Map sorted by keys generated with kotlinx.serialization - kotlin

Question

I need to serialize a class with a map so that the keys in the map are sorted in the json. So if there's a class
#Serializable
class Example(val map: Map<String, Int>)
and it's serialized with
val example = Example(mapOf("b" to 2, "a" to 1, "c" to 3))
println(Json.encodeToString(example))
then the resulting json should be
{
"map": {
"a": 1,
"b": 2,
"c": 3
}
}
I tried to use SortedMap instead of Map, but that throws an exception:
kotlinx.serialization.SerializationException: Class 'TreeMap' is not registered for polymorphic serialization in the scope of 'SortedMap'
How can I get a sorted json using kotlinx.serialization?
(kotlin 1.4.0, kotlinx.serialization 1.0.0-RC)

Answer 1

Figured it out:
import kotlinx.serialization.*
import kotlinx.serialization.json.*
import kotlinx.serialization.builtins.*
import kotlinx.serialization.descriptors.SerialDescriptor
import kotlinx.serialization.encoding.Decoder
import kotlinx.serialization.encoding.Encoder
object SortedMapSerializer: KSerializer<Map<String, Int>> {
private val mapSerializer = MapSerializer(String.serializer(), Int.serializer())
override val descriptor: SerialDescriptor = mapSerializer.descriptor
override fun serialize(encoder: Encoder, value: Map<String, Int>) {
mapSerializer.serialize(encoder, value.toSortedMap())
}
override fun deserialize(decoder: Decoder): Map<String, Int> {
return mapSerializer.deserialize(decoder)
}
}
#Serializable
class Example(
#Serializable(with = SortedMapSerializer::class)
val map: Map<String, Int>
)
fun main() {
val example = Example(mapOf("b" to 2, "c" to 3, "a" to 1))
println(Json.encodeToString(
example
))
}
(Though it would be nice to have an answer for Map<Serializable, Serializable>