java - NullPointerException 尽管 fx :id and fx:controller are correct

Question

我正在为我的局域网应用程序创建包含日志的表。当我尝试运行此应用程序时，它抛出 NullPointerException，但 fx:id 和 fx:controller 是正确的。我正在使用 JavaFX 12。

我尝试使用 Intellij 重构 fx:id 和 fx:controller，Intellij 在 logPane.fxml 和 LogPaneController.class 中也正确更改了这一点，但它仍然无法正常工作。如果我对抛出异常的行进行注释，则所有应用程序都工作正常。我用谷歌搜索短语“FXML 组件返回 NUllPointerException”，但结果始终是“您的 fx:id 或 fx:controller 名称与 Controller 中的名称不同”。

logPane.fxml

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

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<AnchorPane HBox.hgrow="ALWAYS" xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1"
            fx:controller="com.lanssmaker.controller.LogPaneController">
    <children>
        <TableView fx:id="logTable" prefHeight="574.0" prefWidth="382.0" AnchorPane.bottomAnchor="0.0"
                   AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
            <columns>
                <TableColumn maxWidth="140.0" minWidth="70" prefWidth="70.0" text="Time"/>
                <TableColumn text="Content"/>
                <TableColumn maxWidth="140.0" minWidth="70" prefWidth="70.0" text="Category"/>
            </columns>
            <columnResizePolicy>
                <TableView fx:constant="CONSTRAINED_RESIZE_POLICY"/>
            </columnResizePolicy>
        </TableView>
    </children>
</AnchorPane>

LogPaneController.class

public class LogPaneController {

    @FXML
    private TableView<Log> logTable; //it's null

    public TableView<Log> getLogTableView() {
        return logTable;
    }


    public void initialize() {
        //example ussage calling NullPointerException
        logTable.isHover();
    }
}

异常日志

Exception in Application start method
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:567)
	at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
	at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
	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:567)
	at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
	at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
	at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
	at java.base/java.lang.Thread.run(Thread.java:835)
Caused by: javafx.fxml.LoadException: 
/D:/Programming/Java/javastart/ssmaker/target/classes/fxml/buttonsPane.fxml
/D:/Programming/Java/javastart/ssmaker/target/classes/fxml/mainPane.fxml:17

	at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2625)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2595)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
	at javafx.fxml/javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:1154)
	at javafx.fxml/javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:754)
	at javafx.fxml/javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2722)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2552)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3237)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3194)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3163)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3136)
	at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3113)
	at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3106)
	at ssmaker/com.lanssmaker.main.Main.start(Main.java:16)
	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$10(PlatformImpl.java:428)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:389)
	at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$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
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:567)
	at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
	at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	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.loadImpl(FXMLLoader.java:2591)
	... 22 more
Caused by: java.lang.NullPointerException
	at ssmaker/com.lanssmaker.controller.LogPaneController.initialize(LogPaneController.java:25)
	... 33 more

package com.lanssmaker.main;

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



public class Main extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {
        Pane mainPane = FXMLLoader.load(getClass().getResource("/fxml/mainPane.fxml"));
        Scene scene = new Scene(mainPane);
        stage.setScene(scene);
        stage.setTitle("LAN ScreenShots Maker");
        stage.show();
    }
}

mainPane.fxml

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

<?import javafx.scene.layout.*?>
<AnchorPane maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" minHeight="-Infinity"
            minWidth="-Infinity" prefHeight="624.0" prefWidth="813.0" xmlns="http://javafx.com/javafx/11.0.1"
            xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.lanssmaker.controller.MainController">
    <children>
        <HBox layoutX="158.0" layoutY="143.0" prefHeight="624.0" prefWidth="813.0" AnchorPane.bottomAnchor="25.0"
              AnchorPane.leftAnchor="25.0" AnchorPane.rightAnchor="25.0" AnchorPane.topAnchor="25.0">
            <children>
                <AnchorPane HBox.hgrow="NEVER">
                    <children>
                        <VBox prefHeight="574.0" prefWidth="389.0" AnchorPane.bottomAnchor="0.0"
                              AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="15.0" AnchorPane.topAnchor="0.0">
                            <children>
                                <fx:include source="connectionPane.fxml"/>
                                <fx:include source="buttonsPane.fxml"/>
                            </children>
                        </VBox>
                    </children>
                </AnchorPane>
                <fx:include source="logPane.fxml"/>
            </children>
        </HBox>
    </children>
</AnchorPane>

这里是Project structure

Answer 1

仔细查看堆栈跟踪:

...
Caused by: javafx.fxml.LoadException: 
/D:/Programming/Java/javastart/ssmaker/target/classes/fxml/buttonsPane.fxml
/D:/Programming/Java/javastart/ssmaker/target/classes/fxml/mainPane.fxml:17
...
Caused by: java.lang.NullPointerException
    at ssmaker/com.lanssmaker.controller.LogPaneController.initialize(LogPaneController.java:25)

这意味着加载 buttonsPane.fxml 时发生异常当FXMLLoader处理

<fx:include source="buttonsPane.fxml"/>

元素。

此外，它告诉我您不仅使用 LogPaneController对于 logPane.fxml ，也适用于buttonsPane.fxml 。每次加载 fxml 时都会创建单独的 Controller 实例；使用<fx:include>结果是一个单独的 FXMLLoader正在创建实例以加载包含的 fxml。 logTable仅为其中之一注入(inject)字段；对于其他 Controller 实例，它仍然是 null 。 (这甚至忽略了加载 buttonsPane.fxml 在加载 logPane.fxml 之前发生的事实，因此即使在加载 buttonsPane.fxml 完成时使用相同的 Controller 实例， TableView 也不会基于 logPane.fxml 创建。 )

对 buttonsPane.fxml 使用单独的 Controller 类和logPane.fxml 。仅使用相同的 Controller 类无法让两个 fxml 进行“通信”。我建议另一种选择，但你在那里所做的事情没有任何意义(除非你试图激怒 NPE):isHover只是返回 hover 的值属性始终是 false对于尚未属于 Scene 的节点.