Kotlinx Serialization: How to circumvent reified typeargs for deserialization?

Question

Actually, the main problem is still that there are no reified typeargs for classes in Kotlin. But here is why this bothers me in this specific case:

Suppose you have a wrapper class Wrapper that takes in a string content and a class* type and can output an object of class type retrieved by parsing content as JSON by demand by calling the function getObj():

class Wrapper<T>(private val content: String, /*private val type: KClass<*>*/) {
    fun getObj(): T {
        // ?
    }
}

And I want to use kotlinx.serialization. Now, you might have noticed how I put an asterisk after "class" before. Here's the reason: Yes, Wrapper has to take the target class in some way, but how? Should it be just the typearg (won't work because type erausre) or a KClass reference (won't work because I need a reified typearg)?

The thing is that as far as I know, the only way to decode a generic JSON to a serializable target class is to use Json.decodeFromString<T>(content), where T is the target type and content is the JSON string. Now, T is defined to be reified (so that the type can be processed at runtime) and can only be filled with another reified typearg or an actual class reference. I can't use another reified typearg because I am in the context of a class and a class cannot have reified typeargs. I can also not use an actual class reference because the user of the class should be able to construct it with different targets, e.g. they decide what the target is, not me.

So, how do I do this with kotlinx.serialization? Is it even possible?

Sorry, I don't understand what's the problem with using KClass like you're showing in your constructor (which also I think should be KClass<T>). — m0skit0
According to my own knowledge, kotlinx.serialization does not take KClasses because it is a multiplatform library and since reflection is only available on the JVM, it can as well not use reflection. If it would work with KClass, I'd use it. Therefore, if it is possible, please correct me and tell me how to do it — Raphael Tarita
@user AFAIK inline classes are still experimental (although that's not a big problem), but the real problem is that I want to do more than a simple wrapper class, of course. The wrapper class in the question displays just the most simplified explanation of the problem. In reality, the class should, among other things, also do some caching. I think it's obvious that for caching, I'll need more than one field. Therefore I can't use an inline class — Raphael Tarita

Raphael Tarita Raphael Tarita · Accepted Answer · 2020-10-15T15:15:47

Ok so no one answered the question yet, but I also posted this question in the r/Kotlin subreddit. Here it is.

I actually got an answer there (credits to u/JakeWharton), and since you might get across this StackOverflow question because you googled the same question, you might be happy to find an answer here. So here's my try to paraphrase the answer:

So, basically, kotlinx-serialization does indeed not work with KClasses. But when you think about it, you only need the KClass to determine how to serialize it. And since that is determined at compile-time when you work with KXS, you actually just need to pass the serializer (the actual strategy defining how to serialize / deserialize your class). You can obtain a serializer for every class annotated with @Serializable by invoking .serializer() on it; the result will be of the type KSerializer<T>. So, instead of having

class Wrapper<T>(private val content: String, private val type: KClass<T>)

and constructing it via

val wrapper = Wrapper("{}", Foo::class)

You can do it like this:

class Wrapper<T>(private val content: String, private val serializer: KSerializer<T>)

and then construct it like this:

val wrapper = Wrapper("{}", Foo.serializer())

(supposing Foo is annotated with @Serializable)

you can then serialize and deserialize by using the KSerializer instead of a typearg, like this:

val obj: T = Json.decodeFromString(serializer, "[Your JSON String]")
val str: String = Json.encodeToString(serializer, obj)

And that's it! Just swap out your regular (K)Class approach by KSerializer and it'll work with KXS.

Kotlinx Serialization: How to circumvent reified typeargs for deserialization?

1 Answers