How to throw an exception properly when do Flux processing?

Question

Existing code that I have:

private Flux<Integer> testGetFluxTestData() {
    return Flux.just(new TestData(1), new TestData(2))
            .collectList()
            .map(list -> list.stream()
                    .map(TestData::getId)
                    .collect(Collectors.toList()))
            .flatMapMany(Flux::fromIterable);
}

I want to enrich existing code and throw an exception when some not allowed data received, I made the following changes:

    private Flux<Integer> testGetFluxTestData2() {
        return Flux.just(new TestData(1), new TestData(2))
                .collectList()
                .map(list -> {
                    return !list.contains(new TestData(1)) ?
                            list.stream()
                                    .map(TestData::getId)
                                    .collect(Collectors.toList()) :
                            Flux.error(new IllegalTestDataException("illegal test data 1"));
                })
                .flatMapMany(Flux::fromIterable);
    }

but my implementation even noncompilable due to the following line:

Flux.error(new IllegalTestDataException("illegal test data 1"));

Could you please suggest, how to handle exception throwing for my particular scenario?

Answer 1

You are attempting to map from a List<TestData> to either a List<Integer> or a Flux<?> (error), which makes the desired result type ambiguous. Returning a reactive type in a mapping function is generally not desired (you'd want to do that in a flatmapping function).

(side note: even if you were in a flatMap , it wouldn't work either because at that point you're in Mono API due to collectList , so Mono.flatMap expects a Mono result to the Function ).

Note that the map operator catches exceptions from the lambda and turn them into an onError signal, so technically you could replace the Flux.error with a throw .

Otherwise, you'd need to turn the map into a flatMap and the Flux.error into a Mono.error , for the reasons stated above.