Allow ConfigurationProcessor to work with Kotlin Symbol Processor

[kkocel]

The problem:
Currently, ConfigurationProcessor is backed by Annotation Processing. In Kotlin annotation processors can be used thanks to KAPT. Sadly KAPT does not work with JDK 16+ ( https://youtrack.jetbrains.com/issue/KT-45545 ). For Kotlin there are other processors like KSP ( https://github.com/google/ksp ), yet they do not rely on Java Annotation processing.

Proposed solution:
Provide an abstraction layer on ConfigurationMetadataAnnotationProcessor that will allow plugging in other processors.
Additionally provide an adapter for KSP.

@sdeleuze

[wilkinsona]

Thanks for raising this.

Sadly KAPT does not work with JDK 16+ ( https://youtrack.jetbrains.com/issue/KT-45545).

My reading of that Kotlin issue is that KAPT does work with JDK 16+, but that it requires adding some JVM arguments to open up some of the JVM's internals to KAPT. Have I misunderstood the discussion?

[kkocel]

@wilkinsona You understood it better than I :) Yes, it's possible to break javac encapsulation using add-opens flags, but ihmo it's temporary (this flag can be taken away as with --illegal-access=permit in JDK 17).

I know that adding an abstraction layer over processors can add in additional complexity, but it can also be a safer solution (also please note that KSP authors claim that it's up to 2x faster than java annotation processors).

As with everything you should weigh your options as Spring maintainers.

[wilkinsona]

The underlying problem with KAPT has now been fixed and will be part of the Kotlin 1.6 release. Rather than supporting a third-party alternative which, to me at least, has an unclear relationship with Kotlin and JetBrains, I think we should focus our efforts on working via KAPT. Thanks anyway for the suggestion.

[bespaltovyj]

Hi, @wilkinsona.
The new Kotlin documentation says that kapt is in maintenance mode and new features are not planned.
And recommend using ksp(it should work faster).

Will you support ksp after this news?
If you don't mind, then I can start adding ksp support.

[kkocel]

@wilkinsona I asked about the relation of KSP and Kotlin: https://twitter.com/relizarov/status/1455452359669264384

So it looks like KSP is the way to go for Kotlin.