java - Rococoa 位置的内存访问无效

Question

我一直在尝试使用 rococoa(java 到 osx cocoa api 库)编写一个简单的屏幕截图应用程序，并设法实际截取屏幕截图，然后将其保存到文件中。不幸的是，应用程序偶尔会失败，并出现“位置内存访问无效...”错误。我假设这是由于垃圾收集造成的，因为我无法保持引用处于 Activity 状态。导致崩溃的行是: int[] data =pointer.getIntArray(0, bytesPerPlane/4);

我真的没有用 Objective C 编写过任何代码，只是从 rococoa 开始，所以我发现自己对此感到困惑。我已经复制了下面的相关代码，非常感谢任何帮助!

<小时/>

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

发现问题了。

‘imageRep’的最后一次使用上线了“com.sun.jna.Pointer 指针 = imageRep.bitmapData();”。此后，imageRep 对于 Java 垃圾收集器来说是公平的游戏。如果它击中在我们使用完“指针”之前，它指向的后备缓冲区可能/将会得到被释放，导致不好的事情发生。要修复此问题，请为 imageRep 添加一组额外的保留/释放即可完成此工作，或者将对其的任何引用添加到方法的末尾。