Mouse Drag in JDialog produces Mouse Enter & Mouse Exit events in JFrame.

Question

Hi, all.
Do I have a misconception here? When I drag the mouse in a modal JDialog, mouseEntered and mouseExited events are being delivered to JComponents in the parent JFrame that currently happens to be beneath that JDialog. I would not have expected any events to be delivered to any component not in the modal JDialog while that JDialog is displayed.
I submitted this as a bug many months ago, and have heard nothing back from Sun, nor can I find anything similar to this in BugTraq.
Here is sample code:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
* This class demonstrates what I believe are TWO bugs in Mouse Event handling in Swing
* 1.1.3_1, 1.1.3_2, 1.1.3_3, and 1.4.0.
* 1) When a MODAL JDialog is being displayed, and the cursor is DRAGGED from the JDialog
*    into and across the parent JFrame, Mouse Enter and Mouse Exit events are given to
*    the parent JFrame and/or it's child components.  It is my belief that NO such events
*    should be delivered, that modal dialogs should prevent ANY user interaction with any
*    component NOT in the JDialog.  Am I crazy?
*    You can reproduce this simply by running the main() method, then dragging the cursor
*    from the JDialog into and across the JFrame.
* 2) When a MODAL JDialog is being displayed, and the cursor is DRAGGED across the JDialog,
*    Mouse Enter and Mouse Exit events are given to the parent JFrame and/or it's child
*    components.  This is in addition to the problem described above.
*    You can reproduce this by dismissing the initial JDialog displayed when the main()
*    method starts up, clicking on the "Perform Action" button in the JFrame, then dragging
*    the cursor around the displayed JDialog.
* The Mouse Enter and Mouse Exit events are reported via System.err.
public class DragTest
    extends JFrame
    public static void main(final String[] p_args)
        new DragTest();
    public DragTest()
        super("JFrame");
        WindowListener l_windowListener = new WindowAdapter() {
            public void windowClosing(final WindowEvent p_evt)
                DragTest.this.dispose();
            public void windowClosed(final WindowEvent p_evt)
                System.exit(0);
        MouseListener l_mouseListener = new MouseAdapter() {
            public void mouseEntered(final MouseEvent p_evt)
                System.err.println(">>> Mouse Entered: " + ((Component)p_evt.getSource()).getName() );
            public void mouseExited(final MouseEvent p_evt)
                System.err.println(">>> Mouse Exited: " + ((Component)p_evt.getSource()).getName() );
        JPanel l_panel1 = new JPanel();
        l_panel1.setLayout( new BorderLayout(50,50) );
        l_panel1.setName("JFrame Panel");
        l_panel1.addMouseListener(l_mouseListener);
        JButton l_button = null;
        l_button = new JButton("JFrame North Button");
        l_button.setName(l_button.getText());
        l_button.addMouseListener(l_mouseListener);
        l_panel1.add(l_button, BorderLayout.NORTH);
        l_button = new JButton("JFrame South Button");
        l_button.setName(l_button.getText());
        l_button.addMouseListener(l_mouseListener);
        l_panel1.add(l_button, BorderLayout.SOUTH);
        l_button = new JButton("JFrame East Button");
        l_button.setName(l_button.getText());
        l_button.addMouseListener(l_mouseListener);
        l_panel1.add(l_button, BorderLayout.EAST);
        l_button = new JButton("JFrame West Button");
        l_button.setName(l_button.getText());
        l_button.addMouseListener(l_mouseListener);
        l_panel1.add(l_button, BorderLayout.WEST);
        l_button = new JButton("JFrame Center Button");
        l_button.setName(l_button.getText());
        l_button.addMouseListener(l_mouseListener);
        l_panel1.add(l_button, BorderLayout.CENTER);
        JButton l_actionButton = l_button;
        Container l_contentPane = this.getContentPane();
        l_contentPane.setLayout( new BorderLayout() );
        l_contentPane.add(l_panel1, BorderLayout.NORTH);
        JPanel l_panel2 = new JPanel();
        l_panel2.setName("JDialog Panel");
        l_panel2.addMouseListener(l_mouseListener);
        l_panel2.setLayout( new BorderLayout(50,50) );
        l_panel2.add( new JButton("JDialog North Button"),  BorderLayout.NORTH  );
        l_panel2.add( new JButton("JDialog South Button"),  BorderLayout.SOUTH  );
        l_panel2.add( new JButton("JDialog East Button"),   BorderLayout.EAST   );
        l_panel2.add( new JButton("JDialog West Button"),   BorderLayout.WEST   );
        l_panel2.add( new JButton("JDialog Center Button"), BorderLayout.CENTER );
        final JDialog l_dialog = new JDialog(this, "JDialog", true);
        WindowListener l_windowListener2 = new WindowAdapter() {
            public void windowClosing(WindowEvent p_evt)
                l_dialog.dispose();
        l_dialog.addWindowListener(l_windowListener2);
        l_dialog.setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
        l_dialog.getContentPane().add(l_panel2, BorderLayout.CENTER);
        l_dialog.pack();
        Action l_action = new AbstractAction() {
            { putValue(Action.NAME, "Perform Action (open dialog)"); }
            public void actionPerformed(final ActionEvent p_evt)
                l_dialog.setVisible(true);
        l_actionButton.setAction(l_action);
        this.addWindowListener(l_windowListener);
        this.pack();
        this.setLocation(100,100);
        this.setVisible(true);
        l_dialog.setVisible(true);
}(Too bad blank lines are stripped, eh?)
Thanks in advance for any insights you may be able to provide.
---Mark

Answer 1

I guess we can think of this as one problem. When mouse dragged, JFrame also (Parent) receives events. If i understood correctly, what happens here is, Modal dialog creates its own event pump and Frame will be having its own. See the source code of Dialog's show() method. It uses an interface called Conditional to determine whether to block the events or yield it to parent pump.
Here is the Conditional code and show method code from java.awt.dialog for reference
package java.awt;
* Conditional is used by the EventDispatchThread's message pumps to
* determine if a given pump should continue to run, or should instead exit
* and yield control to the parent pump.
* @version 1.3 02/02/00
* @author David Mendenhall
interface Conditional {
    boolean evaluate();
/////show method
    public void show() {
        if (!isModal()) {
            conditionalShow();
        } else {
            // Set this variable before calling conditionalShow(). That
            // way, if the Dialog is hidden right after being shown, we
            // won't mistakenly block this thread.
            keepBlocking = true;
            if (conditionalShow()) {
                // We have two mechanisms for blocking: 1. If we're on the
                // EventDispatchThread, start a new event pump. 2. If we're
                // on any other thread, call wait() on the treelock.
                if (Toolkit.getEventQueue().isDispatchThread()) {
                    EventDispatchThread dispatchThread =
                        (EventDispatchThread)Thread.currentThread();
                       * pump events, filter out input events for
                       * component not belong to our modal dialog.
                       * we already disabled other components in native code
                       * but because the event is posted from a different
                       * thread so it's possible that there are some events
                       * for other component already posted in the queue
                       * before we decide do modal show. 
                    dispatchThread.pumpEventsForHierarchy(new Conditional() {
                        public boolean evaluate() {
                            return keepBlocking && windowClosingException == null;
                    }, this);
                } else {
                    synchronized (getTreeLock()) {
                        while (keepBlocking && windowClosingException == null) {
                            try {
                                getTreeLock().wait();
                            } catch (InterruptedException e) {
                                break;
                if (windowClosingException != null) {
                    windowClosingException.fillInStackTrace();
                    throw windowClosingException;
    }I didn't get exactly what is happening but this may help to think further