Project Reactor - Convert Mono<List<T>> To Flux<T> & Vice Versa

This tutorial gives you examples of how to convert Mono<List<T>> into Flux<T> and vice versa.

Reactor has two reactive types: Mono, which represents an empty or single value result (0..1) and Flux, which represents a sequence of items (0..N). Though Mono can only have maximum of one value, the value can be a List (an Iterable). In some cases, you may need to convert it into a Flux. Or maybe the opposite case, you want to convert Flux into a Mono that contains Iterable. For both cases, you can find the examples below.

Convert Mono<List<T>> Into Flux<T>

For example, you have a Mono<List<Integer> and you want to convert it into Flux<Integer>, you can use .flatMapMany(Flux::fromIterable)

  ArrayList<Integer> list = new ArrayList<>();
  for (int i = 0; i < 10; i++) {
      list.add(i);
  }
  
  Mono<List<Integer>> m = Mono.just(list);
  
  Flux<Integer> f = m.flatMapMany(Flux::fromIterable);

Convert Flux<T> Into Mono<List<T>>

Now,there is a Flux<Integer>. To convert it into Mono<List<Integer>>, use .collectList.

  Flux<Integer> f = Flux.range(1, 10);

  Mono<List<Integer>> m = f.collectList();

That's how to convert between those two reactive types of Reactor.