Expand Collapse Expand JTree Lazy Download Problem

Question

I used a tree using Lazy Loading. First-level nodes are created during the creation of the tree, where when child nodes are created only when the user expands a specific node.

Data comes from the database, and we run a database query to populate the child nodes. Implemented by TreeExpansionListener and uses an overridden implementation of the treeExpanded method. When expanding, I delete all the child nodes for the selected node, make a database query and add records as children to the selected node. Before adding a node to the tree, a dummy child is added to the node. Work with DefaultMutableTreeNode.

So far, so good that it worked fine on hold.

Problem 1 - Like you, its calling the database for decomposition, so if the node crashes and is deployed again, I will disable the database and process it again ... The idea is not to load the nodes fresh if they are already expanded .. .

Problem 2 - If I needed to perform a force update, that is, reload the tree and save the extension state. It is now operational ... How can I achieve this with the fix for problem 1 above?

Appreciate any help on this.

Answer 1

As MadProgrammer explains, here is a small demo showing JTree loading dynamically when a tree expands on nodes (and with a good loading panel as a bonus):

import java.awt.BorderLayout; import java.awt.Dimension; import java.beans.PropertyChangeEvent; import java.beans.PropertyChangeListener; import java.beans.Transient; import java.util.ArrayList; import java.util.List; import javax.swing.JFrame; import javax.swing.JProgressBar; import javax.swing.JScrollPane; import javax.swing.JTree; import javax.swing.SwingUtilities; import javax.swing.SwingWorker; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; import javax.swing.event.TreeExpansionEvent; import javax.swing.event.TreeWillExpandListener; import javax.swing.tree.DefaultMutableTreeNode; import javax.swing.tree.DefaultTreeModel; import javax.swing.tree.ExpandVetoException; import javax.swing.tree.MutableTreeNode; import javax.swing.tree.TreePath; public class TestJTree { public static class MyTreeNode extends DefaultMutableTreeNode { private boolean loaded = false; private int depth; private final int index; public MyTreeNode(int index, int depth) { this.index = index; this.depth = depth; add(new DefaultMutableTreeNode("Loading...", false)); setAllowsChildren(true); setUserObject("Child " + index + " at level " + depth); } private void setChildren(List<MyTreeNode> children) { removeAllChildren(); setAllowsChildren(children.size() > 0); for (MutableTreeNode node : children) { add(node); } loaded = true; } @Override public boolean isLeaf() { return false; } public void loadChildren(final DefaultTreeModel model, final PropertyChangeListener progressListener) { if (loaded) { return; } SwingWorker<List<MyTreeNode>, Void> worker = new SwingWorker<List<MyTreeNode>, Void>() { @Override protected List<MyTreeNode> doInBackground() throws Exception { // Here access database if needed setProgress(0); List<MyTreeNode> children = new ArrayList<TestJTree.MyTreeNode>(); if (depth < 5) { for (int i = 0; i < 5; i++) { // Simulate DB access time Thread.sleep(300); children.add(new MyTreeNode(i + 1, depth + 1)); setProgress((i + 1) * 20); } } else { Thread.sleep(1000); } setProgress(0); return children; } @Override protected void done() { try { setChildren(get()); model.nodeStructureChanged(MyTreeNode.this); } catch (Exception e) { e.printStackTrace(); // Notify user of error. } super.done(); } }; if (progressListener != null) { worker.getPropertyChangeSupport().addPropertyChangeListener("progress", progressListener); } worker.execute(); } } protected void initUI() { JFrame frame = new JFrame(TestJTree.class.getSimpleName()); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); MyTreeNode root = new MyTreeNode(1, 0); final DefaultTreeModel model = new DefaultTreeModel(root); final JProgressBar bar = new JProgressBar(); final PropertyChangeListener progressListener = new PropertyChangeListener() { @Override public void propertyChange(PropertyChangeEvent evt) { bar.setValue((Integer) evt.getNewValue()); } }; JTree tree = new JTree() { @Override @Transient public Dimension getPreferredSize() { Dimension preferredSize = super.getPreferredSize(); preferredSize.width = Math.max(400, preferredSize.width); preferredSize.height = Math.max(400, preferredSize.height); return preferredSize; } }; tree.setShowsRootHandles(true); tree.addTreeWillExpandListener(new TreeWillExpandListener() { @Override public void treeWillExpand(TreeExpansionEvent event) throws ExpandVetoException { TreePath path = event.getPath(); if (path.getLastPathComponent() instanceof MyTreeNode) { MyTreeNode node = (MyTreeNode) path.getLastPathComponent(); node.loadChildren(model, progressListener); } } @Override public void treeWillCollapse(TreeExpansionEvent event) throws ExpandVetoException { } }); tree.setModel(model); root.loadChildren(model, progressListener); frame.add(new JScrollPane(tree)); frame.add(bar, BorderLayout.SOUTH); frame.pack(); frame.setVisible(true); } public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, UnsupportedLookAndFeelException { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); SwingUtilities.invokeLater(new Runnable() { @Override public void run() { new TestJTree().initUI(); } }); } } 

Answer 2

Once a node is populated, just don't populate it. On treeExpanded just check if the parent node node has any children, if so, do not reload anything.

You will need to provide the option to update the parent node. I usually do this with JPopupMenu .