KotlinのEnum の JSON シリアライズ結果をカスタマイズする

kotlinx.serialization

• 文字列に変換するなら @SerialName アノテーションを使う
• 数値に変換するなら custom serializer を実装する
  • 参考: https://github.com/Kotlin/kotlinx.serialization/issues/31#issuecomment-522943969

CommonEnumIntSerializer.kt

open class CommonEnumIntSerializer<T>(
    val serialName: String,
    val choices: Array<T>,
    val choicesNumbers: Array<Int>
) :
    KSerializer<T> {

    override val descriptor: EnumDescriptor = EnumDescriptor(serialName, choicesNumbers.map { it.toString() }.toTypedArray())

    init {
        require(choicesNumbers.size == choices.size) { "There must be exactly one serial number for every enum constant." }
        require(choicesNumbers.distinct().size == choicesNumbers.size) { "There must be no duplicates of serial numbers." }
    }

    final override fun serialize(encoder: Encoder, obj: T) {
        val index = choices.indexOf(obj)
            .also { check(it != -1) { "$obj is not a valid enum $serialName, choices are $choices" } }
        encoder.encodeInt(choicesNumbers[index])
    }

    final override fun deserialize(decoder: Decoder): T {
        val serialNumber = decoder.decodeInt()
        val index = choicesNumbers.indexOf(serialNumber)
        check(index != -1) { "$serialNumber is not a valid serial value of $serialName, choices are $choicesNumbers" }
        check(index in choices.indices)
        { "$index is not among valid $serialName choices, choices size is ${choices.size}" }
        return choices[index]
    }
}

SerialEnum.kt

interface SerialEnum {
    val serialNumber: Int?
}
fun <T> Array<T>.serial() where T : SerialEnum, T : Enum<T> = this.map { it.serialNumber ?: it.ordinal }.toTypedArray()

TestEnum.kt

object TestSerializer :
        CommonEnumIntSerializer<TestEnum>("TestEnum", TestEnum.values(), TestEnum.values().serial())

@Serializable(with = TestSerializer::class)
enum class TestEnum(override val serialNumber: Int? = null) : SerialEnum {
    Foo(123), // Will be serialized to 123
    AnotherConstant // Will be serialized to 1
}

Jackson

@JsonValue アノテーションを使う
https://qiita.com/t-otsuka/items/ef7cd9cac84b2c1915c1