How can I configure Spring Webflux to use Kotlinx Serialization over Jackson, even though both are on classpath? I can't exclude Jackson completely due to using libs which depend on it.

When both are on the classpath, if your class to serialize are properly annotated Kotlinx Serialization should be properly used.

I think so. My class (from gradle module

api

):

@Serializable // from kotlinx.serialization
data class RaceDay(
   val eventCode: EventCode,
   val date: LocalDateAsString, // type-alias configuring LocalDate to use a specific serializer
)

Controller method from different gradle module `app`:

@GetMapping("/link")
   @LogStackCoroutine
   suspend fun getActiveLinks(): ResponseEntity<List<RaceDay>> =
      ResponseEntity.ok(orchestrator.activeLinks())

response:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 49

[
    {
        "date": "1970-03-11",
        "eventCode-U8HkqSU": "PEW"
    }
]

Could be due to combining suspending function with wrapper like

ResponseEntity

or to https://github.com/Kotlin/kotlinx.serialization/issues/2060. Could you please try with/without suspending and with/without

ResponseEntity

and let me know the 4 results?

ok

Hum, "No suspend without ResponseEntity ❌" is weird.

Yes 🙂 I ran it again now though just to double-check, still ❌

Ok, please share a repro and I will have a look later once I have a bit of time.

Looks like there is a bug in

AbstractMessageWriterResultHandler#writeBody

for those use cases with a confusion between leveraging

bodyParameter

versus

actualParam

. Could you please create an issue in https://github.com/spring-projects/spring-framework/issues pinging me (

@sdeleuze

) and linking your repro?

Absolutely. Thanks for taking a look into it 🌟

Fix merged, will be fixed in Spring Boot

2.7.11

and

3.0.6

. If you can test to confirm it works (when CI builds will be done) with snapshots that would be great.

Will do :)