Another conjugate question: What is the process if I want to apply `@Serializable (with = customJava...

# serialization

[Yao-Wen Mei]

Another conjugate question: What is the process if I want to apply @Serializable (with = customJavaSerializer.class) annotation on a Java class? (I want to have both the class itself and the custom serializer defined in Java, but use them in Kotlin)

I have tried define a Java class Hello, and then define a custom serializer HelloSerializer in Java. What is the field name or method name (like the Impl in the previous annotation example) should I give, so that the complier plugin can get access to my custom java serializer with @Serializable? Thank you very much.

[ephemient]

probably you need a static Companion INSTANCE; and a serializer() method inside Companion, but this whole approach looks quite ill-advised to me. what is preventing you from defining the class in Kotlin?

[Yao-Wen Mei]

The thing is we have a Java SDK first (all the annotations and classes are defined in Java), and then we add a Kotlin extension lib for it. Now, I want to provide Kotlin serialization for Kotlin SDK users, there seems no way I can make these Java class @Serializable unless 1) I use @Contextual, 2) Or I find out a way to apply @Serializable to the Java classes. Do you have any suggestions?

[ephemient]

if you are adding references to serialization in your Java SDK, then it'll depend on Kotlin regardless

[Yao-Wen Mei]

Following your suggestion, I am adding the static Companion Instance and serializer() in my Hello.class :

@Serializable(with = HelloSerializer.class)
public final class Hello {
    public static final Hello.Companion INSTANCE = new Hello.Companion((DefaultConstructorMarker)null);

    public static final class Companion {
        private Companion() {
        }

        public Companion(DefaultConstructorMarker $constructor_marker) {
            this();
        }

        @NotNull
        public final KSerializer serializer() {
            return new HelloSerializer();
        }
    }


}

And this is my custom serializer:

public final class HelloSerializer implements KSerializer<Hello> {

    @Override
    public Hello deserialize(@NonNull Decoder decoder) {
        return new Hello();
    }

    @NonNull
    @Override
    public SerialDescriptor getDescriptor() {
        return SerialDescriptorsKt.PrimitiveSerialDescriptor("Hello", (PrimitiveKind) PrimitiveKind.STRING.INSTANCE);
    }

    @Override
    public void serialize(@NonNull Encoder encoder, Hello hello) {
        encoder.encodeString("hello");

When I try to get `Hello`’s serializer from Kotlin, I got NoSuchElementException

[ephemient]

sorry INSTANCE is wrong, it's Companion - but I don't think this is a good idea anyway, it's possible future compiler plugin versions will require valid Kotlin metadata as well which you can't supply

[Yao-Wen Mei]

We are trying to let our Java SDK only depend on the official kotlin-serialization-core library, but independent from our Kotlin extension library.

[ephemient]

it'll depend on kotlin-stdlib transitively, and between that and writing Kotlin in your Java SDK directly there is no observable difference to Java consumers

[Yao-Wen Mei]

I have changed INSTANCE to Companion, but still the same `NoSuchElementException`🤣.

[ephemient]

🤷 I dunno then

[ephemient]

if you really want to keep any reference to Kotlin out of the Java SDK, I think you will have to provide a SerializersModule in your Kotlin SDK to register all the contextual serializers

[Yao-Wen Mei]

Thank you very much for your help, one caveat is that in our Java SDK, we are using compileOnly dependency, so Java user will not pay the size cost.

One more thing to confirm, as you mentioned:

I don't think it can be guaranteed that you  haven't missed anything that it might additionally depend on in the future

Is this plugin supposed to be backward compatible?

[ephemient]

if it's compileOnly, then annotations are ok, but referencing KSerializer will fail when the class loads. you can't do that

[Yao-Wen Mei]

Haha~~~

[ephemient]

it's only expected to be backwards compatible to its own output

[Yao-Wen Mei]

Thank you very much for your help~~