Null Pointer Exceptions after modifying attributes of Document element

[Steve Hannah]

I have been trying to make a source code editor with syntax hilighting
using JTextPane and a custom subclass of AbstractDocument. I have
overridden the insertString() and remove() methods of Abstract
document so that modifications are passed to a formatting thread to
perform syntax hilighting. I beleive that I am using readLock() and
writeLock() appropriately so as to lock the document while I am
coloring it with the formatting thread, but I am frequently getting
the following:

java.lang.NullPointerException
at javax.swing.text.FlowView$FlowStrategy.createView(FlowView.java:569)
at javax.swing.text.FlowView$FlowStrategy.layoutRow(FlowView.java:442)
at javax.swing.text.FlowView$FlowStrategy.layout(FlowView.java:398)
at javax.swing.text.FlowView.layout(FlowView.java:182)
at javax.swing.text.BoxView.setSize(BoxView.java:379)
at javax.swing.text.BoxView.updateChildSizes(BoxView.java:348)
at javax.swing.text.BoxView.setSpanOnAxis(BoxView.java:330)
at javax.swing.text.BoxView.layout(BoxView.java:682)
at javax.swing.text.BoxView.setSize(BoxView.java:379)
at javax.swing.plaf.basic.BasicTextUI$RootView.setSize(BasicTextUI.java:1598)
at javax.swing.plaf.basic.BasicTextUI$RootView.paint(BasicTextUI.java:1317)
at javax.swing.plaf.basic.BasicTextUI.paintSafely(BasicTextUI.java:635)
at apple.laf.AquaTextPaneUI.paintSafely(AquaTextPaneUI.java:36)
at javax.swing.plaf.basic.BasicTextUI.paint(BasicTextUI.java:769)
at javax.swing.plaf.basic.BasicTextUI.update(BasicTextUI.java:748)
at javax.swing.JComponent.paintComponent(JComponent.java:541)
at javax.swing.JComponent.paint(JComponent.java:808)
at javax.swing.JComponent._paintImmediately(JComponent.java:4678)
at javax.swing.JComponent.paintImmediately(JComponent.java:4477)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:410)
at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(SystemEventQueueUtilities.java:117)

The above readout is from jre 1.4.1 on Mac OS 10.2 but I get a similar
readout when running it on windows xp.

The hilighting works when I am just adding text to the end of the
document, but when I try to insert text to the middle or beginning of
the document, these errors come up.

Since I have not made any custom views for this project, I am at a bit
of a dead end on where to look for the source of the problems. What
are the common reasons for such exceptions.

Thanks in advance.

Steve Hannah
sha...@sfu.ca