Asked i serialization channel, nobody replied, maybe someone knows here: Does anyone know if I can s...

# ktor

[Ivan Brko]

Asked i serialization channel, nobody replied, maybe someone knows here:
Does anyone know if I can somehow register a serializer in ktor (using kotlinx.serialization)? I am writing a Ktor sever and I need to return a data class from a 3rd party library, so I can't annotate it with @Serializable. I can write a serializer for it, but how to tell Ktor to use this serializer?

[David Eriksson]

https://ktor.io/servers/features/content-negotiation.html

[Ivan Brko]

Thanks for the answer.
Maybe I'm missing something obvious here but all I can see here is call to register method. If I'm using kotlinx.serialization isn't that handled internally when I call json()?

[David Eriksson]

Maybe I misinterpreted your question 😉

[Ivan Brko]

Sorry, accidentally sent unfinished message above, so I edited it.

[David Eriksson]

application.install(ContentNegotiation) {
            register(ContentType.Application.Json, SerializationConverter())
        }

        application.routing {
            val model = MyEntity(
                777, "Cargo",
                listOf(
                    ChildEntity("Qube", 1),
                    ChildEntity("Sphere", 2),
                    ChildEntity(uc, 3)
                )
            )

            get("/") {
                call.respond(model)
            }'

[David Eriksson]

From https://github.com/ktorio/ktor/blob/d85573f5f1bb1c3d0035804f7db1925258e9f119/ktor-features/ktor-serialization/jvm/test/io/ktor/tests/serialization/SerializationTest.kt#L245

[David Eriksson]

But yes, MyEntity and ChildEntity have @Serializable

[David Eriksson]

So you might need to go this route: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/custom_serializers.md#external-serializers-for-library-classes

[David Eriksson]

"External serializers for library classes"

[Ivan Brko]

Thanks. I know how to write my own serializer, I just don't know how to register that KSerializer in Ktor, so that it uses it when it is trying to serialize or deserialize data in API. For example, when using KMongo with kotlinx.serialization there is a method registerSerializer where I can register any non existing serializer.

[Ivan Brko]

I'll check the samples you gave above in more detail, but I have to admit I still don't understand what will hapen when I write something like this:

install(ContentNegotiation) {
    json()
    register(ContentType.Application.Json, SerializationConverter())
}

Both json() and I call register directly here for Json content. That is OK with Ktor?

[David Eriksson]

Can't say for sure sorry

[Ivan Brko]

Ok, great. Thanks for the help!

[David Eriksson]

But the @Serializable annotations should be picked up by the compiler plugin

[Ivan Brko]

Yes, but the problem is when I can't annotate the data class as it is not in my code.

[David Eriksson]

// Imagine that MyData is a third-party library class.
// Plugin will try to automatically serialize all constructor properties
// and public vars.
@Serializer(forClass = MyData::class)
object DataSerializer {}