JavaFX 时钟小部件随机抛出异常

Question

我编写了一个扩展 JavaFX 的 Text 类的小 Clock Widget。为了更新时间，我使用了一个 Task ，它基本上将文本设置为当前系统时间。当我在 Eclipse 中运行此应用程序时，有时会在我调用 stage.show() 的行中抛出 NullpointerException。

这就是我的小部件源代码的样子:

package clock;

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

import javafx.concurrent.Task;
import javafx.scene.text.Font;
import javafx.scene.text.Text;

public class CurrentClockText extends Text {
    private final DateTimeFormatter formatter;

    public static final int HOUR = 1;
    public static final int HOUR_MINUTE = 2;
    public static final int HOUR_MINUTE_SECOND = 3;
    public static final int HOUR_MINUTE_SECOND_MILLISECOND = 4;

    private final long updateInterval;

    private final Task<Void> updater = new Task<Void>() {
        @Override
        protected Void call() throws Exception {
            while (true) {
                LocalDateTime now = LocalDateTime.now();
                String nowTextual = formatter.format(now);
                setText(nowTextual);

                try {
                    Thread.sleep(updateInterval);
                } catch (InterruptedException ignore) {
                }
            }
        }
    };

    public CurrentClockText() {
        this(HOUR_MINUTE);
    }

    public CurrentClockText(final int detailLevel) {
        String timeFormat = "";
        switch (detailLevel) {
        case HOUR:
            timeFormat = "HH";
            updateInterval = 60000;
            break;
        case HOUR_MINUTE:
            timeFormat = "HH:mm";
            updateInterval = 15000;
            break;
        case HOUR_MINUTE_SECOND:
            timeFormat = "HH:mm:ss";
            updateInterval = 500;
            break;
        case HOUR_MINUTE_SECOND_MILLISECOND:
            updateInterval = 1;
            timeFormat = "HH:mm:ss.S";
            break;
        default:
            throw new IllegalArgumentException(
                    "Unknown detail level for Clock: " + detailLevel);
        }

        setFont(new Font("Verdana", 28));

        formatter = DateTimeFormatter.ofPattern(timeFormat);
        Thread updaterThread = new Thread(updater, "CurrentClockText.updaterThread");
        updaterThread.setDaemon(true);
        updaterThread.start();
    }
}

这是主类:

package clock;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class Boot extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        BorderPane pane = new BorderPane();

        CurrentClockText clock = new CurrentClockText(4);
        pane.setCenter(clock);

        Scene scene = new Scene(pane);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        Boot.launch(args);
    }

}

这是堆栈跟踪:

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplication(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$152(Unknown Source)
    at com.sun.javafx.application.LauncherImpl$$Lambda$50/14845382.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at com.sun.javafx.text.PrismTextLayout.addTextRun(Unknown Source)
    at com.sun.javafx.text.GlyphLayout.addTextRun(Unknown Source)
    at com.sun.javafx.text.GlyphLayout.breakRuns(Unknown Source)
    at com.sun.javafx.text.PrismTextLayout.buildRuns(Unknown Source)
    at com.sun.javafx.text.PrismTextLayout.layout(Unknown Source)
    at com.sun.javafx.text.PrismTextLayout.ensureLayout(Unknown Source)
    at com.sun.javafx.text.PrismTextLayout.getBounds(Unknown Source)
    at javafx.scene.text.Text.getLogicalBounds(Unknown Source)
    at javafx.scene.text.Text.impl_computeLayoutBounds(Unknown Source)
    at javafx.scene.Node$12.computeBounds(Unknown Source)
    at javafx.scene.Node$LazyBoundsProperty.get(Unknown Source)
    at javafx.scene.Node$LazyBoundsProperty.get(Unknown Source)
    at javafx.scene.Node.getLayoutBounds(Unknown Source)
    at javafx.scene.Node.prefWidth(Unknown Source)
    at javafx.scene.Node.minWidth(Unknown Source)
    at javafx.scene.layout.Region.computeChildPrefAreaWidth(Unknown Source)
    at javafx.scene.layout.BorderPane.getAreaWidth(Unknown Source)
    at javafx.scene.layout.BorderPane.computePrefWidth(Unknown Source)
    at javafx.scene.Parent.prefWidth(Unknown Source)
    at javafx.scene.layout.Region.prefWidth(Unknown Source)
    at javafx.scene.Scene.getPreferredWidth(Unknown Source)
    at javafx.scene.Scene.resizeRootToPreferredSize(Unknown Source)
    at javafx.scene.Scene.preferredSize(Unknown Source)
    at javafx.scene.Scene.impl_preferredSize(Unknown Source)
    at javafx.stage.Window$9.invalidated(Unknown Source)
    at javafx.beans.property.BooleanPropertyBase.markInvalid(Unknown Source)
    at javafx.beans.property.BooleanPropertyBase.set(Unknown Source)
    at javafx.stage.Window.setShowing(Unknown Source)
    at javafx.stage.Window.show(Unknown Source)
    at javafx.stage.Stage.show(Unknown Source)
    at clock.Boot.start(Boot.java:19)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$159(Unknown Source)
    at com.sun.javafx.application.LauncherImpl$$Lambda$53/19600960.run(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$172(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$45/18503843.run(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$null$170(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$48/27167109.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$171(Unknown Source)
    at com.sun.javafx.application.PlatformImpl$$Lambda$47/2180324.run(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$145(Unknown Source)
    at com.sun.glass.ui.win.WinApplication$$Lambda$36/3326003.run(Unknown Source)
    ... 1 more
Exception running application clock.Boot

正如我所说，此异常仅在某些运行中发生，但并非总是发生。这种情况主要发生在我更改 detailLevel = 4 的格式模式时，但在再次更改回来后仍保留到另一次运行。我猜测这实际上可能与 Eclipse 有关，但我也无法调试代码，因为在 stage.show 调用中抛出了异常，我绝对不理解。这个随机异常的原因是什么？我该如何解决它？

Answer 1

在 JavaFX 中，与大多数其他 GUI 工具包一样，有一个特定的线程来处理所有 UI 相关操作。应用程序不得在此线程之外更新 UI。如果需要从另一个线程更新 UI，通常有可用的 API 来确保代码在 UI 线程的上下文中执行。如果是 JavaFX，请参阅 Concurrency in JavaFX Tutorial了解更多信息。

就您而言，最简单的解决方案是确保您的 setText() 调用在 JavaFX 应用程序线程上执行，而不是在与您的 Task 关联的线程上执行>:

...
Platform.runLater(() -> setText(nowTextual));
...

JavaFX 中还提供其他 API 来执行动画，这些 API 可用于在特定时间间隔调用处理程序方法 - 这将从循环中删除 Thread.sleep() 调用。请参阅Timeline了解更多信息。