Report from ###@###.###:

// file: TrivialApplication.java

import java.awt.*;
import java.awt.event.*;
import com.sun.java.swing.*;

public class TrivialApplication {

    public static void main(String args[])
    {
        JFrame theFrame = new JFrame("ContainerPanel test");
        theFrame.setSize(100, 100);
        ContainerPanel cp = new ContainerPanel();
        theFrame.getContentPane().add(cp);
        theFrame.show();
        try {
            Thread.currentThread().sleep(10000);
        }
        catch (InterruptedException e) {}
        System.out.println("Removing ContainerPanel");
        theFrame.getContentPane().remove(cp);
        cp.remove(cp.getComponent(0));
        cp = null; // lose our reference to it.
        theFrame.getContentPane().requestFocus();
        theFrame.validate();
        theFrame.repaint();
        System.gc();
        try {
            Thread.currentThread().sleep(10000);
        }            
        catch (InterruptedException e) {}
        System.exit(0);
    }

}

// file: ContainerPanel.java

import java.awt.*;
import java.awt.event.*;
import com.sun.java.swing.*;

public class ContainerPanel extends JPanel {
       public JButton testButton = new JButton("foo");

       public ContainerPanel() {
            setLayout (new BorderLayout());
            add("Center",testButton);
            testButton.addActionListener(new TestButtonListener());
       }

       // The action listener is an inner class
       class TestButtonListener implements ActionListener {
            public void actionPerformed (ActionEvent e) {
                 // Do something, it really doesn't matter what
            }
       }
}

Note that I keep main() into another class so that there won't be any 
extraneous references to ContainerPanel's class on the stack.  Possibly 
paranoid....

I then ran this program under JDK 1.2beta4 with heap profiling turned on:

java -cp AppClasses.jar -Xhprof:file=dump.hprof,format=b TrivialApplication

I took a memory checkpoint (ctrl-break on Windows) during the second 10 
second pause in the code, and ran the output through HAT.
<http://developer.javasoft.com/developer/earlyAccess/hat/>

Here's what I found:

--> java.awt.FocusManager@0xa0acc001 (8 bytes) (field focusOwner:)
--> com.sun.java.swing.JButton@0xb0e0c001 (328 bytes) (field listenerList:)

and:

--> sun.awt.windows.WInputMethod@0x60c7c001 (24 bytes) (field clientComponent:)
--> com.sun.java.swing.JButton@0xb0e0c001 (328 bytes) (field listenerList:)

So it looks like, even though it's been removed from the JFrame (by virtue 
of its parent ContainerPanel being removed), the JButton still has focus. 
And, indeed inserting the line:

        theFrame.getContentPane().requestFocus();

right before the call to theFrame.validate() gets rid of the FocusManager's 
reference to the JButton.  I have thus far been unable to get rid of the 
WInputMethod references, though.  Anyone have any ideas on this?


John Brewer
Jera Design