KotlinSerializationJsonEncoder can't encode java.util.Collections$SingletonList

[ashershnov]

Hello,
could you please look into this issue?

Should org.springframework.http.codec.json.KotlinSerializationJsonEncoder be able to serialize instances of java.util.Collections$SingletonList to JSON?

Here is a piece of code showing that it's not:

The output is:
KotlinSerializationJsonEncoder can encode: false
Jackson2JsonEncoder can encode: true

In org.springframework.http.codec.KotlinSerializationSupport, serializer(ResolvableType) always returns null in this context:

Versions used:
Spring Boot 3.4.6
Spring WebFlux 6.2.7
Spring Web 6.2.7
Kotlinx serialization json 1.8.1

Please find attached the project for reproducing:
webflux.zip

Could it be a bug?

[bclozel]

@ashershnov did you close this by mistake? If this is not an issue after all, care to share what happened?

[ashershnov]

@bclozel , sorry, I think I closed it accidentally.
I'm really not sure about this, but it seems strange that org.springframework.http.codec.json.Jackson2JsonEncoder can serialize collections by default but org.springframework.http.codec.json.KotlinSerializationJsonEncoder cannot...

[sdeleuze]

KotlinSerializationJsonEncoder can serialize such collection but unlike Jackson2JsonEncoder which is mostly a "catch all" converter, Kotlin serialization requires by design full type resolution to determine if it can serialize such type and with ResolvableType.forInstance(data) you get a List<*> due to the type erasure , not a List<String>.

You can fix this by using something like ResolvableType.forClassWithGenerics(List::class.java, String::class.java) in your sample. In practice, WebClient (and RestClient) provides Kotlin extensions allowing to specify such type in an idiomatic way such as bodyValueWithType<List<String>>().

[ashershnov]

@sdeleuze
Merci Sébastien!
Sorry to bother you with this...

[sdeleuze]

No problem, I know this is not totally obvious first, but we depend on Kotlinx Serialization design here.