NetBeans Selection Management Tutorial I—Using a TopComponent’s Lookup

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This tutorial covers how to write components that provide a "selected object" and how to write components that update themselves as the global selection changes.

Optionally, for troubleshooting purposes, you can download the completed sample.

Introduction to Selection

"Selection" is an important concept for any non-trivial application. NetBeans has two basic concepts of selection—the contents of the focused ` TopComponent's ` Lookup, and the focused TopComponent’s activated `Node(s). Here you will deal only with the Lookup portion of selection—doing more advanced things will be covered in a later tutorial.

Selection is used to make possible such things as context sensitive actions (actions that are enabled or disabled depending on what is displayed), and palette windows such as the Property Sheet or Navigator components in the IDE, which each display some aspect of whatever is selected.

Basically, each TopComponent has a bag of objects that it can put things into, and which other code is able to query. That bag of objects is its Lookup—essentially a Map where the keys are class objects and the values are objects that extend or implement the key-class. The thing that makes this approach tremendously useful is the ability to use this mechanism to decouple the components that provide some object and the components that consume those objects—so they can be implemented in separate modules, or new editors for old objects can be provided and the rest of the system will continue to work transparently.

Creating the Module Suite and Projects

The example for this tutorial will contain three modules, contained within a module suite, as illustrated below:

selection 1 project setup 60

Start by creating the module suite to contain all three modules:

  1. Choose File > New Project (Ctrl-Shift-N). Under Categories, select NetBeans Modules. Under Projects, select Module Suite Project and click Next.

  1. In the Name and Location panel, type SelectionSuite in Project Name. Change the Project Location to any directory on your computer. Click Finish.

  1. Choose File > New Project (Ctrl-Shift-N) again. Under Categories, select NetBeans Modules. Under Projects, select Module Project and click Next.

  1. In the Name and Location panel, type MyAPI in Project Name. The default in the wizard should be to create the module underneath the directory where you just created the suite, which is fine. Click Next.

  1. In the Basic Module Configuration panel, replace yourorg in Code Name Base with myorg , so that the whole code name base is org.myorg.myapi . Add spaces to the default Module Display Name, so that it is changed to My API . Leave the location of the localizing bundle and XML layer, so that they will be stored in a package with the name org/myorg/myapi . Click Finish.

  1. You’re going to create two more modules now—follow the same steps above, but using the names "MyEditor" and "MyViewer". The reason you are creating three modules will be clear as you continue.

Creating an API and Setting Up Dependencies

What you’re going to do here is create a trivial API class. In the real world, such an API might represent files or some other kind of data that is being modelled programmatically. For the purposes of this tutorial it will suffice to have simple objects with a couple of properties.

  1. Right click the org.myorg.myapi package and choose New > Java Class, as shown below:

selection 1 create java class 60
  1. Name the class APIObject.

  1. Replace the default code with the following:

public final class APIObject {

   private final Date date = new Date();
   private static int count = 0;
   private final int index;

   public APIObject() {
      index = count++;

   public Date getDate() {
      return date;

   public int getIndex() {
      return index;

   public String toString() {
       return index + " - " + date;


This will be all of the code that this module contains. As you can see, each time a new instance of APIObject is created, a counter is incremented—so there will be some unique attribute to each instance of APIObject.

  1. The next step is to have your API module export the org.myorg.myapi package so other modules can see classes in it. Right click the My API project and choose Properties.

1. In the API Versioning page in the Project Properties dialog box, check the checkbox for org.myorg.api in the Public Packages list, shown below:

selection 1 export package 60
  1. Now you need to set up some dependencies between your modules. The other two modules, My Editor and My Viewer, will use the APIObject class, so each of them needs to say that they depend on the API module. For each of the other module projects in turn, right click the project node and choose Properties.

1. In the Libraries page of each Project Properties dialog box, click the Add Dependency button. In the dialog box that pops up, type APIObject—there should be only one match, which is your API module. Select it and click OK to add the dependency.

selection 1 add deps 60

Creating the Viewer Component

Now you will create a singleton component that will track if there is an APIObject available in the global selection (i.e., if the focused TopComponent has one in its Lookup). If there is one, it will display some data about it. One common use case for this sort of thing is creating master/detail views.

A "singleton component" is a component like the Projects window in the NetBeans IDE, or the Property Sheet or the Navigator—a component that there is only ever one of in the system. The Window Component wizard will automatically generate all of the code needed to create such a singleton component—you just have to use the form designer or write code to provide the contents of your singleton component.

  1. Right click the org.myorg.myviewer package and choose New > Other.

  1. In the resulting dialog, select Module Development > Window Component and click Next (or press Enter).

1. On the "Basic Settings" page of the wizard, select navigator as the location in which to place your viewer component, and check the checkbox to open the component on startup, as shown below:

selection 1 new window 60
  1. Click Next to continue to the "Name, Icon and Location" page of the wizard.

  1. On the following page, name the class MyViewer and click Finish (or press Enter).

You now have a skeleton TopComponent—a singleton component called MyViewerTopComponent. Click the editor tab for MyViewerTopComponent—the form editor should be visible. You will add two labels to the component, which will display some information about the selected APIObject if there is one.

  1. Drag two JLabels to the form from the Palette, one below the other.

selection 1 viewer form editor

Change the text of the first as shown above, so that by default it displays "[nothing selected]".

  1. Click the Source button in the editor toolbar to switch to the code editor

  1. Modify the signature of the class, so that MyViewerTopComponent implements LookupListener:

public class MyViewerTopComponent extends TopComponent implements LookupListener {
  1. Right-click in the editor and choose Fix Imports, so that LookupListener is imported.

1. Put the caret in the signature line as shown below. A lightbulb glyph should appear in the editor margin. Press Alt-Enter, and then Enter again when the popup appears with the text "Implement All Abstract Methods". This will add the LookupListener method to your class.

selection 1 implement methods
  1. You now have a class that implements LookupListener. Now it needs something to listen to. In your case, there is a convenient global Lookup object, which simply proxies the Lookup of whatever component has focus—it can be obtained from the call Utilities.actionsGlobalContext(). So rather than tracking what component has focus yourself, you can simply listen to this one global selection lookup, which will fire appropriate changes whenever focus changes. Edit the source code so that it contains the following methods as shown here:

    private Lookup.Result result = null;
    public void componentOpened() {
        Lookup.Template tpl = new Lookup.Template (APIObject.class);
        result = Utilities.actionsGlobalContext().lookup(tpl);
        result.addLookupListener (this);

    public void componentClosed() {
        result.removeLookupListener (this);
        result = null;

    public void resultChanged(LookupEvent lookupEvent) {
        Lookup.Result r = (Lookup.Result) lookupEvent.getSource();
        Collection c = r.allInstances();
        if (!c.isEmpty()) {
            APIObject o = (APIObject) c.iterator().next();
            jLabel1.setText (Integer.toString(o.getIndex()));
            jLabel2.setText (o.getDate().toString());
        } else {
            jLabel1.setText("[no selection]");
            jLabel2.setText ("");

componentOpened() is called whenever the component is made visible by the window system; componentClosed() is called whenever the user clicks the X button on its tab to close it. So whenever the component is showing, you want it to be tracking the selection—which is what the above code does.

The resultChanged() method is your implementation of LookupListener. Whenever the selected APIObject changes, it will update the two `JLabel`s you put on the form.

Creating the Editor Component

Now you need something to actually provide instances of APIObject, for this code to be of any use. Fortunately this is quite simple.

You will create another TopComponent, this time, one that opens in the editor area and offers an instance of APIObject from its Lookup. You could use the Window Component template again, but that template is designed for creating singleton components, rather than components there can be many of. So you will simply create a TopComponent subclass without the template, and an action which will open additional ones.

  1. You will need to add three dependencies to the My Editor module for it to be able to find the classes you will be using. Right click the My Editor project and choose Properties. On the Library page of the Project Properties dialog box, click the Add Dependency button, and type TopComponent. The dialog should automatically suggest setting a dependency on the Window System API. Do the same thing for Lookups (Utilities API).

  1. Right-click the org.myorg.myeditor package in the My Editor project, and choose New > JPanel Form.

  1. Name it "MyEditor", and finish the wizard.

  1. When the form editor opens, drop two JTextFields on the form, one above the other. On the property sheet, set the Editable property (checkbox) to false for each one.

  1. Click the Source button in the editor toolbar to switch to the code editor.

  1. Change the signature of MyEditor to extends TopComponent instead of javax.swing.JPanel:

public class MyEditor extends TopComponent {
  1. Add the following code to the constructor of MyEditor:

APIObject obj = new APIObject();
associateLookup (Lookups.singleton (obj));
jTextField1.setText ("APIObject #" + obj.getIndex());
jTextField2.setText ("Created: " + obj.getDate());
setDisplayName ("MyEditor " + obj.getIndex());

Right-click in the editor and choose Fix Imports.

The line associateLookup (Lookups.singleton (obj)); will create a Lookup that contains only one object—the new instance of APIObject—and assign that Lookup to be what is returned by MyEditor.getLookup(). While this is an artificial example, you can imagine how APIObject might represent a file, an entity in a database or anything else you might want to edit or view. Probably you can also imagine one component that allowed you to select or edit multiple unique instances of APIObject—that will be the subject of the next tutorial.

To make your editor component at least somewhat interesting (though it doesn’t actually edit anything), you set the text fields' values to values from the APIObject, so you have something to display.

Opening Editor Components

Now you need a way to open MyEditor components in the editor area, so that there will be something to show. To do anything meaningful with selection, you will need more than one editor so that there is more than one APIObject to track. Since you will want multiple editors, you need a simple action on the main menu which will create and open another instance of MyEditor in the window system (as opposed to what the Window Component template would create for us, which is an action that always looks up a singleton component such as the Navigator or Property Sheet components in the IDE).

  1. Right click the org.myorg.myeditor package and choose New > Other.

  1. In the dialog, choose Module Development > Action and click Next.

  1. Accept the defaults ("always enabled") and press Next again.

  1. On the GUI Registration page, accept the defaults by pressing Next again (this will cause your action to appear at the top of the File menu).

  1. On the final page of the wizard, name the action OpenEditorAction and set its display name to say "Open Editor".

  1. Press Finish to generate the action class.

  1. The code editor should now be open over a class called OpenEditorAction, which subclasses CallableSystemAction (a NetBeans subclass of javax.swing.Action which lets you associate context sensitive help with an action). Add the following code to its performAction() method:

MyEditor editor = new MyEditor();;

The above code will simply create a new instance of MyEditor (which in turn will create a new instance of APIObject and put it in its Lookup) and open it in the window system.

Running the Code

Now you’re ready to run the tutorial. Simply right click SelectionSuite, the module suite which owns your three modules, and choose Run from the popup menu. When the IDE opens, simply choose File > Open Editor—invoke your action. Do this a couple of times, so that there are several of your editor components open. Your singleton MyViewer window should also be open. Notice how the MyViewer window’s contents change as you click different tabs, as shown here:

selection 1 result 1 60

If you click in the Projects window, notice that the text changes to "[No Selection]", as shown below:

selection 1 result 2 60

If you do not see the MyViewer window, you probably did not check the checkbox in the wizard to open it on system start—simply go to the Window menu and choose Open MyViewer Window to display it.

So, What’s the Point?

You might be wondering what the point of this exercise is—you’ve just shown that you can handle selection—big deal! The key to the importance of this is the way the code is split into three modules: The My Viewer module knows nothing about the My Editor module—either one can run by itself. They only share a common dependency on My API. That’s important—it means two things: 1. My Viewer and My Editor can be developed and shipped independently, and 2. Any module that wants to provide a different sort of editor than My Editor can do so, and the viewer component will work perfectly with it, as long as the replacement editor offers an instance of APIObject from its Lookup.

To really picture the value of this, imagine APIObject were something much more complex; imagine that MyEditor is an image editor, and ` APIObject` represents an image being edited. The thing that’s powerful here is that you could replace MyEditor with, say, an SVG vector-based editor, and the viewer component (presumably showing attributes of the currently edited image) will work transparently with that new editor. It is this model of doing things that is the reason you can add new tools into the NetBeans IDE that work against Java files, and they will work in different versions of NetBeans, and that you can have an alternate editor (such as the form editor) for Java files and all the components and actions that work against Java files still work when the form editor is used.

This is very much the way NetBeans works with Java and other source files— in their case, the thing that is available from the editor’s Lookup is a ` DataObject, and components like Navigator and the Property Sheet are simply watching what object is being made available by the focused `TopComponent.

Another valuable thing about this approach is that often people are migrating existing applications to the NetBeans platform. The object that is part of the data model, in that case, is probably existing, working code that should not be changed in order to integrate it into NetBeans. By keeping the data model’s API in a separate module, the NetBeans integration can be kept separate from the core business logic.

Changing Selected Objects on the Fly

To make it really evident how powerful this approach can be, you’ll take one more step, and add a button to your editor component that lets it replace the APIObject it has with a new one on the fly.

  1. Open MyEditor in the form editor (click the Design toolbar button in the editor toolbar), and drag a JButton to it.

  1. Set the text property of the JButton to "Replace".

  1. Right click the JButton and choose Events > Action > actionPerformed. This will cause the code editor to open with the caret in an event handler method.

  1. At the head of the class definition, you will add one final field:

public class MyEditor extends TopComponent {
    private final InstanceContent content = new InstanceContent();

InstanceContent is a class which allows us to modify the content of a Lookup (specifically an instance of AbstractLookup) on the fly.

  1. Copy all of the lines you added earlier to the constructor to the clipboard, and delete them from the constructor, except for the line beginning "associateLookup…​". That line of the constructor should be changed as follows:

associateLookup (new AbstractLookup (content));
  1. You will be using the lines that you put on the clipboard in the action handler for the JButton—so you should run this code once when you first initialize the component. Add the following line to the constructor, after the line above:

jButton1ActionPerformed (null);
  1. Modify the event handler method so it appears as follows, pasting from the clipboard and adding the line at the end:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
    APIObject obj = new APIObject();
    jTextField1.setText ("APIObject #" + obj.getIndex());
    jTextField2.setText ("Created: " + obj.getDate());
    setDisplayName ("MyEditor " + obj.getIndex());
    content.set(Collections.singleton (obj), null);
  1. Right-click in the editor and choose Fix Imports.

You’re now ready to run the suite again. Right click SelectionSuite again and choose Run. Notice how, now, when you click the Replace button, all of the components update, including the instance of MyViewer—everything.

selection 1 first run 60

Providing More Than One Object

This is all well and good for decoupling, but isn’t providing this one object from your component a bit like having a Map that only contains one key and one value? The answer is, yes, it is like that. Where this technique becomes even more powerful is when you provide multiple objects from multiple APIs.

As an example, it is very common in NetBeans to provide context sensitive actions. A case in point is the built-in SaveAction that is part of NetBeans' Actions API. What this action actually does is, it simply listens for the presence of something called SaveCookie on the global context— the same way your viewer window listens for APIObject. If a SaveCookie appears (editors typically add one to their lookup when the content of the file is modified but not yet saved), the action becomes enabled, so the Save toolbar button and menu items become enabled. When the Save action is invoked, it calls, which in turn causes the SaveCookie to disappear, so the Save action then becomes disabled until a new one appears.

As you may have noticed, context sensitivity was one option in the New Action wizard. The actions currently generated by the wizard actually use a way of doing this that pre-dates Lookup; the Lookup-based way of doing such context sensitive actions is described in the developer FAQ.

So the pattern in practice is to provide more than just a single object from your component’s Lookup—different auxilliary components and different actions will be interested in different aspects of the object being edited. These aspects can be cleanly separated into interfaces which those auxilliary components and actions can depend on and listen for.

Miscellaneous Things Worth Noticing

While not directly related to the topic of this tutorial, it’s worth noticing that if you open three MyEditor instances, and shut down and restart NetBeans, you end up with three MyEditor instances magically appearing on restart. By default, your editors are serialized to disk on shutdown and restored on restart.

If you do not want this behavior, there are two other choices. Override the following method on MyEditor to cause editors never to be reopened on restart:

public int getPersistenceType() {

If you want to persist the components that are open but discard those that have been closed, return PERSISTENCE_ONLY_OPENED instead. The default (for backward compatibility reasons) is PERSISTENCE_ALWAYS, which is not appropriate for editor-style componenents—it means that even editors that have been closed are kept forever and reloaded on restart.

Note, though, that part of what is serialized to disk is the location of your component in the main window. So singleton TopComponents`s such as the property sheet, or our viewer component, should use `PERSISTENCE_ALWAYS - otherwise if they are closed once by the user, the next time they are opened they will appear in the editor area instead of where they are supposed to be.

Optional Clean-Up

The module template, by default, assumes you will want to use the layer.xml file to install objects. In the case of the My API module, it is actually not used. So a polite thing to do to get a slight improvement in startup time would be the following:

  1. Expand the Important Files node of the My API project

  1. Double click the Module Manifest node

  1. Delete the following line from the manifest:

OpenIDE-Module-Layer: org/myorg/myapi/layer.xml
  1. Then delete the corresponding layer.xml file in org.myorg.myapi

Next Steps

By now you may have noticed that some components have more granular selection logic, and even involve multiple selection. In the next tutorial you will cover how to use the Nodes API to handle that.