GsonConverterFactory.kt

class GsonConverterFactory2(private val gson: Gson = Gson()) : Converter.Factory() {
    override fun responseBodyConverter(type: Type, annotations: Array<Annotation>, retrofit: Retrofit): Converter<ResponseBody, *> {
        return GsonResponseBodyConverter2(gson, gson.getAdapter(TypeToken.get(type)))
    }

    override fun requestBodyConverter(type: Type, parameterAnnotations: Array<Annotation>, methodAnnotations: Array<Annotation>, retrofit: Retrofit): Converter<*, RequestBody> {
        return GsonRequestBodyConverter2(gson, gson.getAdapter(TypeToken.get(type)))
    }
}

class GsonResponseBodyConverter2<T>(
    private val gson: Gson,
    private val adapter: TypeAdapter<T>,
) : Converter<ResponseBody, T> {
    override fun convert(value: ResponseBody): T {
        val jsonReader = gson.newJsonReader(value.charStream())
        return value.use {
            val result = adapter.read(jsonReader)
            if (jsonReader.peek() != com.google.gson.stream.JsonToken.END_DOCUMENT) {
                throw JsonIOException("JSON document was not fully consumed.")
            }
            result
        }
    }
}

class GsonRequestBodyConverter2<T>(private val gson: Gson, private val adapter: TypeAdapter<T>) : Converter<T, RequestBody> {
    companion object {
        val MEDIA_TYPE = "application/json; charset=UTF-8".toMediaType()
    }

    override fun convert(value: T): RequestBody {
        val buffer = Buffer()
        gson.newJsonWriter(OutputStreamWriter(buffer.outputStream(), Charsets.UTF_8)).use { jsonWriter ->
            adapter.write(jsonWriter, value)
        }
        return buffer.readByteString().toRequestBody(MEDIA_TYPE)
    }
}