在Java中取消并重用（SwingWorker）线程

Question

I've inherited a project where we are using java and drawing stuff over an image. As a user moves a slider back and forth to change the threshhold of edges on the image and it runs a method that detects this. This takes like 200-300ms so it locks up the UI for a bit but as the user moves the slider it constantly keeps locking up the interface.

They appear to have moved this to a background thread but it creates a new one every time the slider moves. So as the user moves the slider maybe an inch it spawns like 80 threads and the whole system locks up for a couple of seconds.

I'm not sure how to work around this issue. It's a swingworker thread that posts back to the UI. I tried canceling() and then execute()ing the thread again but it appears that's not possible. I don't want lots of threads when I can just cancel the old one (since we don't need the old preview) so I think only one would work.

Here is the worker thread.

// WORKER
private class ThresholdWorker extends SwingWorker<BufferedImage, Object> {
    // long-running code to be run in a worker thread
    @Override
    public BufferedImage doInBackground() throws Exception {
        @SuppressWarnings("static-access")
        BufferedImage img = new BufferedImage(imagePanel.rect_width, imagePanel.rect_height, BufferedImage.TYPE_3BYTE_BGR);
        img = imagePanel.detectEdges();
        return img;
    } // end method doInBackground

    // code to run on the event dispatch thread when doInBackground returns
    @Override
    protected void done() {
        try {
            @SuppressWarnings("static-access")
            BufferedImage Img = new BufferedImage(imagePanel.rect_width, imagePanel.rect_height, BufferedImage.TYPE_3BYTE_BGR);
            Img = get();
            imagePanel.standardRectEdgesDilated = Img;
            imagePanel.repaint();
        } catch (InterruptedException ignore) {
        } catch (ExecutionException ex) {
            IdentiFrog.LOGGER.writeException(ex);
            System.err.println("Error encountered while performing calculation.");
        }
    }
}

Is this a task for a single thread executor service? I can't seem to find a lot of info on stopping a task but continuing to use the same thread (eg a state pattern I think). I don't want to spin up a bunch of threads if possible.

Answer 1

The problem is probably that the work done in the SwingWorker is not interruptible, that is in doInBackground() Thread.interrupted() is not checked. So even if you cancel the worker, its not actually stopping but still continues to produce a result (that is ultimately thrown away).

One way to possibly solve this is to modify the code (most likely the inside of detectEdges()) to timely respond to interruption.

If thats not a feasible option, change the approach. Keep track if there is an active worker out there; if yes, submit the new parameter set to it, otherwise start one. The worker itself then needs modification to only exit doInBackground() once it has produces a result that matches the current parameters. This approach may be easier to implement by hand, that is, not using SwingWorker.