How do I register a customer serializer for a clas...

# ktor

[crummy]

How do I register a customer serializer for a class using kotlinx serialization in Ktor?

[edenman]

https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/custom_serializers.md

[crummy]

I've followed that, and use it to annotate my data class with @Serializable(with = LocalDateSerializer::class). However, now I'm returning a Map<LocalDate, foo> and I don't know how to annotate this. Is there a way to register a serializer so that it's always used for a specific class?

[crummy]

e.g. I can do this with Jackson because I can get access to the object mapper: https://ktor.io/servers/features/content-negotiation/jackson.html

[edenman]

where are you putting @Serializable(with = LocalDateSerializer::class)? it should just work if you put it on LocalDate afaik

[crummy]

That worked, on a data class. But where do I put it on a Map<LocalDate, foo>?

[edenman]

no i’m saying if you put that annotation on the LocalDate class itself, you shouldn’t need any annotation on the Map

[crummy]

LocalDate is a java class, I can't put it on the class itself

[edenman]

ah hm. not 100% sure but you might have to register a serializer for that Map type

[crummy]

turns out you can add an annotation to a type declaration:

val timesheets: Map<@Serializable(with = LocalDateSerializer::class)LocalDate, DaySummary>

... but it doesn't seem to help

[Sam Garfinkel]

You have a few options: Use a contextual serializer, use the @Serializable(with=<Class>) at each instance of the external class, or use @file:UseSerializers. I prefer the last solution since it’s a bit more clean imo. Both 2 and 3 only work if the serializer is a singleton or has a no-op constructor, otherwise you’ll need to register a contextual serializer with a SerialModule

[Sam Garfinkel]

However in your specific use-case I believe you are being effected by this bug https://github.com/Kotlin/kotlinx.serialization/issues/802

[Sam Garfinkel]

Because there’s no concrete serializer defined for Map, the built in serializer (I think) relies on reflection, which doesn’t have a way of locating contextual serializers. The workaround is to reimplement the typeOf serializer function to accept the serialModule as an argument and try to find a contextual serializer for keys/values

[Sam Garfinkel]

The Ktor client is affected by this bug so I assume the same is true for the server

[crummy]

Thanks for the clarification, that makes sense.

[Sam Garfinkel]

Unfortunately the only actual fix is to implement your own JsonSerializer that uses the patched serializer<T> implementation that I suggested in that issue.