Reactive programming is about building asynchronous, non-blocking, and event-driven applications that can easily scale.

Reactor is a Reactive library for building non-blocking applications. It is based on the Reactive Streams Specification. Java 8 is required to use this library. It is integrated into Java 9.

Reactive Streams are push-based. It is the Publisher that notifies the Subscriber of newly available values as they come, and this push aspect is key to being reactive.


We’ll need reactor-core and reactor-test along with JUnit to go through this tutorial.

Publishers (Mono & Flux)

Mono and Flux are implementations of the Publisher interface. A Flux will observe 0 to N items and eventually terminate successfully or not. A Mono will observe 0 or 1 item, with Mono<Void> hinting at most 0 items.

Let’s see with the help of tests how to use this library.

In this example, we created an empty Mono and a Flux and used a StepVerifier to test them. The Publishers completed without emitting any object.

We initialized the Mono & Flux in different ways and verified that they are emitting the expected objects.

We can use all the Java 8 Stream operations on Mono & Flux.

In the first example, we mapped a Mono emitting a name to a Mono emitting the same name in lower-case. We verified that the resulting Mono emitted the same name in lower-case.

In the second example, we mapped a Flux emitting names to a Flux emitting the names in lower-case after applying a filter that passed only names starting with ‘k’. We verified that the resulting Flux emitted only names starting with ‘k’ in lower-case.

In the first example, we have 3 Fluxes emitting the title, first name and the last name. is combining them in a strict sequence (when all Fluxes have emitted their nth item). We then concatenated them to create a Flux emitting the full names.

In the second example, we created a Flux that generates a long value every 5 ms. We then combined it with the Flux firstNames. Hence, the resulting Flux will emit a value after every 5 ms. We used this Flux similarly as in the previous example and verified that the sequence of combination is maintained despite the delay.

Interleaving is a concept in which data is written non-sequentially to improve performance.

We have two Fluxes, one of them emitting values with a delay. Flux.mergeWith merges them into an interleaved sequence. Hence, we see that the sequence has changed.

Flux.concatWith merges them into a non-interleaved sequence. Hence, we see that the sequence remains the same despite the delay.

We can subscribe to a Publisher indefinitely and get the values in a blocking manner.


I have tried explaining with simple examples the very basics of reactor-core. You can read more about Project Reactor.

To learn how to create Reactive applications using Spring Boot And Reactor you can see these tutorials.

You can find the complete example on Github.