在headless (RepaintManager.currentManager)中调用新JPanel()时的NPE

Question

我正在用openjdk-8-jdk-armhf运行一个无头linux机器，当我运行我的应用程序时，代码行

METRICS = new JPanel().getFontMetrics(FONT);

抛出一个返回到RepaintManager.currentManager(RepaintManager.java:262)的NullPointerException，如下所示：

    public static RepaintManager currentManager(Component c) {
       // Note: DisplayChangedRunnable passes in null as the component, so if
       // component is ever used to determine the current
       // RepaintManager, DisplayChangedRunnable will need to be modified
       // accordingly.
       return currentManager(AppContext.getAppContext());
    }

完整的跟踪如下：

java.lang.NullPointerException
    at javax.swing.RepaintManager.currentManager(RepaintManager.java:262)
    at javax.swing.JComponent.repaint(JComponent.java:4799)
    at java.awt.Component.repaint(Component.java:3303)
    at javax.swing.JComponent.setFont(JComponent.java:2756)
    at javax.swing.LookAndFeel.installColorsAndFont(LookAndFeel.java:208)
    at javax.swing.plaf.basic.BasicPanelUI.installDefaults(BasicPanelUI.java:66)
    at javax.swing.plaf.basic.BasicPanelUI.installUI(BasicPanelUI.java:56)
    at javax.swing.JComponent.setUI(JComponent.java:664)
    at javax.swing.JPanel.setUI(JPanel.java:153)
    at javax.swing.JPanel.updateUI(JPanel.java:126)
    at javax.swing.JPanel.<init>(JPanel.java:86)
    at javax.swing.JPanel.<init>(JPanel.java:109)
    at javax.swing.JPanel.<init>(JPanel.java:117)
    at lukemoll.MyClass.<init>(MyClass.java:15)

我可以通过jsvc和java命令重现这个错误。

Answer 1

多亏了Gyro Gearless指出this answer。

This (Canvas)也将在无头模式下工作。

我最终使用了new Canvas().getFontMetrics(font);，我不确定我上次是如何让它工作的，但它在我的机器(Tm)上工作。