Spring 5 Web Reactive - Web Client - Use of flatmap() on the response stream

Question

I am trying to understand why a flatmap() is used on the response stream, in this post, for example (copy pasted directly from the reference)

final ClientRequest request = ClientRequest.GET(url)
        .accept(MediaType.TEXT_EVENT_STREAM).build();
Flux<Alert> alerts = webClient.exchange(request)
        .flatMap(response -> response.bodyToFlux(Alert.class));

AFAIK, a flatmap applied on a stream produces an arbitrary number of values (0...n) for each input value (in the stream). So a flatmap takes a function that produces a stream.

A flux emits 0 or more items, and then optionally either completing or erroring.

So what exactly is happening on the response stream? Is this the function that takes the response stream, and emits 0 or more objects of the class Alert? So if we subscribe to alerts, we can get them over the web client in a reactive manner. Can someone clarify if I am correct, please?

Simon Baslé Simon Baslé · Accepted Answer · 2017-07-25T08:44:06

Yes that is correct. The receiving of the header is a first async stage, at which point you get a response. You then use flatMap to asynchronously retrieve the body. Said body is a Flux, because in some cases you can get multiple unmarshalled objects (eg SSE). Here you'd probably get only one Alert.

Spring 5 Web Reactive - Web Client - Use of flatmap() on the response stream

1 Answers