RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v3]

[Prasanta Sadhukhan]

On Fri, 14 Nov 2025 22:04:30 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> The NPE being hit by the test program is coming because `setScene(null)`, which is called on the FX app thread, will lead to calling `HostContainer::setEmbeddedStage(null)` and `HostContainer::setEmbeddedScene(null)`. Those methods will set the `stagePeer` and `scenePeer` fields to null. Meanwhile, a repaint operation or a mouse move or ... on the EDT could be trying to access the scene peer.

Yes, the test does this 

for (int i = 0; i < 300; i++) {
            SwingUtilities.invokeLater(contentPane::repaint);
            Platform.runLater(() -> contentPane.setScene(null));
            Thread.sleep(100);
            Platform.runLater(() -> contentPane.setScene(webView.getScene()));
            Thread.sleep(100);
        }

so when FX thread calls `setScene(null)`, the `stagePeer` and `scenePeer` fields are set to null through `HostContainer.setEmbeddedStage(null)` and `HostContainer::setEmbeddedScene(null)` as mentioned and when `HostContainer.setEmbeddedStage` calls `stagePeer.setFocused` on EDT, it crashes 
and if the thread sleep is reduced to 1ms from 100ms, there might be a chance that `repaint` can get scheduled on AWT thread on context switch, it calls `JFXPanel.paintComponent` which tries to access "`scenePeer.getPixels`" which can cause NPE.
So, I have stored the "stagePeer" and "scenePeer" to temp variable whenever the method is supposedly called on EDT like `processXXXEvent `and `sendxxxEventToFX`
Also, additionally I see that `sendxxxEventToFX` is being called on EDT except `sendMoveEventToFX` which is called in couple of places in FX thread, which is now modified to being called in EDT in all places

Also, removed the debug logging

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PR Comment: https://git.openjdk.org/jfx/pull/1968#issuecomment-3542802414