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Spring WebFlux is Spring’s reactive web framework for building non-blocking web applications and HTTP services. It offers two server styles—annotated controllers and functional endpoints—and uses Reactor’s Mono and Flux types to represent asynchronous data. It can suit applications that benefit from non-blocking I/O or streaming, but it is not automatically faster than Spring MVC. In Spring Boot, adding WebFlux alongside the MVC starter still selects MVC for the server by default.
What is Spring WebFlux?
Spring WebFlux is Spring’s reactive-stack web framework. Its non-blocking contracts and Reactive Streams support let components coordinate asynchronous work and signal demand as data flows through an application.
“Non-blocking” describes how work can proceed without tying up a thread while waiting for I/O; it does not mean every operation in an application is automatically non-blocking. Nor does the programming model guarantee higher performance for every workload. The official documentation describes the framework’s behavior, but does not establish a universal performance advantage over Spring MVC.
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WebFlux supports two server programming models: familiar annotated controllers and functional endpoints. Both use the same reactive core, so the choice is primarily about how a team wants to express and organize routes and handlers.
What is the difference between Spring MVC and WebFlux?
Spring MVC and Spring WebFlux are separate Spring web stacks. MVC is the traditional servlet-based model; WebFlux is the reactive stack. Spring’s documentation presents them as alternatives to choose based on application requirements and system boundaries, not as a simple old-versus-new progression.
| Question | Spring MVC | Spring WebFlux |
|---|---|---|
| Programming model | Servlet-based web framework | Reactive web framework built around Reactive Streams |
| Server route styles | Annotated controllers | Annotated controllers or functional endpoints |
| Typical fit | Applications aligned with the servlet model and existing MVC code | Applications that need reactive contracts, non-blocking I/O, or streaming through the web stack |
These distinctions describe the frameworks, not a benchmark. A reactive stack does not make blocking database or other blocking work disappear, and the documentation does not establish that WebFlux is universally faster.
When should I use Mono or Flux?
WebFlux uses Reactor, Spring’s reactive library of choice. Its two central types communicate the number of values an asynchronous operation can produce:
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Mono<T>represents zero or one value of typeT.Flux<T>represents zero to many values of typeT>.
Choose the type that matches the result’s cardinality. A lookup that may return no record or one record naturally has a Mono<T> contract. A collection or stream of results can use Flux<T>. This is more than a style choice: cardinality helps callers understand what a method can produce, and it matters when composing operations and encoding or decoding data.
Reactive Streams back pressure lets a subscriber signal how much data it is ready to receive, helping producers and consumers coordinate demand. It is a flow-control mechanism, not a guarantee against all memory use, latency, or capacity problems.
How do I choose a WebFlux server programming model?
Annotated controllers
Use @Controller or @RestController when you want mappings and request handling expressed in the annotation style familiar to Spring MVC developers. Methods can use reactive return types while retaining a declarative controller structure. The annotated controller reference covers the model.
Functional endpoints
Use WebFlux.fn when you prefer routes and handlers to be explicit in code. A RouterFunction describes routing, while a HandlerFunction handles a request. A handler receives a ServerRequest and returns a delayed ServerResponse, commonly as Mono<ServerResponse>. Request and response contracts are immutable, and their bodies support reactive streams. See Spring’s functional endpoints reference.
Neither style is prescribed as universally better. Consider whether your team prefers annotation-based mappings or explicit router-and-handler composition, and how routes are organized in the application.
How do I use WebClient in Spring Boot?
WebClient is Spring’s fluent, functional HTTP client for non-blocking requests and streaming. It uses the same codecs as WebFlux server applications. Spring’s documentation lists Reactor Netty, JDK HttpClient, Jetty Reactive HttpClient, and Apache HttpComponents as supported underlying clients; other clients can be connected through ClientHttpConnector. The WebClient reference has configuration and usage details.
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You can use WebClient from a Spring MVC application. Adding the WebFlux starter to get the client does not necessarily mean WebFlux is serving your application’s HTTP endpoints; Spring Boot’s default selection when both web starters are present is described below.
How do I create a Spring Boot WebFlux server?
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Add
spring-boot-starter-webfluxto the application’s dependencies. Spring Boot’s reactive web applications guide identifies this as the starter for WebFlux.PerformancePC Slower Than It Used to Be?DriversCrashes, No Sound, or Screen Glitches?PerformanceWindows Errors? Fix Them Before They SpreadSpecial offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. -
Choose annotated controllers or functional endpoints for the server routes, then return reactive types such as
MonoorFluxwhere appropriate.Best Value
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Check the application’s dependencies and Boot configuration to confirm that the intended server stack is selected.
Why does Spring Boot choose MVC when I added WebFlux?
If both spring-boot-starter-web and spring-boot-starter-webflux are on the classpath, Spring Boot auto-configures Spring MVC by default. Boot documents this behavior because developers often add WebFlux to an MVC application just to use WebClient.
If the application is intended to use WebFlux as its server stack, set the application type explicitly to reactive using the configuration documented in the Spring Boot reactive web applications reference. If you only need WebClient in an MVC application, Boot’s default MVC selection is expected.
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How should I customize WebFlux in Spring Boot?
For incremental WebFlux configuration while keeping Boot’s customizations, implement WebFluxConfigurer without adding @EnableWebFlux. Use @EnableWebFlux when you want fuller control over WebFlux configuration rather than Boot’s usual WebFlux customizations. Spring Boot documents this distinction in its reactive web applications guide.
Which Spring version does this guide describe?
The Spring Framework reference reviewed for this guide identifies 7.0.9 as the latest stable release; its 7.1.0-M2 reference describes 7.1 as in development and points readers to 7.0.9 for the latest stable version. Release status changes, so check the Spring Framework reference for the version applicable to your project.
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