Lookup faq:

How to specify that a service in Lookup should be available only on Windows?

How can I specify (in the xml, or programmatically) that this service should only be added to the Lookup if the platform is Windows? >

• It is possible to write a specific module and enable it only on windows. See os specific modules documentation. Then you can put a registration of your instance into your module's META-INF/services directory and it will be available only on Windows.

• Another possibility that does not require new module, but which executes a code on startup (which may have performance implications) is to use methodvalue attribute. Register your instance in layer using your-Object.instance file as described at services documentation and in your factory method either return the instance your want or null depending on result of Utilities.isWindows() call.

• In some cases, the interface for which you will register an implementation permits a no-operation semantics. For example, InstalledFileLocator.locate(...) can return a valid File, or null. You could always register an InstalledFileLocator instance yet disable it on non-Windows platforms (always returning null).

How shall I write an extension point for my module?

Q: I have more modules one of them providing the core functionality and few more that wish to extend it. What is the right way to do it? How does the Netbeans platform declare such extension point?

Start with declaring an extension interface in your core module and put it into the module's public packages. Imagine for example that the core module is in JAR file org-my-netbeans-coremodule.jar and already contains in manifests line like OpenIDE-Module: org.my.netbeans.coremodule/1 and wants to display various tips of the day provided by other modules and thus defines:

 

package org.my.netbeans.coremodule;

public interface TipsOfTheDayProvider {
    public String provideTipOfTheDay ();
}

And in its manifest adds line OpenIDE-Module-Public-Packages: org.my.netbeans.coremodule.* to specify that this package contains exported API and shall be accessible to other modules.

When the core module is about to display the tip of the day it can ask the system for all registered instances of the TipsOfTheDayProvider, randomly select one of them:

import java.util.Collection;
import java.util.Collections;
import org.openide.util.Lookup;

Lookup.Result result = Lookup.getDefault ().lookup (new Lookup.Template (TipsOfTheDayProvider.class));
Collection c = result.allInstances ();
Collections.shuffle (c);
TipsOfTheDayProvider selected = (TipsOfTheDayProvider)c.iterator ().next ();

and then display the tip. Simple, trivial, just by the usage of Lookup interface once creates a registry that other modules can enhance. But such enhancing of course requires work on the other side. Each module that would like to register its TipsOfTheDayProvider needs to depend on the core module - add OpenIDE-Module-Module-Dependencies: org.my.netbeans.coremodule/1 into its manifest and write a class with its own implementation of the provider:

package org.my.netbeans.extramodule;

class ExtraTip implements TipsOfTheDayProvider {
    public String provideTipOfTheDay () {
        return "Do you know that in order to write extension point you should use Lookup?";
    }
}

Then, the only necessary thing is to register such class by using the J2SE standard into plain text file META-INF/services/org.my.netbeans.coremodule.TipsOfTheDayProvider in the module JAR containing just one line:

org.my.netbeans.extramodule.ExtraTip

and your modules are now ready to communicate using your own extension point.