Using GSON to support serialization and deserialization of Kotlin Sealed Classes.

kotlin-sealedclass-serialization.kt

import com.google.gson.Gson
import com.google.gson.GsonBuilder
import com.google.gson.TypeAdapter
import com.google.gson.TypeAdapterFactory
import com.google.gson.reflect.TypeToken
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonWriter
import kotlin.jvm.internal.Reflection
import kotlin.reflect.KClass


object Json {
    val gson =
        GsonBuilder().registerTypeAdapterFactory(
            object : TypeAdapterFactory {
                override fun <T : Any> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T> {
                    val kclass = Reflection.getOrCreateKotlinClass(type.rawType)
                    return if (kclass.sealedSubclasses.any()) {
                        SealedClassTypeAdapter<T>(kclass, gson)
                    } else
                        gson.getDelegateAdapter(this, type)
                }
            }).create()

    inline fun <reified T> fromJson(x: String): T = this.gson.fromJson(x, T::class.java)
    fun <T> fromJsonWithClass(x: String, classObj: Class<T>): T =
            this.serializer.fromJson(x, classObj)
    inline fun <T> toJson(item: T): String = this.gson.toJson(item)
}

sealed class SealedClassOne {
    data class D1(val name: String, val age: Int) : SealedClassOne()
    data class D2(val name: String, val salary: Int) : SealedClassOne()
    data class D3(val p: SealedClassOne) : SealedClassOne()

}

sealed class SealedClassTop {
    data class Container(val p: SealedClassOne) : SealedClassTop()
    object Singleton : SealedClassTop()

}

data class Container(val c: SealedClassTop)

fun main() {
    val h1 = SealedClassOne.D1("Krishna", 34)
    val h2 = SealedClassOne.D2("Archana", 1233)
    println("Running serialization")
    val h1Json = Json.toJson(h1)
    println("JSON: $h1Json")
    val h2Json = Json.toJson(h2)
    println("JSON: $h2Json")
    println("Running deserialization")
    println(Json.fromJson<SealedClassOne.D1>(h1Json))
    println(Json.fromJson<SealedClassOne.D2>(h2Json))

    //super nested model!
    val l1 = SealedClassTop.Container(SealedClassOne.D3(SealedClassOne.D3(h1)))
    val l1Json = Json.toJson(l1)
    println("Json : $l1Json")
    val l1Back = Json.fromJson<SealedClassTop.Container>(l1Json)
    println("L1 : $l1Back")
    println("Are L1 Equal? : ${l1 == l1Back}")

    //more complex shit
    val container1 = Container(l1)
    val container2 = Container(SealedClassTop.Singleton)
    val container1Json = Json.toJson(container1)
    println("Json: $container1Json")
    val container2Json = Json.toJson(container2)
    println("Json: $container2Json")

    val container1Back = Json.fromJson<Container>(container1Json)
    println("Container match : ${container1 == container1Back}")

    val container2Back = Json.fromJson<Container>(container2Json)
    println("Singleton container -> ${container2Back.c}")
    println("Singleton match : ${container2 == container2Back}")

    val container3Back = Json.fromJson<Container>(container2Json)
    println("Extended! : $container3Back")
}

class SealedClassTypeAdapter<T : Any>(val kclass: KClass<Any>, val gson: Gson) : TypeAdapter<T>() {
    override fun read(jsonReader: JsonReader): T? {
        jsonReader.beginObject() //start reading the object
        val nextName = jsonReader.nextName() //get the name on the object
        val innerClass = kclass.sealedSubclasses.firstOrNull {
            it.simpleName!!.contains(nextName)
        } ?: throw Exception("$nextName is not found to be a data class of the sealed class ${kclass.qualifiedName}")
        val x = gson.fromJson<T>(jsonReader, innerClass.javaObjectType)
        jsonReader.endObject()
        //if there a static object, actually return that back to ensure equality and such!
        return innerClass.objectInstance as T? ?: x
    }

    override fun write(out: JsonWriter, value: T) {
        val jsonString = gson.toJson(value)
        out.beginObject()
        out.name(value.javaClass.canonicalName.splitToSequence(".").last()).jsonValue(jsonString)
        out.endObject()
    }

}