How to transform a Mono to a Mono<Void> in case of error

Question

I'm trying to write a method that does something like this

   Mono<A> ma = networkCall(); //this might fail
   Mono<Void> mv  = ma.map( a -> ....)  #some logic to perform with `A`

   return mv;

The trick is, ma might very well fail and then I would want to just log the situation and return a Mono<Void> that completes with no error.

Looking at the Mono api I just found onErrorResume or onErrorReturn but both would take a function that returns an A (which I can't fabricate), while I would like to return a Void. I would imagine the solution is quite simple, but couldn't quite find the right operations for this. So, what operations should I apply to ma to transform it into a Mono<Void> in case of error?

Answer 1

I just found onErrorResume or onErrorReturn but both would take a function that returns an A

onErrorReturn() indeed requires you to return an A, but onErrorResume() just requires you to return a Mono<A>, which can be empty.

So you can use:

• doOnNext() to perform your logic with A if the call is successful;
• doOnError() to log your error if the call is not successful;
• onErrorResume() to return an empty result
• then() to convert the result into a Mono<Void>.

Something like:

networkCall()
        .doOnNext(a -> doSomethingWith(a))
        .doOnError(e -> e.printStackTrace())
        .onErrorResume(e -> Mono.empty())
        .then();