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

user_15318112 0
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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?

spring-webflux spring-boot reactive-programming spring java
2 Answers

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.

user_89092350

Basically you will have the same result in the end and no difference between the two options (maybe performance wise but I have not found anything backing this opinion so I guess it can be negligible. The only “difference” is that Mono.error follows the Reactive Streams specification and throwing an exception as is does not (read more at https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.3/README.md#2.13). However it is not prohibited, but if you like to follow standards and specifications (I guess you do) you should consider using Mono.error.

user_165722950
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