RxJava2 module not working in GraalVM native image

[auke-]

When using the Micronaut RxJava2 module in a Micronaut 3.x application the service always returns an empty json object ({}) when the controller method uses a RxJava2 return type (e.g. io.reactivex.Single).

Since this isn't the case for Reactor and RxJava3 types (they work as expected) I compared the micronaut-rxjava2 and micronaut-rxjava3 projects and it seems like the GraalVM reflection configuration is missing in the micronaut-rxjava2 jar, probably due to the absence of the micronaut-graal annotation processor in micronaut-rxjava2/build.gradle (see micronaut-rxjava3/build.gradle).

The trace logs shows (when returning io.reactivex.Single<List<MyObject>>):

[io-executor-thread-1] TRACE i.m.h.s.netty.RoutingInBoundHandler - Encoding emitted response object [io.micronaut.rxjava2.instrument.RxInstrumentedSingle@1fffc00c] using codec: io.micronaut.json.codec.JsonMediaTypeCodec@1221bd44

While for both Reactor and RxJava3 based return types the actual objects are shown (when returning io.reactivex.rxjava3.core.Single<List<MyObject>> or reactor.core.publisher.Mono<List<MyObject>>):

[io-executor-thread-1] TRACE i.m.h.s.netty.RoutingInBoundHandler - Encoding emitted response object [[<MyObject>,<MyObject>]] using codec: io.micronaut.json.codec.JsonMediaTypeCodec@467a29cd

I was under the impression that Micronaut didn't use reflection for json serialization but uses the BeanIntrospection classes generated based on the @Introspected annotation, but it looks like it is missing reflection information for the RxJava2 classes.

Note that adding @TypeHint(value = io.reactivex.Single.class) to the application works around this issue.

Steps to Reproduce

1. Create a Micronaut project with the GraalVM feature (e.g. using https://guides.micronaut.io/latest/micronaut-creating-first-graal-app-maven-java.html)
2. Add the micronaut-rxjava2 dependency
3. Add a method to the controller which returns a io.reactivex.Single
4. Build the native image and invoke the service

Expected Behaviour

The service should return the objects created in the controller method

Actual Behaviour

An empty json object ({}) is returned

Environment Information

• Operating System: MacOS
• Micronaut Version: v3.2.3
• JDK Version: GraalVM 17.0.1+12-jvmci-21.3-b05