Why does getting the nth child of a Node fail in an ExplorerManager listener?

Question

I'm having problems with the NetBeans Nodes API.

I have this line of code:

Node n = (new MyNode(X)).getChildren().getNodeAt(Y);

The call to new MyNode(X) with the same X always initializes a MyNode the same way, independent of the context.

When I place it by itself (say, in an menu action), it successfully gets the Yth child, but if I put it in an event where other Node/Children stuff happens, it returns null.

MyNode's Children implementation is a trivial subclass of Children.Keys, which is approximately:

// Node
import org.openide.nodes.AbstractNode;

class MyNode extends AbstractNode {

    MyNode(MyKey key) {
        super(new MyNodeChildren(key));
    } 
}


// Children 
import java.util.Collections;
import org.openide.nodes.Children;
import org.openide.nodes.Node;

public class MyNodeChildren extends Children.Keys<MyKey> {
    MyKey parentKey;

    MyNodeChildren(MyKey parentKey) {
        super(true); // use lazy behavior
        this.parentKey = parentKey;
    }

    @Override
    protected Node[] createNodes(MyKey key) {
        return new Node[] {new MyNode(key)};
    }

    @Override
    protected void addNotify() {
        setKeys(this.parentKey.getChildrenKeys());
    }

    @Override
    protected void removeNotify() {
        setKeys(Collections.EMPTY_SET);
    }
}

// MyKey is trivial.

I assume this has something to do with the lazy behavior of Children.Keys. I have the sources for the API, and I've tried stepping through it, but they're so confusing that I haven't figured anything out yet.

NetBeans IDE 7.0.1 (Build 201107282000) with up-to-date plugins.

Edit: More details

The line with the weird behavior is inside a handler for an ExplorerManager selected-nodes property change. The weird thing is that it still doesn't work when the MyNode instance isn't in the heirarchy that the ExplorerManager is using (it's not even the same class as the nodes in the ExplorerManager), and isn't being used for anything else.

Accessing the nodes instead of the underlying model is actually necessary for my use case (I need to do stuff with the PropertySets), the MyNode example is just a simpler case that still has the problem.

Answer 1

Timon Veenstra on the NetBeans Platform Developers mailing list solved this for me.

Actions on the explorerManager are guarded to ensure consistency. A node selection listener on an explorer manager for example cannot manipulate the same explorer manager while handling the selection changed event because that would require a read to write upgrade. The change will be vetoed and die a silent death.

Are you adding the MyNode root node to the explorer manager on initialization, or somewhere else in a listener?

My problem line is in an ExplorerManager selection change listener. I guess the Children.MUTEX lock is getting set by ExplorerManager and preventing the Children.Keys instance from populating its Nodes...?

Anyways, I moved my Node access into a EventQueue.invokeLater(...), so it executes after the selection changed event finishes, and that fixed it.

Answer 2

It is recommended to use org.openide.nodes.ChildFactory to create child nodes unless you have a specific need to use one of the Children APIs. But for the common cases the ChildFactory is sufficient.

One thing to keep in mind when using the Nodes API is that it is only a presentation layer that wraps your model and used in conjunction with the Explorer API makes it available to the various view components in the NetBeans platform such as org.openide.explorer.view.BeanTreeView.

Using a model called MyModel which may look something like:

public class MyModel {

    private String title;
    private List<MyChild> children;

    public MyModel(List<MyChild> children) {
        this.children = children;
    }

    public String getTitle() {
        return title;
    }

    public List<MyChild> getChildren() {
        return Collections.unmodifiableList(children);
    }
}

You can create a ChildFactory<MyModel> that will be responsible for creating your nodes:

public class MyChildFactory extends ChildFactory<MyModel> {

    private List<MyModel> myModels;

    public MyChildFactory(List<MyModel> myModels) {
        this.myModels = myModels;
    }

    protected boolean createKeys(List<MyModel> toPopulate) {
        return toPopulate.addAll(myModels);
    }

    protected Node createNodeForKey(MyModel myModel) {
        return new MyNode(myModel);
    }

    protected void removeNotify() {
        this.myModels= null;
    }  
} 

Then, implementing MyNode which is the presentation layer and wraps MyModel:

public class MyNode extends AbstractNode {

    public MyNode(MyModel myModel) {
        this(myModel, new InstanceContent());
    }

    private MyNode(MyModel myModel, InstanceContent content) {
        super(Children.create(
                new MyChildrenChildFactory(myModel.getChildren()), true),
                new AbstractLookup(content)); // add a Lookup
        // add myModel to the lookup so you can retrieve it latter
        content.add(myModel);
        // set the name used in the presentation
        setName(myModel.getTitle());
        // set the icon used in the presentation
        setIconBaseWithExtension("com/my/resouces/icon.png");
    }
}

And now the MyChildrenChildFactory which is very similar to MyChildFactory except that it takes a List<MyChild> and in turn creates MyChildNode:

public class MyChildFactory extends ChildFactory<MyChild> {

    private List<MyChild> myChildren;

    public MyChildFactory(List<MyChild> myChildren) {
        this.myChildren = myChildren;
    }

    protected boolean createKeys(List<MyChild> toPopulate) {
        return toPopulate.addAll(myChildren);
    }

    protected Node createNodeForKey(MyChild myChild) {
        return new MyChildNode(myChild);
    }

    protected void removeNotify() {
        this.myChildren = null;
    }  
} 

Then an implementation of MyChildNode which is very similar to MyNode:

public class MyChildNode extends AbstractNode {

    public MyChildNode(MyChild myChild) {
        // no children and another way to add a Lookup
        super(Children.LEAF, Lookups.singleton(myChild));
        // set the name used in the presentation
        setName(myChild.getTitle());
        // set the icon used in the presentation
        setIconBaseWithExtension("com/my/resouces/child_icon.png");
    }
}

And we will need the children's model, MyChild which is very similar to MyModel:

public class MyChild {

    private String title;

    public String getTitle() {
        return title;
    }
}

Finally to put it all to use, for instance with a BeanTreeView which would reside in a TopComponent that implements org.openide.explorer.ExplorerManager.Provider:

 // somewhere in your TopComponent's initialization code:
 List<MyModel> myModels = ...
 // defined as a property in you TC
 explorerManager = new ExplorerManager();
 // this is the important bit and we're using true 
 // to tell it to create the children asynchronously 
 Children children = Children.create(new MyChildFactory(myModels), true);
 explorerManager.setRootContext(new AbstractNode(children));

Notice that you don't need to touch the BeanTreeView and in fact it can be any view component that is included in the platform. This is the recommended way to create nodes and as I've stated, the use of nodes is as a presentation layer to be used in the various components that are included in the platform.

If you then need to get a child you can use the ExplorerManager which you can retrieve from the TopComponent using the method ExplorerManager.Provier.getExplorerManager() which was implemented due to the fact that your TopComponent implemented ExplorerManager.Provider and is in fact the way that a view component itself gets the nodes:

ExplorerManager explorerManager = ...
// the AbstractNode from above
Node rootContext = explorerManager.getRootContext();
// the MyNode(s) from above
Children children = rootContext.getChildren().getNodes(true);
// looking up the MyModel that we added to the lookup in the MyNode
MyModel myModel = nodes[0].getLookup().lookup(MyModel.class);

However, you must be aware that using the Children.getNodes(true) method to get your nodes will cause all of your nodes and their children to be created; which weren't created due to the fact that we told the factory that we wanted it to create the children asynchronously. This is not the recommended way to access the data but instead you should keep a reference to the List<MyModel> and use that if at all possible. From the documentation for Children.getNodes(boolean):

...in general if you are trying to get useful data by calling this method, you are probably doing something wrong. Usually you should be asking some underlying model for information, not the nodes for children.

Again, you must remember that the Nodes API is a presentation layer and is used as an adapter between your model and your views.

Where this becomes a powerful technique is when using the same ChildFactory in different and diverse views. You can reuse the above code in many TopComponents without any modifications. You can also use a FilterNode if you need to change only a part of the presentation of a node without having to touch the original node.

Learning the Nodes API is one of the more challenging aspects of learning the NetBeans platform API as you have undoubtedly discovered. Once you have some mastery of this API you will be able to take advantage of much more of the platforms built in capabilities.

Please see the following resources for more information on the Nodes API:

• NetBeans Nodes API Tutorial
• Great introduction to the Nodes API by Antonio Vieiro
• Part 5: Nodes API and Explorer & Property Sheet API by Geertjan Wielenga
• JavaDocs for the Nodes API