import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class App extends Application {

	@Override
    public void start(Stage primaryStage) throws Exception {
        Menu menu = new Menu("Menu");
        menu.setOnShowing(event -> {
            // Fill menu items.
            menu.getItems().add(new MenuItem("MenuItem 1"));
        });

        MenuBar menuBar = new MenuBar(menu);

        BorderPane borderPane = new BorderPane();
        borderPane.setTop(menuBar);

        Scene scene = new Scene(borderPane);
        primaryStage.setScene(scene);
        primaryStage.show();
    } 
}