切换场景时 JavaFX 选择框空指针异常-解网

问：

该程序基本上显示选择框和一个按钮，无论用户选择什么，它都会将其打印在场景 2 的标签中。但是这个错误不断出现。

从选择框菜单中选择项目时发生错误，按下按钮时发生错误。

注意：我对两个fxml文件使用相同的Controller类。

错误：


javafx.fxml.LoadException: 
/C:/Users/yasse/eclipse-workspace/FinalProject/bin/application/exscene2.fxml

   at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2707)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2685)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3331)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3287)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3255)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3227)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3203)
   at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3196)
   at application.BookController.switchscene(BookController.java:57)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   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:8792)
   at javafx.controls/javafx.scene.control.ChoiceBox$4.invalidated(ChoiceBox.java:333)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
   at javafx.controls/javafx.scene.control.ChoiceBox.setValue(ChoiceBox.java:342)
   at javafx.controls/javafx.scene.control.ChoiceBox.lambda$new$1(ChoiceBox.java:194)
   at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:181)
   at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
   at javafx.base/javafx.beans.property.ReadOnlyObjectPropertyBase.fireValueChangedEvent(ReadOnlyObjectPropertyBase.java:80)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:449)
   at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:557)
   at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:943)
   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:184)
   at java.base/java.lang.Thread.run(Thread.java:831)
Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.ChoiceBox.getItems()" because "this.box" is null
   at application.BookController.initialize(BookController.java:44)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2655)
   ... 79 more
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException: Cannot invoke "javafx.scene.control.Label.setText(String)" because "this.label" is null
   at application.BookController.switchscene(BookController.java:66)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   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:8792)
   at javafx.controls/javafx.scene.control.ChoiceBox$4.invalidated(ChoiceBox.java:333)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
   at javafx.controls/javafx.scene.control.ChoiceBox.setValue(ChoiceBox.java:342)
   at javafx.controls/javafx.scene.control.ChoiceBox.lambda$new$1(ChoiceBox.java:194)
   at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:181)
   at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
   at javafx.base/javafx.beans.property.ReadOnlyObjectPropertyBase.fireValueChangedEvent(ReadOnlyObjectPropertyBase.java:80)
   at javafx.base/javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:102)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:113)
   at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
   at javafx.controls/javafx.scene.control.SelectionModel.setSelectedItem(SelectionModel.java:105)
   at javafx.controls/javafx.scene.control.SingleSelectionModel.updateSelectedIndex(SingleSelectionModel.java:223)
   at javafx.controls/javafx.scene.control.SingleSelectionModel.select(SingleSelectionModel.java:149)
   at javafx.controls/javafx.scene.control.ChoiceBox$ChoiceBoxSelectionModel.select(ChoiceBox.java:586)
   at javafx.controls/javafx.scene.control.skin.ChoiceBoxSkin.lambda$addPopupItem$7(ChoiceBoxSkin.java:390)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
   at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
   at javafx.controls/javafx.scene.control.MenuItem.fire(MenuItem.java:459)
   at javafx.controls/com.sun.javafx.scene.control.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1385)
   at javafx.controls/com.sun.javafx.scene.control.ContextMenuContent$MenuItemContainer.lambda$createChildren$12(ContextMenuContent.java:1338)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   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.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$MouseHandler.process(Scene.java:3897)
   at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1878)
   at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2623)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:411)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:301)
   at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:450)
   at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:424)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:449)
   at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:557)
   at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:943)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:184)
   at java.base/java.lang.Thread.run(Thread.java:831)
javafx.fxml.LoadException: 
/C:/Users/yasse/eclipse-workspace/FinalProject/bin/application/exscene2.fxml

   at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2707)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2685)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3331)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3287)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3255)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3227)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3203)
   at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3196)
   at application.BookController.switchscene(BookController.java:57)
   at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
   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:77)
   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:275)
   at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:84)
   at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1852)
   at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1724)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   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:8792)
   at javafx.controls/javafx.scene.control.Button.fire(Button.java:203)
   at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:208)
   at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
   at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
   at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
   at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3897)
   at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1878)
   at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2623)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:411)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:301)
   at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:450)
   at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:424)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:449)
   at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:557)
   at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:943)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:184)
   at java.base/java.lang.Thread.run(Thread.java:831)
Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.ChoiceBox.getItems()" because "this.box" is null
   at application.BookController.initialize(BookController.java:44)
   at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2655)
   ... 65 more
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
   at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1857)
   at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1724)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
   at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
   at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
   at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
   at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
   at javafx.graphics/javafx.scene.Node.fireEvent(Node.java:8792)
   at javafx.controls/javafx.scene.control.Button.fire(Button.java:203)
   at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:208)
   at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
   at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
   at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
   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$MouseHandler.process(Scene.java:3897)
   at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1878)
   at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2623)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:411)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:301)
   at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:450)
   at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:424)
   at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:449)
   at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:557)
   at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:943)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
   at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:184)
   at java.base/java.lang.Thread.run(Thread.java:831)
Caused by: java.lang.reflect.InvocationTargetException
   at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
   at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.base/java.lang.reflect.Method.invoke(Method.java:567)
   at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:77)
   at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
   at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.base/java.lang.reflect.Method.invoke(Method.java:567)
   at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:275)
   at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:84)
   at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1852)
   ... 46 more
Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.Label.setText(String)" because "this.label" is null
   at application.BookController.switchscene(BookController.java:66)
   ... 57 more

我的主要：

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



public class Main extends Application {
    //@Override
    public void start( Stage primaryStage) throws Exception {
        // TODO Auto-generated method stub
        
        AnchorPane mainpane= (AnchorPane) FXMLLoader.load(Main.class.getResource("exscene1.fxml"));
        primaryStage.setScene(new Scene(mainpane));
        
        primaryStage.show();
        
        
        
    }
    
    public static void main(String[] args) {
        Application.launch(args);
    }
}

我的控制器类：

package application;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.control.ChoiceBox;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;

public class BookController implements Initializable {

    @FXML
    private Button button;

    @FXML
    private AnchorPane scene1pane;

    @FXML
    private ChoiceBox<String> box;

    @FXML
    private AnchorPane scene2pane;

    @FXML
    private Label label;
    
    @FXML
    private Label label1;
    
    
    
    ObservableList<String> test = FXCollections.observableArrayList("AAA","BBB","CCC"); 
    
  
    public void initialize(URL arg0, ResourceBundle arg1) {
        // TODO Auto-generated method stub
        box.getItems().addAll(test);
        box.setOnAction(this::switchscene);
        
        
    }
    
    
    
  public  void switchscene(ActionEvent event)  {
        
        
        AnchorPane pane;
        try {
            pane = FXMLLoader.load(getClass().getResource("exscene2.fxml"));
            scene1pane.getChildren().setAll(pane);
            
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
    }
        
        String str = box.getValue();
        label.setText(str);
        
        
        }
    

}

我的第一个场景fxml文件：

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

<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>

<AnchorPane fx:id="scene1pane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.BookController">
   <children>
      <Button fx:id="button" layoutX="223.0" layoutY="162.0" mnemonicParsing="false" onAction="#switchscene" text="Button">
         <font>
            <Font size="36.0" />
         </font>
      </Button>
      <Label fx:id="label1" layoutX="139.0" layoutY="50.0" prefHeight="27.0" prefWidth="148.0" text="Scene 1">
         <font>
            <Font size="42.0" />
         </font>
      </Label>
      <ChoiceBox fx:id="box" layoutX="303.0" layoutY="31.0" prefWidth="150.0" />
   </children>
</AnchorPane>

我的第二个场景fxml文件：

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

<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>

<AnchorPane fx:id="scene2pane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.BookController">
   <children>
      <Label fx:id="label" layoutX="262.0" layoutY="92.0" text="Welcome">
         <font>
            <Font size="33.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

JavaFX NullPointerException

1赞 kleopatra 11/26/2021

fxml：控制器必须是 1：1 - 控制器不能在 fxml 之间共享

1赞 James_D 11/26/2021

第一个场景 FXML 没有的元素，因此在加载该 FXML 时创建的控制器中为 null（如堆栈跟踪状态）。fx:id=“label”label

1赞 Sai Dandem 11/26/2021

当我起草我的答案时，这两个错误都在评论中得到解决:)

答：

2赞 Sai Dandem 11/26/2021 #1

在此示例中，需要考虑许多事项。首先，让我解释一下您面临的错误。

首先，单击按钮或选择选择框时，您将获得 constructLoadException。这是因为您对两个 fxml 使用相同的 Controller 类。并且您期望两个 fxml 共享同一个 Controller 实例。但事实并非如此。使用您的代码，每个 fxml 都有自己的 BookController 实例。因此，在 fxml1 控制器中，所有与 fxml2 相关的实例变量都将为 null，反之亦然。

因此，当您启动 fxml2 时，将创建一个 BookController 的新实例并调用 initialize 方法。由于 fxml2 中没有 “box” 节点，该变量将为 null，并且显然会抛出 NPE，这会导致异常加载 fxml。

NPE 的第二个错误也是一样的，这次你期待 fxml1 控制器中的 fxml2 实例变量 “label”。

溶液：

有两种方法可以解决此问题。这取决于你是否要共享同一个 Controller 类，或者你是否愿意为每个 fxml 使用单独的控制器类。

#1.使用相同的控制器类

这样，您需要进行以下两个更改。

删除 exscene2 fxml 中的控制器声明。
更改 initialize 和 switchScene 方法，如下所示。主要变化是我们手动将相同的控制器实例设置为 fxml2 装载机。并在 initialize 方法中进行一些检查，以免重新添加再次项目。

public void initialize(URL arg0, ResourceBundle arg1) {
    if(box.getItems().isEmpty()) {
        box.getItems().addAll(test);
        box.setOnAction(this::switchscene);
    }
}

public void switchscene(ActionEvent event) {
    AnchorPane pane;
    try {
        // Initiate a loader for the fxml 2
        final FXMLLoader loader = new FXMLLoader(getClass().getResource("exscene2.fxml"));
        // Set this controller instance to fxml 2
        loader.setController(this);
        // And load the fxml 2.
        pane = loader.load();
        // By this line, both fxmls use same instance of controller
        scene1pane.getChildren().setAll(pane);
    } catch (IOException e) {
        e.printStackTrace();
    }

    String str = box.getValue();
    if(str!=null && !str.isEmpty()) {
        label.setText(str);
    }
}

#2.使用单独的控制器类

这样，您需要进行以下两个更改。

使用 fxml 2 相关的实例变量为 fxml 2（比如 BookController2.java）创建一个新的 Controller 类。

import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;

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

public class BookController2 implements Initializable {

    @FXML
    private AnchorPane scene2pane;

    @FXML
    private Label label;


    @Override
    public void initialize(URL url, ResourceBundle resourceBundle) {
        // Empty
    }

    public final Label getLabel(){
        return label;
    }
}

在 fxml 2 中将此 BookController2 设置为控制器。

fx：controller=“应用程序。BookController2”
在 BookController.java 中，删除 fxml 2 的所有实例变量并更改 switchScene 方法，如下所示。

public void switchscene(ActionEvent event) {
    AnchorPane pane;
    try {
        // Initiate the loader for fxml 2. Using this loader, you can get the controller instance of fxml 2
        final FXMLLoader loader = new FXMLLoader(getClass().getResource("exscene2.fxml"));
        pane = loader.load();
        scene1pane.getChildren().setAll(pane);

        String str = box.getValue();
        if (str != null && !str.isEmpty()) {
            // Get the label from the controller and set the text.
            ((BookController2) loader.getController()).getLabel().setText(str);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

2赞 James_D 11/26/2021

第一种解决方案（我并不真正推荐）的作用远不止“共享相同的控制器类”。它实际上共享同一个控制器（即实际控制器实例）。

0赞 Sai Dandem 11/26/2021

@James_D 感谢您的更正。我的实际意图也是一样的。共享同一个控制器实例。

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