Parse a JSON array into Map<String, String> using Kotlinx.serialization

Question

I am writing a Kotlin multiplatform project (JVM/JS) and I am trying to parse a HTTP Json array response into a Map using Kotlinx.serialization

The JSON is something like this:

[{"someKey": "someValue"}, {"otherKey": "otherValue"}, {"anotherKey": "randomText"}]

So far, I am able to get that JSON as String, but I can't find any documentation to help me build a Map or another kind of object. All of it says how to serialize static objects.

I can't use @SerialName because the key is not fixed.

When I try to return a Map<String, String>, I get this error:

Can't locate argument-less serializer for class kotlin.collections.Map. For generic classes, such as lists, please provide serializer explicitly.

At the end, I would like to get either a Map<String, String> or a List<MyObject> where my object could be MyObject(val id: String, val value: String)

Is there a way to do that? Otherwise I am thinking in just writing a String reader to be able to parse my data.

if there is any chance to refactor your json try to put all your objects inside one object like {"someKey": "someValue", "otherKey": "otherValue"} it is a better data structure to use — Mosius
– Mosius, Commented Apr 14, 2019 at 13:07
No, I am writing the client. The server is returning that and I don't have access to it. — Rulo Mejía
– Rulo Mejía, Commented Apr 14, 2019 at 13:08
github.com/Kotlin/kotlinx.serialization/blob/master/docs/… — royB
– royB, Commented Apr 14, 2019 at 13:10

Alexander Egger · Accepted Answer · 2019-04-14 17:29:00Z

10

You can implement you own simple DeserializationStrategy like this:

object JsonArrayToStringMapDeserializer : DeserializationStrategy<Map<String, String>> {

    override val descriptor = SerialClassDescImpl("JsonMap")

    override fun deserialize(decoder: Decoder): Map<String, String> {

        val input = decoder as? JsonInput ?: throw SerializationException("Expected Json Input")
        val array = input.decodeJson() as? JsonArray ?: throw SerializationException("Expected JsonArray")

        return array.map {
            it as JsonObject
            val firstKey = it.keys.first()
            firstKey to it[firstKey]!!.content
        }.toMap()


    }

    override fun patch(decoder: Decoder, old: Map<String, String>): Map<String, String> =
        throw UpdateNotSupportedException("Update not supported")

}


fun main() {
    val map = Json.parse(JsonArrayToStringMapDeserializer, data)
    map.forEach { println("${it.key} - ${it.value}") }
}

answered Apr 14, 2019 at 17:29

Alexander Egger

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2 Comments

Rulo Mejía Over a year ago

Works perfectly!! The workaround I was working on was to use an expect class parser and write both implementations in jvm with klaxon (which I finished) and in js with JSON.parse, but this covers both cases. Thanks!

Richard Domingo Over a year ago

Hey man, how'd you do the SerialClassDescImpl("JsonMap")? Do I have do define that too?

akd005 · Accepted Answer · 2023-03-30 11:24:34Z

As the answer by @alexander-egger looks a bit outdated, here is a modern one:

object ListAsMapDeserializer: KSerializer<Map<String, String>> {

    private val mapSerializer = ListSerializer(MapEntrySerializer(String.serializer(), String.serializer()))

    override val descriptor: SerialDescriptor = mapSerializer.descriptor

    override fun deserialize(decoder: Decoder): Map<String, String> {
        return mapSerializer.deserialize(decoder).associate { it.toPair() }
    }

    override fun serialize(encoder: Encoder, value: Map<String, String>) {
        mapSerializer.serialize(encoder, value.entries.toList())
    }
}

and tests for it :

@Test
fun listAsMap() {
    val jsonElement = json.parseToJsonElement("{ \"map\": [ {\"key1\":\"value1\"}, {\"key2\":\"value2\"} ] }")
    val testWithMap = json.decodeFromJsonElement<TestWithMap>(jsonElement)
    assertEquals(mapOf("key1" to "value1", "key2" to "value2"), testWithMap.map)
}

@Test
fun mapAsList() {
    val jsonElement = json.parseToJsonElement("{ \"map\": [ {\"key1\":\"value1\"}, {\"key2\":\"value2\"} ] }")
    val testWithMap = TestWithMap(mapOf("key1" to "value1", "key2" to "value2"))
    val serialized = json.encodeToJsonElement(TestWithMap.serializer(), testWithMap)
    assertEquals(jsonElement, serialized)
}

@Serializable
data class TestWithMap(
    @Serializable(with = ListAsMapDeserializer::class)
    val map: Map<String, String>
)

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