Mono<T> is a Publisher<T> that supports 0..1 elements.

The definition of Mono is as follows:

public abstract class Mono<T>
    extends Object
    implements Publisher<T>

As detailed in the documentation, the following figure shows the workings of Mono:

Mono<Void> should be used for a Publisher that completes with no value. The documentation explains each method and how it works using a marble diagram, which is self-explanatory. Again, this type is also supported by Spring 5 and Spring Security.

JavaDoc for Mono contains more information: https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html.

Let's have a look at some examples:

• Creating empty Mono:

Mono<String> emptyMono = Mono.empty();

• Creating Mono with a value in it:

Mono<String> itemMono = Mono.just("Spring Security Reactive”);

• Creating Mono that emits an exception:

Mono.error(new CreatedException());