Question

I’m writing a program which has two main phases: determining the region of interest, and then recognizing objects in that region. My interface has a single JProgressBar and I want it to indicate what phase is it currently working on. I noticed that with a simple “linear” approach, only the second message gets displayed. So, following https://stackoverflow.com/a/277048 , I use Runnables and SwingUtilities.invokeLater to set the string of my progress bar.

private class MarkListener implements ActionListener {
    public void actionPerformed(ActionEvent ae) {
        mainFrame.getGlassPane().setCursor(new Cursor(Cursor.WAIT_CURSOR));
        recognitionProgress.setStringPainted(true);
        BlueMarkerTask bmt = new BlueMarkerTask();
        bmt.addPropertyChangeListener(PrismRunnable.this);

        SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                recognitionProgress.setString("Marking ROI...");
            }
        });

        bmt.execute();

        RecognitionTask rt = new RecognitionTask();
        rt.addPropertyChangeListener(PrismRunnable.this);

        SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                recognitionProgress.setString("Segmenting...");
            }
        });

        rt.execute();
    }
}

However, this doesn’t work—only “Segmenting” remains in the progress bar’s text.

I’ve already tried using EventQueue instead of SwingUtilities but to no avail. So, how do I go about this?

Answer 1

You must understand that there is no pause in the code (nor should there be) between your call to

recognitionProgress.setString("Marking ROI...");

and to

recognitionProgress.setString("Segmenting...");

except for the milliseconds required for the first bit of code to complete and reach the second bit of code, and both background tasks are likely occurring concurrently.

Options to solve this include:

• Using two JProgressBars, one for each task,
• or running both tasks from within a single SwingWorker so that both can be done in the same background thread and run sequentially,
• or running the second task after the first task’s property change listener informs you that the first task as been completed (the state property returns SwingWorker.StateValue.DONE).

e.g.,

  public void actionPerformed(ActionEvent ae) {
     mainFrame.getGlassPane().setCursor(new Cursor(Cursor.WAIT_CURSOR));
     recognitionProgress.setStringPainted(true);
     BlueMarkerTask bmt = new BlueMarkerTask();
     bmt.addPropertyChangeListener(PrismRunnable.this);

     SwingUtilities.invokeLater(new Runnable() {
        public void run() {
           recognitionProgress.setString("Marking ROI...");
        }
     });

     bmt.addPropertyChangeListener(new PropertyChangeListener() {

        @Override
        public void propertyChange(PropertyChangeEvent pcEvt) {
           if (pcEvt.getPropertyName().equals("state")) {
              if (pcEvt.getNewValue().equals(SwingWorker.StateValue.DONE)) {
                 // you'd probably have this in a method.
                 RecognitionTask rt = new RecognitionTask();
                 rt.addPropertyChangeListener(PrismRunnable.this);

                 SwingUtilities.invokeLater(new Runnable() {
                    public void run() {
                       recognitionProgress.setString("Segmenting...");
                    }
                 });

                 rt.execute();
              }
           }
        }
     });

     bmt.execute();
  }

Edit:
Note that there is no need to queue the JProgressBar#setString(...) method on the event thread, since all the code above is already in the event thread, the EDT. This is only necessary when the current code is being called off of the EDT, or in a few other peculiar situations (this isn’t one of them).

So your code would be better to look like this:

     // ** no need to queue this on the event thread.
     // ** we're already IN the event thread!
     recognitionProgress.setString("Marking ROI...");

     bmt.addPropertyChangeListener(new PropertyChangeListener() {

        @Override
        public void propertyChange(PropertyChangeEvent pcEvt) {
           if (pcEvt.getPropertyName().equals("state")) {
              if (pcEvt.getNewValue().equals(SwingWorker.StateValue.DONE)) {
                 // you'd probably have this in a method.
                 RecognitionTask rt = new RecognitionTask();
                 rt.addPropertyChangeListener(PrismRunnable.this);

                 // ** no need to queue this on the event thread.
                 // ** we're already IN the event thread!
                 recognitionProgress.setString("Segmenting...");

                 rt.execute();
              }