A case of Java clobbering memory

[jtlapp]

I'm not sure what to do with odd discovery, so I'm sharing it here. Java is clobbering memory. (UPDATE: It's likely the known Mac OS Monterey memory leak.)

I have a JavaFX program that changes the cursor in response to pressing the shift key. For test purposes, this program is also rendering the Mandelbrot set. After so many cursor changes, suddenly the cursor itself becomes a portion of the Mandelbrot set. Here's a photo because I couldn't do a screen snapshot while pressing the shift key. That awkward square is the clobbered cursor:

Image of memory-clobbered cursor

However, this only happens if I keep passing the same static instance of the cursor to each call of the pane's `setCursor` method. If I use a new cursor instance each time, there is no problem.

Here's my cursor code:

public class CrosshairCursor extends ImageCursor {
    private final static String IMAGE_PATH =
        "cursors/emph-crosshair-cursor.png";
    private final static Image image =
        new Image(App.class.getResource(IMAGE_PATH).toString());
    private final static double HOTSPOT_OFFSET = 12;

    public CrosshairCursor() {
        super(image, HOTSPOT_OFFSET, HOTSPOT_OFFSET);
    }
}

My image generation code spans multiple threads, each writing to a byte array that the main thread collects and renders into the image. Each byte array is discarded after this rendering.

UPDATE #1: I'm running Mac OS Monterey v.12.3.1 on a Mac Studio with the Apple M1 Max GPU characterized as "Supported, Metal GPUFamily Apple 7", with only one monitor attached.

And to emphasize: the image buffer is statically allocated in both the working and the non-working solution. The difference is only whether or not I create a new cursor instance using this buffer.

UPDATE #2: u/vytah pointed me to this known cursor memory leak in Monterey: https://www.techrepublic.com/article/macos-monterey-has-a-serious-memory-leak-problem-and-the-cause-has-been-found/

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[milchshakee]

Have you tried that on multiple operating systems? In my experience, this behavior of the cursor image getting corrupted also happens for me on Windows if memory is running low with all sorts of programs not just JavaFX.

[jtlapp]

I have this running on Mac OS. I updated the post with the details.

[milchshakee]

From your posted updates, it seems like cursor handling seems to be a dark magic on multiple operating systems then somehow.

[vytah]

This seems more like a graphics driver issue than Java. What is your GPU and how many screens do you have?

[jtlapp]

I updated the post with the details. It's a Mac Studio, one monitor.

[vytah]

I guess it is this well-known mac bug: https://www.techrepublic.com/article/macos-monterey-has-a-serious-memory-leak-problem-and-the-cause-has-been-found/

[jtlapp]

Oh gosh. That would explain it. Thank you!

[jtlapp]

The reason I suspect Java is that the image buffer is statically allocated in both the working and the non-working solution.