RFR: 8319925: CSS.BackgroundImage incorrectly uses double-checked locking

[Prasanta Sadhukhan]

On Tue, 5 Dec 2023 13:02:36 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

> if the URL is invalid, the image isn't loaded

if (!loadedImage) {
                        URL url = CSS.getURL(base, svalue);
                        if (url != null) {
                            image = new ImageIcon();
                            Image tmpImg = Toolkit.getDefaultToolkit().createImage(url);
                            if (tmpImg != null) {
                                image.setImage(tmpImg);
                            }
                        }
                        loadedImage = true;
                    }

As per your change, if URL is invalid ie url = null, image is not loaded but `loadedImage` is set to true so it will not give another chance to load the URL again via `CSS.getURL` just in case user decides to call setBase with a valid URL (after finding `getImage `returning null)

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PR Comment: https://git.openjdk.org/jdk/pull/16917#issuecomment-1840794783