JavaFX getScene() returns null in initialize method of the controller

Question

I built a small application in JavaFX yesterday. I wanted to get the Scene of the application in the Controller class. I got an error every time I tried to get the scene in the controller class. I could set the OnKeyPressed-method on a Button in the Controller class, works fine. But it works only fine if the Button is selected.. I can get the scene in the Main-class method replaceSceneContent only. I have read this question already, but I call the getScene()-method in the initialize-method?? Thanks for any ideas!

Main class:

public class Main extends Application {

private Stage stage;

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

@Override
public void start(Stage primaryStage) throws Exception {
    stage = primaryStage;
    gotoMenu();
    primaryStage.show();
}

public void gotoMenu() {
    try {
        MenuController menu = new MenuController();
        menu = (MenuController) replaceSceneContent("Menu.fxml");
        menu.setApp(this);
    } catch (Exception ex) {
        Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
    }
}

private Node replaceSceneContent(String fxml) throws Exception {
    FXMLLoader loader = new FXMLLoader();
    @SuppressWarnings("resource")
    InputStream in = Main.class.getResourceAsStream(fxml);
    loader.setBuilderFactory(new JavaFXBuilderFactory());
    loader.setLocation(Main.class.getResource(fxml));
    BorderPane page;
    try {
        page = (BorderPane) loader.load(in);
    } finally {
        in.close();
    }
    page.setOnKeyPressed(event -> {
        switch (event.getCode()) {
        case F11:
            if (stage.isFullScreen()) {
                stage.setFullScreen(false);
            } else {
                stage.setFullScreen(true);
            }
            break;
        default:
            break;
        }
    });
    Scene scene = new Scene(page);
    page.prefWidthProperty().bind(scene.widthProperty());
    page.prefHeightProperty().bind(scene.heightProperty());
    stage.setScene(scene);
    return (Node) loader.getController();
}}

Controller class:

public class MenuController extends BorderPane implements Initializable {

Main application;

@FXML
private Button button;

public void setApp (Main application) {
    this.application = application;
}

@Override
public void initialize(URL location, ResourceBundle resources) {
    button.getScene().setOnKeyPressed(e -> {
        switch(e.getCode()) {
        case A:
            System.out.println("A pressed!");
            break;
        default:
            break;
        }
    });
}}}

The initialize() method is called during the call to loader.load(...), which is (necessarily) before the UI defined in the FXML is placed in a scene. — James_D
(As an aside, you should not both set the location and provide an input stream to the loader. Remove the input stream in entirely, and use the no-argument load method: page = loader.load().) — James_D
2. I tried to use the same code to load the FXML as in the SceneBuilder examples there: link, ist the code there deprecated? — GiantSunflower
1. No. The UI is placed in the scene - and consequently the button's scene property becomes non-null, when you call new Scene(page). The controller knows nothing about the scene, so if you want an event handler on the scene, you should register it in the method where you create the scene. — James_D

James_D James_D · Accepted Answer · 2017-04-21T17:35:04

You can do

private Node replaceSceneContent(String fxml) throws Exception {
    FXMLLoader loader = new FXMLLoader();
    loader.setBuilderFactory(new JavaFXBuilderFactory());
    loader.setLocation(Main.class.getResource(fxml));

    BorderPane page = loader.load();
    MenuController controller = loader.getController();

    page.setOnKeyPressed(event -> {
        switch (event.getCode()) {
        case F11:
            if (stage.isFullScreen()) {
                stage.setFullScreen(false);
            } else {
                stage.setFullScreen(true);
            }
            break;
        default:
            break;
        }
    });
    Scene scene = new Scene(page);
    scene.setOnKeyPressed(event -> {
        if (event.getCode() == KeyCode.A) {
            controller.printA();
        }
    });


    page.prefWidthProperty().bind(scene.widthProperty());
    page.prefHeightProperty().bind(scene.heightProperty());
    stage.setScene(scene);
    return controller ;
}

with

public class MenuController extends BorderPane{

    // existing code...

    public void printA() {
        System.out.println("A!");
    }

}

Just a comment: it makes absolutely no sense for MenuController to be a subclass of BorderPane (or any other UI class). I left that in, in case you need it elsewhere, but it completely violates the MVC pattern.

Additionally, I'm not really sure why you want the key handler for A to be on the scene, and the key handler for F11 to be on the root of the scene. It seems these should both be registered with the scene. But again, I left it as you had it in the question.

JavaFX getScene() returns null in initialize method of the controller

2 Answers