Simple Java Concurrency Issue

Question

I have a class that extends JPanel and draws ~10 images on screen by overriding the paint method (using this method as I want to be able to manipulate the images as I draw each one). What I want to be able to do is have an update method in the class which I pass a list of potential image updates every frame. Here is what I have

List<BufferedImage> imageList = Collections.synchronizedList(new ArrayList());

public void update(list<String> imagePaths) {
    for (String path : imagePaths) {
        synchronized (imageList) {
            //Modify image list adding and removing buffered images
        }
    }
    repaint();
}

@Override
public void paintComponent(Graphics g) {
     synchronized (imageList) {
         g.drawImage(img, 0, 0, this);
     }
}

Currently as you can imagine it runs extremely slow due to the synchronized blocks? How can I drastically improve the performance please?

Don't paint in a sync'd block - just take a snapshot of the list at the start of the method and use that. — BarrySW19
– BarrySW19, Commented Jan 9, 2015 at 11:09

Thorn G · Accepted Answer · 2015-01-09 13:42:11Z

1

You're using a synchronized list, and then also synchronizing on that list when you iterate it. You'll probably see better performance by using something like a CopyOnWriteArrayList, which will always provide a consistent snapshot when iterated, and remove the synchronized blocks entirely.

Although, looking at your snippet, it's not clear to me which list is being iterated -- you have imageList and imagePaths, and I'm not sure where the actual images are retrieved from the list.

answered Jan 9, 2015 at 13:42

Thorn G

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