Why doesn't my Spinner update its bound property the first time it is initialized?

Question

I have a Spinner in controller:

@FXML
private Spinner<Integer> spnMySpinner;

and a SimpleIntegerPropertyin controller:

private static final SimpleIntegerProperty myValue = 
new SimpleIntegerProperty(3); //load a default value

I have bound them together in controller's initialize method:

spnMySpinner.getValueFactory().valueProperty().bindBidirectional(myValueProperty().asObject());

But the bindings work correctly only after second time the controller initializes. Here's how I can reproduce it:

1. I open the stage with the associated controller, it loads the default value, specified in the myValue property correctly (a number 3).
2. I click the increment button on the spinner to make it a 4. It changes the value in the spinner's value property, but the bound property myValue is left intact with the number 3.
3. I close the stage/window.
4. I reopen it, the spinner has again a value of 3.
5. I increment it again. Boom now the binding works and I have a "4" both inside the spinner and the bound property.

Entire minimalistic, but launchable/reproducible code:

Main.java:

package spinnerpoc;

import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) throws IOException {
        Parent root  = FXMLLoader.load(getClass().getResource("MainWindow.fxml"));
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

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

}

MainWindow.fxml:

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.AnchorPane?>


<AnchorPane fx:id="myRoot" id="AnchorPane" prefHeight="231.0" prefWidth="337.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8.0.60" fx:controller="spinnerpoc.MainWindowController">
    <children>
        <Button fx:id="btnOpenSpinnerWindow" layoutX="102.0" layoutY="103.0" mnemonicParsing="false" text="Open SpinnerWindow" onAction="#onOpenSpinnerWindow"/>
    </children>
</AnchorPane>

MainWindowController.java:

package spinnerpoc;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Modality;
import javafx.stage.Stage;

public class MainWindowController implements Initializable {

    @FXML
    private Button btnOpenSpinnerWindow;
    @FXML
    private AnchorPane myRoot;

    @Override
    public void initialize(URL url, ResourceBundle rb) {
    }

    @FXML
    private void onOpenSpinnerWindow(ActionEvent event) throws IOException{
        FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("SpinnerWindow.fxml"));
        Parent root = (Parent) fxmlLoader.load();
        Stage stage = new Stage();
        stage.initOwner(myRoot.getScene().getWindow());
        stage.initModality(Modality.WINDOW_MODAL);
        stage.setTitle("SpinnerWindow");
        stage.setScene(new Scene(root));
        stage.show();
    }

}

SpinnerWindow.fxml:

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.RadioButton?>
<?import javafx.scene.control.ScrollPane?>
<?import javafx.scene.control.Slider?>
<?import javafx.scene.control.Spinner?>
<?import javafx.scene.control.SpinnerValueFactory.DoubleSpinnerValueFactory?>
<?import javafx.scene.control.TitledPane?>
<?import javafx.scene.control.ToggleGroup?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.VBox?>


<ScrollPane xmlns:fx="http://javafx.com/fxml/1" fitToWidth="true" prefHeight="200.0" prefWidth="200.0" xmlns="http://javafx.com/javafx/8.0.60" fx:controller="spinnerpoc.SpinnerWindowController">
    <content>
        <VBox maxWidth="1.7976931348623157E308">
            <children>
                <Spinner fx:id="spnMySpinner" editable="true" prefWidth="50.0" max="10" min="1" />
            </children>
        </VBox>
    </content>
</ScrollPane>

SpinnerWindowController.java:

package spinnerpoc;

import java.net.URL;
import java.util.ResourceBundle;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Spinner;

public class SpinnerWindowController implements Initializable {

    private static final SimpleIntegerProperty myValue = new SimpleIntegerProperty(3);

    public static SimpleIntegerProperty myValueProperty() {
        return myValue;
    }

    public static Integer getMyValue() {
        return myValue.getValue();
    }

    public static void setMyValue(int value) {
        myValue.set(value);
    }

    @FXML
    private Spinner<Integer> spnMySpinner;

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        spnMySpinner.getValueFactory().valueProperty().bindBidirectional(myValueProperty().asObject());
    }

}

(Code also available at a BitBucket repo.)

What am I missing?

Answer 1

You are running into the "premature garbage collection" problem. See a description of this here. You will likely find that it's not always every other time you show the spinner that it fails, but is just sporadic, and that the behavior will vary from one machine to another. If you limit the memory available to the JVM, you might find that it never works.

When you call IntegerProperty.asObject(), it

Creates an ObjectProperty that bidirectionally bound to this IntegerProperty.

Now note that a bidirectional binding has this feature to prevent accidental memory leaks:

JavaFX bidirectional binding implementation use weak listeners. This means bidirectional binding does not prevent properties from being garbage collected.

So the bidirectional binding you explicitly create does not prevent the thing it is bound to (the ObjectProperty<Integer> created by asObject()) from being garbage collected. Since you keep no references to it, it is eligible for garbage collections as soon as you exit the initialize() method in the SpinnerWindow Controller. Obviously, once the value to which your spinner value is bidirectionally bound is garbage collected, the binding will not work any more.

Just for demonstration purposes, you can see this by putting in a hook to force garbage collection. E.g. do

<ScrollPane onMouseClicked="#gc" xmlns:fx="http://javafx.com/fxml/1" ...>

in SpinnerWindow.fxml and

@FXML
private void gc() {
    System.out.println("Invoking GC");
    System.gc();
}

in SpinnerWindowController. If you do this, then clicking in the scroll pane will force garbage collection, and changing the spinner value will not update the property.

To fix this, retain a reference to the property you get from asObject():

public class SpinnerWindowController implements Initializable {

    private static final SimpleIntegerProperty myValue = new SimpleIntegerProperty(3);

    public static SimpleIntegerProperty myValueProperty() {
        return myValue;
    }

    public static Integer getMyValue() {
        return myValue.getValue();
    }

    public static void setMyValue(int value) {
        myValue.set(value);
    }

    @FXML
    private Spinner<Integer> spnMySpinner;

    private ObjectProperty<Integer> spinnerValue = myValueProperty().asObject();

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        spnMySpinner.getValueFactory().valueProperty().bindBidirectional(spinnerValue);
    }

}