⛓ Convert event-based API to Flux

java Nov 16, 2019

When working with reactive streams in Java, you may encounter a situation when there is an event-based API that must somehow return data as a stream.

In my case for Reactive Streams, I use Reactor. Let's say we have a simple listener object with onUpdate callback method, like in the code snippet below.

public class CustomListener implements BaseListener {
    private final FluxConnector<String[]> namesFluxConnector;;

    public MarketDataListener() {
        namesFluxConnector = new FluxConnector<>();

    public void onUpdate(String[] names) {

    public Flux<String[]> getFlux() {
      return namesFluxConnector.getFlux();

And we need to work with data as with stream, in the code above CustomListener returns Flux<String[]> object to consume the stream later. The main transformations are done in FluxConnector. FluxConnector creates a Flux and uses FluxSink to push objects into stream. A simple implementation to convey the main idea in the code snippet below.

public class FluxConnector<T> {
    private final Flux<T> flux;
    private FluxSink<T> fluxSink;

    public FluxConnector() {
        this.flux = Flux
            .create(new SinkAdapter<T>(this::setFluxSink))

    public void next(T data) {
        if (Objects.nonNull(fluxSink))

    public Flux<T> getFlux() {
        return flux;

    private void setFluxSink(FluxSink<T> fluxSink) {
        if (Objects.isNull(this.fluxSink))
            this.fluxSink = fluxSink;

    private static class SinkAdapter<T> implements Consumer<FluxSink<T>> {
        private final Consumer<FluxSink<T>> consumer;

        SinkAdapter(Consumer<FluxSink<T>> consumer) {
            this.consumer = consumer;

        public void accept(FluxSink<T> sink) {

Photo by Denys Nevozhai on Unsplash