Kotlinx Serialization with GraalVM Native Images

[kdabir]

This may not necessarily be bug, any reference in documentation would be helpful. When compiling the example as per the blog announcement

@Serializable
data class Project(
   val name: String,
   val owner: Account,
   val group: String = "R&D"
)

@Serializable
data class Account(val userName: String)

val moonshot = Project("Moonshot", Account("Jane"))
val cleanup = Project("Cleanup", Account("Mike"), "Maintenance")

fun main() {
   val string = Json.encodeToString(listOf(moonshot, cleanup))
   println(string)
 
   val projectCollection = Json.decodeFromString<List<Project>>(string)
   println(projectCollection)
}

it works with Gradle run task (application plugin applied). However When I convert application to a graalvm native image, the compilation succeeds (and binary native image is generated) but when I run the native image, I get the following error:

Exception in thread "main" kotlinx.serialization.SerializationException: Serializer for class 'Project' is not found.
Mark the class as @Serializable or provide the serializer explicitly.
        at kotlinx.serialization.internal.Platform_commonKt.serializerNotRegistered(Platform.common.kt:91)
        at kotlinx.serialization.internal.PlatformKt.platformSpecificSerializerNotRegistered(Platform.kt:29)
        at kotlinx.serialization.SerializersKt__SerializersKt.serializer(Serializers.kt:59)
        at kotlinx.serialization.SerializersKt.serializer(Unknown Source)
        at kotlinx.serialization.SerializersKt__SerializersKt.builtinSerializerOrNull$SerializersKt__SerializersKt(Serializers.kt:79)
        at kotlinx.serialization.SerializersKt__SerializersKt.serializerByKTypeImpl$SerializersKt__SerializersKt(Serializers.kt:69)
        at kotlinx.serialization.SerializersKt__SerializersKt.serializer(Serializers.kt:54)
        at kotlinx.serialization.SerializersKt.serializer(Unknown Source)

To Reproduce
Just run compile the example using graalvm native image.

Expected behavior
Since I assume, kotlinx.serialization does not use reflection at runtime, the native image should have worked properly.

If there is something needed in reflection config, please update the docs.

Environment

• Kotlin version: e.g. 1.4.10
• Library version: e.g. 1.0.0
• Kotlin platforms: JVM
• Gradle version: 6.6.1
• GraalVM: 20.2.0

[qwwdfsad]

Thanks!

Since I assume, kotlinx.serialization does not use reflection at runtime,

kotlinx.serialization uses reflection is exactly one place -- serializer() function to lookup a serializer.
This function is used in reified methods that do not accept serializer as a parameter.
We are going to change that soon (around Kotlin 1.4.30), but now as a temporary workaround I'd suggest to use Json.encodeToString(ListSerializer(Project.serializer()), listOf(moonshot, cleanup)) and Json.decodeFromString(ListSerializer(Project.serializer(), string) respectively.

[kdabir]

Thanks @qwwdfsad for quick response. I can confirm that the workaround works with GraalVM Native Image 👍

Looking forward for kotlin 1.4.30 though :)

[nanodeath]

Just ran into this, and the workaround is mildly annoying -- can we keep this open until

We are going to change that soon

this happens? Thanks!

[rescribet]

Just bumped into this while using ktor client, tried to add the serializer manually, but that still fails with 'serializer not found'. So I'm contextual serialization uses reflection as well.

val serializer = Json {
    serializersModule = SerializersModule {
        contextual(MySerializableClass::class, MySerializableClass.serializer())
    }
}

val client = HttpClient(CIO) {
    install(JsonFeature) {
        serializer = KotlinxSerializer(serializer)
    }
}

val data = client.get<HttpResponse>("...").receive<MySerializableClass>()

When using an explicit serializer it bumps into KotlinReflectionInternalError: Unresolved class: class java.lang.String.

val body = client.get<HttpResponse>("...").receive<String>()
val data = Json.decodeFromString(MySerializableClass.serializer(), data)

Though I guess I'll just postpone building a native image since it mostly creates unnecessary future refactor work

[qwwdfsad]

It will be fixed with #1348, hopefully in Kotlin 1.8.0

[Scui1]

kotlinx.serialization uses reflection is exactly one place -- serializer() function to lookup a serializer. This function is used in reified methods that do not accept serializer as a parameter. We are going to change that soon (around Kotlin 1.4.30), but now as a temporary workaround I'd suggest to use Json.encodeToString(ListSerializer(Project.serializer()), listOf(moonshot, cleanup)) and Json.decodeFromString(ListSerializer(Project.serializer(), string) respectively.

I just used graalvms tracing agent which generated a config for me that works without this ugly workaround. I suspect this is the part that's making it work:

{
   "name":"package.name.YourClassName",
   "fields":[{"name":"Companion"}]
},
{
   "name":"package.name.YourClassName$Companion",
   "methods":[{"name":"serializer","parameterTypes":[] }]
}

[thombergs]

I can confirm that @Scui1's suggestion above works. When you add the above JSON snippet (wrapped in an array) to, e.g. META-INF/native-image/groupid/artifactid/reflect-config.json (see GraalVM docs), you don't need to specify the serializer manually anymore.

Or, if you're using Spring, you can register your @Serializable classes like this:

class RuntimeHints : RuntimeHintsRegistrar {
    override fun registerHints(hints: RuntimeHints, classLoader: ClassLoader?) {
        hints.reflection()
            .registerField(YourSerializable::class.java.getField("Companion"))
        hints.reflection()
            .registerMethod(YourSerializable.Companion::class.java.getMethod("serializer"), ExecutableMode.INVOKE)
       }
}