Why doesn't the ProgressMonitor display on second call?

I have written a program which uses the progressbar class from the default java runtime. It works perfectly except for the fact that it only works once. I use it to display the loading of various images into my GUI and it works fine on startup. However, once I call it again it just displays a JFrame with no content. I can drag the JFrame around and everything, it's just that it won't display anything. This is peculiar considering it works when it runs the first time, but not there after. Here is my code:

    public void prefetch (){
    try{
        pageURLs = mangaEngine.getPageList();
        monitor = new ProgressMonitor(parent, "Loading Chapter..,", null, 0, mangaEngine.getPageList().length);
        pages = new StretchIconHQ[pageURLs.length];
        //Resets page counter to first page by fetching pages in reverse order
        monitor.setMaximum(pageURLs.length-1);
        for(int i = pageURLs.length-1; i>=0; i--){
            pages[i] = mangaEngine.loadImg(pageURLs[i]);
            monitor.setProgress(pageURLs.length-1-i);
            monitor.setNote("Loading Image:" + (pageURLs.length-1-i) + " of " + (pageURLs.length-1));
        }

    }
    catch(Exception ex){
        ex.printStackTrace();
    }
}

Any ideas on how to fix it?

Solution

You need to run the long-running code in a background thread such as that created by a SwingWorker.

In fact, the Progress Monitor Tutorial describes how to do this exactly:

Create a SwingWorker
Add a PropertyChangeListener to it
Listen to the "progress" property
Update your monitor's progress with that of the SwingWorker's.