Run the provided test case. With 5.0 it will throw an exception and show a modal dialog with the message when it catches it in ThreadGroup.uncaughtExceptionHandler. With 6.0 this doesn't happen, exception is shown in the console instead.

import javax.swing.*;
import java.awt.*;

public class ExceptionGroup extends ThreadGroup {
  public ExceptionGroup() {
    super("ExceptionGroup");
  }
  public void uncaughtException(Thread t, Throwable e) {
     
      JOptionPane.showMessageDialog(findActiveFrame(),
          e.toString(), "Exception Occurred", JOptionPane.ERROR_MESSAGE);
  
  }
  /**
   * I hate ownerless dialogs. With this method, we can find the
   * currently visible frame and attach the dialog to that, instead
   * of always attaching it to null.
   */
  private Frame findActiveFrame() {
    Frame[] frames = JFrame.getFrames();
    for (int i = 0; i < frames.length; i++) {
      Frame frame = frames[i];
      if (frame.isVisible()) {
        return frame;
      }
    }
    return null;
  }
}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;

public class Gui extends JFrame {
  private static final int[] DAYS_PER_MONTH =
      {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
  public Gui() {
    super("GUI Example");
    final JTextArea text = new JTextArea(14, 30);
    getContentPane().add(new JScrollPane(text));
    getContentPane().add(new JButton(new AbstractAction("Calculate") {
      public void actionPerformed(ActionEvent e) {
          if (SwingUtilities.isEventDispatchThread()) {
              System.out.println("AP in EventDispatchThread()");
              System.out.println("Thread: " + Thread.currentThread());
              System.out.println("TG: " + Thread.currentThread().getThreadGroup());
          }
        text.setText("");
        for (int i=0; i<=DAYS_PER_MONTH.length; i++) {
          text.append("Month " + (i+1) + ": " +
              DAYS_PER_MONTH[i] + "\n");
        }
      }
    }), BorderLayout.NORTH);
  }
  public static void main(String[] args) {
  ThreadGroup exceptionThreadGroup = new ExceptionGroup();
    new Thread(exceptionThreadGroup, "Init thread") {
      public void run() {
        Gui gui = new Gui();
        gui.pack();
        gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        gui.show();
      }
    }.start();

  }
}