Treat Kotlin nullable as non-required

[rocketraman]

Modify the core MethodParameter class to return the Kotlin nullability
of a parameteter (cached for performance).

Where isOptional is used, also check for isKotlinNullable i.e.
values are not considered required if they are Kotlin nullables:

• spring-messaging: named value method arguments
• spring-web: named value method arguments
• spring-webmvc: request parts

This means that Kotlin client code no longer has to explicity specify
"required=false" for Kotlin nullables -- this information is inferred
automatically by the framework.

Issue: SPR-14165

[rocketraman]

Notes on submission / pending work:

• I put the kotlin-specific code into a separate module spring-core-kotlin to avoid adding the kotlin compiler to the spring-core module (inspired from spring-beans-groovy).
• I wasn't sure where to add tests for this. My first though was MethodParameterTests, but currently it contains very little in the way of tests. Guidance is appreciated.

Otherwise the implementation turned out to be quite simple and concise.

[sdeleuze]

Thanks for this PR, I only had a quick look but I would be in favor of not creating a new spring-core-kotlin, instead you could add kotlin dependency as an optional one. After that I will make a detailed review of your PR.

[rocketraman]

Thanks for this PR, I only had a quick look but I would be in favor of not creating a new spring-core-kotlin, instead you could add kotlin dependency as an optional one. After that I will make a detailed review of your PR.

Done!

[rocketraman]

NOTE: Another place we could use this functionality is when @Inject or @Autowired is used, the Kotlin nullable information could be used to determine if the value is required or not.

[rocketraman]

@sdeleuze Ping. Looks like it needs another rebase, but would appreciate a review.

[sdeleuze]

Let's target 5.0 for this feature

[sdeleuze]

Sorry for the delay and thanks for your updated PR. I have added comments on the relevant lines of your commit.

Could you also rebase that on top of the new master (Spring Framework 5.0) and add some Kotlin unit tests to check that works?

Good idea to support that also for @Inject, feel free to add that to your PR and update the title of this JIRA issue to make is less specific.

[rocketraman]

Good idea to support that also for @Inject, feel free to add that to your PR and update the title of this JIRA issue to make is less specific.

It looks more complicated to do this [1], so I'd prefer to have this pull request reviewed as-is for now, just to keep things as simple as possible. We can add the @Inject functionality in a subsequent pull.

Sorry for the delay and thanks for your updated PR. I have added comments on the relevant lines of your commit.

Could you also rebase that on top of the new master (Spring Framework 5.0) and add some Kotlin unit tests to check that works?

I have updated the pull with all of your comments taken into account. Thanks!

[1] It looks like a variation of RequiredAnnotationBeanPostProcessor is needed.

[rocketraman]

Ping @sdeleuze for review.

[sdeleuze]

Superseeded by #1242