Non @FXML instance variable of an FXML controller becomes null

Question

I'm trying to design a class (TabPaneController) that controls adding and removing Tabs to a given TabPane instance.

Here's my AbstractView class:

public abstract class AbstractView implements TabUser {
    private Tab tab;
    private boolean loaded;

    void load(String tabName, String fxmlPath) {
        try {
            Parent root = FXMLLoader.load(getClass().getResource(fxmlPath));
            tab = new Tab(tabName, root);
            loaded = true;
        }
        catch(IOException e) {
            System.out.println("Failed to load tab:\n"+e);
        }
    }

    public abstract void load();

    @FXML
    public abstract void initialize();

    public boolean isLoaded() { return loaded; }

    @Override
    public Tab getTab() { return tab; }
}

Here's the DashboardView class:

public class DashboardView extends AbstractView {
    private String fxmlPath = "fxml/dashboard-view.fxml";
    private String tabName = "Dashboard";

    // becomes null during onButtonClick()
    private TabPaneController tabPaneController;

    @FXML
    private Button btnSearch, btnSettings, btnAbout;

    @Override
    public void load() {
        load(tabName, fxmlPath);
        getTab().setClosable(false);
    }

    @Override
    public void initialize() { }

    @FXML
    private void onButtonClick(ActionEvent event) {
        if(event.getSource() == btnSearch)
            tabPaneController.openNewTab(AppViews.SEARCH_VIEW);
    }

    @Override
    public void onTabOpened(TabPaneController tabPaneController) {
        this.tabPaneController = tabPaneController;
    }
}

The tabPaneController is assigned during the onTabOpened() call. This method is called by the only active TabPaneController instance. But during the onButtonClick() call, it somehow becomes null again.

[ EDIT: The tabPaneController does not become null if declared as static ]

Here's the TabPaneController class:

public class TabPaneController {
    private TabPane tabPane;

    public TabPaneController() { }

    public void setTabPane(TabPane tabPane) {
        this.tabPane = tabPane;
    }

    public void openNewTab(AbstractView targetView) {
        if(!targetView.isLoaded())
            targetView.load();

        Tab targetTab = targetView.getTab();

        // avoids occasional addition of duplicate tabs
        if(!tabPane.getTabs().contains(targetTab)) {
            targetView.onTabOpened(this);
            tabPane.getTabs().add(targetTab);
        }
        tabPane.getSelectionModel().select(targetTab);
    }
}

And if I click the btnSearch, which invokes the onButtonClick() method, i get an exception:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8411)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:394)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$353(GlassViewEventHandler.java:432)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at com.sun.glass.ui.gtk.GtkApplication.lambda$null$48(GtkApplication.java:139)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
    ... 52 more
Caused by: java.lang.NullPointerException
    at app.borax.view.DashboardView.onButtonClick(DashboardView.java:41)
    ... 62 more

Now my questions are:

Why is the tabPaneController in DashboardView becoming null?
How to prevent this?
What is a better way to design the TabPaneController?
I just want a mechanism to open any AbstractView as a tab in the TabPane from any other tabs or any other control (like menubar etc)

Thank you.

See [this question](https://stackoverflow.com/questions/50389361/changing-sceenes-with-intellij-2018-1-and-scene-builder-9-0-1) as well as the linked https://stackoverflow.com/questions/33303167/javafx-can-application-class-be-the-controller-class - the issue is that `FXMLLoader::load` creates a new instance of the controller, so it is no longer the instance on which you called `onTabOpened`. — Itai, Jun 11 '18 at 11:51
Indeed that was the problem. I took away the `load` method from `AbstractView` and put it inside another class, which also passes the `FXMLLoader::getController` value to my `TabPaneController` instance. This fixes the problem — Raj, Jun 15 '18 at 15:43

Non @FXML instance variable of an FXML controller becomes null

0 Answers0