ADDITIONAL SYSTEM INFORMATION :
JDK23.0.2 on MacOS 15.5 (aarch64)

A DESCRIPTION OF THE PROBLEM :
If an object property is bound to another object property with an FXML binding expression (e.g. ${textField1.length*2}), and the source object comes later in the XML than the target object, a NullPointerException occurs in FXMLLoader.

If the binding is a simple 1:1 binding (e.g. ${textField1.length}) there is no error.



STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Set up a project with the test case code below and run it.



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should show a stage where the Label text is set to 2 x the length of the TextField, and is updated as text is typed into the field.
ACTUAL -
A null pointer exception, with this line:
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Number.longValue()" because "<parameter1>" is null
at javafx.fxml@24.0.1/com.sun.javafx.fxml.expression.Expression.lambda$multiply$2(Expression.java:932)


Full trace:

Exception in Application start method
Exception in thread "main" java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:118)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1164)
Caused by: java.lang.RuntimeException: Exception in Application start method
at javafx.graphics@24.0.1/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:894)
at javafx.graphics@24.0.1/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$0(LauncherImpl.java:197)
at java.base/java.lang.Thread.run(Thread.java:1575)
Caused by: javafx.fxml.LoadException:
/Users/peterhull/Projects/jfx-bug-proj/target/classes/com/mycompany/jfxbugproj/main.fxml:5

at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2692)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2670)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2533)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2503)
at com.mycompany.jfxbugproj.App.start(App.java:21)
at javafx.graphics@24.0.1/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$7(LauncherImpl.java:840)
at javafx.graphics@24.0.1/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$5(PlatformImpl.java:444)
at javafx.graphics@24.0.1/com.sun.javafx.application.PlatformImpl.lambda$runLater$4(PlatformImpl.java:419)
at javafx.graphics@24.0.1/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.Number.longValue()" because "<parameter1>" is null
at javafx.fxml@24.0.1/com.sun.javafx.fxml.expression.Expression.lambda$multiply$2(Expression.java:932)
at javafx.fxml@24.0.1/com.sun.javafx.fxml.expression.BinaryExpression.evaluate(BinaryExpression.java:55)
at javafx.fxml@24.0.1/com.sun.javafx.fxml.expression.ExpressionValue.getValue(ExpressionValue.java:191)
at javafx.base@24.0.1/com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:64)
at javafx.base@24.0.1/javafx.beans.value.ObservableValueBase.addListener(ObservableValueBase.java:57)
at javafx.fxml@24.0.1/com.sun.javafx.fxml.expression.ExpressionValue.addListener(ExpressionValue.java:200)
at javafx.base@24.0.1/javafx.beans.property.StringPropertyBase.bind(StringPropertyBase.java:171)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader$Element.processPropertyAttribute(FXMLLoader.java:318)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader$Element.processInstancePropertyAttributes(FXMLLoader.java:235)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader$ValueElement.processEndElement(FXMLLoader.java:767)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.processEndElement(FXMLLoader.java:2939)
at javafx.fxml@24.0.1/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2624)
... 7 more


---------- BEGIN SOURCE ----------
Example FXML:

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
    <Label text="${textField1.length*2}" />
    <TextField fx:id="textField1" />
</VBox>

Example Java:

package com.mycompany.jfxbugproj;

import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

public class App extends javafx.application.Application {

    @Override
    public void start(Stage stage) throws Exception {
        FXMLLoader loader = new FXMLLoader(App.class.getResource("main.fxml"));
        Pane root = loader.load();
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }
}
---------- END SOURCE ----------