Publisher
.double
.Flux
that emits the provided elements and then completes.Flux
that will only emit a single element then onComplete.Mono
that emits the specified item, which is captured at
instantiation time.Optional
创建一个Mono,如果值存在就返回一个有元素的Mono,否则就返回一个 Mono.empty()
Create a new Mono
that emits the specified item if Optional.isPresent()
otherwise only emits
onComplete.Mono.empty()
Create a new Mono
that emits the specified item if non null otherwise only emits
onComplete.