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    <p>Swing is a single threaded environment. That is, there is a single thread responsible for managing all the interactions and updates to the Swing UI - the Event Dispatching Thread.</p> <p>Among the golden rules of Swing are...</p> <ul> <li>DON'T block the EDT (<code>Thread.sleep</code>, <code>Thread#join</code>, <code>Object#wait</code>, block IO and/or time consuming tasks (among others) should never be called from within the EDT), doing so will stop the EDT from dispatching events and paint updates (amongst other things)</li> <li>ONLY create/update Swing UI elements from within the EDT.</li> </ul> <p>This raises a question...how do you "wait" for a thread?</p> <p>The best way is use an Observer pattern. Basically, you provide the <code>Thread</code> with some kind of reference that it will call to provide notification of events, such as errors and completion...</p> <p>This will require you to think very carefully about the design of your applications, as you can not rely on a simple A to B execution of your code.</p> <p>For example...</p> <pre><code>public class TestThreadCallBack { public static void main(String[] args) { new TestThreadCallBack(); } public TestThreadCallBack() { EventQueue.invokeLater(new Runnable() { @Override public void run() { try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); } catch (Exception ex) { } JFrame frame = new JFrame("Testing"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setLayout(new BorderLayout()); frame.add(new TestPane()); frame.pack(); frame.setLocationRelativeTo(null); frame.setVisible(true); } }); } public interface ThreadCallBack { public void threadCompleted(Runnable source); public void threadFailed(Runnable source); } public class TestPane extends JPanel implements ThreadCallBack { private JLabel message; private JLabel dots; private int count; private Timer timer; public TestPane() { setLayout(new GridBagLayout()); message = new JLabel("Running background task, please wait"); dots = new JLabel(" "); add(message); add(dots); timer = new Timer(250, new ActionListener() { @Override public void actionPerformed(ActionEvent e) { count++; if (count &gt; 3) { count = 0; } StringBuilder sb = new StringBuilder(3); for (int index = 0; index &lt; count; index++) { sb.append("."); } for (int index = count; index &lt; 3; index++) { sb.append(" "); } dots.setText(sb.toString()); } }); timer.setRepeats(true); timer.setCoalesce(true); timer.start(); Thread thread = new Thread(new BackgroundTask(this)); thread.start(); } @Override public void threadCompleted(Runnable source) { timer.stop(); message.setText("Task completed successfully"); } @Override public void threadFailed(Runnable source) { timer.stop(); message.setText("Task failed"); } } public class BackgroundTask implements Runnable { private ThreadCallBack callBack; public BackgroundTask(ThreadCallBack callBack) { this.callBack = callBack; } @Override public void run() { System.out.println("Background task underway..."); try { Thread.sleep(2000); } catch (InterruptedException interruptedException) { } int result = (int) Math.round((Math.random() * 1)); if (result == 0) { callBack.threadCompleted(this); } else { callBack.threadFailed(this); } } } } </code></pre> <p>Updating the UI from within a <code>Thread</code> other then the EDT is, well, messy. An easier solution would actually be to use a <a href="http://docs.oracle.com/javase/tutorial/uiswing/concurrency/worker.html" rel="nofollow"><code>SwingWorker</code></a>. This has publish/process methods that make easy to update the UI and progress methods that can be used to provide feedback about the progress of the current task.</p> <p>You can use it's <code>done</code> method to notify interested parties when the worker has completed.</p>
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