Handling checked exception in Mono flow - mono

Question

Not sure how to handle checked exception in the Mono flow.
return Mono.when(monoPubs)
.zipWhen((monos) -> repository.findById(...))
.map((tuple) -> tuple.getT2())
.zipWhen((org) -> createMap(org))
.map((tuple) -> tuple.getT2())
.zipWhen((map) -> emailService.sendEmail(...))
.flatMap(response -> {
return Mono.just(userId);
});
Here, the sendEmail method is declared with throws Exception.
public Mono<Boolean> sendEmail(...)
throws MessagingException, IOException
So, How to handle this checked exception in the zipWhen flow.
Also, How to handle
.zipWhen((map) -> emailService.sendEmail(...))
if the method returns void.

Answer 1

You need to review implementation of the sendEmail. You cannot throw checked exceptions from the publisher and need to wrap any checked exception into an unchecked exception.
The Exceptions class provides a propagate method that could be used to wrap any checked exception into an unchecked exception.
try {
...
}
catch (SomeCheckedException e) {
throw Exceptions.propagate(e);
}
As an alternative, you could use lombok #SneakyThrows to wrap non-reactive method.

Answer 2

Exceptions are thrown from Mono method, you can use onError* methods to handle the exception the way you like
var result = Mono.just("test")
.zipWhen((map) -> sendEmail())
.onErrorMap(SendEmailException.class, e -> new RuntimeException(e.getMessage()))
.flatMap(Mono::just);
Also it is not very clear from your post that if sendEmail takes param or not, sendEmail is not taking any input, I would just use doOnNext as it is a void method.