JavaFX FileChooser引发错误（可能很容易修复，但仍然困惑） [英] JavaFX FileChooser Throws Error (probably easy fix, but still confused)

问题描述

我正在尝试使用以下代码打开FileChooser：

I'm trying to open a FileChooser with this code:

package back_end.controllers;

import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.control.ListView;
import javafx.scene.layout.BorderPane;
import javafx.stage.FileChooser;

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

/**
 * Created by Quinn McHugh on 4/15/2014.
 */
public class StartScreen_Controller implements Initializable {

    @FXML
    private ListView lstRecents;
    @FXML
    private Button btnBrowse;
    @FXML
    private BorderPane borderPane;

    @Override
    public void initialize(URL fxmlFileLocation, ResourceBundle resources){
        lstRecents = new ListView();
        btnBrowse = new Button();
        borderPane = new BorderPane();
    }

    public void openFile(){
        /* Presents a file chooser but does nothing functionally.
        */
        FileChooser fc = new FileChooser();
        File f = fc.showOpenDialog(btnBrowse.getScene().getWindow());
    }
}

但它在openFile（）中搞砸了方法。编译器抛出NullPointerException并显示以下错误消息：

but it is getting screwed up within the openFile() method. The compiler is throwing a NullPointerException with these error messages:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1760)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1643)
    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.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.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.Node.fireEvent(Node.java:8196)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:97)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:90)
    at com.sun.javafx.event.CompositeEventHandler$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.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$MouseHandler.process(Scene.java:3712)
    at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3440)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1663)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2455)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:326)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:251)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:360)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
    at com.sun.glass.ui.View.notifyMouse(View.java:898)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
    at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
    at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1757)
    ... 47 more
Caused by: java.lang.NullPointerException
    at back_end.controllers.StartScreen_Controller.openFile(StartScreen_Controller.java:38)
    ... 57 more

我非常感谢任何帮助，因为我是JavaFX的新手和使用Scene Builder。

Any help would be greatly appreciated, as I'm brand new to JavaFX and the use of Scene Builder.

推荐答案

这是我在两天内看到的第三个问题是人们重新初始化的字段@ FXML - 由 FXMLLoader 注入。

This is the third question I've seen in two days that's been caused by people re-initializing fields that have been @FXML-injected by the FXMLLoader.

@FXML 是你告诉 FXMLLoader 使用通过解析FXML文件创建的对象初始化该字段。当你执行

The point of @FXML is that you tell the FXMLLoader to initialize that field with the object that was created by parsing the FXML file. When you then execute

btnBrowse = new Button();

创建一个新按钮，并且不再引用在解析FXML时为您创建的按钮。

in your initialize() method, you create a new button and no longer have a reference to the one that was created for you when the FXML was parsed.

所以，现在应该很明显 btnBrowse.getScene（）将返回 null （因为该按钮从未添加到场景中）和 btnBrowse.getScene（）。getWindow（）将生成 NullPointerException 。

So, now it should be pretty obvious that btnBrowse.getScene() will return null (because the button was never added to a scene) and btnBrowse.getScene().getWindow() will generate a NullPointerException.

只需从 initialize（）方法中取出这三行。

Just take those three lines out of your initialize() method.

我可以问你在哪里看到重新初始化那些字段吗？如果有一些流行的资源正在传播错误的代码，则需要将其删除。

Can I ask where you saw to re-initialize those fields like that? If there's some popular resource out there that's spreading incorrect code, it needs to be taken down.

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