如何创建具有可选软件包分辨率的有效OSGi捆绑包？

Question

I would like my bundle to be either configurable via System properties or via the OSGi Compendium Config Admin.

I am compiling my bundle against the org.osgi:osgi.cmpn:6.0.0 bundle, which as made clear in the OSGi Alliance blog is meant to be used at compile-time only, with the framework providing the actual implementations at runtime.

My code obviously needs to use the ConfigAdmin package (to handle the case in which the ConfigAdmin Service is present)... which means that if the runtime does not export the ConfigAdmin package, my bundle will not resolve properly.

But I wanted this resolution to be optional... so I added this to the manifest:

org.osgi.service.cm;resolution:=optional;version="[1.5,2)"

Now, the bundle will resolve but will crash at runtime with a java.lang.NoClassDefFoundError: org/osgi/service/cm/ManagedService even if the user will not actually use ConfigAdmin for configuration. So this forces the user to install the config-admin bundle just to make my bundle work.

I guess the secret here is to not instantiate any classes that force the JVM to load a class that uses the org.osgi.service.cm package... but I can't see how I can achieve that without ugly hacks with reflection...

Does anyone know how I can check if the package is available at runtime, and if not, avoid the java.lang.NoClassDefFoundError at runtime, making this package dependency truly optional?

Answer 1

The way to tell if the package is available at runtime is to attempt to load a class from it and be prepared for the NoClassDefFoundError. You could do this in a central place and then decide to avoid code paths which require the optional but absent package.

Answer 2

The PackageAdmin can be used to inspect package metadata of the system. It has been deprecated and you are expected to use the BundleWiring instead. So, in your DS, you would have a dependency to the PackageAdmin/BundleWiring and check if the cm package is exported...