kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.

These codes works on 1.0.1, but on 1.1.0 it throws kotlinx.serialization.SerializationException: Serializer for class ‘Any’ is not found.

fun main() {
    val result = json.encodeToString(Encode(ClassA()))
    println(result)
}

val json = Json {
    serializersModule = SerializersModule {
        contextual(ClassA::class, ClassASerializer)
    }
}

@Serializable
data class Encode(val data: @Contextual Any)

abstract class NotSerializableClass

@Serializable(with = ClassASerializer::class)
class ClassA : NotSerializableClass() {
    fun convertTo() = ClassB()
}

@Serializable
class ClassB {
    fun convertTo() = ClassA()
}

@Serializer(forClass = ClassA::class)
object ClassASerializer : KSerializer<ClassA> {
    override val descriptor = PrimitiveSerialDescriptor("WithCustomDefault", PrimitiveKind.STRING)

    override fun serialize(encoder: Encoder, value: ClassA) {
        encoder.encodeSerializableValue(ClassB.serializer(), value.convertTo())
    }

    override fun deserialize(decoder: Decoder): ClassA {
        return decoder.decodeSerializableValue(ClassB.serializer()).convertTo()
    }
}

Environment

• Kotlin version: [ 1.4.30 ]
• Library version: [ 1.1.0 ]
• Kotlin platforms: [ Android / JVM]

[sandwwraith]

Hi, thanks for the report!

Unfortunately, contextual serialization was never meant to work this way. It worked due to a mistake in implementation, that was fixed in 1.1.0 (see https://github.com/Kotlin/kotlinx.serialization/pull/1277 for details). As documentation for ContextualSerializer says, 'Serializers are being looked for in a [SerializersModule] from the target [Encoder] or [Decoder], using statically known [KClass]". Here statically known KClass is Any, so it is being looked up in module instead of ClassA.

If you need to determine serializer dynamically for subtypes of Any, I suggest you to use polymorphic serialization.

If you want only serialization (because deserialization never worked in your example even in 1.0.0), I can suggest a workaround serializer for Any:

@ExperimentalSerializationApi
class DynamicLookupSerializer: KSerializer<Any> {
    override val descriptor: SerialDescriptor = ContextualSerializer(Any::class, null, emptyArray()).descriptor

    override fun serialize(encoder: Encoder, value: Any) {
        val actualSerializer = encoder.serializersModule.getContextual(value::class) ?: value::class.serializer()
        encoder.encodeSerializableValue(actualSerializer as KSerializer<Any>, value)
    }

    override fun deserialize(decoder: Decoder): Any {
        error("Unsupported")
    }
}

[OnwukaDaniel]

Hi, thanks for the report! Unfortunately, contextual serialization was never meant to work this way. It worked due to a mistake in implementation, that was fixed in 1.1.0 (see #1277 for details). As documentation for ContextualSerializer says, 'Serializers are being looked for in a [SerializersModule] from the target [Encoder] or [Decoder], using statically known [KClass]". Here statically known KClass is Any, so it is being looked up in module instead of ClassA. If you need to determine serializer dynamically for subtypes of Any, I suggest you to use polymorphic serialization. If you want only serialization (because deserialization never worked in your example even in 1.0.0), I can suggest a workaround serializer for Any:

@ExperimentalSerializationApi
class DynamicLookupSerializer: KSerializer<Any> {
    override val descriptor: SerialDescriptor = ContextualSerializer(Any::class, null, emptyArray()).descriptor

    override fun serialize(encoder: Encoder, value: Any) {
        val actualSerializer = encoder.serializersModule.getContextual(value::class) ?: value::class.serializer()
        encoder.encodeSerializableValue(actualSerializer as KSerializer<Any>, value)
    }

    override fun deserialize(decoder: Decoder): Any {
        error("Unsupported")
    }
}

it works ~

Everyone is accepting the solution. Please tell how to use it.