kotlinx.serialization should not be used for Java interfaces implemented by Java classes

[mickael-coquer]

Affects: 5.3.2

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Having both Jackson and kotlinx.serialization in the classpath, kotlinx.serialization will try to deserialize Java interfaces implemented by Java classes resulting in the following exception:

org.springframework.http.converter.HttpMessageNotReadableException: Could not read JSON: Polymorphic serializer was not found for missing class discriminator ('null')
... nested exception is kotlinx.serialization.json.internal.JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null')

The issue comes from the KotlinSerializationJsonHttpMessageConverter.candRead() method that returns true when the Type is a Java interface.

In case of Java concrete implementations, this method should return false and let Jackson handle the conversion.

[quaff]

You could try https://github.com/FasterXML/jackson-modules-base/tree/master/mrbean , It will create an proxy implemented interface.

[sdeleuze]

Good catch, I would expect SerializersKt.serializer(type) provided by kotlinx.serialization to throw an exception in that case, but indeed for some reasons it does not. I have raised that point to the Kotlin team, and will add a related check on Spring side and related tests as well.

[sdeleuze]

I will wait for Kotlin team feedback since the check to use is not obvious because some Java interfaces should be supported (collections) while some others don't.

[tomfi]

@scadgek will this solve the problem mentioned in #26321?

In my use-case I have both kotlin serialization and jackson in the classpath, but kotlin serialization was not configured for spring. in previous versions it did not affect anything but since this version spring try to use kotlin serialization in web requests which is not what i expected (or at least i expect to control it somehow)