[英]Null pointer exception while using the setCenter method of a MapController in Android
[英]Getting Null Pointer Exception with setCenter() method in JavaFX with Scene Builder 8.0
我創建了一個小程序來重現我在實際項目中遇到的錯誤。 我有一個名為MainWindow.java
的控制器類,它負責兩個.fxml文件: MainWindow.fxml
和AnchorTest.fxml
。
控制器類的代碼:
package projecterror.controller;
import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Rectangle2D;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;
import javafx.stage.Screen;
import javafx.stage.Stage;
public class MainWindow extends Application {
private Stage primaryStage;
private BorderPane mainWindow;
@FXML
Menu menuFile, menuAnalysis;
@FXML
MenuItem menuNew;
@FXML
private void initialize() {
menuNew.setOnAction((event) -> {
try {
FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource("../view/AnchorTest.fxml"));
AnchorPane anchorTest = (AnchorPane) loader.load();
mainWindow.setCenter(anchorTest);
} catch (IOException e) {
e.printStackTrace();
}
});
}
@Override
public void start(Stage primaryStage) throws Exception {
this.primaryStage = primaryStage;
this.primaryStage.setTitle("Test Project");
initMainWindow();
}
public void initMainWindow() {
try {
FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource("../view/MainWindow.fxml"));
mainWindow = (BorderPane) loader.load();
Scene scene = new Scene(mainWindow);
primaryStage.setScene(scene);
//Fullscreen
Screen screen = Screen.getPrimary();
Rectangle2D bounds = screen.getVisualBounds();
primaryStage.setX(bounds.getMinX());
primaryStage.setY(bounds.getMinY());
primaryStage.setWidth(bounds.getWidth());
primaryStage.setHeight(bounds.getHeight());
primaryStage.setResizable(false);
primaryStage.show();
} catch (IOException e) {
e.printStackTrace();
}
}
}
MainWindow.fxml
是BorderPane
, top
具有MenuBar
。
碼:
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.Menu?>
<?import javafx.scene.control.MenuBar?>
<?import javafx.scene.control.MenuItem?>
<?import javafx.scene.control.SeparatorMenuItem?>
<?import javafx.scene.layout.BorderPane?>
<BorderPane style="-fx-background-color: #DCDCDC;" xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="projecterror.controller.MainWindow">
<top>
<MenuBar BorderPane.alignment="CENTER">
<menus>
<Menu fx:id="menuFile" mnemonicParsing="false" text="File">
<items>
<Menu fx:id="menuAnalysis" mnemonicParsing="false" text="Analysis">
<items>
<MenuItem fx:id="menuNew" mnemonicParsing="false" text="New" />
</items>
</Menu>
</items>
</Menu>
</menus>
</MenuBar>
</top>
</BorderPane>
AnchorTest.fxml
是一個AnchorPane
,具有兩個子AnchorPane
。
碼:
<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>
<AnchorPane xmlns:fx="http://javafx.com/fxml/1">
<children>
<AnchorPane layoutX="6.0" layoutY="450.0" />
<AnchorPane layoutX="6.0" layoutY="569.0" prefHeight="116.0" prefWidth="730.0">
<Button layoutX="6.0" layoutY="26.0" mnemonicParsing="false" prefHeight="25.0" prefWidth="102.0" text="Button 1" />
<Button layoutX="115.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 2" />
<Button layoutX="226.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 3" />
<Button layoutX="115.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 4" />
<Button layoutX="383.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 5" />
<Button layoutX="491.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 6" />
<Button layoutX="600.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 7" />
<Button layoutX="491.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 8" />
<Button layoutX="600.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 9" />
<TextField alignment="TOP_CENTER" layoutX="341.0" layoutY="26.0" prefHeight="25.0" prefWidth="28.0" text="10" />
</AnchorPane>
</children>
</AnchorPane>
在MainWindow.fxml
,單擊菜單File > Analysis > New
,應該將AnchorTest.fxml
內容放入MainWindow.fxml
中心。 但是,我得到以下堆棧跟蹤錯誤:
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at projecterror.controller.MainWindow.lambda$0(MainWindow.java:35)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1405)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$343(ContextMenuContent.java:1358)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:380)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:294)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:416)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:415)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)
標記NPE的行是:
mainWindow.setCenter(anchorTest);
我已經閱讀了與此類似的其他問題,但大多數問題都與fxml文件的路徑有關。 of path for other views and I don't have problem so far. 我堅信這不是問題,因為在我的實際項目中,我對其他視圖使用相同的路徑,到目前為止,我還沒有遇到任何問題。
如果有人要運行該程序,我已經在這里上傳了該項目。
項目結構如下:
在此先感謝您,任何幫助將不勝感激!
當您啟動JavaFX應用程序時,將創建您的應用程序類的實例,並且(在其他情況下)將在FX Application線程上的該Application
實例上調用start()
方法。
加載FXML文件時,如果FXML文件指定了控制器類, @FXML
創建該控制器類的實例,將@FXML
字段注入該實例中,並調用initialize()
方法。 所有這些都在調用FXMLLoader.load()
過程中發生。
因此,在您的代碼中,您最終得到MainWindow
兩個不同實例。 在一個實例上調用start()
方法,該實例將初始化mainWindow
字段,在另一個實例上調用initialize()
方法。 由於mainWindow
從未在第二個實例中初始化,因此當您嘗試在initialize()
方法中對其進行取消引用時,會出現空指針異常。
將控制器類與Application
類分開。 Application
類除了管理整個應用程序的生命周期外別無所求(通常,它應實現啟動應用程序所需的代碼):
package projecterror.controller;
import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Rectangle2D;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Screen;
import javafx.stage.Stage;
public class MainWindow extends Application {
@Override
public void start(Stage primaryStage) throws Exception {
primaryStage.setTitle("Test Project");
initMainWindow(primaryStage);
}
public void initMainWindow(Stage primaryStage) {
try {
FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource("../view/MainWindow.fxml"));
BorderPane mainWindow = loader.load();
Scene scene = new Scene(mainWindow);
primaryStage.setScene(scene);
//Fullscreen
Screen screen = Screen.getPrimary();
Rectangle2D bounds = screen.getVisualBounds();
primaryStage.setX(bounds.getMinX());
primaryStage.setY(bounds.getMinY());
primaryStage.setWidth(bounds.getWidth());
primaryStage.setHeight(bounds.getHeight());
primaryStage.setResizable(false);
primaryStage.show();
} catch (IOException e) {
e.printStackTrace();
}
}
}
package projecterror.controller;
import java.io.IOException;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;
public class MainWindowController {
@FXML
private BorderPane mainWindow;
@FXML
Menu menuFile, menuAnalysis;
@FXML
MenuItem menuNew;
@FXML
private void initialize() {
menuNew.setOnAction((event) -> {
try {
FXMLLoader loader = new FXMLLoader();
loader.setLocation(getClass().getResource("../view/AnchorTest.fxml"));
AnchorPane anchorTest = (AnchorPane) loader.load();
mainWindow.setCenter(anchorTest);
} catch (IOException e) {
e.printStackTrace();
}
});
}
}
為了使控制器訪問BorderPane
,您需要從FXML注入它(請注意上面的控制器代碼中的注釋)。 將fx:id
添加到FXML文件中的元素:
<BorderPane style="-fx-background-color: #DCDCDC;" fx:id="mainWindow"
xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1"
fx:controller="projecterror.controller.MainWindowController">
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