JavaFX - 如何修复'fx：controller只能应用于根元素'

Question

I have created a simple borderless application with JavaFX. Now I created a Label which should replace the missing "close" button. Actually, my attempt is not working because I am not able to interact with my FXMLController.

I have already tried to add the FXMLController to my *.fxml file. I tried to insert the controller to the VBox and the AnchorPane but both are not working and I don't really get it.

LabelCloseTest.fxml

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

<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<VBox prefHeight="720.0" prefWidth="1025.0" 
    xmlns="http://javafx.com/javafx/10.0.1" xmlns:fx="http://javafx.com/fxml/1">
      <children>
        <AnchorPane maxHeight="-1.0" maxWidth="-1.0" prefHeight="-1.0" prefWidth="-1.0" style="-fx-background-color: #303136;" VBox.vgrow="ALWAYS" fx:controller="com.mycompany.testing.FXMLDocumentController">
         <children>
            <Label fx:id="labelTest" layoutX="1007.0" layoutY="5.0" prefHeight="10.0" prefWidth="5.0" text="X" textFill="WHITE">
               <font>
                  <Font name="System Bold" size="13.0" />
               </font>
            </Label>
         </children>
    </AnchorPane>
  </children>
</VBox>

FXMLDocumentController.java

package com.mycompany.testing;

import javafx.event.EventHandler;
import javafx.fxml.FXML;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;

public class FXMLDocumentController {

@FXML
private Label labelTest;

@FXML
public void closeLabelPressed() {

       labelTest.setOnMouseClicked(new EventHandler<MouseEvent>() {

        public void handle(MouseEvent event) {
            System.exit(0);

             }
        });
    }
}

LabelTest.java

package com.mycompany.testing;


import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class LabelTest extends Application {

    public static void main (String[] args) {

        launch(args);

    }

    @Override
    public void start(Stage primaryStage) throws Exception {


       primaryStage.initStyle(StageStyle.UNDECORATED);
       FXMLLoader loader = new FXMLLoader(getClass().getResource("/fxml/LabelCloseTest.fxml"));
       Parent root = loader.load();

       FXMLDocumentController fdc = new FXMLDocumentController();

       fdc.closeLabelPressed();


       Scene scene = new Scene(root);

       primaryStage.setScene(scene);
       primaryStage.show();
    }

}

Stacktrace

Exception in Application start method
Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: javafx.fxml.LoadException: fx:controller can only be applied to root element.
/D:/Workspace/SocketClient/target/classes/fxml/LabelCloseTest.fxml:10

    at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2621)
    at javafx.fxml.FXMLLoader$ValueElement.processAttribute(FXMLLoader.java:917)
    at javafx.fxml.FXMLLoader$InstanceDeclarationElement.processAttribute(FXMLLoader.java:980)
    at javafx.fxml.FXMLLoader$Element.processStartElement(FXMLLoader.java:227)
    at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:752)
    at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2722)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2552)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2435)
    at com.mycompany.testing.LabelTest.start(LabelTest.java:25)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    ... 1 more

I would expect that the application starts and when I press the "X" Label that the Application would close.

Quick reminder: I am new to JavaFX so I would also be thankful for every JavaFX tutorial recommendation.

Answer 1

I tried to insert the controller to the VBox and the AnchorPane but both are not working and I don't really get it.

You mention trying to put the fx:controller attribute both on the VBox element as well as the AnchorPane element. The code you show is your attempt to add it to the AnchorPane element. Running your code on my computer as is results in the LoadException you observe. This problem is solved by putting the fx:controller attribute in the VBox element (the root element ):

<VBox prefHeight="720.0" prefWidth="1025.0" 
      xmlns="http://javafx.com/javafx/10.0.1" xmlns:fx="http://javafx.com/fxml/1" 
      fx:controller="com.mycompany.testing.FXMLDocumentController">
    <!--- Rest of FXML file omitted for brevity -->

---

Note : I initially noticed another problem where you weren't closing the <VBox> or <children> elements. However, those closing tags were there the whole time; they just weren't rendering in your question, as pointed out by fabian . I neglected to check for this.

---

However , this results in a different exception.

// Beginning of stack trace omitted for brevity...
Caused by: java.lang.NullPointerException
        at com.mycompany.testing.FXMLDocumentController.closeLabelPressed(FXMLDocumentController.java:16)
        at com.mycompany.testing.LabelTest.start(LabelTest.java:28)
        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

This problem is ultimately caused by this code in LabelTest#start :

FXMLLoader loader = new FXMLLoader(getClass().getResource("/fxml/LabelCloseTest.fxml"));
Parent root = loader.load();

FXMLDocumentController fdc = new FXMLDocumentController(); // the real problem

fdc.closeLabelPressed(); // accesses the labelTest field which is null

Since you're using fx:controller the FXMLLoader will create its own instance of FXMLDocumentController . It is this instance where the @FXML annotated fields get injected. You, on the other hand, create your own instance which will not have had the fields injected. You should be using the FXMLDocumentController instance created by the FXMLLoader .

FXMLLoader loader = new FXMLLoader(getClass().getResource("/fxml/LabelCloseTest.fxml"));
Parent root = loader.load();

// gets instance from FXMLLoader, must be called after load()
FXMLDocumentController fdc = loader.getController(); 

fdc.closeLabelPressed();

However , while your code (with the fixes so far) will do what you want it is not the correct way to do it. The method closeLabelPressed looks like you intend it to be an event handler since you annotated it with @FXML . You don't link this method with the FXML file, however, and instead the method adds an EventHandler to the Label . Set up correctly, you shouldn't be calling closeLabelPressed from LabelTest at all.

The first thing you should do is link the method to the Label via the FXML file.

<!-- previous omitted for brevity -->

<Label fx:id="labelTest" layoutX="1007.0" layoutY="5.0" prefHeight="10.0" prefWidth="5.0" text="X"
       textFill="WHITE" onMousePressed="#closeLabelPressed">

<!-- rest omitted for brevity -->

Notice the onMousePressed attribute .

The second thing to do is to change closeLabelPressed method in FXMLDocumentController .

package com.mycompany.testing;

import javafx.application.Platform;
import javafx.fxml.FXML;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;


public class FXMLDocumentController {

    @FXML
    private Label labelTest;

    @FXML
    private void closeLabelPressed(MouseEvent event) {
        Platform.exit();
    }
}

Now the method is called when the Label is pressed. Internally, the FXMLLoader sets the onMousePressed property (of labelTest ) to an EventHandler that invokes closeLabelPressed . I've also changed the use of System.exit(0) to Platform.exit() . The latter will allow the JavaFX runtime to gracefully shutdown (allowing you to do clean up in places like Application#stop ) while the former immediately terminates the JVM (and doesn't return normally). Note: Injecting the Label may no longer be necessary .

The third thing to do is to remove the call to closeLabelPressed in the LabelTest.start method.

After these changes your code should be working properly.

---

As for asking for tutorials (which is off-topic on Stack Overflow, see the help center ) you can likely find many by searching Google. For "official" sources, see JavaFX: Getting started with JavaFX (last updated for JavaFX 8) and Introduction to FXML . There are also many questions related to FXML here on Stack Overflow.