this is a demonstration of what seemed to be a bug in either micronaut-core or micronaut-rabbitmq module.

a connection to a RabbitMQ broker is required, configurable in application.yml. the command to expose the bug: mvn test

sending a message to RabbitMQ fails with:

java.lang.ClassCastException: class reactor.core.publisher.FluxOnErrorResume cannot be cast to class reactor.core.publisher.Mono (reactor.core.publisher.FluxOnErrorResume and reactor.core.publisher.Mono are in unnamed module of loader 'app')

raised as an issue.

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turned out i was missing a dependency.