Hi there. I need to parse a JSON where one field i...

# serialization

[Mike Wong]

Hi there. I need to parse a JSON where one field is described as "can be either a String or an Array of Strings". Unfortunately, this is an external specification and cannot be changed.
I've tried implementing a custom KSerializer for a Sealed Interface, where I would call decoder.decodeString() in the deserialize-Method and check if it starts with [ . But that throws a JsonDecodingException (Expected JsonPrimitive at ... but found ...) when it encounters the "Array of Strings" option.
What would be the correct approach for this scenario?

[Mike Wong]

Is it safe to cast the Decoder to a JsonDecoder and then call decodeJsonElement() to check if it is a JsonPrimitive or a JsonArray?

[glureau]

Did you try with JsonTransformingSerializer ? It allow you to operate on JsonElement directly, nice to play with those weird formats 🙂

[Mike Wong]

Haven't looked at that class yet, but from the documentation it doesn't look like it achieves what I need to do. Transforming from one JsonElement to another still leaves me with the problem that I need to decode it into a Sealed Interface with two different data structures. Unless I'm understanding that class wrong?

[Vampire]

You would transform the primitive into array and in the data object always have array, and optionally transform a one element array into primitive

[glureau]

I was thinking about always decoding with a List<String>, reading your message again, I guess you don't have only this field but other fields which are different? Can you share those classes?

[Mike Wong]

Yes, I could transform it to an array with a single element, but since the specification distinguishes the two options, I would prefer to keep my data model in line with that.
My sealed interface looks like this:

@Serializable(with = EntryValueSerializer::class)
sealed interface EntryValue {
	@Serializable
	data class Reference(val reference: String) : EntryValue

	@Serializable
	data class Predefined(val mapping: Map<String, String>) : EntryValue
}

[Mike Wong]

And I'm using that sealed interface like this:

@Serializable
data class EntryRegistry(
	val entries: Map<String, EntryValue>,
	// More properties
)

[Mike Wong]

The serializer I have right now and is working looks like this:

override fun deserialize(decoder: Decoder): EntryValue {
		val jsonDecoder = decoder as? JsonDecoder ?: throw IllegalArgumentException("This class can only be deserialized from JSON")
		return when (val jsonElement = jsonDecoder.decodeJsonElement()) {
			is JsonPrimitive -> EntryValue.Reference(jsonElement.content)
			is JsonObject -> {
				EntryValue.Predefined(jsonElement.toMap().mapValues { it.value.jsonPrimitive.content })
			}
			else -> throw IllegalArgumentException("Excpected either JsonPrimitive or JsonArray but was $jsonElement")
		}
	}

Since I'm always serializing from/to JSON, the cast to JsonDecoder should be fine, right?

[ephemient]

https://pl.kotl.in/nqNOvr-cX

[Vampire]

Or maybe a content polymorphic serializer?
https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-content-polymorphic-serializer/

[Mike Wong]

JsonContentPolymorphicSerializer looks like the more elegant solution to the JsonDecoder solution I'm using right now and which @ephemient also suggested. I'll try it out, thanks

[Mike Wong]

Actually, that doesn't seem to work either, I get Expected class kotlinx.serialization.json.JsonObject as the serialized body of EntryValue.Predefined, but had class kotlinx.serialization.json.JsonArray
Since the serializer for that sealed subclass expects to parse the string or string array into an object