[FIXED] Throwing an exception vs Mono.error() in Spring webflux

Issue

I'm working on a Spring webflux project and I want to understand the difference between throwing an exception vs using Mono.error().

If there is a validation class like this for example:

 public class NameValidator {

   public static boolean isValid(String name) {
     if(StringUtils.isEmpty(name)) {throw new RuntimeException("Invalid name");}
     return true;
  }
}



public class NameValidator2 {

   public static Mono<Object> isValid(String name) {
     if(StringUtils.isEmpty(name)) {
          return Mono.error(new RuntimeException("Invalid name"));}
     return Mono.just(true);

  }

 }

What are the pros & cons with each approach. When to use one over the other while working with reactive streams using spring webflux?

Solution

As @Joao already stated, the recommended way to deal with an error is to call the error method on a Publisher(Mono.error/Flux.error).

I would like to show you an example in which the traditional throw does not work as you may expect:

public void testErrorHandling() {
    Flux.just("a", "b", "c")
        .flatMap(e -> performAction()
            .onErrorResume(t -> {
                System.out.println("Error occurred");
                return Mono.empty();
            }))
        .subscribe();
}

Mono<Void> performAction() {
    throw new RuntimeException();
}

The onErrorResume operator will never be executed because the exception is thrown before Mono is assembled.

Answered By - lkatiforis