Components in JDialog not showing

First of all: I know this question seems to have been asked a few million times, but none of the answers given to other questions seem to work with me.

Sometimes, when I run Message.popup(String, int) in the code below, the text displays correctly, but sometimes the JDialog is empty, like if the component wasn't added at all.

public class Message extends JDialog {
    private int width;
    private int height;

    private JLabel content;

    public Message(String _content, int _margin) {
        super();
        this.content = new JLabel(_content);
        content.setFont(new Font("Monospaced", Font.BOLD, 20));
        this.margin = _margin;
        this.width = content.getPreferredSize().width + _margin;
        this.height = content.getPreferredSize().height + _margin;

        createComponents();
        setProperties();
    }

    public static void popup(String _content, int _time) {
      if (SwingUtilities.isEventDispatchThread()) {
        runPopup(_content, _time);
      }
      else {
        SwingUtilities.invokeLater(new Runnable() {
          @Override
          public void run() {
            runPopup(_content, _time);
          }
        });
      }
    }

    private static void runPopup(String _content, int _time) {
      final Message message = new Message(_content);
      new Timer(_time, new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {
          message.dispose();
        }
      }).start();
    }

    private void createComponents() {
        setLayout(new BorderLayout());

        Box box = Box.createHorizontalBox();
        box.add(Box.createHorizontalGlue());
        box.add(content, BorderLayout.CENTER);
        box.add(Box.createHorizontalGlue());
        add(box);
    }

    private void setProperties() {
        setSize(width, height);
        setLocation(Coordinator.calculateCenteredWindowLocation(width, height));
        setUndecorated(true);
        setResizable(false);
        setTitle(content.getText());
        setVisible(true);
        update(getGraphics());
    }
}

Without the update(getGraphics());, the frame is always emp开发者_如何学Goty, but with it, it depends of what direction the wind is blowing... (go figure!)

As mentioned by @Riduidel, it is important that anything Swing-related occur on the Event Dispatch Thread, or EDT. This is because Swing is not thread-safe. When invoking popup(), you ought to do the following

if(SwingUtilities.isEventDispatchThread()){
    Message.popup(...);
}
else{
    SwingUtilities.invokeLater(new Runnable(){
        @Override
        public void run(){
            Message.popup(...);
        }
    });
}

This will ensure that the JFrame is created on the EDT. Also, from the code snippet you've posted, it would seem likely that Message should have a private constructor. In addition, since you're not doing any custom rendering, why not just make a JFrame member variable instead of extending the class? -- seems a bit superfluous to me.

Regardless, you should also never sleep in the EDT either, since this will make the GUI appear to "freeze" and blocks execution of other queued events. When performing long-running tasks, use either SwingWorker, or as @Riduidel mentioned, javax.swing.Timer. But if you prefer to use the java.util.Timer, use the SwingUtilities utility class as shown above to post the Runnable task on the EventQueue to be executed in the EDT.

EDIT

Here is what I'd do (and yes, it works)

public class Message {

    // Private constructor to prevent external instantiation
    private Message(){
    }

    public static void createAndShowDialog(final String content, final int time){
        final JDialog dialog = new JDialog();
        dialog.setLayout(new BorderLayout());
        dialog.setUndecorated(true);

        JLabel label = new JLabel(content);
        label.setFont(new Font("Monospaced", Font.BOLD, 20));

        Box b = Box.createHorizontalBox();
        b.add(Box.createHorizontalGlue());
        b.add(label, BorderLayout.CENTER);
        b.add(Box.createHorizontalGlue());

        dialog.add(b);
        dialog.pack();
        dialog.setLocationRelativeTo(null);
        dialog.setVisible(true);

        // kick-off timer
        Timer t = new Timer(time, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
              dialog.dispose();
            }
        });
        t.setRepeats(false);
        t.start();
    }
}

And wherever you invoke createAndShowDialog(...), do the following

SwingUtilities.invokeLater(new Runnable(){
    @Override
    public void run(){
        Message.createAndShowDialog("Content", 5000); // wait 5 seconds before disposing dialog
    }
});

Are you sure your code is executing in the EDT ? indeed, if not (which is what I expect to be, since you sleep the current thread, what Swing would typically don't like), your frame will have trouble rendering.

To avoid those typical Swing threading issues, please take a look at the SwingUtilities class, which provide you methods to ensure you're running in EDT. Additionnaly, instead of directly sleeping your thread, you could repalce it with a Swing javax.swing.Timer (beware not to confuse it with the java.util.Timer).

update(getGraphics()); 

Never use the update() method or the getGraphics() method.

Invoking update() is used for AWT NOT Swing.

If you need to do custom painting then you override the paintComponent() method of your component which already has access to the graphics object.