How to Call launch() More Than Once in Java

The JavaFX application launch method, launch(), is designed to be called only once per application. Attempting to call launch() more than once results in an "IllegalStateException" error.

Solution: Wrap Subsequent Window Creation in Platform.runLater()

Instead of calling launch() multiple times, consider the following approach:

1. Call launch() once to initialize the JavaFX runtime and create the main window.
2. Keep the JavaFX runtime running in the background using Platform.setImplicitExit(false). This prevents JavaFX from shutting down automatically when the main window is closed.
3. When you need to display a new window, wrap its creation and display in a Platform.runLater() block. This ensures that JavaFX operations are executed on the application thread.

Example Code:

public class Wumpus extends Application {
    private static final Insets SAFETY_ZONE = new Insets(10);
    private Label cowerInFear = new Label();
    private Stage mainStage;

    @Override
    public void start(final Stage stage) {
        mainStage = stage;
        mainStage.setAlwaysOnTop(true);
        Platform.setImplicitExit(false);
        cowerInFear.setPadding(SAFETY_ZONE);

        cowerInFear.setOnMouseClicked(event -> Platform.exit());
        stage.setScene(new Scene(cowerInFear));
    }

    public static void main(String[] args) {
        launch(args);

        // Another window can be created here by wrapping its creation
        // and display in a Platform.runLater() block.
        Platform.runLater(() -> {
            Stage newStage = new Stage();
            newStage.setScene(new Scene(new Label("Another Window")));
            newStage.show();
        });
    }
}

Note:

• The Wumpus class above demonstrates the approach with a custom JavaFX application.
• For use with Swing components, a JFXPanel can be used instead of an Application.
• Calling Platform.exit() will terminate the JavaFX runtime, so it is important to call this method when all JavaFX operations are complete.