Question

I have a JFrame with a CardLayout set as its layout manager. This has two JPanel subclasses in it. One is a panel, WordsLoadingPanel, which displays the text “Loading words…” and has a JProgressBar. The other has to actually load the words. This takes a while (about 10-14 seconds for 100 words; it’s a pretty selective algorithm), so I want to assure the user that the program is still working. I made the loading algorithm in the panel fire a property change with firePropertyChange(String, int, int), and the WordsLoadingPanel is catching the change just fine – I know this because I added a listener for this event to perform a println, and it works. However, when I change the println to actually changing the JProgressBar‘s value, it doesn’t do anything. I know I’m changing the value right, because if I set the value before the algorithm starts, it works, and it works on the last iteration of the loop. I’m guessing this is because my algorithm is eating the computing power and won’t let JProgressBar update.

So, my question is: How do I make my algorithm wait for Swing (would this be the AWT Dispatching Thread?) to finish updating the progress bar before continuing? I’ve tried:

• Thread.yield in each iteration of the loop
• Thread.sleep(1000L) in each iteration of the loop, in a try/catch
• putting everything in the loop in a SwingUtilities.invokeLater(Runnable)
• putting only the CPU-intensive algorithm in a SwingUtilities.invokeLater(Runnable)

EDIT: To further support my hypothesis of the CPU-eating algorithm (sounds like a children’s story…), when I set the JProgressBar to indeterminate, it only starts moving after the algorithm finishes.

Does anyone have any suggestions?

Thanks!

Answer 1

To do expensive operations in background, consider using the SwingWorker class. The documentation has examples on how to use it to do tasks that interact with the user interface in a separate thread, including progress display in JProgressBars.

If you have trouble understanding how the class works, consider:

• SwingWorker is a generic class that takes two parameters: T, and V
• The doInBackground method returns T and is executed in a separate thread.
• Since Swing may only be manipulated in the Event Dispatch Thread, you may not manipulate Swing in doInBackground.
• The process method takes a List<V> as a parameter and is called asynchronously on the Event Dispatch Thread.
• The publish method takes V... arguments and sends them for processing in the process method.

In conclusion:

• T is the type of the result of the computation, if any.
• V is the type of the data needed to manipulate the user interface.
• Your algorithm should run entirely in doInBackground.
• The user interface should be manipulated in the process method.
• Your algorithm should use publish to send data to the process method.