Cannot successfully init controller by ControllerFactory that returns a kotlin singleton object in javafx

Question

I have a javafx project and I want to access the main controller by kotlin singleton reference.

But the injection cannot succeed.

A minimal example on the below:

App.kt

class App: Application() {
    override fun start(primaryStage: Stage) {

        val loader = FXMLLoader(javaClass.getResource("Window.fxml")).also { it.setControllerFactory { Controller } }
        val root = loader.load<Parent>()
        val scene = Scene(root, 400.0, 600.0)

        primaryStage.scene = scene
        primaryStage.show()
    }

    fun main(vararg args: String) {
        launch(*args)
    }
}

Controller.kt

object Controller : Initializable {

    @FXML private lateinit var button: Button
    @FXML fun buttonAction() { println("Clicked") }

    override fun initialize(location: URL?, resources: ResourceBundle?) {
        button.text = "Click Me"
    }
}

Window.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<AnchorPane xmlns="http://javafx.com/javafx"
            xmlns:fx="http://javafx.com/fxml"
            fx:controller="Controller"
            prefHeight="400.0" prefWidth="600.0">
    <Button fx:id="button" onAction="#buttonAction"/>
</AnchorPane>

Then I got the Exception:

Exception in Application start method
Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: javafx.fxml.LoadException: 
/D:/WorkPlace/Kotlin/LabelPlusFX/target/test-classes/window.fxml

    at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2625)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2603)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
    at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2435)
    at DemoApp.start(DemoApp.kt:16)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    ... 1 more
Caused by: kotlin.UninitializedPropertyAccessException: lateinit property button has not been initialized
    at Controller.initialize(Controller.kt:20)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2573)
    ... 12 more

Why cannot fxml do injection successfully?

I don’t speak Kotlin, but it looks like you are trying to access cMenuBar in the constructor, which of course is before it’s initialised. The whole premise to this question seems wrong though. It doesn’t make sense to make a JavaFX controller a singleton. — James_D
@James_D Thanks for the advice, I removed the useless code blocks. — Meodinger Wang
Actually, I suspect what happens here is that the fields in a Kotlin singleton are compiled to static fields in the Java class. The FXMLLoader uses reflection (of course) to initialized the @FXML-annotated fields, and only looks for instance fields (not static ones). See stackoverflow.com/questions/23105433/…. It doesn't really make sense to have static @FXML-annotated fields (for the same reason it doesn't make sense to make your controller a singleton). — James_D
So I suppose a workaround is to make the controller a class, then define a singleton wrapper which has a single instance of the controller class. Define your controller factory to access the controller instance of the singleton. But again, I'd strongly caution you that creating a controller as a singleton is a really bad idea, and it's not clear why you'd want to do it. — James_D

Meodinger Wang Meodinger Wang · Accepted Answer · 2021-08-24T00:47:58

Thanks to James_D.

The key to this problem is how kotlin compiles object singleton.

Kotlin chooses to make the singleton fields static, but FXMLLoader only looks for instance fields (not static) since JavaFX8. That's why cannot FXMLLoader does injection successfully.

Cannot successfully init controller by ControllerFactory that returns a kotlin singleton object in javafx

1 Answers