Die SSCCE des Problems ist wie folgt. Ich bevölkere dynamischJTree, aber nichts passiert.

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Vector;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreePath;

public class DynamicTreeDemo extends JFrame
                        implements ActionListener {
private int newNodeSuffix = 1;
private static String ADD_COMMAND = "add";
private static String REMOVE_COMMAND = "remove";
private static String CLEAR_COMMAND = "clear";

private JTree jtrMainMenu;
DefaultMutableTreeNode   rootNode = null;
DefaultTreeModel treeModel;
public DynamicTreeDemo() {
    super(new BorderLayout());

    //Create the components.
    jtrMainMenu = new JTree();
    populateTree();

    JButton addButton = new JButton("Add");
    addButton.setActionCommand(ADD_COMMAND);
    addButton.addActionListener(this);

    JButton removeButton = new JButton("Remove");
    removeButton.setActionCommand(REMOVE_COMMAND);
    removeButton.addActionListener(this);

    JButton clearButton = new JButton("Clear");
    clearButton.setActionCommand(CLEAR_COMMAND);
    clearButton.addActionListener(this);

    //Lay everything out.
    jtrMainMenu.setPreferredSize(new Dimension(300, 150));
    add(jtrMainMenu, BorderLayout.CENTER);

    JPanel panel = new JPanel(new GridLayout(0,3));
    panel.add(addButton);
    panel.add(removeButton); 
    panel.add(clearButton);
add(panel, BorderLayout.SOUTH);
}


    public void populateTree() {
  Vector<Vector<String>> data =GetDataFromDB();           
 java.util.List<String> list =GetListFromDB();
        int i=0;;
 int size=list.size();
 treeModel=(DefaultTreeModel)jtrMainMenu.getModel();  

 ((DefaultMutableTreeNode)  (treeModel.getRoot())).removeAllChildren();
 treeModel.reload();
 rootNode = new DefaultMutableTreeNode("JewelleryERPApplication");
 treeModel=new DefaultTreeModel(rootNode);


 DefaultMutableTreeNode p1=null, p2=null,p3=null,p4=null,p5=null;


     while(i<size){

        if(list.get(i).length()==2){
          p1 = addObject(null, data.get(i).elementAt(1),true);  

        }else 
        if(list.get(i).length()==4){
           p2 = addObject(p1, data.get(i).elementAt(1),true);   

     }else if
       (list.get(i).length()==6){
             p3 = addObject(p2, data.get(i).elementAt(1),true);   

     }
     else if
       (list.get(i).length()==8){
             p4 = addObject(p3, data.get(i).elementAt(1),true);    

     }  
     else if
       (list.get(i).length()==10){
             p5 = addObject(p4, data.get(i).elementAt(1),true);    

     }      
         i++;
     }

    jtrMainMenu.setModel(treeModel);
   ((DefaultMutableTreeNode)  (treeModel.getRoot())).getChildCount();
    jtrMainMenu.revalidate();
    jtrMainMenu.repaint();
}


public DefaultMutableTreeNode addObject(DefaultMutableTreeNode parent,
                                        Object child, 
                                        boolean shouldBeVisible) {


          DefaultMutableTreeNode childNode = 
            new DefaultMutableTreeNode(child);



    if (parent == null) {
        parent = rootNode;
    }

//It is key to invoke this on the TreeModel, and NOT DefaultMutableTreeNode
    treeModel.insertNodeInto(childNode, parent, 
                             parent.getChildCount());

     jtrMainMenu.setModel(treeModel);

    //Make sure the user can see the lovely new node.
    if (shouldBeVisible) {
        jtrMainMenu.scrollPathToVisible(new TreePath(childNode.getPath()));
    }

    return childNode;
} 

/**
 * Create the GUI and show it.  For thread safety,
 * this method should be invoked from the
 * event-dispatching thread.
 */
private static void createAndShowGUI() {
    //Create and set up the window.
    JFrame frame = new JFrame("DynamicTreeDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Create and set up the content pane.
    DynamicTreeDemo newContentPane = new DynamicTreeDemo();
    newContentPane.setOpaque(true); //content panes must be opaque
    frame.setContentPane(newContentPane);

    //Display the window.
    frame.pack();
    frame.setVisible(true);
}

public static void main(String[] args) {
    //Schedule a job for the event-dispatching thread:
    //creating and showing this application's GUI.
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}
}