NPE in GC: Cannot read field "nativeZoom" because "this.data" is null

[ptziegler]

I haven't found a small reproducer yet, but the problem occurs when opening and closing one of our dialogs via the context menu.

Caused by: java.lang.NullPointerException: Cannot read field "nativeZoom" because "this.data" is null
	at org.eclipse.swt.graphics.GC.getZoom(GC.java:5139)
	at org.eclipse.swt.graphics.GC.drawImage(GC.java:934)
	at org.eclipse.swt.widgets.Composite.WM_PAINT(Composite.java:1656)
	at org.eclipse.swt.widgets.Control.windowProc(Control.java:4835)
	at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:336)
	at org.eclipse.swt.widgets.Display.windowProc(Display.java:5106)
	at org.eclipse.swt.internal.win32.OS.RedrawWindow(Native Method)
	at org.eclipse.swt.widgets.Control.update(Control.java:4615)
	at org.eclipse.swt.widgets.Display.update(Display.java:5076)
	at org.eclipse.jface.window.Window.runEventLoop(Window.java:830)
	at org.eclipse.jface.window.Window.open(Window.java:799)

Tested with org.eclipse.swt.win32.win32.x86_64 3.130.0.v20250507-1813

[ptziegler]

The paint method raises a lot of questions...

Here, the paintGC instance is assigned to gc

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Composite.java

Lines 1581 to 1583 in 8b45dd8

	if ((style & (SWT.DOUBLE_BUFFERED | SWT.TRANSPARENT)) != 0) {
	paintGC = gc;
	int originalStyle = gc.getStyle();

Later, the gc instance (and therefore also the paintGC instance) is explicitly disposed. However, the paintGC instance is then immediately used for painting.

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Composite.java

Lines 1649 to 1660 in 8b45dd8

	if ((style & (SWT.DOUBLE_BUFFERED | SWT.TRANSPARENT)) != 0) {
	if (!gc.isDisposed ()) {
	GCData gcData = gc.getGCData ();
	if (gcData.focusDrawn && !isDisposed ()) updateUIState ();
	}
	gc.dispose();
	if (!isDisposed ()) {
	paintGC.drawImage (image, DPIUtil.scaleDown(ps.left, zoom), DPIUtil.scaleDown(ps.top, zoom));
	}
	image.dispose ();
	gc = paintGC;
	}

This is what's causing the NullPointerException. I just don't see why this wasn't an issue before...

[ptziegler]

I have reverted 807b159 locally and the issue no longer shows up. Funny stuff seems to be happening when using the ImageGcDrawer.

[fedejeanne]

Hello @ptziegler, thank you for reporting it!

Is this still reproducible in the latest master?

Can you provide a series of steps to reproduce it so we can check too?

[ptziegler]

Is this still reproducible in the latest master?

Yes. I've loaded the latest version in our application and the issue still exists.

Can you provide a series of steps to reproduce it so we can check too?

The dialog is sadly deeply embedded into our application. But I hope I can extract a small reproducer, now that I have isolated the problem.

[fedejeanne]

Oh boy, oh boy: I missed this in my review

So before 807b159, gc and paintGC were 2 different instances, which is why disposing gc and drawing on paintGC 2 lines later did work

eclipse.platform.swt/bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Composite.java

Lines 1654 to 1656 in 8b45dd8

	gc.dispose();
	if (!isDisposed ()) {
	paintGC.drawImage (image, DPIUtil.scaleDown(ps.left, zoom), DPIUtil.scaleDown(ps.top, zoom));

After that commit, they are the same instance, as you pointed out.

[fedejeanne]

I'm on it

[ptziegler]

I'm on it

Awesome! 😄

So before 807b159, gc and paintGC were 2 different instances, which is why disposing gc and drawing on paintGC 2 lines later did work

I just noticed that as well. I also managed to construct a small reproducer:

package test;

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Canvas;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class Snippet {
	public static void main(String[] args) {
		Shell shell = new Shell(SWT.SHELL_TRIM | SWT.DOUBLE_BUFFERED);
		Display display = shell.getDisplay();
		
		Canvas c = new Canvas(shell, SWT.TRANSPARENT);
		c.addPaintListener(e -> {});
		
		shell.setLayout(new FillLayout());
		shell.setSize(200, 200);
		shell.open();
		
		while(!display.isDisposed()) {
			display.readAndDispatch();
		}
	}
}

which should fail with

Exception in thread "main" java.lang.NullPointerException: Cannot read field "nativeZoom" because "this.data" is null
	at org.eclipse.swt.graphics.GC.getZoom(GC.java:5185)
	at org.eclipse.swt.graphics.GC.drawImage(GC.java:938)
	at org.eclipse.swt.widgets.Composite.WM_PAINT(Composite.java:1657)
	at org.eclipse.swt.widgets.Control.windowProc(Control.java:4836)
	at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:336)
	at org.eclipse.swt.widgets.Display.windowProc(Display.java:5106)
	at org.eclipse.swt.internal.win32.OS.UpdateWindow(Native Method)
	at org.eclipse.swt.widgets.Decorations.setVisible(Decorations.java:1325)
	at org.eclipse.swt.widgets.Shell.setVisible(Shell.java:2133)
	at org.eclipse.swt.widgets.Shell.open(Shell.java:1297)
	at test.Snippet.main(Snippet.java:19)