Invalid memory access of location with Rococoa

Question

I've been trying to code a simple screenshot application using rococoa (java to osx cocoa api library), and managed to get as far as actually taking the screenshot, and then saving it to a file. Unfortunately, once in a while, the application fails with an 'Invalid memory access of location...' error. I'm assuming this is due to something being garbage collected, because I'm failing to keep a reference alive. The line that is causing the crash is: int[] data = pointer.getIntArray(0, bytesPerPlane / 4);

I really haven't coded anything with Objective C, and just starting up with rococoa, so I'm finding myself just confused with this. I've copied the relevant code below, and would really appreciate any help with this!

---

public interface QuartzLibrary extends Library {

    QuartzLibrary INSTANCE = (QuartzLibrary) Native.loadLibrary("Quartz", QuartzLibrary.class);

    class CGPoint extends Structure {
        public double x;
        public double y;
    }

    class CGSize extends Structure {
        public double width;
        public double height;
    }

    class CGRect extends Structure implements Structure.ByValue {
        public static class CGRectByValue extends CGRect { }

        public CGPoint origin;
        public CGSize size;
    }

    int kCGWindowListOptionIncludingWindow = (1 << 3);
    int kCGWindowImageBoundsIgnoreFraming = (1 << 0);

    ID CGWindowListCreateImage(CGRect screenBounds, int windowOption, int windowId, int imageOption);
}

public interface NSBitmapImageRep extends NSObject {

    public static final _Class CLASS = Rococoa.createClass("NSBitmapImageRep", _Class.class);

    public interface _Class extends NSClass {
        NSBitmapImageRep alloc();
    }

    NSBitmapImageRep initWithCGImage(ID imageRef);
    com.sun.jna.Pointer bitmapData();
    NSSize size();
}

public class Screenshot {

    public static void getScreenshot(int windowId) throws IOException {
        QuartzLibrary.CGRect bounds = new QuartzLibrary.CGRect.CGRectByValue();
        bounds.origin = new QuartzLibrary.CGPoint();
        bounds.origin.x = 0;
        bounds.origin.y = 0;
        bounds.size = new QuartzLibrary.CGSize();
        bounds.size.width = 0;
        bounds.size.height = 0;
        ID imageRef = QuartzLibrary.INSTANCE.CGWindowListCreateImage(bounds, QuartzLibrary.kCGWindowListOptionIncludingWindow, windowId, QuartzLibrary.kCGWindowImageBoundsIgnoreFraming);

        NSBitmapImageRep imageRep = NSBitmapImageRep.CLASS.alloc();
        imageRep = imageRep.initWithCGImage(imageRef);
        NSSize size = imageRep.size();
        com.sun.jna.Pointer pointer = imageRep.bitmapData();

        int width = size.width.intValue();
        int height = size.height.intValue();

        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
        // The crash always happens when calling 'getIntArray' in the next line.
        int[] data = pointer.getIntArray(0, bytesPerPlane / 4);
        int idx = 0;
        for(int y = 0; y < height; y++)
            for(int x = 0; x < width; x++)
                image.setRGB(x, y, data[idx++]);

        ImageIO.write(image, "png", new File("foo.png"));
    }
}

Answer 1

Possibly related, possibly not: NSBitmapImageRep descends from NSImageRep, not directly from NSObject.

Answer 2

When you're trying to learn Cocoa, do it with Objective-C. Trying to do it from another language is like trying to deal cards with mittens on.

Answer 3

Found the problem.

The last use of 'imageRep' is on the line "com.sun.jna.Pointer pointer = imageRep.bitmapData();". After this, imageRep is fair game for the Java garbage collector. And if it hits in before we are done using 'pointer', the backing buffer it's pointing to may/will get freed, causing bad things to happen. To fix it, adding an extra set of retain/release for imageRep does the job, or alternatively adding any reference to it to the end of the method.