Reactive Programming library based on the Java 9 Flow API and a 4th generation ReactiveX-style architecture.
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Reactive4JavaFlow Maven Central

Reactive Programming library based on the Java 9 Flow API and a 4th generation ReactiveX-style architecture.


compile "com.github.akarnokd:reactive4javaflow:0.1.5"

Getting Started

The base package of Reactive4JavaFlow is hu.akarnokd.reactive4javaflow where the main reactive classes are located. Note that the library requires a JDK 9 compatible runtime, thus it is not compatible with most of what is currently available on Android.

The library doesn't declare modules (Project Jigsaw) and should be usable from both classpath and modularized environments.


The Folyam base reactive type is the 0 .. N element, backpressure aware, Java 9 Flow.Publisher implementation. The word means flow or stream in hungarian and was chosen to avoid name clashing with existing libraries and types (such as Flowable, Flux, Stream and unfortunately Flow too). The name is also one of the few words describing the target concept that doesn't use accented letters.

Folyam features most of the typical ReactiveX operators commonly found in RxJava and Reactor and couple of the less common extension operators from RxJava 2 Extensions.

Hello world!

import hu.akarnokd.reactive4javaflow.*;

Folyam.just("Hello world!")

Operators that work with multiple sources and may need to delay errors now have separate methods with DelayError postfix: concatDelayError, flatMapDelayError.


The Esetleg base reactive type is the 0 .. 1 element, backpressure aware, Java 9 FLow.Publisher implementation. It resembles the RxJava 2 Maybe type and is similar to Reactor's Mono type. The word itself means maybe or perhaps.

Hello world!

AutoDisposable d = Esetleg.fromCallable(() -> "Hello world")
.subscribeWith(new AbstractFolyamSubscriber<>() {
    @Override public void onNext(String s) {
    @Override public void onError(Throwable ex) {
    @Override public void onComplete() {



The resource manager type is the AutoDisposable interface, which extends from java.util.AutoCloseable and thus can be used in try-with-resources. Unlike RxJava 2 Disposable, the interface doesn't offer any means to check if the resource is disposed.

The AbstractFolyamSubscriber shown in the previous section's example also implements AutoDisposable and together with hu.akarnokd.reactive4javaflow.disposables.CompositeAutoDisposable can be used to track and mass-close resources.

The disposables.BooleanAutoDisposable can help in testing and SequentialAutoDisposable may help with custom operators or individual resource tracking.


Just like RxJava 2 and Reactor 3, the library offers parallel operations through the Folyam.parallel() method. The ParallelFolyam features the same operators as RxJava 2 does plus several of the sequential and less common operators such as the asynchronous mapWhen().

Folyam.range(1, 100)


The library offers the standard set of global schedulers in SchedulerServices (named to avoid too much conflict with RxJava and Reactor): computation, io, newThread, single, trampoline and newExecutor. Testing can be done with the TestSchedulerService class.

In addition, the SchedulerServices allows creating custom schedulers, parameterized by name, priority and daemon-ness via newParallel, newIO, newThread and newSingle. The utility class also supports creating blocking schedulers via newBlocking and sharing an existing worker via newShared.

The SchedulerService interface implemented by the scheduler offers the ability to schedule tasks of Runnable directly or on a Worker; immediately, delayed or periodically. The Worker interface implements AutoDisposable and all of the pending tasks can be cancelled via Worker.close().

The current API design in Folyam and Esetleg doesn't offer operators with default schedulers; the SchedulerService to be used must be specified as parameter to these operators.

Folyam.intervalRange(1, 5, 100, 100, TimeUnit.MILLISECONDS, SchedulerServices.single())

Functional interfaces

Unfortunately, Java's default functional interfaces don't support throwing checked exceptions, therefore a set of new interfaces were added and used throughout the API:

import hu.akarnokd.reactive4javaflow.functionals.*;

CheckedFunction<Integer, Integer> f = v -> { throw new IOException(); };


Processors (hot sources)

The library features almost all standard "subject" types from RxJava in the processors subpackage:

  • DirectProcessor: emit items directly to multiple subscribers, signals error if a particular subscriber is not ready to receive items (similar to RxJava's PublishProcessor),
  • MulticastProcessor: coordinates the backpressure between subscribers (no standard RxJava equivalent),
  • FirstProcessor: takes and caches the first item or terminal signal and exposes it as an Esetleg reactive type (similar to MaybeSubject),
  • LastProcessor: waits for and caches the last item or terminal signal and exposes it as a Folyam (similar to AsyncProcessor),
  • CachingProcessor: caches items (unbounded, size and/or time bound) and replays them to current and future subscribers (similar to ReplayProcessor),
  • SolocastProcessor: buffers items until a single subscriber is able to consume them (similar to UnicastProcessor).

The BehaviorProcessor is currently not replicated in this library and can be emulated via new CachingProcessor<>(1).

Connectable flows

The cold-to-hot conversion is available via the Folyam.publish() and Folyam.replay() operators which return a ConnectableFolyam instance.

Unlike RxJava and Reactor, the ConnectableFolyam has 3 states: fresh, running, terminated. In order to get back to the fresh state, one has to call ConnectableFolyam.reset() in the terminated state.

The reason for this design difference is that the RxJava behavior often causes trouble in preparing subscribers after the first round has completed, which can lead to data loss in case of publish().

ConnectableFolyam<Integer> cf = Folyam.range(1, 5).publish();



// the source has run to completion and
// publish will complete any latecommers


// go back to the fresh state


// prepare the next set of subscribers without
// the source to rush ahead



A more prominent effect is when replay or replayLast() is used. Once the source completed, all or the last items are still available to late subscribers until reset is called.

The usual autoConnect() and refCount operators are supported along with refCount overloads that allows specifying the minimum subscriber count and/or a grace period before closing the connection after the last subscriber has cancelled.


Testing the Folyam and Esetleg types can be done via the convenient .test() method which uses a TestConsumer class behind the scenes (similar to TestSubscriber).

Scheduler-dependent operators can be tested with the help of the TestSchedulerService and its advanceTimeBy method as the mean to move time forward.

TestSchedulerService sch = new TestSchedulerService();

TestConsumer<Integer> tc = Folyam.just(1).delay(5, TimeUnit.MILLISECONDS, sch).test();


tc.advanceTimeBy(5, TimeUnit.MILLISECONDS);


Plugins & hooks

The FolyamPlugins offers the ability to hook the assembling of operators (onAssembly), when they get subscribed onSubscribe and when there is an undeliverable exception onError.

The FolyamPluins also allows overriding the initial and current schedulers returned by SchedulerServices. Note that accessing SchedulerServices - for example, to create a custom scheduler via one of the newXXX methods may prematurely initialize the other schedulers. Therefore, it is recommended to hook the current schedulers only (setOnComputationSchedulerService).

Operator fusion

The Reactive4JavaFlow library is a 4th generation reactive solution that supports the operator fusion concept more extensively than RxJava 2 does at the moment. Since there is no standard, multi-library API for operator fusion anyway, this library defines the core interfaces in the hu.akarnokd.reactive4javaflow.fused package:

  • ConditionalSubscriber: with its tryOnNext method, such subscriber can avoid request(1) calls and keep draining longer without costly atomic decrements
  • FusedQueue: base interface for queues with minimal number of operations and a poll() method that can throw a Throwable
  • FusedSubscription: the combination of FusedQueue and the Java 9 Flow.Subscription interfaces to establish and run fused queues
  • FusedDynamicSource: represents a source that can produce an item, null or throw an exception at the runtime of the flow
  • FusedStaticSource: special variant of the dynamic source that can hosts a constant (or is empty) and allows assembly time optimizations.