## Network File-Sharing chat System (Java)

### Overview
This project is a multi-client network chat system with file sharing, implemented using **Java sockets**, **MUltithreading** and **Object Serialization**.
Clients connect to a central server and can:

* send public chat message
* send private messages
* request the list of online users
* transfer files to other users

communication between client and server is implemented using Java Object Streams(`ObjectInputStream` / `ObjectOutputStream`) so that structured objects can be transferred directly over the network.


---
### Architecture

1. **ChatServer** opens a `ServerSocket` and waits for connections.
2. Each connecting client gets its own **ClientSession** thread.
3. A shared **UserManager** tracks who's online and routes messages.
4. Everything is exchanged as serialized objects (`ChatMessage` / `FileMessage`).

```
Client  →  Socket  →  Server  →  ClientSession (thread)  →  UserManager  →  Broadcast/Route  →  Clients
```

---

### Message Protocol
Two serializable classes carry everything over the wire:

* **ChatMessage** – text-based: logins, public/private messages, user-list requests, server replies.
* **FileMessage** – file transfers, carrying the file as a `byte[]`.

`MessageType` is an enum (`LOGIN`, `LOGIN_SUCCESS`, `LOGIN_FAILED`, `PUBLIC_MESSAGE`, `PRIVATE_MESSAGE`, `USER_LIST`, …) so both sides agree on what each message means.

---

### Package Structure
```
com.university.chat
├── common  → shared message classes (ChatMessage, FileMessage, MessageType)
├── server  → server-side logic
└── client  → client application
```

#### `common`
Already complete — these define the protocol and don't need any changes.

#### `server`
* **ChatServer** – entry point. Opens the server socket and creates a `ClientSession` per client.
* **ClientSession** – one thread per client. Handles login, reads incoming messages, and dispatches them (broadcast, private message, user list, file transfer).
* **UserManager** – thread-safe registry of online users (`ConcurrentHashMap<String, ClientSession>`). Already complete.
* **FileManager** – manages per-user folders on disk (`server_data/<username>/sent` and `received`). Already complete.

#### `client`
* **chatClient** – entry point. Connects to the server, logs in, starts a listener thread, and runs the input loop for user commands.
* **ServerListener** – background thread that continuously reads and displays messages/files coming from the server.
* **TransferProgress** – small helper that prints a progress bar during file uploads. Already complete.

---

### Client Commands
```
/msg <username> <message>       → private message
/users                          → list online users
/sendfile <username> <filepath> → send a file
<anything else>                 → public message
```

---

### File Transfer Flow
1. Client reads the file and converts it to `byte[]`.
2. It's wrapped in a `FileMessage` and sent to the server.
3. The server saves a copy (in `sent/` for the sender, `received/` for the receiver) and forwards it to the recipient.

---

### Threading Model
* **Server**: one `ClientSession` thread per connected client.
* **Client**: main thread handles user input, a `ServerListener` thread handles incoming messages — so you can chat and receive **at the same time**.

---

## Your Task

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.

| File                         | What to implement                                                                                                                                                                                                                         |
|------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `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.                                                                                             |
| `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. |
| `Client/chatClient.java`     | Connect to the server, set up streams, log in, start `ServerListener`, and implement the command loop (`/msg`, `/users`, `/sendfile`, plain messages).                                                                                    |
| `Client/ServerListener.java` | Continuously read objects from the server and print chat messages / file-received notifications.                                                                                                                                          |

**Tips:**
* Always create the `ObjectOutputStream` *before* the `ObjectInputStream` on both ends — this avoids a stream-handshake deadlock.
* Don't forget `out.flush()` after `writeObject(...)`.
* Test with two or more client instances to verify broadcasting, private messages, and file transfer all work correctly.

#### Running multiple clients in IntelliJ
1. Click the **⋮** icon next to the run button → **Edit Configurations**
2. Open **Modify options** (Alt+M)
3. Enable **Allow multiple instances** (Alt+U)

---

## Optional Bonus: JavaFX UI

If you want to go further replace the console client with a simple **JavaFX** GUI (chat window, online-user list, file-send button, etc.).

1. **Add the JavaFX dependencies** to your `pom.xml` —  for Maven:
   ```xml
   <dependency>
       <groupId>org.openjfx</groupId>
       <artifactId>javafx-controls</artifactId>
       <version>21</version>
   </dependency>
   <dependency>
       <groupId>org.openjfx</groupId>
       <artifactId>javafx-fxml</artifactId>
       <version>21</version>
   </dependency>
   ```
   You'll also need the `javafx-maven-plugin` to run the app from Maven.


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`.


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**).

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.