绑定嵌套Controller属性时遭遇NullPointerException问题求助

我需要在main.fxml视图中获取nested.fxml界面内TextField的修改值，已编写对应控制器和FXML文件，但更新TextField后，点击按钮调用testVal()时抛出NullPointerException，提示attVal.getValue()返回null，预期能获取到修改后的值。

现有代码 #

NestedController.java #

package reproduced;
import java.net.URL;
import java.util.ResourceBundle;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.TextField;

public class NestedController implements Initializable{
    @FXML
    private TextField changeValue;

    public ObjectProperty<ObjectStringToUpdate> attVal= new SimpleObjectProperty<>();

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        this.changeValue.textProperty().addListener(new ChangeListener<Object>() {
     
            public void changed(ObservableValue<?> ov, Object t, Object t1) {
                ObjectStringToUpdate newValue = new ObjectStringToUpdate();  
                newValue.setAttValNested(t1.toString());
                attVal.setValue(newValue);
            }
 
        });
    }
}

MainController.java #

package reproduced;
import java.io.IOException;
import java.net.URL;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.Property;
import javafx.beans.property.SimpleObjectProperty;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.fxml.FXMLLoader;
import javafx.scene.layout.AnchorPane;

public class MainController implements Initializable{
    @FXML
    public BorderPane mainBorderPane;
    Property<ObjectStringToUpdate> attVal =  new SimpleObjectProperty<>();
     
    @FXML public NestedController nestedController;//injected nested controller
    @Override
    public void initialize(URL location, ResourceBundle resources) {
        ObjectProperty<ObjectStringToUpdate> binding = nestedController.attVal;
        ((Property<ObjectStringToUpdate>) this.attVal).bind(binding);
    }

    @FXML
    public void testVal() {
        System.out.println(this.attVal.getValue().attValNested);
        // 抛出NPE: Cannot read field "attValNested" because getValue() returns null
    }

    @FXML 
    public void openNestedView() throws IOException {
        FXMLLoader loaderOfTabs = new FXMLLoader(getClass().getResource("/nested.fxml"));
        AnchorPane tabPane = (AnchorPane) loaderOfTabs.load();
        mainBorderPane.setCenter(tabPane);
    }
}

ObjectStringToUpdate.java #

public class ObjectStringToUpdate {
    public String attValNested;
    
    public String getAttValNested() {
        return attValNested;
    }
    
    public void setAttValNested(String attVal) {
        this.attValNested = attVal;
    }   
}

UInterface2.java（主入口） #

package reproduced;
import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class UInterface2 extends Application{
    @Override
    public void start(Stage stage) throws IOException {
        String fxmlFile = "main.fxml";
        FXMLLoader loader = new FXMLLoader();
        Parent rootNode = (Parent) loader.load(getClass().getClassLoader().getResource(fxmlFile));
        Scene scene = new Scene(rootNode, 1024, 843);
        stage.setTitle("reproduced");
        stage.setResizable(false);
        stage.setScene(scene);
        stage.show();
    }

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

main.fxml #

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.layout.*?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:id="rootAnchorPane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="843.0" prefWidth="1224.0" fx:controller="reproduced.MainController">
   <children>
      <BorderPane fx:id="mainBorderPane" layoutY="-1.0" prefHeight="843.0" prefWidth="1024.0">
         <top>
            <Button mnemonicParsing="false" onAction="#openNestedView" text="open nested View" BorderPane.alignment="CENTER" />
         </top>
      </BorderPane>
      <Button fx:id="testVal" mnemonicParsing="false" onAction="#testVal" text="test" />
      <Label fx:id="showValue" text="Label" />
   </children>
   <fx:include fx:id="nested" source="nested.fxml" visible="false" />
</AnchorPane>

nested.fxml #

<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane xmlns="http://javafx.com/javafx/17" xmlns:fx="http://javafx.com/fxml/1" fx:id="rootAnchorPaneInner" fx:controller="reproduced.NestedController">
   <children>
      <TextField xmlns="http://javafx.com/javafx/8" fx:id="changeValue" />
   </children>
</AnchorPane>

问题原因 #

1. 重复加载FXML实例：main.fxml中通过<fx:include>已经加载了nested.fxml并注入了nestedController，但openNestedView()方法里又用FXMLLoader重新加载了一次nested.fxml，导致界面显示的是新的NestedController实例，而绑定属性的是<fx:include>注入的旧实例。修改TextField时，旧实例的attVal根本不会更新，所以mainController.attVal始终为null。
2. 属性定义冗余：MainController中的attVal定义为Property类型，后续又强制转换为ObjectProperty，没必要，直接用ObjectProperty更简洁。
3. 初始值缺失：attVal初始为null，若用户未修改TextField就点击test按钮，也会触发NPE。

修复方案 #

1. 修改MainController，复用已加载的嵌套实例 #

去掉openNestedView()中的重新加载逻辑，直接使用<fx:include>加载的实例：

package reproduced;
import java.net.URL;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.layout.AnchorPane;

public class MainController implements Initializable{
    @FXML
    public BorderPane mainBorderPane;
    // 直接定义为ObjectProperty，避免类型转换
    private ObjectProperty<ObjectStringToUpdate> attVal =  new SimpleObjectProperty<>();
     
    @FXML public NestedController nestedController;
    // 获取<fx:include>加载的嵌套视图根节点
    @FXML private AnchorPane nested;

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        // 直接绑定，无需强制转换
        attVal.bind(nestedController.attVal);
        // 设置初始值，避免未修改时的NPE
        attVal.setValue(new ObjectStringToUpdate());
    }

    @FXML
    public void testVal() {
        // 先判断是否为null，再取值
        if (attVal.getValue() != null) {
            System.out.println(attVal.getValue().getAttValNested());
        } else {
            System.out.println("值未初始化");
        }
    }

    @FXML 
    public void openNestedView() {
        // 直接显示已加载的嵌套视图，添加到BorderPane的center
        mainBorderPane.setCenter(nested);
        nested.setVisible(true);
    }
}

2. 优化NestedController的监听器（可选） #

将监听器的泛型改为String，更符合TextField的textProperty类型：

@Override
public void initialize(URL location, ResourceBundle resources) {
    this.changeValue.textProperty().addListener(new ChangeListener<String>() {
        @Override
        public void changed(ObservableValue<? extends String> ov, String oldVal, String newVal) {
            ObjectStringToUpdate newValue = new ObjectStringToUpdate();  
            newValue.setAttValNested(newVal);
            attVal.setValue(newValue);
        }
    });
}

3. 调整main.fxml的fx:include（可选） #

如果不需要初始隐藏，也可以去掉visible="false"，在代码中控制显示：

<fx:include fx:id="nested" source="nested.fxml" />

验证效果 #

1. 启动程序，点击"open nested View"按钮，会显示nested.fxml的TextField。
2. 在TextField中输入内容，点击"test"按钮，控制台会输出输入的内容，不再抛出NPE。

内容的提问来源于stack exchange，提问作者ites