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General improvements e.g. make use of Java 8 Stream. The main reason for the refactoring however to tighten error handling. To that extent: InvocableHandlerMethod turns all exceptions into Reactive Streams error signals, in effect never allowing any Exceptions to bubble up. HandlerMethodArgumentResolver may throw an Exception for sync resolution or produce an error signal via the returned Publisher. Either way the exception is consistently wrapped with helpful method argument details. For the latter case using a custom mapError operator. HandlerMethodArgumentResolver no longer needs to return Optional for nullable argument values. Instead (for now) the defaultIfEmpty operator of reactor.rx.Stream operator is used to ensure a default constant value (called "NO_VALUE") is produced. That way an argument resolver may produce 0..1 values where 0 means it did not resolve to any value and that results in null passed as the argument value. If a HandlerMethodArgumentResolver produces more than one value, all additional values beyond the first one will be ignored with the help of a custom "first" operator. As HandlerMethod is invoked within the map operator, checked exceptions are not allowed but instead of wrapping it in a runtime exception what we really need is to unwrap the target exception for exception resolution purposes. To this end concatMap is used to produce a nested Publisher or an error Publisher with the unwrapped target exception. Related to that InvocableHandlerMethod now returns Publisher<HandlerResult> instead of Publisher<Object> so that no longer needs to be externally mapped from Object to HandlerResult. InvocableHandlerMethodTests provides tests for the above scenarios and verifies the details of resulting error signals.