KAFKA-8410: Migrating stateless operators to new Processor API

[jeqo]

Migrating KStream stateless operators to new Processor API, first. Following PRs will complete migration of KStream stateful operators and KTable.

Testing strategy: Keep the current tests green.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[vvcephei]

Thanks so much for picking this up, @jeqo !

It looks great to me, although I'd like to tighten the screws slightly on AbstractProcessor and the generic parameters when we don't need to forward anything.

[vvcephei]

Just FYI, @jeqo , during my POC, I tried to fix this right away, and it dragged me into migrating the entire DSL at once. I think we should do what you are doing instead: just migrate the individual processors first, and then come back and drop the unsafe casts in a later PR.

[vvcephei]

One minor thing that bugged me about the old PAPI was the AbstractProcessor class. It was widely used in places where it provided no value. I'm worried that we will just perpetuate here.

Since the new API has default implementations for init and close, the only value this new abstract class provides is when the subclass needs the context. In that case, we save them from the boilerplace of saving off the context in a field.

Therefore, I'd like to call this class ContextualProcessor instead of AbstractProcessor. This should make it a little harder to depend on this class unnecessarily.

Suggested change

	public abstract class AbstractProcessor<KIn, VIn, KOut, VOut> implements Processor<KIn, VIn, KOut, VOut> {
	public abstract class ContextualProcessor<KIn, VIn, KOut, VOut> implements Processor<KIn, VIn, KOut, VOut> {

[jeqo]

Really like this suggestion. Thanks @vvcephei!

[vvcephei]

Thanks, @jeqo !

[vvcephei]

Looks like the builds failed due to a checkstyle problem.

FYI, you can run ./gradlew :streams:testAll to run all the tests in all the Streams modules, as well as checkstyle and findbugs.

[vvcephei]

LGTM! Thank you, @jeqo .