java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

Does this problem occur on J2SE 5.0.x or 6.0? Yes / No (pick one)
Yes

Operating System Configuration Information (be specific):
Client: Microsoft Windows XP [Version 5.1.2600]

Bug Description:
BoxLayout uses invalid Layout Information.
Lots of methods cause BoxLayout to cache information, which it keeps until invalidateLayout is called.
When the label becomes invalid it calls parent.invalidateIfValid(); which does nothing as the panel is already invalid and hence invalidateLayout is not getting called on the BoxLayout.

Expected: the Preferred Size of 'Test' to be greater than 0
Actual: the Preferred Size of 'Test' is 0

import static java.lang.System.*;
import static javax.swing.BoxLayout.*;
import javax.swing.*;

public class Test {
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      @Override
      public void run() {
        JLabel label = new JLabel();
        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, X_AXIS));
        panel.add(label);
        out.println("Preferred Size of '': " + panel.getPreferredSize());
        label.setText("Test");
        out.println("Preferred Size of 'Test': " + panel.getPreferredSize());
      }
    });
  }
}