Map sorted by keys generated with kotlinx.serialization

user_390928 0
Viewed 1631

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)

kotlinx.serialization kotlin
1 Answers

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>

user_3909280
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