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Lesson 15 - Java Chat - Client - Introducing the Application Structure

In previous lessons, we created a basic Java server, which we completed in the lesson Java Server - Plugin System Improvements.

Starting with today's lesson, we'll be creating the promised chat as a sample application using our universal Java server.


We'll write the client in Java and the JavaFX framework. We'll stick to the MVC architecture to keep the code readable. We'll write all views as separate .fxml files. The logic will be in the model and everything will be wired via the controller.


The chat will include the most basic communication features:

  • sending a message
  • notification that the user started to write a message

We won't implement adding friends to contacts here. Every user will be able to write to everyone. There will also be no registration / login process. When connecting to the server, the user will only enters their name/nickname which is how they'll appear in the chat. If you like the course, we'll also implement end2end encrypted messaging.

Client Structure

To save us some time, I created a basic structure of the client app to which we're going to add our code. Please download the archive attached at the end of this lesson. It contains the client structure (and the code from today's lesson in case something goes wrong for you).

The archive contains:

  • code from previous lessons on creating the server
  • the skeleton of the client in the client module

The client module has the following file structure:

Client module structure for Java server

The controller package contains classes representing controllers for each functionality. MainController will contain the logic of the main window which is displayed at the application start. ConnectController will manage the window in which we'll connect to the server. The model package will contain all the models we're going to create and use during the lessons. The service package will include classes for advanced functionality, such as ChatService, which will include methods for sending messages.

The ChatApp class initializes the JavaFX application. We've already created the LanServerFinder class and we'll use it in the future to find servers in the local network. The resources/ folder contains additional files necessary to run the application. For now, there are only .fxml files that serve as views for each controller.

Introducing the App Structure

If we run the application with the command ./gradlew :client:jfxRun, we'll see the basic application window.

Main window of client application for Java server

This window is defined in the main.fxml file and its controller is in the MainController class.

The window consists of three parts:

  • menu: contains only buttons to connect to the server and exit the application
  • on the left there's a listView, which will show the connected users
  • on the right is a chat window with separated tabs for each user

Displaying Local Servers

The second separate window that exists in the application takes care of connecting/dis­connecting to/from the server. In this window, servers from the local network will be displayed. The window the user will see is defined in the connect.fxml file. The controller of this window is in the ConnectController class. The image of the resulting window is shown below:

Window for connecting to/finding a Java server

The window is divided into three parts again:

  • at the top there's information about the current server and a disconnect button
  • at the bottom, there're text boxes for the server address, port and nickname that we'll use; at the bottom right is a button to connect to the specified server
  • all servers found in the local network will be displayed in the middle

There are three controllers in the controller package. We've already seen two of them so logically the last one should be ChatController. This controller won't have its own window, but will show user conversation and a text box to create a new message with a send button in the main window.


Before we begin implementing specific chat client logic, we'll create a ThreadPool class to run background tasks easily. We'll place the class in the root package next to the client's main class:


public final class ThreadPool {

    public static final ExecutorService COMMON_EXECUTOR = ForkJoinPool.commonPool();
    public static final Executor JAVAFX_EXECUTOR = Platform::runLater;
    public static final ScheduledExecutorService SCHEDULER = Executors.newSingleThreadScheduledExecutor();

    static void shutDown() {

        try {
            COMMON_EXECUTOR.awaitTermination(5, TimeUnit.SECONDS);
        } catch (InterruptedException e) {

        try {
            SCHEDULER.awaitTermination(5, TimeUnit.SECONDS);
        } catch (InterruptedException e) {

    private ThreadPool() {
        throw new AssertionError();

There are three public constants in the class:

  • COMMON_EXECUTOR - A general threadpool for standard operations
  • JAVAFX_EXECUTOR - An executor for executing code in the main thread
  • SCHEDULER - An executor for running a task later

The shutdown() method terminates the activities of individual executors safely. We'll call this method when closing the main application window. We'll call the shutdown() methods on individual executors to start the terminating process. If, for some reason, the termination isn't done within five seconds, they'll be killed using the awaitTermination() method. In the ChatApp class where the main window is created, we need to add a listener on closing the window:

stage.setOnCloseRequest(windowEvent -> ThreadPool.shutDown());

By this we should be familiar with the structure of the client application.

Next, in the lesson Java Chat - Client - View Local Servers, we'll implement displaying servers found in the local network and connecting to the them.


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Java Server - Plugin System Improvements
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Server for Client Applications in Java
Article has been written for you by Petr Štechmüller
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Activities (4)




Ramy:12/26/2020 18:06

Where is the attached project skeleton for this lesson as mentioned ?

12/26/2020 18:06
Replies to Ramy
Petr Štechmüller:12/28/2020 14:24

Hi, I'm glad you read all the articles up to this one. There had to be some mistake when translation happened. I will check the archive and try to add it to the article ASAP.

Edited 12/28/2020 14:25
12/28/2020 14:24
korkorna:11. July 3:40

how can download a basic structure of the client app?

11. July 3:40
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