You do know you can't do that in an unsigned Applet.
See http://mindprod.com/jgloss/applet.html

Try putting in some debug code. Your problem may have nothing to do
with serializing.  Try System.out.println("here");
just before you start to serialize.

Swing components uninstall their UI delegate when serialising. Possibly
it is a problem relating to that (I don't think it is tolerant of
exceptions, for instance).

I've tried to reproduce your problem with no lock. My code is below.
Does it work for you? Can you make it not work (in the appropriate style)?

Tom Hawtin

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

public class MyTree extends JTree {
    private void writeObject(
        ObjectOutputStream out
    ) throws IOException {
        out.defaultWriteObject();
    }
    private void readObject(
        ObjectInputStream in
    ) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
    }
    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {
             public void run() {
                JFrame frame = new JFrame("SerialTree");
                frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                final JTree tree = new MyTree();
                frame.add(tree);
                JButton save = new JButton("Save");
                save.addActionListener(new ActionListener() {
                        public void actionPerformed(ActionEvent event) {
                            try {
                                OutputStream fileOut =
                                    new FileOutputStream("test.ser");
                                try {
                                    ObjectOutputStream objectOut =
                                        new ObjectOutputStream(
                                            fileOut
                                        );
                                    objectOut.writeObject(tree);
                                    objectOut.flush();
                                } finally {
                                    fileOut.close();
                                }
                            } catch (IOException exc) {
                                exc.printStackTrace();
                            }
                        }
                });
                frame.add(save, BorderLayout.SOUTH);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }
}

Use less JPaper!

( sorry, I just couldn't resist that )