Kotlin 实现密封接口的值 class 的序列化

Question

I am trying to use Kotlin serialization (Kotlin 1.7.2, kotlinx.serialization 1.4.1) for value classes that implement a sealed interface:

@Serializable
sealed interface Power {
    val value: Int
}

@Serializable
@JvmInline
value class PowerWatt(override val value: Int) : Power

@Serializable
@JvmInline
value class PowerHp(override val value: Int) : Power

When attempting serialization to Json, like so:

    @Test
    fun `Correctly serialize and deserialize a value class that implements a sealed interface`() {
        val power: Power = PowerWatt(123)
        val powerSerialized = Json.encodeToString(power)
        val powerDeserialized = Json.decodeFromString<Power>(powerSerialized)
        assertEquals(power, powerDeserialized)
    }

I run into the following error:

kotlinx.serialization.json.internal.JsonDecodingException: Expected class kotlinx.serialization.json.JsonObject as the serialized body of Power, but had class kotlinx.serialization.json.JsonLiteral

    at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:24)
    at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:94)
    at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:81)
    at kotlinx.serialization.json.Json.decodeFromString(Json.kt:95)

How to make this work? Am I missing something?

Answer 1

The answer was provided in a Kotlin Serialization GitHub Issue here . For value classes, the wrapped underlying type is serialized directly. Hence, there is not wrapping JSON object where the type field for the polymorphic serializer could be inserted.