Support dynamic classloader-hook registration by framework extensions

[HannesWell]

As discussed in

• Implement OSGi Service Loader Mediator Specification #295 (comment)

being able to register classloader-hooks dynamically in the extension activator avoids the need to add the extension to the boot-classpath via the osgi.framework.extensions system-property.

Immediately use it in equinox.spi.

[github-actions]

Test Results

  234 files  ±0    234 suites  ±0   49m 45s ⏱️ - 1m 31s
2 231 tests ±0  2 182 ✅ ±0   49 💤 ±0  0 ❌ ±0 
6 456 runs  ±0  6 343 ✅ ±0  113 💤 ±0  0 ❌ ±0 

Results for commit 78c5916. ± Comparison against base commit 8294c6b.

♻️ This comment has been updated with latest results.

[tjwatson]

I pushed a change to use ActivatorHookFactory to manage the tracker instance instead.

[tjwatson]

One subtle thing I changed in my commit is to not restrict the registrations to only when the framework is STARTING. I believe you will want that for the spi implementation when it is installed and resolved by p2. When a framework extension is resolved after framework start then it will dynamically call the extensions bundle activator.

[HannesWell]

Thank you Tom for the update. The ActivatorHookFactory was the piece I missed and that makes it smoother.

One subtle thing I changed in my commit is to not restrict the registrations to only when the framework is STARTING. I believe you will want that for the spi implementation when it is installed and resolved by p2. When a framework extension is resolved after framework start then it will dynamically call the extensions bundle activator.

OK, that's fine for me as well. I assumed that P2 would require a framework restart anyway. But at the same time I'm not sure there are no cases where it's not happening later.

I squashed your commit into the first one and made a few more adjustments, mainly in the ServiceLoaderMediatorHookConfigurator and ServiceLoaderMediatorHook to initialize it more directly.

With that, this is ready from my POV.

[tjwatson]

LGTM

[HannesWell]

Just noticed that the hookconfigurators.properties file is now obsolete as well and removed it.

Thanks again for your review and help.