Threads and Swing

Anyone who has done some serious Swing development has (hopefully) heard about the “single thread rule”:

Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.

The rule is well explained in the classical threads and swing article.

SwingUtilities.invokeLater() is your friend, so this rule is not particularly difficult to follow. However, until a few minutes (read: some posts) ago, i wasn’t aware that the rule has changed in 2004:

To avoid the possibility of deadlock, you must take extreme care that Swing components and models are created, modified, and queried only from the event-dispatching thread.

It is the subtle difference between realized (setVisible(true), show(), or pack() has been called) and created. While it was totally fine to launch your UI like this:

public static void main(String[] args) {
  JFrame frame = new JFrame("HelloWorldSwing");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  JLabel label = new JLabel("Hello World");
  frame.getContentPane().add(label);
  frame.pack();
  frame.setVisible(true);
}

you are now required to create and show all swing components in the EDT (from the official sun swing tutorial):

public class HelloWorldSwing {

  private static void createAndShowGUI() {
    //Create and set up the window.
    JFrame frame = new JFrame("HelloWorldSwing");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Add the ubiquitous "Hello World" label.
    JLabel label = new JLabel("Hello World");
    frame.getContentPane().add(label);

    //Display the window.
    frame.pack();
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    //Schedule a job for the event-dispatching thread:
    //creating and showing this application's GUI.
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
          createAndShowGUI();
        }
    });
  }
}

I am a bit puzzled — can the real swing experts please stand up?