Java Swing - 如何展开JScrollPane中的JTree路径

import java.awt.BorderLayout;
import java.util.Enumeration;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreePath;

public class Main {
  Enumeration<TreePath> expansionState;

  public Main() {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  
    JTree tree = new JTree(buildDemoModel());

    JPanel buttonsPanel = new JPanel();

    JButton saveButton = new JButton("Capture state");
    saveButton.addActionListener(e->expansionState = saveExpansionState(tree));

    JButton loadButton = new JButton("Load state");
    loadButton.addActionListener(e -> {
      loadExpansionState(tree, expansionState);
      expansionState = saveExpansionState(tree);
    });

    buttonsPanel.add(saveButton);
    buttonsPanel.add(loadButton);

    JPanel content = new JPanel(new BorderLayout());
    content.add(buttonsPanel, BorderLayout.SOUTH);
    content.add(new JScrollPane(tree), BorderLayout.CENTER);

    frame.add(content);
    frame.pack();
    frame.setVisible(true);
  }

  public static Enumeration<TreePath> saveExpansionState(JTree tree) {
    return tree.getExpandedDescendants(new TreePath(tree.getModel().getRoot()));
  }

  public static void loadExpansionState(JTree tree,
      Enumeration<TreePath> enumeration) {
    if (enumeration != null) {
      while (enumeration.hasMoreElements()) {
        TreePath treePath = enumeration.nextElement();
        tree.expandPath(treePath);
      }
    }
  }

  public static DefaultTreeModel buildDemoModel() {
    DefaultMutableTreeNode root = new DefaultMutableTreeNode("Root");

    DefaultMutableTreeNode childNode = new DefaultMutableTreeNode("A");
    childNode.add(new DefaultMutableTreeNode("A1"));
    childNode.add(new DefaultMutableTreeNode("A2"));
    root.add(childNode);

    childNode = new DefaultMutableTreeNode("B");
    childNode.add(new DefaultMutableTreeNode("B1"));
    childNode.add(new DefaultMutableTreeNode("B2"));
    root.add(childNode);

    return new DefaultTreeModel(root);
  }
  public static void main(String[] args) {
    new Main();
  }
}