How to avoid unnesary naming with Kotlin native serialization? - json

Question

Situation
Say I have this JSON, which I get from some server:
{
"useless_info": "useless info",
"data": {
"useless_info2": "useless info 2",
"children": [
{
"kind": "Car",
"data": {
"id": 1,
"transmission": "manual"
}
},
{
"kind": "Boat",
"data": {
"id": 2,
"isDocked": true
}
}
]
}
}
This JSON is an example. It represent a much larger and complex one, but with similar structure. I have no say in it's structure, so I must adapt to it.
What I Want
The JSON has deep inside it an array of vehicle objects. Say I only care about the ID.
I could model it like this:
#Serializable
data class VehicleResponse(
#Serializable(with = VehicleResponseSerializer::class)
#SerialName("data")
val vehicles: List<Vehicle>
)
#Serializable
data class Vehicle(val id: Int)
In order to avoid modeling the response exactly by writing many neste data classes (which would be awful with the real one, which is more nested and complex), I can use a JsonTransformingSerializer to cut right to part I want, like this:
object VehicleResponseSerializer : JsonTransformingSerializer<List<Vehicle>>(ListSerializer(Vehicle.serializer())) {
override fun transformDeserialize(element: JsonElement): JsonElement {
val vehicles = mutableListOf<JsonElement>()
val vehicleArray = element.jsonObject["children"]!!.jsonArray
// equals: [{"kind":"Car","data":{"id":1,"totalWheels":"4"}},{"kind":"Boat","data":{"id":2,"isDocked":true}}]
vehicleArray.forEach { vehicle ->
val vehicleData = vehicle.jsonObject["data"]!!
// equals: {"id":1,"totalWheels":"4"}}
vehicles.add(vehicleData)
}
return JsonArray(vehicles.toList())
}
}
Calling it from main:
import kotlinx.serialization.decodeFromString
import kotlinx.serialization.json.Json
fun main() {
val vehicles: VehicleResponse = configuredJson.decodeFromString(vehicleJson)
println(vehicles)
}
val configuredJson = Json {
ignoreUnknownKeys = true
}
Prints:
VehicleResponse(vehicles=[Vehicle(id=1), Vehicle(id=2)])
So if it works perfectly, what's even the problem
It's this part:
#Serializable
data class VehicleResponse(
#Serializable(with = VehicleResponseSerializer::class)
#SerialName("data")
val vehicles: List<Vehicle>
)
I have a problem with the #SerialName. It feels like a hack to me. All this accomplishes is give the custom serializer the data object rather than the full original object from the response. If you remember from the custom serializer, I cut right to the array I want by walking down the structure. It would be trivial to get to the data object. So, instead of
val vehicleArray = element.jsonObject["children"]!!.jsonArray
I would have to do:
val vehicleArray = element.jsonObject["data"]!!.jsonObject["children"]!!.jsonArray
Currently I feel like what I'm doing doesn't really describe the code. In my VehicleResponse data class, I write that my vehicles list does has a serial name of "data", when it doesn't at all. This could lead me to misinterpret the structure of JSON in the future, or even confuse me as to what I'm trying to do. With this simple example it's OK, but what about more complex ones?
What I Tried
Changing VehicleResponse to this:
#Serializable(with = VehicleResponseSerializer::class)
data class VehicleResponse(
val vehicles: List<Vehicle>
)
And altering the line that gets the array inside the custom serializer to this:
override fun transformDeserialize(element: JsonElement): JsonElement {
...
val vehicleArray = element.jsonObject["data"]!!.jsonObject["children"]!!.jsonArray
...
}
Running this code gives the following error:
Exception in thread "main" java.lang.ClassCastException: class java.util.ArrayList cannot be cast to class VehicleResponse (java.util.ArrayList is in module java.base of loader 'bootstrap'; VehicleResponse is in unnamed module of loader 'app')
at MainKt.main(Main.kt:5)
at MainKt.main(Main.kt)
Process finished with exit code 1
For reference, I tried to print the vehicles array inside the serializer, to see it by any chance I messed up navigating the JSON.
override fun transformDeserialize(element: JsonElement): JsonElement {
...
println(vehicles)
return JsonArray(vehicles.toList())
}
It prints:
[{"id":1,"transmission":"manual"}, {"id":2,"isDocked":true}]
So everything seems to be fine. Not sure what I'm doing wrong.