The latest articles on DEV Community by Nouha50215 (@nouha50215). https://dev.to/nouha50215 https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.us-east-2.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F4201173%2F78535713-3372-439b-91cb-050ddbf4bee9.png https://dev.to/nouha50215 en Nouha50215 Thu, 22 Oct 2026 20:05:00 +0000 https://dev.to/nouha50215/building-a-multi-client-chat-app-with-java-sockets-virtual-threads-and-javafx-53nk https://dev.to/nouha50215/building-a-multi-client-chat-app-with-java-sockets-virtual-threads-and-javafx-53nk <h1> Building a Multi-Client Chat App with Java Sockets, Virtual Threads and JavaFX </h1> <p>For my Advanced Distributed Programming course, I built a group chat application: one server, many clients, real-time messages over TCP, and a JavaFX interface on both sides.</p> <h2> What I built </h2> <ul> <li> <strong>TCPServer</strong>: accepts many clients at the same time, adds the sender's name and time to every message (<code>[alice 14:05] hello</code>) and sends it to everyone. Its window shows an activity log and a live list of connected users, each with a random colour.</li> <li> <strong>TCPClient</strong>: asks for a username, then opens the chat. Users send with a SEND button or Enter, type <code>allUsers</code> to see who is online, and leave with <code>bye</code>, <code>end</code> or a LEAVE button. An "Online" label with a green circle shows the connection status. Joining without a username gives <strong>read-only mode</strong>: you can read but not send.</li> </ul> <h2> Development process </h2> <p><strong>1. Planning from the requirements.</strong> I turned the requirements document into a checklist and a short plan, then chose the architecture before writing code.</p> <p><strong>2. Architecture.</strong> Both programs have the same layers:</p> <ul> <li> <strong>Model</strong> (business logic): sockets, threads and the message protocol, with <strong>no JavaFX imports</strong>.</li> <li> <strong>View</strong>: the JavaFX windows (GridPane + CSS).</li> <li> <strong>Config</strong>: reads the server address and port from <code>config.properties</code> at runtime, so nothing needs recompiling.</li> </ul> <p>The model reports events through small listener interfaces (<code>ServerListener</code>, <code>MessageListener</code>) that the view implements. This is the Observer pattern, and it keeps the logic independent of the UI.</p> <p><strong>3. Threading.</strong> The server uses <strong>one virtual thread per client</strong> (Java 21). Each client's thread spends most of its time waiting in <code>readLine()</code>, and virtual threads make that waiting almost free: the JVM parks them and frees the underlying OS thread. Shared data uses <code>ConcurrentHashMap</code>, and writes to each client are <code>synchronized</code> so lines never get mixed.</p> <p><strong>4. Tools.</strong> Two Maven projects (built into executable "fat" JARs with the shade plugin), IntelliJ IDEA, Git/GitHub with small commits, and PlantUML for the UML diagrams.</p> <p>I used an AI assistant (Claude) to help plan the project and generate a first version of the code. Then I ran and tested everything, built the JARs, and spent time understanding how each part works, especially the threading.</p> <h2> Technical challenges </h2> <ul> <li> <strong>Running the JARs from the command line.</strong> The app worked in IntelliJ but not from cmd at first. I learned to build with Maven <code>clean</code> + <code>package</code>, use the right JAR (not <code>original-…jar</code>), and run with a Java 21+ JDK. IntelliJ's terminal made this easy.</li> <li> <strong>Understanding virtual threads.</strong> At first I thought each virtual thread was tied to one platform thread. Actually, many virtual threads share a few "carrier" threads and only hold one while they are running.</li> <li> <strong>Thread safety.</strong> Several threads broadcast at once, so I had to understand why a plain <code>HashMap</code> isn't safe and why each client's writer needs a lock.</li> <li> <strong>Exporting UML diagrams.</strong> PlantUML's online editor wouldn't save a PNG directly, so I used a screenshot or the IntelliJ PlantUML plugin.</li> <li> <strong>README images not showing on GitHub.</strong> My screenshot file names had spaces, which break Markdown image links. Renaming the files fixed it.</li> </ul> <h2> Final implementation </h2> <ul> <li>Multi-client server with virtual threads</li> <li>Broadcast messages with username + time</li> <li> <code>allUsers</code> command (reply only to the requester)</li> <li>Read-only mode</li> <li> <code>bye</code> / <code>end</code> / LEAVE with notifications to the others</li> <li>Online status indicator, coloured user list, server activity log</li> <li>Host and port loaded from <code>config.properties</code> </li> <li>Model fully independent of the JavaFX view</li> </ul> <p><strong>Source code:</strong> [<a href="https://github.com/Nouha50215/Multi-Client-Chat" rel="noopener noreferrer">GitHub repo</a>]</p> <h2> What I learned </h2> <p>Java sockets and I/O streams, multithreading with virtual threads, thread-safe collections, client-server architecture, separating logic from UI, and building and packaging a Java app with Maven.</p> java networking programming softwaredevelopment