The Javadoc for this API resides in org.openide.nodes. In particular, the class Node is the logical starting point.


Nodes API

The Nodes API controls the usage and creation of nodes, which are a variant of JavaBeans that may have adjustable property sets; provide cookies and actions; be visually displayed in the Explorer with full hierarchy support; and other features.

What is a Node?

A node provides the visual representation and apparent behavior of most objects in NetBeans. It may be used to represent a data object from the Datasystems API (and provide a more UI-friendly wrapper around that data object), or it may be created separately for a special purpose. For example, every icon displayed in the Explorer is a node, and this is the primary UI interface to the entire hierarchy of nodes. (Many dialog boxes, the Component Palette, and so on, also operate based on nodes, via the Explorer API.)

Nodes themselves ought not be used to hold actual data; that should be stored in a data object, or in some other appropriate storage mechanism. Rather, they provide a presentation device for existing data.

JavaBeans and nodes

A node is a sort of extension to the JavaBeans concept, adding some features that were necessary for the full functioning of explorer views. Some key components that were missing from the JavaBeans specification: That said, the JavaBeans architecture was by no means ignored while designing the Nodes system; it was used as a conceptual foundation. To blur the distinction, special node implementations are available which wrap around standard JavaBeans and present them as nodes, including handling any Bean Context available, introspected properties, and so on.

Nodes in the Explorer

Using the Explorer API, you can create a variety of different presentations for any tree or subtree of the Nodes hierarchy.

The Explorer interactions with nodes include actions that the node provides (generally available in a right-click context menu); cookie-based action enabling (so that the node selection affects the availability of system actions, like Compile); cut, copy, and paste support, as well as reordering of children, deletion, and creation of new children; displayable names and icons, which may be sensitive to the state of the node; and so on.

Importantly, nodes are not dead data - they are live components. So, actions taken in one part of the system will frequently cause open Explorer views to refresh to display the new node structure (for example, after pasting a component onto a form); and conversely, actions that seem natural to do in the Explorer will usually be accessible there through its interface and update the rest of the system accordingly (for example, deleting a toolbar button in the Environment subtree has immediate effect). These capabilities owe to the rich event notification supported by the Nodes API.

Common node types

This is a partial list of common types of nodes, some more apparent than others, to give an idea of what is possible with the API: The point is that most aspects of UI integration can be handled well with appropriate use of nodes; modules which use them to good effect will blend into the rest of NetBeans naturally from a user's perspective, and probably be easier to write as well.

Creating Custom Nodes

This section details the steps you must take to create various kinds of customized nodes. In simple cases, you need do very little, as there exist prebuilt default implementations. For more sophisticated nodes, for example containing a structured list of children of a particular type (as is the case with e.g. the Active Processes list), some more work is necessary, but again much of it will involve subclassing existing supports.

General Aspects

All nodes must subclass the general Node abstract class. However, in practice it is most common to actually subclass a convenient base class, AbstractNode, or one of its subclasses. Your node class needs to specify a list of child nodes in the constructor (the contents of which may be changed later, but not the identity of the Children object itself); for the case of a leaf node, just pass in Children.LEAF.

As this class is not abstract, there are no strict requirements on what needs to be overridden. However, the following general methods you are likely to want to override:

Properties, sets, and sheets

There are three levels of organization for node properties:
  1. Properties, which represent individual settable properties of the node object, with a definition class and so on, very similar to JavaBean properties (and in fact extending Bean features).
  2. Property sets, which are just collections of properties that typically would be displayed together - for example, all the event handlers for a form component.
  3. Sheets, which group together several named property sets - for example, the same form component may also have a normal Bean property set (foreground, etc.), and an expert set (with rarely-used properties).
Nodes are only required to cluster properties into sets, by implementing Node.getPropertySets(); but sheets are convenient to handle, and they may be used from an AbstractNode, for example by overriding AbstractNode.createSheet() to provide the basic list of property sets desired for the node. (You should get the sheet set you need from it, checking whether it really exists yet, add properties to the sheet set, and then replace it into the sheet to be sure your changes take effect.)

Each property has a few interesting aspects to it:

There are a number of support classes from Node.Property which provide useful refinements: You may group whatever properties are needed for your node into different sets as desired (or put them all into one set, if there is no useful distinction); note that the property sets are also feature descriptors, and so may have internal and display names, etc.

Naturally, common node implementation classes such as DataNode may automatically create a sheet with some useful properties on it; then this sheet should generally be appended to by overriding AbstractNode.createSheet() and calling the superclass method first.

For complex nodes, such as a system option controlling the appearance of an entire editor, it may be cumbersome for the user to edit individual properties, especially without getting a holistic preview. If this is the case, a customizing GUI component may be returned from Node.getCustomizer(), and Node.hasCustomizer() turned on. The exact way in which the customizer will be displayed is determined by NetBeans, but typically it will be popped up in a dialog or non-modal window; it should be tied to the node's properties however appropriate.

If a full customizer is not required, individual properties may still have a custom editing style associated with them; Node.Property.getPropertyEditor() is used to look for a property editor, defaulting to the standard JavaBeans property editor for the appropriate type.

Note that the Nodes API, unlike JavaBeans, permits a specific instance of PropertyEditor to be associated with the node, not just its class - so if you override getPropertyEditor(), it is possible to select an editor based on the current state of the node (for example, a table may have a completely different editor when it is bound to a SQL rowset), or to keep an initialized editor associated with a node that may have some UI state not kept in the node itself.

Hierarchy nodes and their children

Creating a leaf node - a node with no children - is fairly straightforward, since the child list may simply be specified as Node.EMPTY. To create a hierarchy node that will act as a parent for other nodes, you must consider both what type of children it will have, and how those children should be managed.

The basic data structure for managing a child list is Children, which is not likely to be subclasses directly but rather used in the form of one of the support classes available for it. Note that the node must keep the same children object throughout its lifetime, and the children object is responsible for managing the addition, removal, and structure of children under it.

A simple child list may be created with Children.Array. You need only create it with the default constructor, and add child nodes to it (at any time, or remove them later for that matter) using Children.add(...).

If it is desirable that the children be sorted when displayed, you can use e.g. Children.SortedArray to do this. In this case, the comparator (i.e. sort criteria) can be changed at any time.

If the children need to be accessed based on keys, as in a hashtable, this is possible with Children.Map (and also Children.SortedMap). Along similar lines, Children.Keys permits clustering of the children by key, where several children may be associated with one key. This class may be especially useful when mirroring an external hierarchical system into a node hierarchy, such as Java class hierarchies, which need the children to be partitioned in a certain way (e.g. methods vs. fields).

This document will not go into the details of subclassing children lists, since doing so is not likely to be required very frequently - the provided support classes should handle the common cases. If it is necessary to subclass, the documentation for Children should suffice.

Indexing and reordering children

Many structural constraints on children are probably satisfied by the children class itself - i.e. using Children.SortedArray guarantees that your children will be properly sorted without any work beyond providing the comparator. However, for an unsorted child list it may be useful to provide support for directed reordering of the children.

Generally you will want to make the children rearrangeable by the user, as well as by external code. To do so, you should implement the Index cookie on your node, which exists to handle this case. This cookie provides ways for the user to move particular children around, or to undertake a complete rearrangement using a dialog box. There is a generic support class which implements the raw requirements of the cookie, but this is usually used in a more friendly form by using a special children implementation such as Index.ArrayChildren. This implementation stores a list of children and makes it straightforward for the user to manipulate the order in several ways.

Cookie and action support

One important ability provided by the Nodes API is to associate cookies and actions with nodes. What cookies are and how to create them is explained in the Datasystems API Actions in general are likewise described by the Actions API.

(If your node is actually a DataNode representing a data object, there are already some conventions for attaching actions and cookies to the node, which prepopulate certain entries based on the data loader and/or data object. The Datasystems API describes these defaults.)

Attaching cookies to a node, so that it will be considered to implement certain behaviors, is quite straightforward. The basic interface for retrieving a cookie is Node.getCookie(...). However, this is abstract in Node, and also Node itself does not set any policy for settings up the cookies for a node or changing them.

Rather, if you are subclassing AbstractNode, you may use AbstractNode.setCookieSet(...) to specify a set of cookies to be returned by the node (and you should merge your cookies with those provided by the superclass, as a rule). The CookieSet is a simple container for cookies looked up by their representation class. The AbstractNode will then use this as an index for implementing getCookie(...).

To attach actions to a node, which are listed by Node.getActions() (and sometimes a primary and obvious action in Node.getDefaultAction()), you should merge the superclass' actions into your own (if desired), and override e.g. AbstractNode.createActions(), which is called to set up the actions list when getActions() is first called.

These actions may be used by various UI components to clearly associate commands with the node, e.g. by providing them in a pop-up menu. Node.getDefaultAction() and Node.getContextActions() provide more refined variants of the actions list which may be appropriate for different presentations. Nodes with unusual needs for action presentation can override Node.getContextMenu() to define a particular UI for this presentation.

Installing special nodes from modules

Frequently nodes will be created secondarily, especially as a result of being delegates to data objects (in which case their creation is under the control of the data loader pool). However, for some modules it is appropriate to specially install nodes into defined places in the system.

Currently, system gives you ability to automatically install a node of your choice into (currently three) common places in the IDE:

The basic definition of how settings in layers work is given in the Services API.

Special Node Usage

There are a few sorts of special operations and techniques which it may be useful to apply to nodes, either in the course of implementing a node or node hierarchy, or just using nodes from other code.

Serialization and traversal

If you need to store (serialize) a node for any reason, this is generally impossible due to the welter of Java-level references connecting it to the rest of the system. Rather, you must use a special serializable handle which represents the node by its position in the hierarchy, and permits finding the original node again after deserialization (if it still exists). To create a handle, just call Node.getHandle(), and to restore the node call Node.Handle.getNode().

Creation of a usable handle is implemented in AbstractNode, and you should not need to override it. However, note that a handle consists of a handle for the root node of the target node's hierarchy together with a path (by system name) down to the target node; so if you are creating a root node, and want it or its children to be serializable, then you should create a specific implementation of Node.Handle capable of reconstructing your root from scratch, and return it from Node.getHandle().

The methods in NodeOp such as NodeOp.findPath(...) may also be used for general-purpose navigation along the hierarchy, should this be necessary.

JavaBean bridging

It is possible to create a node which picks up its node behavior from an underlying JavaBean. That is, Bean introspection will be used to determine its properties (also categorizing them into normal, expert, and hidden property sets), find a customizer for it if the Bean specifies one, look for an icon from the BeanInfo, implement copying via serialization, create children according to Bean Context, and so on.

Since most of this behavior is automatic and driven by the JavaBeans API, you need do little to use it: just create a node using new BeanNode(...).

Do not confuse such a bean node, which may be any sort of node that just happens to use the JavaBeans API to implement its behavior, with the specific kind of node created to represent a data object whose file is found to be a JavaBean (serialized, or as a class) - this latter type of node behaves in most respects like any other data node, and just adds a couple of features like a Customize action.

Data object delegates

Many nodes serve primarily to represent a data object, which would otherwise be invisible to the user. While such data nodes may be customized like any other node, including creating node hierarchies (even for non-folder data objects), there are special considerations and supports for creating these. Please refer to the Datasystems API for details.

Filters and cloning

Under some circumstances, it may be useful to create a node which does nothing except serve as a sort of symbolic link to another primary node; this may be used when the visual organization of a hierarchy requires one object to appear in more than one place. In such a case, you may use new FilterNode(...) to create such a proxy.

Or, you may use AbstractNode.cloneNode() to create the filter if the node does not intrinsically support Cloneable, or to really clone it if it does. Note that a properly-designed node does not actually store real data, but just provides an interface to that data; and so it is reasonable to implement Cloneable to provide a new node attached to the same data, if that behavior is desired. Some nodes, such as DataNodes, do not do this, as such behavior would be contrary to the UI goal of having a data node live in one place in the Repository according to the position of the data object and primary file object.

Event model

Every interesting aspect of nodes may be listened to using the Java Event Model, as is routine in the IDE:

Edit operations on nodes

Nodes can support a variety of mechanisms for the basic edit operations.

Simple operations

There are some simple node-level operations which do not need to use data transfer. AbstractNode.setName(...) and Node.destroy() may simply be overridden to handle customized renames and deletes. (Or, you could attach a NodeListener to take action after the fact, if that suffices.)

Supporting creation of fresh children is possible by overriding Node.getNewTypes() to provide a list of new types of data which can be created under your node. Each of these should implement NewType.create() to actually create a new child. Make sure that you include NewAction in your list of actions.

Data transfer

By default, nodes provide hooks to permit other objects to receive them as pastes, and to permit other objects to paste to them, but do not provide any particular hookup between the two sides.

Certain standard subclasses of AbstractNode (such as the DataNode commonly used to represent data objects) already have special implementations of data transfer appropriate to your task (such as actually moving a file object to a new folder), which may eliminate the need to deal with it directly.

Flow of control during node data transfer
If you want to do more complex customization of node cut-copy-paste, or if you are debugging such an implementation, you will want to understand the flow of control, which is fairly subtle. This section will also be helpful for understanding the NetBeans data-transfer system in general.

This flow assumes a copy-and-paste operation. Cut-and-paste is rather similar (the source node would be destroyed rather than cloned, typically). Also, use of AbstractNodes is assumed; otherwise the nodes involved would have to implement more.

The scenario is that Node B permits other nodes to be pasted into it, creating shortcuts; the user wants to create a shortcut to some arbitrary Node A.

  1. The user selects node A. The action Copy is enabled (from a context menu, the Edit menu, etc.), because node A indicated it could be copied using Node.canCopy() (turned on in AbstractNode). Note that ExplorerUtils provides the regular implementation of CopyAction for any TopComponent.
  2. The user invokes the Copy action. AbstractNode.clipboardCopy() is called. It creates a transferable supporting only one flavor, which is invisible to the APIs. The creation of this transferable is done by a special utility method which hides the data flavor and transferables contents: NodeTransfer.transferable(...). The copy action sets that transferable to the clipboard.
  3. Some time later, the user selects node B, a node capable of holding children. One effect of the change in selection is that the Paste action checks to see if it should be enabled. To do so, it checks node B's AbstractNode.getPasteTypes(...), which in turns calls AbstractNode.createPasteTypes(...) to do the work.

    Now, AbstractNode's implementation of createPasteTypes(...) only allows one data flavor to be accepted by the node (so-called "intelligent pastes"); this flavor is hidden from the APIs but can be tested for in a transferable using NodeTransfer.findPaste(Transferable). This is not the flavor that was provided by the copy, so no paste type is created in the super method. However, Node B in this example was specifically expecting to get copied nodes pasted into it, so it overrode createPasteTypes(...) like this:

    public class Shortcuts extends AbstractNode {
      public Shortcuts () {
        super (new Index.ArrayChildren ());
        setName ("Shortcuts");
        getCookieSet ().add (ch);
      protected SystemAction[] createActions () {
        return new SystemAction[] {
          SystemAction.get (ReorderAction.class),
          SystemAction.get (PasteAction.class)
      protected void createPasteTypes(Transferable t, List ls) {
        final Node[] ns = NodeTransfer.nodes (t, NodeTransfer.COPY);
        if (ns != null) {
          ls.add (new PasteType () {
            public Transferable paste () throws IOException {
              Node[] nue = new Node[ns.length];
              for (int i = 0; i < nue.length; i++)
                nue[i] = ns[i].cloneNode ();
              getChildren ().add (nue);
              return null;
        // Also try superclass, but give it lower priority:
        super.createPasteTypes(t, ls);
    Nothing is actually pasted yet. However, one paste type, that provided by Node B, has been added to the set of paste types. So, the Paste action sees that there is an option to paste, and provides a context menu item (by default labelled "Paste"), enables the toolbar button, etc.
  4. The user selects this action. The paste type's paste() method is actually called, making an alias of Node A and inserting it as one of B's children. The method returns null, so the clipboard is left alone.
Directions of implementation
The data transfer process, here looking at nodes, may be implemented from various directions:

UML Diagrams

First class diagram listed shows structure of the API in general, all other diagrams provides specialized view of specific section of the API.

General node structure class diagram

general UML

Node children class diagram

children UML

Node events class diagram

events UML

Node ordering class diagram

ordering UML

Node properties class diagram

properties UML