不确定如何使用初始化从 SceneBuilder 注入 FXML 变量而不抛出空异常

Question

我正在设计一个 JavaFX 程序，我试图让一个

Slider volumeSlider

控制在

MediaPlayer mediaPlayer

类中初始化的

Main

的音量。

volumeSlider

在名为

settings.fxml

的场景中，通过单击程序的第一个场景

interface.fxml

中的按钮访问该场景。我有设置 volumeSlider 值的代码，并根据我的

initialize

方法中滑块的值设置 mediaPlayer。如果我在第一个场景中添加一个名为

volumeSlider

的滑块，则该滑块和

settings.fxml

中的滑块都可以工作。但是，当我的程序更改为没有具有该名称的滑块的场景时，它会抛出

Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.Slider.setValue(double)" because "this.volumeSlider" is null

@FXML Slider volumeSlider;

    @Override
    public void initialize(URL location, ResourceBundle resources){
        volumeSlider.setValue(mediaPlayer.getVolume() * 100);
        volumeSlider.valueProperty().addListener(new InvalidationListener() {
            @Override
            public void invalidated(Observable observable) {
                mediaPlayer.setVolume(volumeSlider.getValue() / 100);
            }
        });

    }

为了解决这个问题，我从初始化方法中删除了代码，而是将其放在

onClickSettings

中，这是当用户点击设置按钮时抛出的方法。但是，似乎 volumeSlider 没有初始化，因为单击设置按钮会抛出与之前相同的错误消息

Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.Slider.setValue(double)" because "this.volumeSlider" is null at com.example.cribbage/com.example.cribbage.MainController.onClickSettings(MainController.java:934) ... 57 more

onClickSettings

方法：

@FXML
    public void onClickSettings(ActionEvent event) throws IOException {
        volumeSlider.setValue(mediaPlayer.getVolume() * 100);
        volumeSlider.valueProperty().addListener(new InvalidationListener() {
            @Override
            public void invalidated(Observable observable) {
                mediaPlayer.setVolume(volumeSlider.getValue() / 100);
            }
        });
        root = FXMLLoader.load(getClass().getResource("settings.fxml"));
        root.setId("pane");
        stage = (Stage) ((Node) event.getSource()).getScene().getWindow();
        scene = new Scene(root);
        scene.getStylesheets().addAll(this.getClass().getResource("style.css").toExternalForm());
        stage.setScene(scene);

        stage.show();


    }

settings.fxml

文件：

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.CheckBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.Slider?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="600.0" prefWidth="1000.0" xmlns="http://javafx.com/javafx/18" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.example.cribbage.MainController">
   <children>
      <Label layoutX="354.0" layoutY="75.0" text="Settings">
         <font>
            <Font name="Copperplate Gothic Bold" size="60.0" />
         </font></Label>
      <Button layoutX="51.0" layoutY="37.0" mnemonicParsing="false" onAction="#onClickMenu" prefHeight="38.0" prefWidth="82.0" text="Back">
         <font>
            <Font name="Copperplate Gothic Light" size="19.0" />
         </font></Button>
      <CheckBox layoutX="534.0" layoutY="262.0" mnemonicParsing="false" onAction="#onClickFullScreen" prefHeight="91.0" prefWidth="169.0" />
      <Label layoutX="354.0" layoutY="262.0" prefHeight="91.0" prefWidth="145.0" text="Fullscreen">
         <font>
            <Font size="29.0" />
         </font>
      </Label>
      <Slider fx:id="volumeSlider" layoutX="534.0" layoutY="371.0" value="50.0" />
      <Label layoutX="354.0" layoutY="340.0" prefHeight="91.0" prefWidth="145.0" text="Volume">
         <font>
            <Font size="29.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

Main

班级：

package com.example.cribbage;



import java.io.IOException;
import javafx.application.Application;

import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;

import javafx.scene.media.MediaPlayer;
import javafx.stage.Stage;
import javafx.scene.media.Media;




public class Main extends Application {



    public static MediaPlayer mediaPlayer;
    @Override

    public void start(Stage stage) {

        Parent root;

        try {
            Media media = new Media("https://soundimage.org/wp-content/uploads/2017/05/Car-Theft-101.mp3");
            mediaPlayer = new MediaPlayer(media);
            mediaPlayer.setAutoPlay(true);
            mediaPlayer.setCycleCount(MediaPlayer.INDEFINITE);
            mediaPlayer.setVolume(50);
            mediaPlayer.play();
            
            root = FXMLLoader.load(getClass().getResource("/com/example/cribbage/interface.fxml"));
            root.setId("pane");
            Scene scene = new Scene(root);
            scene.getStylesheets().addAll(this.getClass().getResource("style.css").toExternalForm());
            stage.setScene(scene);
            stage.show();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            
           System.out.println(e);
        }

    }

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

    }
}

initialize

方法：

@FXML Slider volumeSlider;

    @Override
    public void initialize(URL location, ResourceBundle resources){
        
    }

interface.fxml

文件：

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

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

<AnchorPane prefHeight="600.0" prefWidth="1000.0" xmlns="http://javafx.com/javafx/18" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.example.cribbage.MainController">
   <children>
      <Button layoutX="407.0" layoutY="201.0" mnemonicParsing="false" onAction="#onClickPlay" prefHeight="69.0" prefWidth="188.0" text="Play">
         <font>
            <Font name="Copperplate Gothic Bold" size="40.0" />
         </font>
      </Button>
      <Button layoutX="374.0" layoutY="314.0" mnemonicParsing="false" onAction="#onClickTutorial" prefHeight="71.0" prefWidth="254.0" text="Chart">
         <font>
            <Font name="Copperplate Gothic Bold" size="40.0" />
         </font>
      </Button>
      <Button layoutX="374.0" layoutY="428.0" mnemonicParsing="false" onAction="#onClickSettings" prefHeight="71.0" prefWidth="254.0" text="Settings">
         <font>
            <Font name="Copperplate Gothic Bold" size="40.0" />
         </font>
      </Button>
      <Label layoutX="251.0" layoutY="49.0" prefHeight="108.0" prefWidth="656.0" text="Cribbage">
         <font>
            <Font name="Copperplate Gothic Bold" size="96.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

如果这很混乱，我非常抱歉，这是我的第一个问题，我不确定如何格式化它以便于理解。如果需要更多文件或代码，请告诉我。