Wait for thread to finish in Java

Question

I have some code which executes a download in a separate thread, created so that the JFrame GUI will continue to update during the download. But, the purpose is completely defeated when I use Thread.join(), as it causes the GUI to stop updating. I need a way to wait for the thread to finish and still update the GUI.

Answer 1

You can have the task that does the download also fire an event to the GUI.

For example:

Runnable task = new Runnable() {
   public void run() {
      // do your download

      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            // call some method to tell the GUI that the download finished.
         }
      });
   }
};

and then to run it, either use an Executor (preferred method) or a raw thread:

executor.execute(task);

or

new Thread(task).start();

---

As pointed out in the comments, you'd generally use a SwingWorker to do this kind of thing but you can also do the manual approach outlined above.

SwingWorker provides a doInBackground method where you would stick your download logic in, a done method where you would stick in code to notify the GUI that the download finished and a get method to get the result of doInBackground (if there was one).

E.g.,

class Downloader extends SwingWorker<Object, Object> {
   @Override
   public Object doInBackground() {
       return doDownload();
   }

   @Override
   protected void done() {
       try { 
         frame.downloadDone(get());
       } catch (Exception ignore) {
       }
   }
}

(new Downloader()).execute();