翻譯自：https://tech.io/playgrounds/929/reactive-programming-with-reactor-3/Mono
Mono
Description
A Mono<T> is a Reactive Streams Publisher, also augmented with a lot of operators
that can be used to generate, transform, orchestrate Mono sequences.
Mono是一種反應(yīng)流發(fā)布者，還增加了許多運(yùn)算符，可用于生成、轉(zhuǎn)換和編排Mono序列。
It is a specialization of Flux that can emit at most 1 <T> element:
a Mono is either valued (complete with element), empty (complete without element)
or failed (error).
它是Flux的特例，最多可以發(fā)射1個(gè)元素：Mono要么有值（帶元素完成），要么為空（無(wú)元素完成），要么為失敗（錯(cuò)誤）。
A Mono<Void> can be used in cases where only the completion signal is interesting
(the Reactive Streams equivalent of a Runnable task completing).
在只對(duì)完成信號(hào)感興趣的情況下（相當(dāng)于Runnable任務(wù)完成的反應(yīng)流），可以使用Mono<Void>。
Like for Flux, the operators can be used to define an asynchronous pipeline
which will be materialized anew for each Subscription.
與Flux一樣，可以使用運(yùn)算符定義異步管道，該管道將針對(duì)每個(gè)訂閱重新具體化。
Note that some API that change the sequence's cardinality will return a Flux
(and vice-versa, APIs that reduce the cardinality to 1 in a Flux return a Mono).
請(qǐng)注意，一些更改序列基數(shù)的API將返回Flux（反之亦然，在Flux中將基數(shù)減少到1的API將返回Mono）。
Mono in action:
Mono.firstWithValue(
Mono.just(1).map(integer -> "foo" + integer),
Mono.delay(Duration.ofMillis(100)).thenReturn("bar")
)
.subscribe(System.out::println);
Practice
As for the Flux let's return a empty Mono using the static factory.
就像Flux，讓我們使用靜態(tài)工廠返回一個(gè)空Mono。
// Return an empty Mono
Mono<String> emptyMono() {
return Mono.empty();
}
Now, we will try to create a Mono which never emits anything.
Unlike empty(), it won't even emit an onComplete event.
現(xiàn)在，我們將嘗試創(chuàng)建一個(gè)從不發(fā)射任何東西的Mono。與empty()不同，它甚至不會(huì)發(fā)出onComplete事件。
// Return a Mono that never emits any signal
Mono<String> monoWithNoSignal() {
return Mono.never();
}
Like Flux, you can create a Mono from an available (unique) value.
與Flux一樣，您可以從可用（唯一）值創(chuàng)建Mono。
// Return a Mono that contains a "foo" value
Mono<String> fooMono() {
return Mono.just("foo");
}
And exactly as we did for the flux, we can propagate exceptions.
正如我們對(duì)Flux所做的那樣，我們可以傳播異常。
// Create a Mono that emits an IllegalStateException
Mono<String> errorMono() {
return Mono.error(new IllegalStateException());
}
