Reactive Streams and Spring Integration

Spring Integration extends the Spring programming model into messaging and enterprise integration patterns — channels, endpoints, gateways, and adapters. Since Spring Integration 5, the framework participates in the reactive world: message channels can be backed by Reactor Fluxes, inbound endpoints can serve HTTP through WebFlux, and outbound handlers are driven by WebClient.

This chapter shows how the pieces fit together, using two runnable samples:

• WebFlux endpoints with Spring Integration — an HTTP inbound endpoint wired to a reactive channel and a reactive outbound handler, assembled with annotations.
• Spring Integration Java DSL — the same flow assembled with the fluent IntegrationFlow DSL.
• Reactive Message Channels — reactive message channels across the ecosystem: Spring Integration's FluxMessageChannel, Redis pub/sub, and Reactor Sinks.

How reactive support works

Spring Integration introduces a reactive channel type, FluxMessageChannel, which implements both MessageChannel and Publisher<Message<?>>. Producers send messages to it through the ordinary send() contract; consumers subscribe to it as a Reactive Streams Publisher and get backpressure for free.

On the web side, two components bridge Integration to WebFlux:

• WebFluxInboundEndpoint — receives HTTP requests, maps them to Integration messages, and writes the (possibly streamed) reply back to the response.
• WebFluxRequestExecutingMessageHandler — the outbound counterpart; it executes HTTP requests through a WebClient and produces reactive replies.

Together they form a full reactive processing pipeline: HTTP in → message channel → HTTP out, without a blocking call anywhere.

How the chapter's examples are structured

Both samples share the same skeleton — a WebFlux app serving GET /posts from an in-memory repository — and add an Integration layer on top that exposes GET /all. A request to /all travels through the Integration flow, which calls back into the same app's /posts endpoint through a WebClient, and streams the result to the caller.

GET /all ──▶ WebFluxInboundEndpoint ──▶ FluxMessageChannel ──▶ WebFluxRequestExecutingMessageHandler ──▶ GET /posts

The samples are plain Spring applications (no Spring Boot auto-configuration) built with the manual wiring from the Getting Started chapter, with one extra dependency:

<dependency>
    <groupId>org.springframework.integration</groupId>
    <artifactId>spring-integration-webflux</artifactId>
</dependency>

Where to go next

• WebFlux endpoints with Spring Integration — annotation-based assembly.
• Spring Integration Java DSL — the same flow with IntegrationFlow.from(...).
• Reactive Message Channels — channel patterns across Spring Integration, Spring Data Redis, and Reactor.