Side effect operator (or a like) in case if empty flux

How can I call a method (consumer) in case of Flux is empty, for example i want to log message if the flux is empty

I see two ways of achieving it:

1. Utilise switchIfEmpty()

Then your code snippet will have the following representation:

.switchIfEmpty(Flux.empty().doOnComplete(() -> log.info("there were no elements")))

2. Another option is hasElements().

Try to place the code snippet below somewhere at the end of your chain:

.hasElements()
.doOnNext(hasElements -> {
    if (!hasElements) {
        log.info("there were no elements");
    }
})

This one is there for you:

/**
 * Add behavior (side-effect) triggered when the {@link Flux} completes successfully.
 * <p>
 * <img class="marble" src="doc-files/marbles/doOnComplete.svg" alt="">
 *
 * @param onComplete the callback to call on {@link Subscriber#onComplete}
 *
 * @return an observed  {@link Flux}
 */
public final Flux<T> doOnComplete(Runnable onComplete) {

See this JavaDocs:

/**
 * Represents an empty publisher which only calls onSubscribe and onComplete.
 * <p>
 * This Publisher is effectively stateless and only a single instance exists.
 * Use the {@link #instance()} method to obtain a properly type-parametrized view of it.
 * @see <a href="https://github.com/reactor/reactive-streams-commons">Reactive-Streams-Commons</a>
 */
final class FluxEmpty extends Flux<Object>