Replace Jackson with kotlinx.serialization in Javalin framework

Question

I'm trying to replace the default Javalin JSON serializer Jackson by Kotlinx.serialization.

The documentation show how to do it with GSON serializer.
Unfortunately kotlinx serializer has a different function signature and I can't figure out how to pass arguments through.

Serialization is OK but deserialization with decodeFromString function require to be passed a type given by the mapping function as targetClass.

I'm stuck here:

val kotlinx = Json { coerceInputValues = true }
    
JavalinJson.toJsonMapper = object : ToJsonMapper {
    override fun map(obj: Any): String = kotlinx.encodeToString(obj)
}
    
JavalinJson.fromJsonMapper = object : FromJsonMapper {
    override fun <T> map(json: String, targetClass: Class<T>): T = kotlinx.decodeFromString(json)
}

But I get: Cannot use 'T' as reified type parameter. Use a class instead.

I also tried:

JavalinJson.fromJsonMapper = object : FromJsonMapper {
    override inline fun <reified T> map(json: String, targetClass: Class<T>): T = kotlinx.decodeFromString(json)
}

But I get a warning: Override by an inline function and an error: Override by a function with reified type parameter.

I'm new to kotlin and I'm struggling understanding what's wrong with this override.

Михаил Нафталь · Accepted Answer · 2020-10-09T20:41:27.223

2

Try this one:

JavalinJson.toJsonMapper = object : ToJsonMapper {
    override fun map(obj: Any): String {
        val serializer = serializer(obj.javaClass)
        return kotlinx.encodeToString(serializer, obj)
    }
}

JavalinJson.fromJsonMapper = object : FromJsonMapper {
    override fun <T> map(json: String, targetClass: Class<T>): T {
        @Suppress("UNCHECKED_CAST")
        val deserializer = serializer(targetClass) as KSerializer<T>
        return kotlinx.decodeFromString(deserializer, json)
    }
}

edited Oct 09 '20 at 20:41

answered Oct 09 '20 at 18:28

Михаил Нафталь

7,063
2
11
22

Thanks a lot, it looks better than before. I'm always getting some errors though. `Serializer for class 'Any' is not found`. The problem is that `serializer(targetClass)` returns a `KSerializer` and not a `KSerializer`. – Freezystem Oct 09 '20 at 20:18
I tried `val deserializer: KSerializer = serializer()` that seems more appropriate but I'm having the `reified` issue again.. – Freezystem Oct 09 '20 at 20:28
Seems, this error is related to `toJsonMapper`. Edited answer, added this one too. – Михаил Нафталь Oct 09 '20 at 20:42
Now I get this error: `Serializer for class 'GenericResponse' is not found. Mark the class as @Serializable or provide the serializer explicitly` which is better because `GenericResponse` is my serialized data class. The strange thing is that this class is already tagged as `Serializable`. – Freezystem Oct 09 '20 at 21:17
Are you sure you added Kotlin Serialization compiler plugin to your project? – Михаил Нафталь Oct 10 '20 at 07:31
yes I followed the setup procedure of kotlinx README.md and add `kotlin("plugin.serialization") version "1.4.10"` to plugins and `implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.0.0")` to dependencies – Freezystem Oct 10 '20 at 07:45
Are you sure you are using `kotlinx.serialization.Serializable`, not `java.io.Serializable`? – Михаил Нафталь Oct 10 '20 at 08:12
Can't reproduce in JVM... What platform are you targeting? – Михаил Нафталь Oct 10 '20 at 09:21
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/222814/discussion-between-freezystem-and--). – Freezystem Oct 10 '20 at 09:38

Replace Jackson with kotlinx.serialization in Javalin framework

1 Answers1