💬 Multi-User Chat — Client/Server (Java, TLS)

A real-time multi-user chat over TLS 1.3 TCP sockets, with a custom binary-header + JSON protocol — built in Java as a multi-module client/server.

📋 Overview

A client/server chat where the server handles many clients concurrently and broadcasts each message to everyone else connected. Communication runs over encrypted TCP sockets with a small, explicit wire protocol.

┌──────────┐        TLS/TCP         ┌──────────┐
│  Client  │ ◄────────────────────► │  Server  │
└──────────┘       port 12345       └──────────┘
                                         ▲
                                         │
                                    ┌──────────┐
                                    │  Client  │
                                    └──────────┘

🚀 Tech Stack

Layer	Technology
Language	Java 21 (LTS)
Build	Maven (multi-module: comum, servidor, cliente)
Transport	TCP sockets over TLS 1.3
Payload	JSON (Gson)

🏗 Architecture & Protocol

Three Maven modules: comum (shared protocol), servidor (the server), and cliente (the client). The server runs one thread per connected client and keeps a shared, guarded map of active clients for broadcasting.

Each message is a 4-byte length header + a UTF-8 JSON payload:

┌─────────────────┬──────────────────────────────┐
│  Header 4 bytes │       Payload JSON UTF-8     │
│  (length N)     │           (N bytes)          │
└─────────────────┴──────────────────────────────┘

The full contract is specified in PROTOCOL.md.

🧠 Technical Decisions

• TLS 1.3 with a self-signed certificate — the whole channel is encrypted; the server identifies itself with a locally generated certificate.
• Custom binary framing — a length-prefixed header makes message boundaries explicit over a stream socket, instead of relying on delimiters.
• Thread-per-client with a synchronized client registry — simple concurrency that keeps broadcast and connect/disconnect consistent.

🔐 Security

The channel is fully encrypted with TLS 1.3. servidor.keystore (private key) is git-ignored; servidor.crt (public certificate) is shipped to the client module.

🔧 How to Run Locally

Prerequisites: Java 21, Maven 3.8+, OpenSSL.

1. Generate the TLS certificate (first time only) — inside servidor/:

keytool -genkeypair -alias servidor -keyalg RSA -keysize 2048 -validity 365 \
  -keystore servidor.keystore -storepass senhaKeystore -keypass senhaKeystore \
  -dname "CN=localhost, OU=Chat, O=Chat, L=Brasil, ST=Brasil, C=BR"

keytool -exportcert -alias servidor -keystore servidor.keystore \
  -storepass senhaKeystore -file ../cliente/src/main/resources/servidor.crt -rfc

2. Build everything (from the repo root): mvn install

3. Start the server: cd servidor && mvn exec:java -Dexec.mainClass="chat.Server"

4. Start a client (in another terminal): cd cliente && mvn exec:java -Dexec.mainClass="chat.Client"

Open more terminals to simulate additional users. Authentication is static (demo users defined in the server).

👤 Author

João Barbosa — Software Engineer (backend / platform). LinkedIn · GitHub