Java Garbage Collection: Memory leak from array (with OpenCV)

Question

I just started to play with OpenCV in Java and have problem with memory leaks. 30 times per second I read an image and saves it in a monitor class. At the same frequency my main thread clones the image and calls the moving method, in the class written bellow, with the clone as a attribute.

This method should basically do a low-pass filtering in the time domain why I want to constantly store the latest 10 images. I do this in an array which I shift back every iteration. The code attached is simplified to the problem.

The next step is that the the returned value is sent as an argument to a set-method in another object where is it assigned to a Mat object.

In this case I get a huge memory leak and the memory consumption increases with around 100MB per minute until it gets an out-of-memory-exception. The main problem seams to be the cloning, if I send the reference instead of cloning it then the problem is gone. The for-loop in the code given below makes the problem much worse (if i use cloning). Without the for-loop the memory leak is much smaller but still there. The same goes if I remove the movement.moving() call and just sends the clone to the set-method directly.

But i don't get why there is a memory leak. When i assign my variable to the new cloned object the old clone should be unreferenced and hence cleaned in the next GC?

private void update(){
    if (videoMon.haveImage()) {
        // Takes the image from the monitor
        Mat m = videoMon.getImageMat();
        // Sends a clone to the method stated given further down
        // and then to the setImage where it is assigned to the local object
        videoPanel.setImage(movement.moving(m.clone()));
        videoPanel.repaint();
    }
}

The other method

public class Movement {
    private static final int NBR_OLD_IMAGES = 10;

    Mat [] lastImages;

    public Movement(){
        lastImages = new Mat[NBR_OLD_IMAGES];
    }

    public Mat moving(Mat image){
        for (int i = NBR_OLD_IMAGES-1; i > 0; i--){
            lastImages[i] = lastImages[i-1];
        }
        lastImages[0] = image;

        return image;
    }
}

java's GC thinks, a Mat is a 20 byte entity (it does not see the c++ allocated bytes), so you have to call mat.release() manually wherever possible. — berak
– berak, Commented Nov 14, 2015 at 13:05

Dave Keane · Accepted Answer · 2016-01-16 10:10:15Z

1

Use copyTo() instead of clone(), clone() doesn't release native memory, I had this same problem

answered Jan 16, 2016 at 10:10

Dave Keane

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