FXML控制器中FXML字段为null但事件响应正常的问题求助

在为selection_bar.fxml编写的SelectionBarController控制器类中，发现@FXML注解标记的home_btn、patient_btn、setting_btn字段均为null，但按钮的Action事件可以正常响应。

相关代码 #

控制器代码 #

public class SelectionBarController {
   @FXML
    private Button home_btn;
    @FXML
    private Button patient_btn;
    @FXML
    private Button setting_btn;

    private MainView mainView;
    private UserModel viewModel;

    public void initSelectionBarController(MainView mainView, UserModel viewModel) {
        this.mainView = mainView;
        this.viewModel = viewModel;
        if (home_btn == null && patient_btn == null && setting_btn == null) {
            System.out.println("fxml field values are null");
        }
    }

    public void switchHomePage(Event e) {
        System.out.println("switchHomePage clicked");
    }

    public void switchPatientPage(Event e) {
        System.out.println("switchPatientPage clicked");
    }
}

调用类代码 #

public class MainView {
    private final Stage stage;
    private final BorderPane root;
    private final UserModel viewModel;
    private ToolBar selectionBar;
    private SelectionBarController selectionBarController;
    private PatientPageController patientPageController;
    private AnchorPane homePage;
    private BorderPane patientPage;
    //Size of the window
    private static final double WINDOW_X = 800;
    private static final double WINDOW_Y = 600;

    public MainView(Stage stage, UserModel viewModel) {
        this.stage = stage;
        this.viewModel = viewModel;
        root = new BorderPane();
        stage.show();
        initializeUserInterface();
    }
public void initializeUserInterface() {
        drawBackground();
        drawToolBar();
        stage.show();
    }

    public void drawBackground() {
        Scene scene = new Scene(root, WINDOW_X, WINDOW_Y);
        stage.setScene(scene);
    }

    public void drawToolBar() {
        try {
            // linking fxml to controller
            FXMLLoader loader = new FXMLLoader(getClass().getResource("/view/selection_bar.fxml"));
            selectionBar = loader.load();
            selectionBarController = loader.<SelectionBarController>getController();
            selectionBarController.initSelectionBarController(this, viewModel);
        } catch (IOException io) {
            System.out.println("selection bar not loaded");
        }
        root.setTop(selectionBar);
    }
}

FXML文件代码 #

<ToolBar prefHeight="30.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/18" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.java.clinic.controller.SelectionBarController">
    <items>
        <Button id="home_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Home"
                onAction="#switchHomePage">
            <opaqueInsets>
                <Insets/>
            </opaqueInsets>
        </Button>
        <Button id="patient_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Patient"
                onAction="#switchPatientPage">
            <opaqueInsets>
                <Insets/>
            </opaqueInsets>
        </Button>
        <Button id="setting_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Settings"
                onAction="#switchSettingPage">
            <opaqueInsets>
                <Insets/>
            </opaqueInsets>
        </Button>
    </items>
</ToolBar>

运行结果 #

fxml field values are null

解决方案 #

问题出在FXML文件中按钮的属性使用错误：FXML中实现控制器字段注入需要使用fx:id属性，而你当前用的是id属性。id只是节点的通用标识符，和FXML注入无关；fx:id才是FXML用来和控制器中@FXML标记的字段建立绑定的属性。

修改FXML文件中每个Button的id为fx:id即可解决字段为null的问题，修改后的FXML按钮部分如下：

<Button fx:id="home_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Home"
        onAction="#switchHomePage">
    <opaqueInsets>
        <Insets/>
    </opaqueInsets>
</Button>
<Button fx:id="patient_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Patient"
        onAction="#switchPatientPage">
    <opaqueInsets>
        <Insets/>
    </opaqueInsets>
</Button>
<Button fx:id="setting_btn" mnemonicParsing="false" style="-fx-background-color: transparent;" text="Settings"
        onAction="#switchSettingPage">
    <opaqueInsets>
        <Insets/>
    </opaqueInsets>
</Button>

另外，事件能正常响应是因为onAction绑定的是控制器中的方法名，不需要依赖fx:id，所以不受影响。

内容的提问来源于stack exchange，提问作者shonn Li