Custom JavaFX control - "root value already specified" in Scene Builder 2.0

Question

I have implemented a custom control, using an fxml file and a Java class, similar to what is explained in this official tutorial (see the code bellow). Note that the fxml root element is defined with fx:root and I call setRoot programmatically.

I have tried including the control in the FXML layout of an application, and the application loads fine (and displays the control as expected).

However, if I try to import a jar file containing my control in Scene Builder 2.0, the control doesn't appear in the list of components to import (unlike some other controls in the same jar). If I select "Show JAR Analysis Report", it shows an error caused by javafx.fxml.LoadException: Root value already specified.

Do you know why I get this error when loading in Scene Builder, even though it loads correctly in a real application?

Here is the FXML :

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

<?import javafx.scene.control.*?>
<?import javafx.scene.image.ImageView?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.text.Font?>

<fx:root type="javafx.scene.layout.GridPane" id="MediaMetadataDisplay" hgap="20.0" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="200.0"
         prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
    <columnConstraints>
        <ColumnConstraints fillWidth="false" hgrow="NEVER" maxWidth="-Infinity" minWidth="-Infinity" prefWidth="200.0"/>
        <ColumnConstraints halignment="LEFT" hgrow="ALWAYS"/>
    </columnConstraints>
    <rowConstraints>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="30.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="40.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="30.0" vgrow="SOMETIMES"/>
    </rowConstraints>
    <children>
        <ImageView id="coverView" fx:id="coverView" fitHeight="200.0" fitWidth="200.0" pickOnBounds="true" preserveRatio="true" GridPane.rowSpan="7"/>
        <Label id="trackName" fx:id="trackName" maxWidth="1.7976931348623157E308" text="trackName" GridPane.columnIndex="1" GridPane.rowIndex="1">
            <font>
                <Font name="System Bold" size="16.0"/>
            </font>
        </Label>
        <Label id="artist" fx:id="artist" maxWidth="1.7976931348623157E308" text="artist" GridPane.columnIndex="1" GridPane.rowIndex="2"/>
        <Label id="album" fx:id="album" maxWidth="1.7976931348623157E308" text="album" GridPane.columnIndex="1" GridPane.rowIndex="3"/>
        <Label id="genre" fx:id="genre" maxWidth="1.7976931348623157E308" text="genre" GridPane.columnIndex="1" GridPane.rowIndex="4"/>
        <Label id="trackNumber" fx:id="trackNumber" maxWidth="1.7976931348623157E308" text="trackNumber" GridPane.columnIndex="1" GridPane.rowIndex="5"/>
    </children>
</fx:root>

And the Java controller/root element:

package customjavafx.scene.control;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.control.Label;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.layout.GridPane;
import javafx.scene.media.Media;

import java.io.IOException;
import java.util.Map;

public class MediaMetadataDisplay extends GridPane {

    public MediaMetadataDisplay() {
        final FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("MediaMetadataDisplay.fxml"));
        fxmlLoader.setRoot(this);
        fxmlLoader.setController(this);
        try {
            fxmlLoader.load();
        } catch (IOException exception) {
            throw new RuntimeException(exception);
        }

        media.addListener((obs, oldVal, newVal) -> updateMedia(newVal));
    }

    private final ObjectProperty<Media> media = new SimpleObjectProperty<>((Media) null, "media");
    @FXML private ImageView coverView;
    @FXML private Label trackName;
    @FXML private Label artist;
    @FXML private Label album;
    @FXML private Label genre;
    @FXML private Label trackNumber;

    public void updateMedia(Media media) {
        // TODO show updated metadata
    }

    public ObjectProperty<Media> mediaProperty() {
        return media;
    }
    public Media getMedia() {
        return mediaProperty().get();
    }
    public void setMedia(final Media media) {
        mediaProperty().set(media);
    }
}

The cause of the error, in the stacktrace shown by Scene Builder :

Caused by: javafx.fxml.LoadException: Root value already specified.
file:/Users/guillaumegaly/Library/Application%20Support/Scene%20Builder/Library/custom-controls_2.11.jar!/customjavafx/scene/control/MediaMetadataDisplay.fxml

    at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2613)
    at javafx.fxml.FXMLLoader.createElement(FXMLLoader.java:2771)
    at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2720)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2527)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2441)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2409)
    at customjavafx.scene.control.MediaMetadataDisplay.<init>(MediaMetadataDisplay.java:26)
    ... 18 more

Answer 1

Remove the line

fxmlLoader.setRoot(this);

Your FXML defines the root to be an AnchorPane (and you can't set the root twice, which is why you are getting the error).

<fx:root type="javafx.scene.layout.GridPane" id="MediaMetadataDisplay" hgap="20.0" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="200.0"
         prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">

I am really thankful to this post

Root value already specified