How can I create a custom serializer for a string ...

# serialization

[martmists]

How can I create a custom serializer for a string value that counts as null if the string is empty?
In my case I have a class UuidSerializer : KSerializer<UUID> but I want "" to be decoded as null rather than a UUID value.

[ephemient]

I don't think it's possible, there's issues like https://github.com/Kotlin/kotlinx.serialization/issues/1838 in the issue tracker

[hfhbd]

object MyEmptyUuidSerializer : KSerializer<Uuid?> {
    override fun deserialize(decoder: Decoder): Uuid? {
        val uuidString = decoder.decodeString()
        return if (uuidString.isEmpty()) {
            null
        } else {
            Uuid.parse(uuidString)
        }
    }
}

[hfhbd]

What’s wrong with this custom serializer? 🤔

[martmists]

Afaik that won't allow serializing values that are guaranteed non-null, no?

[martmists]

Yeah that isn't allowed: Type '@Serializable(...) UUID' is non-nullable and therefore can not be serialized with serializer for nullable type 'UuidSerializer'

[martmists]

I did try using a JsonTransformingSerializer:

class StringAsNullUuidSerializer : JsonTransformingSerializer<UUID>(UuidSerializer()) {
    override fun transformDeserialize(element: JsonElement): JsonElement {
        if (element is JsonPrimitive && element.jsonPrimitive.contentOrNull == "") return JsonNull
        return element
    }
}

but that errors at runtime with

Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Expected string value for a non-null key 'primitive', got null literal instead at element: $.primitive

[hfhbd]

But how should it work? If the value is empty, what value do you expect to be decoded?

[martmists]

I expect it to be decoded as null when it's empty, but there are also fields which are guaranteed non-null.

[hfhbd]

Yeah, so you need to change your Kotlin type to Uuid?

[hfhbd]

And use the custom serializer on these fields only

[martmists]

so I need a serializer for UUID and a duplicate serializer with null support for UUID? ? That seems kinda convoluted