Mono, an Asynchronous 0-1 Result

The following image shows how a Mono transforms an item:

A Mono<T> is a specialized Publisher<T> that emits at most one item via the onNext signal then terminates with an onComplete signal (successful Mono, with or without value), or only emits a single onError signal (failed Mono).

Most Mono implementations are expected to immediately call onComplete on their Subscriber after having called onNext. Mono.never() is an outlier: it doesn’t emit any signal, which is not technically forbidden although not terribly useful outside of tests. On the other hand, a combination of onNext and onError is explicitly forbidden.

Mono offers only a subset of the operators that are available for a Flux, and some operators (notably those that combine the Mono with another Publisher) switch to a Flux. For example, Mono#concatWith(Publisher) returns a Flux while Mono#then(Mono) returns another Mono.

Note that you can use a Mono to represent no-value asynchronous processes that only have the concept of completion (similar to a Runnable). To create one, you can use an empty Mono<Void>.