What are the ways a module can influence the system?

There are four basic ways a module can install configuration data or objects. Three of the ways are declarative (DevFaqModulesDeclarativeVsProgrammatic); these mechanisms are preferred.

If you are writing a module that has an API you want other modules to plug in to, you probably want to read DevFaqWhenToUseWhatRegistrationMethod.


For global services, singletons and such, using this annotation is the preferred technique.

What exactly you register is a contract between you and whatever module is providing the interface and will, presumably, do something with what you put there. What's really happening is that you are adding your implementation of this interface to the default Lookup. At build-time, registration files are generated into META-INF/services in your module's JAR file. The default lookup (or JDK 6's ServiceLoader) knows how to read such files. Typically the classes need to be public and have a public no-argument constructor.

Any module can specify interfaces or classes that it would like other modules to implement and register instances of. For example, the Project API module asks that each module that implements a project type (the things you see in the New Project wizard in NetBeans) register their ProjectFactorys in default lookup.

To get an instance of something registered this way, call

TheInterface i = Lookup.getDefault().lookup(TheInterface.class);

If there might be more than one registered object of this type, you can get them all as follows:

for (TheInterface i : Lookup.getDefault().lookupAll(TheInterface.class)) {...}

Registering objects in the System Filesystem

The system filesystem (see DevFaqSystemFilesystem) allows for more detailed configuration when registering objects. It is a virtual filesystem composed of XML fragments (see DevFaqModulesLayerFile) from modules in the system. The top layer of the system filesystem is $USERDIR/config which is where changes that are written at runtime are put.

The system filesystem is composed of folders. Some folders have special meanings to the system; which folders exist and are meaningful depends on which modules you have installed. For example, the core window system defines the folder Menu/, which contains one subfolder for each menu in the main window's menu bar. If you add a file to the folder Menu/File called com-foo-mymodule-MyAction.instance, an instance of will be created, and a menu item will be put on the menu for it.

For more details on registering objects, defining an order in which they should appear, etc., see DevFaqModulesLayerFile. In the short form, a module registers a layer by including a line in its manifest:

OpenIDE-Module-Layer: com/foo/mymodule/resources/layer.xml

which points to an actual XML file by that name inside the module JAR file. A layer file is an XML file defining a mini-filesystem:

    <folder name="SomeFolder">
        <file name="SomeFile"/>

More and more layer registrations can be made by using various source code annotations. If you use these exclusively, you will not need to declare a layer in your module's sources at all.

Registering objects in the module's manifest

Some types of objects used to be installed by adding a section to the module manifest. This is now deprecated.

Programmatic registration - ModuleInstall classes

The module system allows you to provide a ModuleInstall class, which runs some code during startup or when the module is loaded, and can run cleanup code when it is uninstalled or disabled. This is the least desirable way to do things, because running code on startup means slowing down startup. Before you use such a class, be sure there is no declarative way to do what you're trying to do; see: DevFaqModulesDeclarativeVsProgrammatic

To have some code run on startup/installation/uninstallation/etc., add a line like the following to your module's manifest file:

OpenIDE-Module-Install: org/netbeans/modules/paintcatcher/PaintCatcherModule.class

This line should be part of the group of lines at the top of the manifest, with no blank lines before it. It is a pointer to a class file inside the module. The class file must extend the class org.openide.modules.ModuleInstall. There is a wizard in the development support to create and register such a class for you.

Applies to: NetBeans 6.7 and later

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