Mastering Reactor Error Handling: 5 Essential Operators in Spring WebFlux

Reactor Error Handling Operators Overview

Reactor, the core engine of Spring WebFlux, provides several error‑handling operators. This article explains five common ones—onErrorReturn, onErrorResume, doOnError, retry, blockOptional—detailing their behavior, appropriate use cases, and pitfalls.

Reactor is one of the mainstream implementations of the Reactive Streams specification in the Java ecosystem and the core engine of Spring WebFlux.

Usage Examples

1. onErrorReturn (default value)

Mono.just("hello")
    .map(s -> { throw new RuntimeException(); })
    .onErrorReturn("default value");

2. onErrorResume (lambda support)

Mono.just("hello")
    .map(s -> { throw new IllegalArgumentException(); })
    .onErrorResume(e -> {
        if (e instanceof IllegalArgumentException) {
            return Mono.just("fallback");
        }
        return Mono.error(e);
    });

3. doOnError (logging only)

Mono.just("hello")
    .map(s -> { throw new RuntimeException(); })
    .doOnError(e -> log.error("Exception occurred", e));

4. retry (automatic retries)

Mono.fromCallable(() -> callThirdParty())
    .retry(3);

5. blockOptional (blocking for final result)

Mono<String> mono = Mono.just("hello");
Optional<String> result = mono.blockOptional();

Key Takeaways

onErrorReturn : simple fallback that treats all errors equally; rarely recommended.

onErrorResume : flexible; decides fallback based on exception type; most commonly used.

doOnError : only logs; does not alter the stream; useful for diagnostics.

retry : retries on transient failures; beware of infinite loops or added load.

blockOptional : blocking call suitable only for testing or end‑of‑pipeline scenarios; avoid in reactive flows.

Understanding these operators helps you build robust, non‑blocking reactive applications with Spring WebFlux.