Custom data

You can pass custom objects through JNI boundary by marking them with JniSerializable annotation. The KNI processor will generate serializers to convert your data from and to a ByteArray.

import com.dshatz.kni.serialization.JniSerializable

@JniSerializable
data class ColorfulObject(
    val color: String,
    val weight: Double
)

@JniSerializable
value class Amount(val amountCents: Long) {
    val amount: Double get() = amountCents / 100.0
}

You can then use this type in your JNI calls:

// commonMain
@JniCall
expect fun passObject(obj: ColorfulObject): Amount


// nativeMain
@JniCall 
actual fun passObject(obj: ColorfulObject): Amount {
    return if (obj.color == "orange") Amount(150)
    else if (obj.color == "green") Amount(100)
    else Amount(80)
}

Supported class types:

data class
value class
sealed class - polymorphic serializer will be generated

Customizing serialization for properties

Apply a @JniSerializable(with = ...) annotation to a property of data class to supply your own implementation of a JniSerializer.

Last modified: 16 July 2026