Starting JavaFX from random Java code

I write a lot of prototype routines - too many to keep track of in separate projects so I end up with a ton of mains(). Best practice? Who gives a shit: its a prototype, played with for a day or a week and then forgotten.

So far for graphical output i've just been using Swing: actually there's probably not much reason not to use it for that because it does the job but once you need to add some interactivity it becomes a bit of a pain if you've been playing with JavaFX. I might add a 'display intermediate image' anywhere in the code and up it comes.

But JavaFX doesn't let you just call Platform.runLater() or new Stage() from anywhere as with Swing: the system needs initialising within an Application context.

Here's a solution. I have no claims it's a good one but it works for me so far.

// This code is placed in the public domain
public class FXUtils {

    static FXApplication app;
    static Semaphore sem = new Semaphore(0);

    public static void startFX(Runnable r) {
        if (app == null) {
            try {
                Thread t = new Thread(() -> {
                    FXApplication.start(r);
                });
                t.start();

                sem.acquire();
            } catch (InterruptedException ex) {
            }
        } else {
            Platform.runLater(r);
        }
    }

    public static class FXApplication extends Application {

        WritableImage image;
        static Runnable run;

        public FXApplication() {
        }

        @Override
        public void start(Stage stage) throws Exception {
            app = this;
            run.run();
            sem.release();
        }

        public static void start(Runnable r) {
            run = r;
            // Application.launch() can only be called from a static
            // method from a class that extends Application
            Application.launch();
            // no windows - no app!
            System.exit(0);
        }
    }
}

Whether start() calls System.exit() or not is up to you - personally when I close a window i'm prototyping stuff on I want everything else to fuck off for good.

And this is how to use it:

    public static void main(String[] args) {
        FXApplication.start(() -> {
            // Now on javafx thread
            Stage s = new Stage();

            s.setScene(new Scene(new VBox(new Label("foobar!"))));
            
            s.show();
        });

        // Will wait for javafx to start, but then continue here
        // exiting will leave the windows open, till they're closed
    }

This uses a thread to launch javafx so that the original main thread can continue; Application.launch() doesn't return until the last window is closed so would otherwise block. The thread could be made a daemon too for some different behaviours.

If you just want to launch a full JavaFX application from multiple mains then none of this is required, just create a static start() method which calls Application.launch().