How can I configure Spring Webflux to use Kotlinx Serialization over Jackson, even though both are o...

# spring

[Emil Kantis]

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.

[sdeleuze]

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

[sdeleuze]

Are you seeing a different behavior?

[Emil Kantis]

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"
    }
]

[Emil Kantis]

app has dependency on org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.0

[Emil Kantis]

If I change the controller method to use Kotlinx Serialization directly it gets rid of the -U8HkqSU which I believe comes from jackson trying to serialize a value class.
``````

[sdeleuze]

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?

[Emil Kantis]

ok

[Emil Kantis]

Suspend with ResponseEntity ❌
Suspend without ResponseEntity ✅

No suspend with ResponseEntity ❌
No suspend without ResponseEntity ❌

[sdeleuze]

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

[Emil Kantis]

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

[sdeleuze]

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

[Emil Kantis]

https://github.com/Kantis/kxs-repro

[sdeleuze]

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?

[Emil Kantis]

Absolutely. Thanks for taking a look into it 🌟

[Emil Kantis]

In case anyone else comes looking into this convo.. issue is here: https://github.com/spring-projects/spring-framework/issues/30214

[sdeleuze]

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.

[Emil Kantis]

Will do :)

[Emil Kantis]

Nice work 🙌