使用 kotlinx.serialization 库反序列化具有不同值类型的 JSON 数组

Question

我正在尝试反序列化以下字符串：

 val stringJson = "{\"decomposed\":[\", \",{\"id\":4944372,\"name\":\"Johny\",\"various\":false,\"composer\":false,\"genres\":[]}]}"
   

使用以下代码反序列化工作正常

@Serializable
data class Artist(
    val decomposed: JsonArray
)

fun main() {
    val jsonString = "{\"decomposed\":[\", \",{\"id\":4944372,\"name\":\"Johny\",\"various\":false,\"composer\":false,\"genres\":[]}]}"
    println(Json.decodeFromString<Artist>(jsonString))
}

但我想做类似的事情

@Serializable
class Decomposed {
    @Serializable
    class DecomposedClassValue(val value: DecomposedClass)

    @Serializable
    class StringValue(val value: String)
}


@Serializable
data class DecomposedClass(
    val id: Long? = null,
    val name: String? = null,
    val various: Boolean? = null,
    val composer: Boolean? = null,
    val genres: JsonArray? = null
)

@Serializable
data class Artist(
    val decomposed: List<Decomposed>
)

fun main() {
    val jsonString = "{\"decomposed\":[\", \",{\"id\":4944372,\"name\":\"Johny\",\"various\":false,\"composer\":false,\"genres\":[]}]}"
    println(Json.decodeFromString<Artist>(jsonString))
}

但kotlinx.serialization预期失败了JsonDecodingException: Unexpected JSON token at offset 15: Expected '{, kind: CLASS' 而且我不知道如何重写我的Decomposed反序列化工作。你能帮我一下吗？

Answer 1

您正在尝试做的称为多态反序列化。它要求反序列化的目标类有一个共同的超类（最好是密封的）：

@Serializable
data class Artist(
    val decomposed: List<Decomposed>
)

@Serializable
sealed class Decomposed

@Serializable
class StringValue(val value: String) : Decomposed() //Can't add superclass to String, so we have to create a wrapper class which we could make extend Decomposed

@Serializable
data class DecomposedClass(
    val id: Long? = null,
    val name: String? = null,
    val various: Boolean? = null,
    val composer: Boolean? = null,
    val genres: JsonArray? = null
) : Decomposed() //DecomposedClassValue is redundant, we may extend DecomposedClass from Decomposed directly

这将允许您反序列化以下格式的 JSON：

val jsonString = "{\"decomposed\":[{\"type\":\"StringValue\", \"value\":\",\"}, {\"type\":\"DecomposedClass\", \"id\":4944372,\"name\":\"Johny\",\"various\":false,\"composer\":false,\"genres\":[]}]}" 

由于原始 JSON 中没有类鉴别器，序列化库无法确定应该用于反序列化 Kotlin 类的实际序列化器。您必须编写自定义JsonContentPolymorphicSerializer并将其连接到Decomposed类；您还必须为StringValue类编写自定义序列化程序，因为它在 JSON 中表示为String，而不是具有类型value字段的JSONObject String：

object DecomposedSerializer : JsonContentPolymorphicSerializer<Decomposed>(Decomposed::class) {
    override fun selectDeserializer(element: JsonElement) = when {
        element is JsonPrimitive -> StringValue.serializer()
        else -> DecomposedClass.serializer()
    }
}

object StringValueSerializer : KSerializer<StringValue> {
    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("StringValue")

    override fun deserialize(decoder: Decoder): StringValue {
        require(decoder is JsonDecoder)
        val element = decoder.decodeJsonElement()
        return StringValue(element.jsonPrimitive.content)
    }

    override fun serialize(encoder: Encoder, value: StringValue) {
        encoder.encodeString(value.value)
    }
}

@Serializable(with = DecomposedSerializer::class)
sealed class Decomposed

@Serializable(with = StringValueSerializer::class)
class StringValue(val value: String) : Decomposed()

这将允许您反序列化原始格式的 JSON。