Add a Kotlin Serialization entry

[sdeleuze]

As a follow-up of spring-projects/spring-boot#46546, after Spring Boot 4.0.0.M3 has been released, would it be possible to add a Kotlin Serialization entry on start.spring.io for Spring Boot 4+ project using Kotlin language?

When this entry is selected, spring-boot-starter-kotlin-serialization would be added to the dependencies and spring.http.converters.preferred-json-mapper=kotlin-serialization would be set in the generated application.properties.

This would be a very valuable improvement for Kotlin DevXP.

[bclozel]

Setting the property is rather unusual. At least we don't do that for "spring-boot-starter-gson" nor spring-boot-starter-jsonb"

[snicoll]

Also, unless I've missed something the entry makes no sense if the project isn't Kotlin-based. That would be a first as far as I remember.

If this was to be added then I wonder why we didn't add gson and jsonb. To me, the right way to do this would be to exclude Jackson to replace it with such an entry. Unfortunately we can't do that with certain entries requiring Jackson.

[mhalbritter]

This might actually be a shortcoming of the Spring Boot starters. I'd expect that, if I add spring-boot-starter-gson or spring-boot-starter-jsonb or spring-boot-starter-kotlin-serialization that Spring Boot will use Gson / JSON-B / Kotlin Serialization as the preferred JSON mapper. I'll open a Spring Boot issue to discuss that with the team after some experimentation.

[bclozel]

Now that we have separate starters, the issue is a bit different, but if I remember correctly lots of apps are getting gson or kotlin serialization on their classpath without them opting in.

[mhalbritter]

It appears that Kotlin Serialization works only with Kotlin types. I don't think we can grey out a dependency based on the chosen language.

And it also needs a Kotlin Compiler plugin to work: https://kotlinlang.org/docs/serialization.html#example-json-serialization

Here's the same for Maven: https://github.com/Kotlin/kotlinx.serialization?tab=readme-ov-file#maven

[mhalbritter]

After some changes on Framework/Boot side, having org.springframework.boot:spring-boot-starter-kotlin-serialization on the classpath is now enough, there's no need to set a property to get kotlin serialization.