reactor-core/src/main/java/reactor/core/publisher/Mono.java
public Optional<T> blockOptional() {
		BlockingOptionalMonoSubscriber<T> subscriber = new BlockingOptionalMonoSubscriber<>();
		Operators.onLastAssembly(this).subscribe(Operators.toCoreSubscriber(subscriber));
		return subscriber.blockingGet();
	}
public Optional<T> blockOptional() {
		BlockingOptionalMonoSubscriber<T> subscriber = new BlockingOptionalMonoSubscriber<>();
		Operators.onLastAssembly(this).subscribe(Operators.toCoreSubscriber(subscriber));
		return subscriber.blockingGet();
	}
Subscribe to this {@link Mono} and <strong>block indefinitely</strong> until a next signal is
received or the Mono completes empty. Returns an {@link Optional}, which can be used
to replace the empty case with an Exception via {@link Optional#orElseThrow(Supplier)}.
In case the Mono itself errors, the original exception is thrown (wrapped in a
{@link RuntimeException} if it was a checked exception).

<p>
<img class="marble" src="doc-files/marbles/blockOptional.svg" alt="">
<p>
Note that each blockOptional() will trigger a new subscription: in other words, the result
might miss signal from hot publishers.

@return T the result