream` on both ends — this avoids a stream-handshake deadlock.
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* Don't forget `out.flush()` after `writeObject(...)`.
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* Test with two or more client instances to verify broadcasting, private messages, and file transfer all work correctly.
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#### Running multiple clients in IntelliJ
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1. Click the **⋮** icon next to the run button → **Edit Configurations**
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2. Open **Modify options** (Alt+M)
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3. Enable **Allow multiple instances** (Alt+U)
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---
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## Optional Bonus: JavaFX UI
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If you want to go further replace the console client with a simple **JavaFX** GUI (chat window, online-user list, file-send button, etc.).
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1. **Add the JavaFX dependencies** to your `pom.xml` — for Maven:
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```xml
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<dependency>
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<groupId>org.openjfx</groupId>
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<artifactId>javafx-controls</artifactId>
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<version>21</version>
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</dependency>
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<dependency>
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<groupId>org.openjfx</groupId>
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<artifactId>javafx-fxml</artifactId>
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<version>21</version>
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</dependency>
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```
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You'll also need the `javafx-maven-plugin` to run the app from Maven.
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2. **Create a JavaFX entry point**, e.g. `Client/ChatClientApp.java`, extending `javafx.application.Application` and implementing `start(Stage stage)`. This becomes your new launch class instead of (or alongside) `chatClient`.
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3. Reuse your existing socket/streams logic — just move the "send" actions to button handlers and update the UI from `ServerListener` using `Platform.runLater(...)` (**since UI updates must happen on the JavaFX Application Thread**).
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This part is **completely optional** and won't affect your core grade — just a fun way to practice connecting a GUI to networking code and multithreading.
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