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Reactor by Example

Key takeaways

  • Reactor is a reactive streams library targeting Java 8 and providing an Rx-conforming API
  • It uses the same approach and philosophy as RxJava despite some API differences
  • It is a 4th generation reactive library that allows operator fusion, like RxJava 2
  • Reactor is a core dependency in the reactive programming model support of Spring Framework 5.


RxJava recap

Reactor, like RxJava 2, is a fourth generation reactive library. It has been launched by Spring custodian Pivotal, and builds on the Reactive Streams specification, Java 8, and the ReactiveX vocabulary. Its design is the result of a savant mix fueled by designs and core contributors from Reactor 2 (the previous major version) and RxJava.

In previous articles in this series, "RxJava by Example" and "Testing RxJava", you learned about the basics of reactive programming: how data is conceptualized as a stream, the Observable class and its various operators, the factory methods that create Observables from static and dynamic sources.

Observable is the push source and Observer is the simple interface for consuming this source via the act of subscribing. Keep in mind that the contract of an Observable is to notify its Observer of 0 or more data items through onNext, optionally followed by either an onError or onComplete terminating event.

To test an Observable, RxJava provides a TestSubscriber, which is a special flavor of Observer that allows you to assert events in your stream.

In this article we'll draw a parallel between Reactor and what you already learned about RxJava, and showcase the common elements as well as the differences.

Reactor's types

Reactor's two main types are the Flux<T> and Mono<T>. A Flux is the equivalent of an RxJava Observable, capable of emitting 0 or more items, and then optionally either completing or erroring.

A Mono on the other hand can emit at most once. It corresponds to both Single and Maybe types on the RxJava side. Thus an asynchronous task that just wants to signal completion can use a Mono<Void>.

This simple distinction between two types makes things easy to grasp while providing meaningful semantics in a reactive API: by just looking at the returned reactive type, one can know if a method is more of a "fire-and-forget" or "request-response" (Mono) kind of thing or is really dealing with multiple data items as a stream (Flux).

Both Flux and Mono make use of this semantic by coercing to the relevant type when using some operators. For instance, calling single() on a Flux<T> will return a Mono<T>, whereas concatenating two monos together using concatWith will produce a Flux. Similarly, some operators will make no sense on a Mono (for example take(n), which produces n > 1 results), whereas other operators will only make sense on a Mono (e.g. or(otherMono)).

One aspect of the Reactor design philosophy is to keep the API lean, and this separation into two reactive types is a good middle ground between expressiveness and API surface.

"Build on Rx, with Reactive Streams at every stage"

As expressed in "RxJava by Example", RxJava bears some superficial resemblance to Java 8 Streams API, in terms of concepts. Reactor on the other hand looks a lot like RxJava, but this is of course in no way a coincidence. The intention is to provide a Reactive Streams native library that exposes an Rx-conforming operator API for asynchronous logic composition. So while Reactor is rooted in Reactive Streams, it seeks general API alignment with RxJava where possible.

Reactive Libraries and Reactive Streams adoption

Reactive Streams (abbreviated RS in the remainder of this article) is "an initiative to provide a standard for asynchronous stream processing with non-blocking back pressure". It is a set of textual specifications along with a TCK and four simple interfaces (Publisher, Subscriber, Subscription and Processor), which will be integrated in Java 9.

It mainly deals with the concept of reactive-pull back-pressure (more on that later) and how to interoperate between several implementing reactive sources. It doesn't cover operators at all, focusing instead exclusively on the stream's lifecycle.

A key differentiator for Reactor is its RS first approach.Both Flux and Mono are RS Publisher implementations and conform to reactive-pull back-pressure.

In RxJava 1 only a subset of operators support back-pressure, and even though RxJava 1 has adapters to RS types, its Observable doesn't implement these types directly. That is easily explained by the fact that RxJava 1 predates the RS specification and served as one of the foundational works during the specification's design.

That means that each time you use these adapters you are left with a Publisher, which again doesn't have any operator. In order to do anything useful from there, you'll probably want to go back to an Observable, which means using yet another adapter. This visual clutter can be detrimental to readability, especially when an entire framework like Spring 5 directly builds on top of Publisher.

Another difference with RxJava 1 to keep in mind when migrating to Reactor or RxJava 2 is that in the RS specification, null values are not authorized. It might turn out important if your code base uses null to signal some special cases.

RxJava 2 was developed after the Reactive Streams specification, and thus has a direct implementation of Publisher in its new Flowable type. But instead of focusing exclusively on RS types, RxJava 2 also keeps the "legacy" RxJava 1 types (Observable, Completable, and Single)  and introduces the "RxJava Optional", Maybe. Although they still provide the semantic differentiation we talked about earlier, these types have the drawback of not implementing RS interfaces. Note that unlike in RxJava 1, Observable in RxJava 2 does not support the backpressure protocol in RxJava 2 (a feature now exclusively reserved to Flowable). It has been kept for the purpose of providing a rich and fluent API for cases, such as user interface eventing, where backpressure is impractical or impossible. Completable, Single and Maybe have by design no-need for backpressure support, they will offer a rich API as well and defer any workload until subscribed.

Reactor is once again leaner in this area, sporting its Mono and Flux types, both implementing Publisher and both backpressure-ready. There's a relatively small overhead for Mono to behave as a Publisher, but it is mostly offsetted by other Mono optimizations. We'll see in a later section what backpressure means for Mono.

An API similar but not equal to RxJava's

The ReactiveX and RxJava vocabulary of operators can be overwhelming at times, and some operators can have confusing names for historical reasons. Reactor aims to have a more compact API and to deviate in some cases, e.g. in order to choose better names, but overall the two APIs look a lot alike. In fact the latest iterations in RxJava 2 actually borrow some vocabulary from Reactor as well, a hint of the ongoing close collaboration between the two projects. Some operators and concepts first appear in one library or the other, but often end up in both.

For instance, Flux has the same familiar just factory method (albeit having only two just variants: one element and a vararg). But from, has been replaced by several explicit variants, most notable being fromIterable. Flux also has all the usual suspects in term of operators: map, merge, concat, flatMap, take…, etc.

One example of an RxJava operator name that Reactor eschewed was the puzzling amb operator, which has been replaced with the more appropriately named firstEmitting. Additionally, to introduce greater consistency in the API, toList has been renamed collectList. In fact all collectXXX operators now aggregate values into a specific type of collection but still produce a Mono of said collection, while toXXX methods are reserved for type conversions that take you out of the reactive world, eg. toFuture().

One more means by which Reactor can be leaner, this time in terms of class instantiation and resource usage, is fusion: Reactor is capable of merging multiple sequential uses of certain operators (eg. calling concatWith twice) into a single use, only instantiating the operator's inner classes once (macro-fusion). That includes some data source based optimization which greatly helps Mono offset the cost of implementing Publisher. It is also capable of sharing resources like inner queues between several compatible operators (micro-fusion). These capabilities make Reactor a fourth-generation reactive library. But that is a topic for a future article.

Let's take a closer look at a few Reactor operators. (You will notice the contrast with some of the examples in the earlier articles in our series.)

A few operator examples

(This section contains snippets of code, and we encourage you to try them and experiment further with Reactor. To that effect, you should open your IDE of choice and create a test project with Reactor as a dependency.)

To do so in Maven, add the following to the dependencies section of your pom.xml:


To do the same in Gradle, edit the dependencies section to add reactor, similarly to this:

dependencies {
    compile "io.projectreactor:reactor-core:3.0.3.RELEASE"

Let's play with examples used in the previous articles in this series!

Very similarly to how you would create your first Observable in RxJava, you can create a Flux using the just(T…) and fromIterable(Iterable<T>) Reactor factory methods. Remember that given a List, just would just emit the list as one whole, single emission, while fromIterable will emit each element from the iterable list:

public class ReactorSnippets {
  private static List<String> words = Arrays.asList(

  public void simpleCreation() {
     Flux<String> fewWords = Flux.just("Hello", "World");
     Flux<String> manyWords = Flux.fromIterable(words);


Like in the corresponding RxJava examples, this prints


In order to output the individual letters in the fox sentence we'll also need flatMap (as we did in RxJava by Example), but in Reactor we use fromArray instead of from. We then want to filter out duplicate letters and sort them using distinct and sort. Finally, we want to output an index for each distinct letter, which can be done using zipWith and range:

public void findingMissingLetter() {
  Flux<String> manyLetters = Flux
        .flatMap(word -> Flux.fromArray(word.split("")))
        .zipWith(Flux.range(1, Integer.MAX_VALUE),
              (string, count) -> String.format("%2d. %s", count, string));


This helps us notice the s is missing as expected:

1. a
2. b
18. r
19. t
20. u
25. z

One way of fixing that is to correct the original words array, but we could also manually add the "s" value to the Flux of letters using concat/concatWith and a Mono:

public void restoringMissingLetter() {
  Mono<String> missing = Mono.just("s");
  Flux<String> allLetters = Flux
        .flatMap(word -> Flux.fromArray(word.split("")))
        .zipWith(Flux.range(1, Integer.MAX_VALUE),
              (string, count) -> String.format("%2d. %s", count, string));


This adds the missing s just before we filter out duplicates and sort/count the letters:

1. a
2. b
18. r
19. s
20. t
26. z

The previous article noted the resemblance between the Rx vocabulary and the Streams API, and in fact when the data is readily available from memory, Reactor, like Java Streams, acts in simple push mode (see the backpressure section below to understand why). More complex and truly asynchronous snippets wouldn't work with this pattern of just subscribing in the main thread, primarily because control would return to the main thread and then exit the application as soon as the subscription is done. For instance:

public void shortCircuit() {
  Flux<String> helloPauseWorld = 


This snippet prints "Hello", but fails to print the delayed "world" because the test terminates too early. In snippets and tests where you only sort of write a main class like this, you'll usually want to revert back to blocking behavior. To do that you could create a CountDownLatch and call countDown in your subscriber (both in onError and onComplete). But then that's not very reactive, is it? (and what if you forget to count down, in case of error for instance?)

The second way you could solve that issue is by using one of the operators that revert back to the non-reactive world. Specifically, toIterable and toStream will both produce a blocking instance. So let's use toStream for our example:

public void blocks() {
  Flux<String> helloPauseWorld = 


As you would expect, this prints "Hello" followed by a short pause, then prints "world" and terminates.

As we mentioned above, RxJava amb() operator has been renamed firstEmitting (which more clearly hints at the operator's purpose: selecting the first Flux to emit). In the following example, we create a Mono whose start is delayed by 450ms and a Flux that emits its values with a 400ms pause before each value. When firstEmitting() them together, since the first value from the Flux comes in before the Mono's value, it is the Flux that ends up being played:

public void firstEmitting() {
  Mono<String> a = Mono.just("oops I'm late")
  Flux<String> b = Flux.just("let's get", "the party", "started")

  Flux.firstEmitting(a, b)

This prints each part of the sentence with a short 400ms pause between each section.

At this point you might wonder, what if you're writing a test for a Flux that introduces delays of 4000ms instead of 400? You don't want to wait 4s in a unit test! Fortunately, we'll see in a later section that Reactor comes with powerful testing facilities that nicely cover this case.

But for now, we have sampled how Reactor compares for a few common operators, so let's zoom back and have a look at other differentiating aspects of the library.

A Java 8 foundation

Reactor targets Java 8 rather than previous Java versions. This is once again aligning with the goal of reducing the API surface: RxJava targets Java 6 where there is no java.util.function package so classes like Function or Consumer can't be leveraged. Instead they had to add specific classes like Func1, Func2, Action0, Action1, etc. In RxJava 2 these classes mirror java.util.function the way Reactor 2 used to do when it still had to support Java 7.

The Reactor API also embraces types introduced in Java 8. Most of the time-related operators will be about a duration (eg. timeout, interval, delay, etc.), so using the Java 8 Duration class is appropriate.

The Java 8 Stream API and CompletableFuture can also both be easily converted to a Flux/Mono, and vice-versa. Should we usually convert a Stream to a Flux though? Not really. The level of indirection added by Flux or Mono is a negligible cost when they decorate more costly operations like IO or memory-bound operations, but most of the time a Stream doesn't imply that kind of latency and it is is perfectly ok to use the Stream API directly. Note that for these use cases in RxJava 2 we'd use the Observable, as it is not backpressured and thus becomes a simple push use case once you've subscribed. But Reactor is based on Java 8, and the Stream API is expressive enough for most use cases. Note also that even though you can find Flux and Mono factories for literal or simple Objects, they mostly serve the purpose of being combined in higher level flows. So typically you wouldn't want to transform an accessor like "long getCount()" into a "Mono<Long> getCount()" when migrating an existing codebase to reactive patterns.

The Backpressure story

One of the main focuses (if not the main focus) of the RS specification and of Reactor itself is backpressure. The idea of backpressure is that in a push scenario where the producer is quicker than the consumer, there's value in letting the consumer signal back to the producer and say "Hey! Slow down a little, I'm overwhelmed". This gives the producer a chance to control its pace rather than having to resort to discarding data (sampling) or worse, risking a cascading failure.

You may wonder at this point where backpressure comes into the picture with Mono: what kind of consumer could possibly be overwhelmed by a single emission? Short answer is "probably none". However, there's still a key difference between how a Mono works and how a CompletableFuture works. The latter is push only: if you have a reference to the Future, it means the task processing an asynchronous result is already executing. On the other hand, what a backpressured Flux or Mono enables is a deferred pull-push interaction:

  1. Deferred because nothing happens before the call to subscribe()
  2. Pull because at the subscription and request steps, the Subscriber will send a signal upstream to the source and essentially pull the next chunk of data
  3. Push from producer to consumer from there on, within the boundary of the number of requested elements

For Mono, subscribe() is the button that you press to say "I'm ready to receive my data". For Flux, this button is request(n), which is kind of a generalization of the former.

Realizing that Mono is a Publisher that will usually represent a costly task (in terms of IO, latency, etc.) is critical to understanding the value of backpressure here: if you don't subscribe, you don't pay the cost of that task. Since Mono will often be orchestrated in a reactive chain with regular backpressured Flux, possibly combining results from multiple asynchronous sources, the availability of this on-demand subscribe triggering is key in order to avoid blocking.

Having backpressure helps us differentiate that last use case from another Mono broad use case: asynchronously aggregating data from a Flux into a Mono. Operators like reduce and hasElement are capable of consuming each item in the Flux, aggregating some form of data about it (respectively the result of a reduce function and a boolean) and exposing that data as a Mono. In that case, the backpressure signalled upstream is Long.MAX_VALUE, which lets the upstream work in a fully push fashion.

Another interesting aspect of backpressure is how it naturally limits the amount of objects held in memory by the stream. As a Publisher, the source of data is most probably slow (at least slowish) at producing items, so the request from downstream can very well start beyond the number of readily available items. In this case, the whole stream naturally falls into a push pattern where new items are notified to the consumer. But when there is a production peak and the pace of production accelerates, things fall nicely back into a pull model. In both cases, at most N data (the request() amount) is kept in memory.

You can reason about the memory used by your asynchronous processing by correlating that demand for N with the number of kilobytes an item consumes, W: you can then infer that at most W*N memory will be consumed. In fact, Reactor will most of the time take advantage of knowing N to apply optimizations: creating queues bounded accordingly and applying prefetching strategies where it can automatically request 75% of N every time that same ¾ amount has been received.

Finally, Reactor operators will sometimes change the backpressure signal to correlate it with the expectations and semantics they represent. One prime example of this behavior would be buffer(10): for every request of N from downstream, that operator would request 10N from upstream, which represents enough data to fill the number of buffers the subscriber is ready to consume. This is called "active backpressure", and it can be put to good use by developers in order to explicitly tell Reactor how to switch from an input volume to a different output volume, in micro-batching scenarios for instance.

Relation to Spring

Reactor is the reactive foundation for the whole Spring ecosystem, and most notably Spring 5 (through Spring Web Reactive) and Spring Data "Kay" (which corresponds to spring-data-commons 2.0).

Having a reactive version for both of these projects is essential, in the sense that this enables us to write a web application that is reactive from start to finish: a request comes in, is asynchronously processed all the way down to and including the database, and results come back asynchronously as well. This allows a Spring application to be very efficient with resources, avoiding the usual pattern of dedicating a thread to a request and blocking it for I/O.

So Reactor is going to be used for the internal reactive plumbing of future Spring applications, as well as in the APIs these various Spring components expose. More generally, they'll be able to deal with RS Publishers, but most of the time these will happen to be Flux/Mono, bringing in the rich feature set of Reactor. Of course, you will be able to use your reactive library of choice, as the framework provides hooks for  adapting between Reactor types and RxJava types or even simpler RS types.

At the time of writing of this article, you can already experiment with Spring Web Reactive in Spring Boot by using Spring Boot 2.0.0.BUILD-SNAPSHOT and the spring-boot-starter-web-reactive dependency (eg. by generating such a project on


This lets you write your @Controller mostly as usual, but replaces the underlying Spring MVC traditional layer with a reactive one, replacing many of the Spring MVC contracts by reactive non-blocking ones. By default, this reactive layer is based on top of Tomcat 8.5, but you can also elect to use Undertow or Netty.

Additionally, although Spring APIs are based on Reactor types, the Spring Web Reactive module lets you use various reactive types for both the request and response:

  • Mono<T>: as the @RequestBody, the request entity T is asynchronously deserialized and you can chain your processing to the resulting mono afterward. As the return type, once the Mono emits a value, the T is serialized asynchronously and sent back to the client. You can combine both approaches by augmenting the request Mono and returning that augmented chain as the resulting Mono.
  • Flux<T>: Used in streaming scenarios (including input streaming when used as @RequestBody and Server Sent Events with a Flux<ServerSentEvent> return type)
  • Single/Observable: Same as Mono and Flux respectively, but switching to an RxJava implementation.
  • Mono<Void> as a return type: Request handling completes when the Mono completes.
  • Non-reactive return types (void and T): This now implies that your controller method is synchronous, but should be non-blocking (short-lived processing). The request handling finishes once the method is executed. The returned T is serialized back to the client asynchronously.

Here is a quick example of a plain text @Controller using the experimental web reactive module:

public class ExampleController {

   private final MyReactiveLibrary reactiveLibrary;

   //Note Spring Boot 4.3+ autowires single constructors now
   public ExampleController(MyReactiveLibrary reactiveLibrary) {
      this.reactiveLibrary = reactiveLibrary;

   public Mono<String> hello(@PathVariable String who) {
      return Mono.just(who)
                 .map(w -> "Hello " + w + "!");

   public Mono<String> helloDelay(@PathVariable String who) {
      return reactiveLibrary.withDelay("Hello " + who + "!!", 2);

   public Flux<String> hey(@RequestBody Mono<Sir> body) {
      return Mono.just("Hey mister ")
                .flatMap(sir -> Flux.fromArray(sir.getLastName().split("")))
            ).concatWith(Mono.just(". how are you?"));

The first endpoint takes a path variable, transforms it into a Mono<String> and maps that name to a greeting sentence that is returned to the client.

By doing a GET on /hello/Simon we get "Hello Simon!" as a text/plain response.

The second endpoint is a bit more complicated: it asynchronously receives a serialized Sir instance (a class simply made up of a firstName and lastName attributes) and flatMaps it into a stream of the last name's letters. It then takes the first of these letters, maps it to upper case and concatenates it into a greeting sentence.

So POSTing the following JSON object to /heyMister

	"firstName": "Paul",
	"lastName": "tEsT"

Returns the string "Hello mister T. How are you?".

The reactive aspect of Spring Data is also currently being developed in the Kay release train, which for spring-data-commons is the 2.0.x branch. There is a first Milestone out that you can get by adding the Spring Data Kay-M1 bom to your pom:


Then for this simplistic example just add the Spring Data Commons dependency in your pom (it will take the version from the BOM above):


Reactive support in Spring Data revolves around the new ReactiveCrudRepository<T, ID> interface, which extends Repository<T, ID>. This interface exposes CRUD methods, using Reactor input and return types. There is also an RxJava 1 based version called RxJava1CrudRepository. For instance, in the classical blocking CrudRepository, retrieving one entity by its id would be done using "T findOne(ID id)". It becomes "Mono<T> findOne(ID id)" and "Observable<T> findOne(ID id)" in ReactiveCrudRepository and RxJava1CrudRepository respectively. There are even variants that take a Mono/Single as argument, to asynchronously provide the key and compose on that.

Assuming a reactive backing store (or a mock ReactiveCrudRepository bean), the following (very naive) controller would be reactive from start to finish:

public class DataExampleController {

   private final ReactiveCrudRepository<Sir, String> reactiveRepository;

   //Note Spring Boot 4.3+ autowires single constructors now
   public DataExampleController(ReactiveCrudRepository<Sir, String> repo) {
      this.reactiveRepository = repo;

   public Mono<ResponseEntity<Sir>> hello(@PathVariable String who) {
      return reactiveRepository.findOne(who)

Notice how the data repository usage naturally flows into the response path: we asynchronously fetch the entity and wrap it as a ResponseEntity using map, obtaining a Mono we can return right away. If the Spring Data repository cannot find data for this key, it will return an empty Mono. We make that explicit by using defaultIfEmpty and returning a 404.

Testing Reactor

The article "Testing RxJava" covered techniques for testing an Observable. As we saw, RxJava comes with a TestScheduler that you can use with operators that accept a Scheduler as a parameter, to manipulate a virtual clock on these operators. It also features a TestSubscriber class that can be leveraged to wait for the completion of an Observable and to make assertions about every event (number and values for onNext, has onError triggered, etc.) In RxJava 2, the TestSubscriber is an RS Subscriber, so you can test Reactor's Flux and Mono with it!

In Reactor, these two broad features are combined into the StepVerifier class. It can be found in the addon module reactor-test from the reactor-addons repository. The StepVerifier can be initialized by creating an instance from any Publisher, using the StepVerifier.create builder. If you want to use virtual time, you can use the StepVerifier.withVirtualTime builder, which takes a Supplier<Publisher>. The reason for this is that it will first ensure that a VirtualTimeScheduler is created and enabled as the default Scheduler implementation to use, making the need to explicitly pass the scheduler to operators obsolete. The StepVerifier will then configure if necessary the Flux/Mono created within the Supplier, turning timed operators into "virtually timed operator". You can then script stream expectations and time progress: what the next elements should be, should there be an error, should it move forward in time, etc. Other methods include verifying that data matches a given Predicate or even consume onNext events, allowing you to do more advanced interactions with the value (like using an assertion library). Any AssertionError thrown by one of these will be reflected back in the final verification result. Finally, call verify() to check your expectations, this will truly subscribe to the defined source via StepVerifier.create or StepVerifier.withVirtualTime.

Let's take a few simple examples and demonstrate how StepVerifier works. For these snippets, you'll want to add the following test dependencies to your pom:



First, imagine you have reactive class called MyReactiveLibrary that produces a few Flux that you want to test:

public class MyReactiveLibrary {

  public Flux<String> alphabet5(char from) {
     return Flux.range((int) from, 5)
           .map(i -> "" + (char) i.intValue());

  public Mono<String> withDelay(String value, int delaySeconds) {
     return Mono.just(value)

The first method is intended to return the 5 letters of the alphabet following (and including) the given starting letter. The second method returns a flux that emits a given value after a given delay, in seconds.

The first test we'd like to write ensures that calling alphabet5 from x limits the output to x, y, z. With StepVerifier it would go like this:

public void testAlphabet5LimitsToZ() {
  MyReactiveLibrary library = new MyReactiveLibrary();
        .expectNext("x", "y", "z")

The second test we'd like to run on alphabet5 is that every returned value is an alphabetical character. For that we'd like to use a rich assertion library like AssertJ:

public void testAlphabet5LastItemIsAlphabeticalChar() {
  MyReactiveLibrary library = new MyReactiveLibrary();
              .consumeNextWith(c -> assertThat(c)
                    .as("first is alphabetic").matches("[a-z]"))
              .consumeNextWith(c -> assertThat(c)
                    .as("second is alphabetic").matches("[a-z]"))
              .consumeNextWith(c -> assertThat(c)
                    .as("third is alphabetic").matches("[a-z]"))
              .consumeNextWith(c -> assertThat(c)
                    .as("fourth is alphabetic").matches("[a-z]"))

Turns out both of these tests fail :(. Let's have a look at the output the StepVerifier gives us in each case to see if we can spot the bug:

java.lang.AssertionError: expected: onComplete(); actual: onNext({)


java.lang.AssertionError: [fourth is alphabetic] 
to match pattern:

So it looks like our method doesn't stop at z but continues emitting characters from the ASCII range. We could fix that by adding a .take(Math.min(5, 'z' - from + 1)) for instance, or using the same Math.min as the second argument to range.

The last test we want to make involves virtual time manipulation: we'll test the delaying method but without actually waiting for the given amount of seconds, by using the withVirtualTime builder:

public void testWithDelay() {
  MyReactiveLibrary library = new MyReactiveLibrary();
  Duration testDuration =
     StepVerifier.withVirtualTime(() -> library.withDelay("foo", 30))
  System.out.println(testDuration.toMillis() + "ms");

This tests a flux that would be delayed by 30 seconds for the following scenario: an immediate subscription, followed by 3x10s where nothing happens, then an onNext("foo") and completion.

The System.out output prints the actual duration the verification took, which in my latest run was 8ms :)

Note that when using the create builder instead, the thenAwait and expectNoEvent methods would still be available but would actually block for the provided duration.

StepVerifier comes with many more methods for describing expectations and asserting state of a Publisher (and if you think about new ones, contributions and feedback are always welcome in the github repository).

Custom Hot Source

Note that the concept of hot and cold observables discussed at the end of "RxJava by Example" also applies to Reactor.

If you want to create a custom Flux, instead of the RxJava AsyncEmitter class, you'd use Reactor's FluxSink. This will cover all the asynchronous corner cases for you and let you focus on emitting your values.

Use Flux.create and get a FluxSink in the callback that you can use to emit data via next. This custom Flux can be cold, so in order to make it hot you can use publish() and connect(). Building on the example from the previous article with a feed of price ticks, we get an almost verbatim translation in Reactor:

SomeFeed<PriceTick> feed = new SomeFeed<>();
Flux<PriceTick> flux =
     Flux.create(emitter ->
        SomeListener listener = new SomeListener() {
           public void priceTick(PriceTick event) {
              if (event.isLast()) {

           public void error(Throwable e) {
     }, FluxSink.OverflowStrategy.BUFFER);

ConnectableFlux<PriceTick> hot = flux.publish();

Before connecting to the hot Flux, why not subscribe twice?  One subscription will print the detail of each tick while the other will only print the instrument:

hot.subscribe(priceTick -> System.out.printf("%s %4s %6.2f%n", priceTick
     .getDate(), priceTick.getInstrument(), priceTick.getPrice()));

hot.subscribe(priceTick -> System.out.println(priceTick.getInstrument()));

We then connect to the hot flux and let it run for 5 seconds before our test snippet terminates:


(note that in the example repository, the feed would also terminate on its own if the isLast() method of PriceTick is changed).

FluxSink also lets you check if downstream has cancelled its subscription via isCancelled(). You can also get feedback on the outstanding requested amount via requestedFromDownstream(), which is useful if you want to simply comply with backpressure. Finally, you can make sure any specific resources your source uses are released upon Cancellation via setCancellation.

Note that there's a backpressure implication of using FluxSink: you must provide an OverflowStrategy explicitly to let the operator deal with backpressure. This is equivalent to using onBackpressureXXX operators (eg. FluxSink.OverflowStrategy.BUFFER is equivalent to using .onBackpressureBuffer()), which kind of overrides any backpressure instructions from downstream.


In this article, you have learned about Reactor, a fourth-generation reactive library that builds on the Rx language but targets Java 8 and the Reactive Streams specification. We've shown how the concepts you might have learned in RxJava also apply to Reactor, despite a few API differences. We've also shown how Reactor serves as the foundation for Spring 5, and that it offers resources for testing a Publisher/Flux/Mono.

If you want to dig deeper into using Reactor, the snippets presented in this article are available in our github repository. There is also a workshop, the "Lite Rx API hands-on", that covers more operators and use cases.

Finally, you can reach the Reactor team on Gitter and provide feedback there or through github issues (and of course, pull-requests are welcomed as well).

About the Author

Simon Basle is a software development aficionado, especially interested in reactive programming, software design aspects (OOP, design patterns, software architecture), rich clients, what lies beyond code (continuous integration, (D)VCS, best practices), and also a bit of CompSci, concurrent programming. He works at Pivotal on Reactor.

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