JavaFX - TableView 的问题(NullPointerException)

Question

我在使用 JavaFX 中的 TableView 控件时遇到了一个大问题。那是我第一次使用它，我不知道我做错了什么。

我的代码:

主类:

package sample;

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

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}

Controller 类:

package sample;

import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.TableView;
import javafx.stage.Modality;
import javafx.stage.Stage;

import java.io.IOException;

public class Controller {

    @FXML
    private TableView<Person> peopleTableView;

    @FXML
    public void openNewWindow(){
        try {
            Parent root = FXMLLoader.load(getClass().getResource("mainWindow.fxml"));
            Stage mainWindow = new Stage();
            mainWindow.setTitle("App");
            mainWindow.setScene(new Scene(root, 1200, 600));
            mainWindow.initModality(Modality.APPLICATION_MODAL);
            mainWindow.show();

            ObservableList<Person> people = FXCollections.observableArrayList();

            people.add(new Person("Walter", "White"));
            people.add(new Person("Gus", "Fring"));

            peopleTableView.setItems(people);
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

人类:

package sample;

import javafx.beans.property.SimpleStringProperty;

public class Person {

    private SimpleStringProperty firstName = new SimpleStringProperty("");
    private SimpleStringProperty lastName = new SimpleStringProperty("");

    public Person(String firstName, String lastName) {
        this.firstName.set(firstName);
        this.lastName.set(lastName);
    }

    public String getFirstName() {
        return firstName.get();
    }

    public SimpleStringProperty firstNameProperty() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName.set(firstName);
    }

    public String getLastName() {
        return lastName.get();
    }

    public SimpleStringProperty lastNameProperty() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName.set(lastName);
    }
}

还有两个 FXML 代码，一个用于运行代码时显示的窗口:

示例.fxml:

<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.BorderPane?>
<BorderPane fx:controller="sample.Controller"
            xmlns:fx="http://javafx.com/fxml">
    <center>
        <Button fx:id="openWindowButton" text="Open new window" onAction="#openNewWindow"/>
    </center>
</BorderPane>

还有一个用于 TableView 的新阶段:

主窗口.fxml:

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

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<?import javafx.scene.control.cell.PropertyValueFactory?>
<BorderPane xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
    <center>
        <TableView fx:id="peopleTableView">
            <columns>
                <TableColumn text="First Name">
                    <cellValueFactory>
                        <PropertyValueFactory property="firstName"/>
                    </cellValueFactory>
                </TableColumn>
                <TableColumn text="Last Name">
                    <cellValueFactory>
                        <PropertyValueFactory property="lastName"/>
                    </cellValueFactory>
                </TableColumn>
            </columns>
        </TableView>
    </center>
</BorderPane>

所以问题是 - 我无法设置 TableView 的项目，因为我得到一个异常，它是空的，即使它清楚地显示在(我相信)它实际存在的新阶段。

错误代码:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1787)
    at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1670)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Node.fireEvent(Node.java:8879)
    at javafx.controls/javafx.scene.control.Button.fire(Button.java:200)
    at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:206)
    at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3851)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1200(Scene.java:3579)
    at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1849)
    at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2588)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:397)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:434)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:390)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:433)
    at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
    at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1784)
    ... 47 more
Caused by: java.lang.NullPointerException
    at TableView.issue/sample.Controller.openNewWindow(Controller.java:35)
    ... 59 more

Answer 1

问题是您正在尝试对两个 FXML 文件使用相同的 Controller 类(我认为，假设这意味着您两次都将拥有相同的 Controller 实例，但事实并非如此)。

openNewWindow() 方法在加载 sample.fxml 时创建的 Controller 实例上调用。该 FXML 文件没有名为 peopleTableView 的字段，因此 peopleTableView 在该实例中为 null，并且 peopleTableView.setItems(...) 导致空指针异常。

peopleTableView 字段在加载 mainWindow.fxml 时创建的 Controller 实例中初始化，但没有调用任何方法实例。

解决方案是为每个 FXML 文件使用不同的 Controller 类(您基本上应该始终这样做)。您可以在 mainWindow.fxml 的 Controller 中使用 initialize() 方法来初始化表格数据。

这是 sample.fxml 的 Controller :

public class LoginController {

    @FXML
    public void openNewWindow(){
        try {
            Parent root = FXMLLoader.load(getClass().getResource("mainWindow.fxml"));
            Stage mainWindow = new Stage();
            mainWindow.setTitle("App");
            mainWindow.setScene(new Scene(root, 1200, 600));
            mainWindow.initModality(Modality.APPLICATION_MODAL);
            mainWindow.show();

        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这是 mainWindow.fxml 的 Controller :

public class MainController {

    @FXML
    private TableView<Person> peopleTableView ;

    @FXML
    private void initialize() {
        ObservableList<Person> people = FXCollections.observableArrayList();

        people.add(new Person("Walter", "White"));
        people.add(new Person("Gus", "Fring"));


        peopleTableView.setItems(people);
    }
}

现在只需更新两个 FXML 文件中的 fx:controller 属性。

如果您需要将数据从一个 Controller 传递到下一个 Controller ，请参阅 Passing Parameters JavaFX FXML

例如，如果你想在第一个 Controller 的openNewWindow()方法中加载表格数据(这看起来不自然，但作为一个例子)，你可以这样做:

public class LoginController {

    @FXML
    public void openNewWindow(){
        try {

            // Note using FXMLLoader instance, not static load(URL) method:
            FXMLLoader loader = new FXMLLoader(getClass().getResource("mainWindow.fxml"));
            Parent root = loader.load();

            Stage mainWindow = new Stage();
            mainWindow.setTitle("App");
            mainWindow.setScene(new Scene(root, 1200, 600));
            mainWindow.initModality(Modality.APPLICATION_MODAL);
            mainWindow.show();

            ObservableList<Person> people = FXCollections.observableArrayList();

            people.add(new Person("Walter", "White"));
            people.add(new Person("Gus", "Fring"));

            MainController mainController = loader.getController();
            mainController.initializeTableData(people);

        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

和

public class MainController {

    @FXML
    private TableView<Person> peopleTableView ;

    public void initializeTableData(ObservableList<Person> people) {

        peopleTableView.setItems(people);
    }
}

但是，关于 Controller 之间各种通信方式的完整讨论，请再次参阅 Passing Parameters JavaFX FXML