What is then, thenEmpty, thenMany and flatMapMany in spring webflux?

• flatMap vs flatMapMany

In functional programming, flatMap returns the same type than the type that bear the method, so for Mono<T>, flatMap returns a Mono. Which means that only one element can be emitted by the inner Publisher (or that it is truncated). We enforced that by having Mono#flatMap take a Function<T, Mono<R>>.

As a consequence, we needed an alternative for more arbitrary Publisher that could emit more than one element. Hence Mono#flatMapMany(Function<T, Publisher<R>>) which returns a Flux<R>.

TL;DR: Mono#flatMap is for asynchronous but 1-to-1 transformation of the element in the source Mono, Mono#flatMapMany is for 1-to-N asynchronous transformation (like Flux#flatMap).

• then, thenEmpty and thenMany

All the thenXXX methods on Mono have one semantic in common: they ignore the source onNext signals and react on completion signals (onComplete and onError), continuing the sequence at this point with various options. As a consequence, this can change the generic type of the returned Mono:

1. then will just replay the source terminal signal, resulting in a Mono<Void> to indicate that this never signals any onNext.
2. thenEmpty not only returns a Mono<Void>, but it takes a Mono<Void> as a parameter. It represents a concatenation of the source completion signal then the second, empty Mono completion signal. In other words, it completes when A then B have both completed sequentially, and doesn’t emit data.
3. thenMany waits for the source to complete then plays all the signals from its Publisher<R> parameter, resulting in a Flux<R> that will “pause” until the source completes, then emit the many elements from the provided publisher before replaying its completion signal as well.