Question: How to create ser/des for generics with List and Map?

[ArcherEmiya05]

I cannot find any sample converting Map<K,V> and List<T> to JSON and vice-versa with generics. Planning to migrate from Moshi to KotlinX Serialization as part of adapting KMM. Currently this is what we had.

class MoshiJsonParserRepositoryImpl(private val moshi: Moshi) : JsonParserRepository {

    override fun <T> fromJsonString(jsonString: String?, type: Class<T>): T? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            moshi.adapter(type).fromJson(jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> fromJsonStringToList(jsonString: String?, type: Class<T>): List<T>? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            moshi.adapter<List<T>>(Types.newParameterizedType(List::class.java, type)).fromJson(jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <K, V> fromJsonStringToMap(
        jsonString: String?,
        keyType: Class<K>,
        valueType: Class<V>
    ): Map<K, V>? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        val jsonAdapter: JsonAdapter<Map<K, V>> = moshi.adapter(
            Types.newParameterizedType(
                Map::class.java,
                keyType,
                valueType
            ))

        return try {
            jsonAdapter.fromJson(jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> toJsonString(obj: T, type: Class<T>): String? {

        return try {
            moshi.adapter(type).toJson(obj)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> toJsonString(obj: List<T>, type: Class<T>): String? {

        return try {
            moshi.adapter<List<T>>(Types.newParameterizedType(List::class.java, type)).toJson(obj)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <K, V> toJsonString(value: Map<K, V>, keyType: Class<K>, valueType: Class<V>): String? {

        val jsonAdapter: JsonAdapter<Map<K, V>> = moshi.adapter(
            Types.newParameterizedType(
                Map::class.java,
                keyType,
                valueType
            ))

        val buffer = Buffer()
        val jsonWriter: JsonWriter = JsonWriter.of(buffer)
        jsonWriter.serializeNulls = true
        jsonAdapter.toJson(jsonWriter, value)

        return try {
            buffer.readUtf8()
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

}

Attempt to work with simple data type like data class.

    override fun <T> fromJsonString(jsonString: String?, type: Class<T>): T? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            @Suppress("UNCHECKED_CAST")
            Json.decodeFromString(Json.serializersModule.serializer(type) as KSerializer<T>, jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> toJsonString(obj: T, type: Class<T>): String? {

        return try {
            @Suppress("UNCHECKED_CAST")
            Json.encodeToString(Json.serializersModule.serializer(type) as KSerializer<T>, obj)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

[sandwwraith]

See https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#constructing-collection-serializers

Normally, you shouldn't handle generic types manually since json.decodeFromString<List<MyDataClass>>(jsonString) works just fine. In rare case you need manual construction of serializer, use factory functions, e.g. ListSerializer(serializer(type)). You may also find serializer( kClass: KClass<*>, typeArgumentsSerializers: List<KSerializer<*>>, isNullable: Boolean) overload (https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-core/kotlinx.serialization/serializer.html) helpful.

[ArcherEmiya05]

See https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#constructing-collection-serializers

Normally, you shouldn't handle generic types manually since json.decodeFromString<List<MyDataClass>>(jsonString) works just fine. In rare case you need manual construction of serializer, use factory functions, e.g. ListSerializer(serializer(type)). You may also find serializer( kClass: KClass<*>, typeArgumentsSerializers: List<KSerializer<*>>, isNullable: Boolean) overload (https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-core/kotlinx.serialization/serializer.html) helpful.

Does this mean that KotlinX Serialization is not generic friendly? Always needing to specify data types will result on bunch of boilerplate.

[sandwwraith]

Does this mean that KotlinX Serialization is not generic friendly?

It means the opposite. kotlinx-serialization has first-class support for generics, without the need for type adapters

[ArcherEmiya05]

Does this mean that KotlinX Serialization is not generic friendly?

It means the opposite. kotlinx-serialization has first-class support for generics, without the need for type adapters

Does this mean we no longer need to create the above interface? What if we want to create an extension function?

So far this is what we ended up doing (not tested yet)

    override fun <T> fromJsonString(jsonString: String?, type: Class<T>): T? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            @Suppress("UNCHECKED_CAST")
            Json.decodeFromString(Json.serializersModule.serializer(type) as KSerializer<T>, jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> fromJsonStringToList(jsonString: String?, type: Class<T>): List<T>? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            Json.decodeFromString<List<T>>(jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <K, V> fromJsonStringToMap(
        jsonString: String?,
        keyType: Class<K>,
        valueType: Class<V>
    ): Map<K, V>? {

        if (jsonString.isNullOrBlank()) {
            return null
        }

        return try {
            Json.decodeFromString(serializer<Map<K, V>>(), jsonString)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> toJsonString(obj: T, type: Class<T>): String? {

        return try {
            @Suppress("UNCHECKED_CAST")
            Json.encodeToString(Json.serializersModule.serializer(type) as KSerializer<T>, obj)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <T> toJsonString(obj: List<T>, type: Class<T>): String? {

        return try {
            Json.encodeToString(serializer<List<T>>(), obj)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

    override fun <K, V> toJsonString(value: Map<K, V>, keyType: Class<K>, valueType: Class<V>): String? {

        return try {
            Json.encodeToString(serializer<Map<K, V>>(), value)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

But we are still manually handling types here.

[ArcherEmiya05]

Just tested the code above and we are already getting exception at

override fun <K, V> toJsonString(value: Map<K, V>, keyType: Class<K>, valueType: Class<V>): String? {

        return try {
            Json.encodeToString(serializer<Map<K, V>>(), value)
        }
        catch (e: Exception) {
            e.printStackTrace()
            null
        }

    }

java.lang.IllegalStateException: Captured type parameter K from generic non-reified function. Such functionality cannot be supported as K is erased, either specify serializer explicitly or make calling function inline with reified K

[sandwwraith]

Use MapSerializer(serializer(keyType), serializer(valueType)).

I can also see that you are using java.lang.Class. It won't be available in common multiplatform code. I don't know where you are getting it from, but migrate to kotlin.reflect.KType if possible. Since KType also contains generic arguments, there wouldn't be any need in different overloads for Map/List/value. See example:

fun foo(): Map<String, Int> {
    return mapOf("a" to 1)
}

fun getKType(): KType = typeOf<Map<String, Int>>()

fun <T> toJson(kType: KType, value: T): String = Json.encodeToString(serializer(kType), value)

@Test
fun serializationExample() {
    // Prints {"a":1}
    println(toJson(getKType(), foo()))
}

[ArcherEmiya05]

Use MapSerializer(serializer(keyType), serializer(valueType)).

I can also see that you are using java.lang.Class. It won't be available in common multiplatform code. I don't know where you are getting it from, but migrate to kotlin.reflect.KType if possible. Since KType also contains generic arguments, there wouldn't be any need in different overloads for Map/List/value. See example:

fun foo(): Map<String, Int> {
    return mapOf("a" to 1)
}

fun getKType(): KType = typeOf<Map<String, Int>>()

fun <T> toJson(kType: KType, value: T): String = Json.encodeToString(serializer(kType), value)

@Test
fun serializationExample() {
    // Prints {"a":1}
    println(toJson(getKType(), foo()))
}

Sorry but may I know if this approach will work with multiplatform as well? Thanks!

[sandwwraith]

Yes, typeOf / KType are multiplatform functions. Reflection, though, is available only on JVM