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Web on Reactive Stack

Table of Contents

This part of the documentation covers support for reactive stack, web applications built on a Reactive Streams API to run on non-blocking servers such as Netty, Undertow, and Servlet 3.1+ containers. Individual chapters cover the Spring WebFlux framework, the reactive WebClient, support for Testing, and Reactive Libraries. For Servlet stack, web applications, please see Web on Servlet Stack.

Testing

The spring-test module provides mock implementations of ServerHttpRequest, ServerHttpResponse, and ServerWebExchange. See Spring Web Reactive mock objects.

The WebTestClient builds on these mock request and response objects to provide support for testing WebFlux applications without and HTTP server. The WebTestClient can be used for end-to-end integration tests too.

Threading model

Reactive Libraries

spring-webflux depends on reactor-core and uses it internally to compose asynchronous logic and to provide Reactive Streams support. Generally WebFlux APIs return Flux or Mono — since that’s what’s used internally, and leniently accept any Reactive Streams Publisher implementation as input. The use of Flux vs Mono is important because it helps to express cardinality — e.g. whether a single or multiple async values are expected, and that can be essential for making decisions, for example when encoding or decoding HTTP messages.

For annotated controllers, WebFlux transparently adapts to the reactive library chosen by the application. This is done with the help of the ReactiveAdapterRegistry which provides pluggable support for reactive library and other asynchronous types. The registry has built-in support for RxJava and CompletableFuture, but others can be registered too.

For functional APIs such as [webflux-fn], the WebClient, and others, the general rules for WebFlux APIs apply — Flux and Mono as return values, and Reactive Streams Publisher as input. When a Publisher, whether custom or from another reactive library, is provided, it can only be treated as a stream with unknown semantics (0..N). If however the semantics are known, you can wrap it with Flux or Mono.from(Publisher) instead of passing the raw Publisher.

Note

For example, given a Publisher that is not a Mono, the Jackson JSON message writer expects multiple values. If the media type implies an infinite stream — e.g. "application/json+stream", values are written and flushed individually; otherwise values are buffered into a list and rendered as a JSON array.