Mono switchIfEmpty() is always called

For those who, despite the well voted answer, do not still understand why such a behaviour:

Reactor sources (Mono.xxx & Flux.xxx) are either:

• Lazily evaluated : the content of the source is evaluated/triggered only when a subscriber subscribes to it;

• or eagerly evaluated : the content of the source is immediately evaluated even before the subscriber subscribes.

Expressions like Mono.just(xxx), Flux.just(xxx), Flux.fromIterable(x,y,z) are eager.

By using defer(), you force the source to be lazily evaluated. That's why the accepted answer works.

So doing this:

 someMethodReturningAMono()
  .switchIfEmpty(buildError());

with buildError() relying on an eager source to create an alternative Mono will ALWAYS be evaluated before the subscription:

Mono<String> buildError(){
       return Mono.just("An error occured!"); //<-- evaluated as soon as read
}

To prevent that, do this:

 someMethodReturningAMono()
  .switchIfEmpty(Mono.defer(() -> buildError()));

Read this answer for more.

It's because switchIfEmpty accepts Mono "by value". Meaning that even before you subscribe to your mono, this alternative mono's evaluation is already triggered.

Imagine a method like this:

Mono<String> asyncAlternative() {
    return Mono.fromFuture(CompletableFuture.supplyAsync(() -> {
        System.out.println("Hi there");
        return "Alternative";
    }));
}

If you define your code like this:

Mono<String> result = Mono.just("Some payload").switchIfEmpty(asyncAlternative());

It'll always trigger alternative no matter what during stream construction. To address this you can defer evaluation of a second mono by using Mono.defer

Mono<String> result = Mono.just("Some payload")
        .switchIfEmpty(Mono.defer(() -> asyncAlternative()));

This way it will only print "Hi there" when alternative is requested

UPD:

Elaborating a little on my answer. The problem you're facing is not related to Reactor but to Java language itself and how it resolves method parameters. Let's examine the code from the first example I provided.

Mono<String> result = Mono.just("Some payload").switchIfEmpty(asyncAlternative());

We can rewrite this into:

Mono<String> firstMono = Mono.just("Some payload");
Mono<String> alternativeMono = asyncAlternative();
Mono<String> result = firstMono.switchIfEmpty(alternativeMono);

These two code snippets are semantically equivalent. We can continue unwrapping them to see where the problem lies:

Mono<String> firstMono = Mono.just("Some payload");
CompletableFuture<String> alternativePromise = CompletableFuture.supplyAsync(() -> {
        System.out.println("Hi there");
        return "Alternative";
    }); // future computation already tiggered
Mono<String> alternativeMono = Mono.fromFuture(alternativePromise);
Mono<String> result = firstMono.switchIfEmpty(alternativeMono);

As you can see future computation was already triggered at the point when we start composing our Mono types. To prevent unwanted computations we can wrap our future into a defered evaluation:

Mono<String> result = Mono.just("Some payload")
        .switchIfEmpty(Mono.defer(() -> asyncAlternative()));

Which will unwrap into

Mono<String> firstMono = Mono.just("Some payload");
Mono<String> alternativeMono = Mono.defer(() -> Mono.fromFuture(CompletableFuture.supplyAsync(() -> {
        System.out.println("Hi there");
        return "Alternative";
    }))); // future computation defered
Mono<String> result = firstMono.switchIfEmpty(alternativeMono);

In second example the future is trapped in a lazy supplier and is scheduled for execution only when it will be requested.