Return result from javafx platform runlater

Question

I am working on JavaFX application, in my scenario is to show a password prompt created in JavaFX which takes password with two option OK and Cancel. I have returned the password entered by user.

My class of showing password dialog is -

public static String showPasswordDialog(String title, String message, Stage parentStage, double w, double h) {
    try {
        Stage stage = new Stage();
        PasswordDialogController controller = (PasswordDialogController) Utility.replaceScene("Password.fxml", stage);
        passwordDialogController.init(stage, message, "/images/password.png");
        if (parentStage != null) {
            stage.initOwner(parentStage);
        }
        stage.initModality(Modality.WINDOW_MODAL);
        stage.initStyle(StageStyle.UTILITY);
        stage.setResizable(false);
        stage.setWidth(w);
        stage.setHeight(h);                
        stage.showAndWait();
        return controller.getPassword(); 
    } catch (Exception ex) {
         return null;
    }

My code where to show password prompt is below, actually this prompt will be shown over other UI, so I need to inclose this inside Platform.runlater(), otherwise it throws Not on FX application thread. I need this password prompt to be shown until I get correct one. How can I get value of password if I inclosed showing password inside runlater.

Is there any other better way?

final String sPassword = null;

          do {
            Platform.runLater(new Runnable() {

                @Override
                public void run() {
                     sPassword = JavaFXDialog.showPasswordDialog(sTaskName + "Password", "Enter the password:", parentStage, 400.0, 160.0);
                }
            });

            if (sPassword == null) {
                System.out.println("Entering password cancelled.");
                throw new Exception("Cancel");
            }
        } while (sPassword.equalsIgnoreCase(""));

Answer 1

I'd recommend wrapping the code within a FutureTask object. FutureTask is a construct useful (among other things) for executing a portion of code on one thread (usually a worker, in your case the event queue) and safely retrieving it on another. FutureTask#get will block until FutureTask#run has been invoked, therefore your password prompt could look like this:

final FutureTask query = new FutureTask(new Callable() {
    @Override
    public Object call() throws Exception {
        return queryPassword();
    }
});
Platform.runLater(query);
System.out.println(query.get());

As FutureTask implements Runnable, you can pass it directly to Platform#runLater(...). queryPassword() will be inokved on the event queue, and the subsequent call to get block until that method completes. Of course, you will want to invoke this code in a loop until the password actually matches.