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\":[]}]}"

反序列化适用于以下代码

@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))
}

但是 kotlinx.serialization 预计会失败 JsonDecodingException: Unexpected JSON token at offset 15: Expected '{, kind: CLASS' 而且我不知道如何重写我的 Decomposed 所以反序列化工作。你能帮帮我吗？

Answer 1

您正在尝试做的事情叫做 polymorphic deserialization。要求反序列化的目标classes有一个共同的superclass（最好是sealed）：

@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\":[]}]}"

由于没有class descriminator in original JSON, serialization library can't determine the actual serializer which should be used to deserialize Kotlin class. You will have to write custom JsonContentPolymorphicSerializer，连线到Decomposed class；您还必须为 StringValue class 编写自定义序列化程序，因为它在 JSON 中表示为 String，而不是带有 [=16] 的 JSON 对象=] String 类型的字段：

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 JSON 原始格式。

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

Deserialize JSON array with different values type with kotlinx.serialization library

serialization

json

kotlin

kotlinx.serialization