Ktor: Deserialize a list of generics inside a data class with kotlinx.serialization

[lamba92]

Say you have:

@Serializable
data class PagedData<T>(val data: T, val page: Int, val totalPages: Int, val pageSize: Int)

Where you know for sure that T is a class that has the annotations @Serializable or a list of a type that is annotated with @Serializable.

Why the serialization runtime is not able to infer what to use? It would be kind of pointless to explicitly build a serializer before every response as such.

Cross reference on kotlin.serialization issue board

[e5l]

Hi, @lamba92. I reproduced the problem on the server, it will be fixed in the next minor release.

[lamba92]

Any ETA for the next minor release?

[oleg-larshin]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.

[monxalo]

Don't know if this is new or not but with a combination of Plugin generated generic serializer with Serializers module this is possible.

@Serializable
data class Item(val entry: String)

@Serializable
data class PagedData<T>(val data: T, val page: Int, val totalPages: Int, val pageSize: Int)

// in Application.module()

json(json = Json {
    serializersModule = SerializersModule {
        contextual(PagedData.serializer(Item.serializer()))
    }
})

With these changes on your response in ktor you no longer need to serialize the object in call.response() method to a String

[graycatdeveloper]

@monxalo after set contextual(PagedData.serializer(Item.serializer()))

Exception in thread "main" java.lang.NoSuchMethodError: kotlinx.serialization.modules.SerializersModuleBuilder.build()Lkotlinx/serialization/modules/SerializersModule;

Solved!

After added to dependencies org.jetbrains.kotlinx:kotlinx-serialization-runtime:1.0-M1-1.4.0-rc

before that there was only io.ktor: ktor-serialization:1.4.1

[graycatdeveloper]

@monxalo
how do you apply your solution to an array?

contextual(PagedData.serializer(List<Item>.serializer())) ???

[monxalo]

@graycatdeveloper

If you are using a list might better define it in the PagedData class like so:

@Serializable
data class PagedData<T>(val data: List<T>, val page: Int, val totalPages: Int, val pageSize: Int)

The contextual remains the same:

contextual(PagedData.serializer(Item.serializer()))

[monxalo]

Drawback of this approach is that SerializersModule only allows one serializer for each type. So you if do:

contextual(PagedData.serializer(Item.serializer()))
contextual(PagedData.serializer(SubItem.serializer()))

It will crash at runtime with SerializerAlreadyRegistered.

[graycatdeveloper]

@monxalo
No!

I am trying to create a serializer. Can you help

class ArrayListSerializer<T>(
    private val serializer: KSerializer<T>
) : KSerializer<ArrayList<T>> {

    override val descriptor: SerialDescriptor = serializer.descriptor

    override fun serialize(encoder: Encoder, value: ArrayList<T>) {
        encoder.encodeSerializableValue<ArrayList<T>>(ListSerializer<T>(serializer), value)
    }

    override fun deserialize(decoder: Decoder): ArrayList<T> {
        return decoder.decodeSerializableValue<List<T>>(ListSerializer<T>(serializer)).toList() as ArrayList<T>
    }
}

serializersModule = SerializersModule {
    //contextual(ApiResponse.serializer(StoreProductDao.Entity.serializer()))
    contextual(ApiResponse.serializer(ArrayListSerializer(StoreProductDao.Entity.serializer())))
}

But doesn't work

Caused by: java.lang.NullPointerException: Parameter specified as non-null is null: method kotlinx.serialization.internal.LinkedHashMapSerializer.collectionSize, parameter $this$collectionSize

[graycatdeveloper]

I have a class like this

The data is always different. Or object or array of objects or primitive.
I am trying to implement something that works well with Gson. But with this everything is complicated :(

[graycatdeveloper]

Drawback of this approach is that SerializersModule only allows one serializer for each type. So you if do:

contextual(PagedData.serializer(Item.serializer()))
contextual(PagedData.serializer(SubItem.serializer()))

It will crash at runtime with SerializerAlreadyRegistered.

How can this problem be solved? Any other way? I'm already tired of looking for answers (

[monxalo]

I did not find any other way, i ended up using ApplicationCall extensions like so:

suspend inline fun <reified T> ApplicationCall.respondWithPaged(message: PagedData<T>) {
    response.pipeline.execute(this, Json.encodeToString(message))
}

[kutoman]

@monxalo an improvement in order to respond with the proper content type (JSON)

suspend inline fun <reified T> ApplicationCall.respondWithPaged(message: PagedData<T>) {
    val encoded = Json.encodeToString(message)
    val status = HttpStatusCode.OK
    response.pipeline.execute(this, TextContent(encoded, ContentType.Application.Json, status))
}