In order not to instantiate singletons with gson

gson-singletons.kt

import com.google.gson.*

fun <T : Any> GsonBuilder.registerSingletonInstance(instance: T): GsonBuilder =
        registerTypeAdapter(instance.javaClass, InstanceCreator { instance })

fun main(args: Array<String>) {
//    val gsonBuilder = GsonBuilder().registerTypeAdapterFactory(object : TypeAdapterFactory {
//            override fun <T: Any> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
//                val c: Class<T> = type.rawType as Class<T>
//                return c.kotlin.objectInstance?.let { instance ->
//                    object : TypeAdapter<T>() {
//                        override fun read(`in`: JsonReader): T {
//                            `in`.beginObject()
//                            `in`.endObject()
//                            return instance
//                        }
//
//                        override fun write(out: JsonWriter, value: T) {
//                            out.beginObject()
//                            out.endObject()
//                        }
//                    }
//                }
//            }
//        })

    val gson = GsonBuilder().registerSingletonInstance(Unit).create()
    val u1 = Unit
    val u2 = gson.fromJson("{}", Unit::class.java)
    val u3 = Unit

    println(u1 == u2)
    println(u1 == u3)
}