The resteasy-client and resteasy-client-api both have dependencies on io.reactivestreams:reactive-streams. These should not be dependencies on either module and should be in the org.jboss.resteasy:resteasy-reactive module. This will likely break backwards compatibility, however I really think this needs to be cleaned up.

The org.jboss.resteasy.client.jaxrs.internal.ClientInvocation has the following method as well:

public Optional<ReactiveInvocation> reactive() {
   if (client.httpEngine() instanceof ReactiveClientHttpEngine) {
      return Optional.of(new ReactiveInvocation((ReactiveClientHttpEngine)client.httpEngine()));
   }
   return Optional.empty();
}

However, ReactiveInvocation is package-private which is also strange. This whole part needs to be examined.

As part of this we may want to introduce JDK reactive API's. This might be better done in a separate commit. It will depend on how this can and will be broken.