Mono.just("some") // `①`
                .flatMap(x -> {
                    if (x.length() > 5) return Mono.just(x);
                    else return Mono.empty();
                //want to get `some` from `①`
                .switchIfEmpty(Mono.just("empty"))
                .subscribe(System.out::print);

Mono.just("some") // `①`
                .flatMap(x -> {
                    if (x.length() > 5) return Mono.just(x);
                    else return Mono.empty();
                //want to get `some` from `①`
                .onEmptyResume((x)->{ // onEmptyResume not exist
                    //this x is `some` from `①`
                .switchIfEmpty(Mono.just("empty"))
                .subscribe(System.out::print);