Problem rendering tree nodes of tree sent to applet from servlet


I have an applet and a servlet and a DefaultMutableTreeNode being sent serialized from the servlet to the applet. In the servlet, the root has children of DefaultMutableTreeNodes with Result objects as UserObjects. The JTree in the applet should display the nodes of the root as different colored icons, see MyRenderer. The problem is that in the code in MyRenderer, the getUserObject object is not instanceOf Result, and only displays the dummy (yellow) icon. The getUserObject is null...

When I test with a root node with DFMTreeNode children with Result objects as UserObjects in the applet code, the tree displays with correct colored icons.

The Result.class exists both on server and client side in my deployment.
Why is getUserObject null? How can I get the nodes to render correctly?
Greatful for help - thanks in advance!


package result;


import javax.swing.*;
import javax.swing.tree.DefaultTreeCellRenderer;
import javax.swing.tree.DefaultMutableTreeNode;
import java.awt.*;

public class TreePanel extends JPanel {

  private JTree _tree;
  private TestApplet _parent;

  public TreePanel(TestApplet parent) {
    _parent = parent;
    setLayout(new BorderLayout());

    DefaultMutableTreeNode root = new DefaultMutableTreeNode(new Result("Root", 2));
    DefaultMutableTreeNode child_1 = new DefaultMutableTreeNode(new Result("Result 1", 2));
    DefaultMutableTreeNode child_2 = new DefaultMutableTreeNode(new Result("Result 2", 3));
    DefaultMutableTreeNode child_11 = new DefaultMutableTreeNode(new Result("Result 4", 1));
    DefaultMutableTreeNode child_12 = new DefaultMutableTreeNode(new Result("Result 5", 2));
    DefaultMutableTreeNode child_21 = new DefaultMutableTreeNode(new Result("Result 6", 1));
    DefaultMutableTreeNode child_22 = new DefaultMutableTreeNode(new Result("Result 7", 1));
    _tree = new JTree(root);
    _tree.setCellRenderer(new MyRenderer(_parent.getGreenIcon(), _parent.getRedIcon(), _parent.getYellowIcon()));

  public void updateTree(DefaultMutableTreeNode root) {
    if (root != null) {
      if (_tree != null) {
      _tree = new QCTree(root);
      _tree.setCellRenderer(new MyRenderer(_parent.getGreenIcon(), _parent.getRedIcon(), _parent.getYellowIcon()));
    } else {
      JLabel label = new JLabel("No result tree structure is available");

  private class MyRenderer extends DefaultTreeCellRenderer {

    private ImageIcon _greenIcon;
    private ImageIcon _redIcon;
    private ImageIcon _yellowIcon;

    public MyRenderer(ImageIcon greenIcon, ImageIcon redIcon, ImageIcon yellowIcon) {
      _greenIcon = greenIcon;
      _redIcon = redIcon;
      _yellowIcon = yellowIcon;

    public Component getTreeCellRendererComponent(JTree tree, Object value, boolean sel, boolean expanded,
                                                  boolean leaf, int row, boolean hasFocus) {

      ResultComponent item;

      super.getTreeCellRendererComponent(tree, value, sel, expanded, leaf, row, hasFocus);
      DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
      Object temp = node.getUserObject();

      // Debug
      if (temp == null) {
        System.out.println("temp = null"); // This is run for nodes of root from servlet
      } else {
        System.out.println("temp getClass = " + temp.getClass()); // This is run for nodes of root in applet (getClass = result.Result)

      if (temp instanceof Result) {
        item = (Result)temp;

        if (item.getValue() == 1) {
          if (_greenIcon != null) {
        if (item.getValue() == 2) {
          if (_redIcon != null) {
        if (item.getValue() == 3) {
          if (_yellowIcon != null) {

      } else {
        if (_yellowIcon != null) {
          setIcon(_yellowIcon);     // Only this statement is run
      return this;


package result;

import javax.servlet.*;
import javax.servlet.http.*;
import java.util.Vector;
import javax.swing.tree.*;

public class TestServlet extends HttpServlet {

  public void init() {

  public void doPost(HttpServletRequest request, HttpServletResponse response)
          throws ServletException, IOException {  

    // Hanle request for applet

    // Tree to return to applet
    Vector roots = new Vector();
    DefaultMutableTreeNode root = new DefaultMutableTreeNode(new Result("Root", 0));
    DefaultMutableTreeNode n1 = new DefaultMutableTreeNode(new Result("A", 0));
    DefaultMutableTreeNode n2 =  new DefaultMutableTreeNode(new Result("B", 1));
    DefaultMutableTreeNode n3 = new DefaultMutableTreeNode(new Result("C", 2));
    n1.add(new DefaultMutableTreeNode(new Result("A 1", 0)));
    n1.add(new DefaultMutableTreeNode(new Result("A 2", 1)));
    n2.add(new DefaultMutableTreeNode(new Result("B 1", 2)));
    n3.add(new DefaultMutableTreeNode(new Result("C 1", 3)));
    // Vector with the root object sent to applet
    try {
       ObjectOutputStream outputToApplet = new ObjectOutputStream(response.getOutputStream());
   } catch(Exception x) {
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hm...just a suggestion...may be you have different JVM on client an server, so desirialization doesn't work correctly?
    may be you can try use XMLEncoder and XMLDecoder for serialization objects in XML, is should be compatibly between different JVMs. (this featere included in JDK since 1.4.0)
Mick BarryJava DeveloperCommented:
is that just a summaryt of your code?
The servlet returns the vector roots but I don't see where that vector is populated.
Mick BarryJava DeveloperCommented:
Ignore last comment, have found the problem.

the user object member variable in DMTN is declared transient so it does not get serialized.

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MarteJAuthor Commented:

Yes, it is just a summary, forgot roots.add(root); .

<the user object member variable in DMTN is declared transient so it does not get serialized.>

What does it mean that the userObjcet is transient? The userObject is null because it does not get serialized then? How should I solve this?

Mick BarryJava DeveloperCommented:
A member var that is declared transient is not included in the serialization process.
Mick BarryJava DeveloperCommented:
to solve it you need to use a different structure to pass your tree between servlet and applet.
Writew your own class which extends DefaultMutableTreeNode and add your own "Object" field to it...

(make sure you implement Serializable)

Transient is explained here:


"Transient instance fields are neither saved nor restored by the standard serialisation mechanism. You have to handle restoring them yourself."



public class DefaultMutableTreeNode extends Object implements Cloneable,
       MutableTreeNode, Serializable

    /** optional user object */
    transient protected Object      userObject;

Mick BarryJava DeveloperCommented:
just what i was about to suggest :)
Or write your own TreeNode class, or make Result implement TreeNode.
MarteJAuthor Commented:
Thanks for helping!!

I'm trying to extend DMTreeNode, but I think I got it wrong:

package result;

import javax.swing.tree.*;

public class MyTreeNode extends DefaultMutableTreeNode implements Serializable {

  protected Object userObject;

  public MyTreeNode(Result result) {

  public void setUserObject(Object object) {
    userObject = object;

  public Object getUserObject() {
    return userObject;

Since this is used both on client and server side, I copy MyTreeNode.class to the servlet's result directory (as with Result.class)

When running the applet, I get the following error: result.MyTreeNode; local class incompatible: stream classdesc serialVersionUID = -6854483571538912795, local class serialVersionUID = 4884205795974306009

Mick BarryJava DeveloperCommented:
you'll get that error if the class doing the serializing is different from the class doing the unserializing. ie. if you update one you need to update the other, and restart webapp and reload applet.
You must place copies of your latest class versions at both ends or you'll get binary incompatibilities with serialization
MarteJAuthor Commented:
Yessss! It works! I am now able to transfer a tree root from my servlet to my applet, displayed according to the objects each node represents! I'm not totally done with my objectives, but I'm getting there thanks to your help! Probably need some more assistance pretty soon...

Thanks a lot!!
Mick BarryJava DeveloperCommented:
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