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Spring WebFlux

Introduction

The original web framework included in the Spring Framework, Spring Web MVC, was purpose built for the Servlet API and Servlet containers. The reactive stack, web framework, Spring WebFlux, was added later in version 5.0. It is fully non-blocking, supports Reactive Streams back pressure, and runs on servers such as Netty, Undertow, and Servlet 3.1+ containers.

Both web frameworks mirror the names of their source modules spring-webmvc and spring-webflux and co-exist side by side in the Spring Framework. Each module is optional. Applications may use one or the other module, or in some cases both — e.g. Spring MVC controllers with the reactive WebClient.

Motivation

Why was Spring WebFlux created?

Part of the answer is the need for a non-blocking web stack to handle concurrency with a small number of threads and scale with less hardware resources. Servlet 3.1 did provide an API for non-blocking I/O. However, using it leads away from the rest of the Servlet API where contracts are synchronous (Filter, Servlet) or blocking (getParameter, getPart). This was the motivation for a new common API to serve as a foundation across any non-blocking runtime. That is important because of servers such as Netty that are well established in the async, non-blocking space.

The other part of the answer is functional programming. Much like the addition of annotations in Java 5 created opportunities — e.g. annotated REST controllers or unit tests, the addition of lambda expressions in Java 8 created opportunities for functional APIs in Java. This is a boon for non-blocking applications and continuation style APIs — as popularized by CompletableFuture and ReactiveX, that allow declarative composition of asynchronous logic. At the programming model level Java 8 enabled Spring WebFlux to offer functional web endpoints alongside with annotated controllers.

Define "reactive"

We touched on non-blocking and functional but what does reactive mean?

The term "reactive" refers to programming models that are built around reacting to change — network component reacting to I/O events, UI controller reacting to mouse events, etc. In that sense non-blocking is reactive because instead of being blocked we are now in the mode of reacting to notifications as operations complete or data becomes available.

There is also another important mechanism that we on the Spring team associate with "reactive" and that is non-blocking back pressure. In synchronous, imperative code, blocking calls serve as a natural form of back pressure that forces the caller to wait. In non-blocking code it becomes important to control the rate of events so that a fast producer does not overwhelm its destination.

Reactive Streams is a small spec, also adopted in Java 9, that defines the interaction between asynchronous components with back pressure. For example a data repository — acting as Publisher, can produce data that an HTTP server — acting as Subscriber, can then write to the response. The main purpose of Reactive Streams is to allow the subscriber to control how fast or how slow the publisher will produce data.

Note

Common question: what if a publisher can’t slow down?
The purpose of Reactive Streams is only to establish the mechanism and a boundary. If a publisher can’t slow down then it has to decide whether to buffer, drop, or fail.

Reactive API

Reactive Streams plays an important role for interoperability. It is of interest to libraries and infrastructure components but less useful as an application API because it is too low level. What applications need is a higher level and richer, functional API to compose async logic — similar to the Java 8 Stream API but not only for collections. This is the role that reactive libraries play.

Reactor is the reactive library of choice for Spring WebFlux. It provides the Mono and Flux API types to work on data sequences of 0..1 and 0..N through a rich set of operators aligned with the ReactiveX vocabulary of operators. Reactor is a Reactive Streams library and therefore all of its operators support non-blocking back pressure. Reactor has a strong focus on server-side Java. It is developed in close collaboration with Spring.

WebFlux requires Reactor as a core dependency but it is interoperable with other reactive libraries via Reactive Streams. As a general rule WebFlux APIs accept a plain Publisher as input, adapt it to Reactor types internally, use those, and then return either Flux or Mono as output. So you can pass any Publisher as input and you can apply operations on the output, but you’ll need to adapt the output for use with another reactive library. Whenever feasible — e.g. annotated controllers, WebFlux adapts transparently to the use of RxJava or other reactive library. See [webflux-reactive-libraries] for more details.

Programming models

The spring-web module contains the reactive foundation that underlies Spring WebFlux including HTTP abstractions, Reactive Streams adapters for supported servers, codecs, and a core WebHandler API comparable to the Servlet API but with non-blocking contracts.

On that foundation Spring WebFlux provides a choice of two programming models:

  • Annotated Controllers — consistent with Spring MVC, and based on the same annotations from the spring-web module. Both Spring MVC and WebFlux controllers support reactive (Reactor, RxJava) return types and as a result it is not easy to tell them apart. One notable difference is that WebFlux also supports reactive @RequestBody arguments.

  • [webflux-fn] — lambda-based, lightweight, functional programming model. Think of this as a small library or a set of utilities that an application can use to route and handle requests. The big difference with annotated controllers is that the application is in charge of request handling from start to finish vs declaring intent through annotations and being called back.

Applicability

Spring MVC or WebFlux?

A natural question to ask but one that sets up an unsound dichotomy. It’s actually both working together to expand the range of available options. The two are designed for continuity and consistency with each other, they are available side by side, and feedback from each side benefits both sides. The diagram below shows how the two relate, what they have in common, and what each supports uniquely:

spring mvc and webflux venn

Below are some specific points to consider:

  • If you have a Spring MVC application that works fine, there is no need to change. Imperative programming is the easiest way to write, understand, and debug code. You have maximum choice of libraries since historically most are blocking.

  • If you are already shopping for a non-blocking web stack, Spring WebFlux offers the same execution model benefits as others in this space and also provides a choice of servers — Netty, Tomcat, Jetty, Undertow, Servlet 3.1+ containers, a choice of programming models — annotated controllers and functional web endpoints, and a choice of reactive libraries — Reactor, RxJava, or other.

  • If you are interested in a lightweight, functional web framework for use with Java 8 lambdas or Kotlin then use the Spring WebFlux functional web endpoints. That can also be a good choice for smaller applications or microservices with less complex requirements that can benefit from greater transparency and control.

  • In a microservice architecture you can have a mix of applications with either Spring MVC or Spring WebFlux controllers, or with Spring WebFlux functional endpoints. Having support for the same annotation-based programming model in both frameworks makes it easier to re-use knowledge while also selecting the right tool for the right job.

  • A simple way to evaluate an application is to check its dependencies. If you have blocking persistence APIs (JPA, JDBC), or networking APIs to use, then Spring MVC is the best choice for common architectures at least. It is technically feasible with both Reactor and RxJava to perform blocking calls on a separate thread but you wouldn’t be making the most of a non-blocking web stack.

  • If you have a Spring MVC application with calls to remote services, try the reactive WebClient. You can return reactive types (Reactor, RxJava, or other) directly from Spring MVC controller methods. The greater the latency per call, or the interdependency among calls, the more dramatic the benefits. Spring MVC controllers can call other reactive components too.

  • If you have a large team, keep in mind the steep learning curve in the shift to non-blocking, functional, and declarative programming. A practical way to start without a full switch is to use the reactive WebClient. Beyond that start small and measure the benefits. We expect that for a wide range of applications the shift is unnecessary. If you are unsure what benefits to look for, start by learning about how non-blocking I/O works (e.g. concurrency on single-threaded Node.js) and its effects.

Servers

Spring WebFlux is supported on Tomcat, Jetty, Servlet 3.1+ containers, as well as on non-Servlet runtimes such as Netty and Undertow. All servers are adapted to a low-level, common API so that higher level programming models can be supported across servers.

Spring WebFlux does not have built-in support to start or stop a server. However it is easy to assemble an application from Spring configuration, and WebFlux infrastructure, and run it with a few lines of code.

Spring Boot has a WebFlux starter that automates these steps. By default the starter uses Netty but it is easy to switch to Tomcat, Jetty, or Undertow simply by changing your Maven or Gradle dependencies. Spring Boot defaults to Netty because it is more widely used in the async, non-blocking space, and provides a client and a server share resources.

Tomcat and Jetty can be used with both Spring MVC and WebFlux. Keep in mind however that the way they’re used is very differently. Spring MVC relies on Servlet blocking I/O and allows applications to use the Servlet API directly if they need to. Spring WebFlux relies on Servlet 3.1 non-blocking I/O and uses the Servlet API behind a low-level adapter and not exposed for direct use.

For Undertow, Spring WebFlux uses Undertow APIs directly without the Servlet API.

Performance vs scale

Performance has many characteristics and meanings. Reactive and non-blocking generally do not make applications run faster. They can, in some cases, for example if using the WebClient to execute remote calls in parallel. On the whole it requires more work to do things the non-blocking way and that can increase slightly the required processing time.

The key expected benefit of reactive and non-blocking is the ability to scale with a small, fixed number of threads and less memory. That makes applications more resilient under load because they scale in a more predictable way. In order to observe those benefits however you need to have some latency including a mix of slow and unpredictable network I/O. That’s where the reactive stack begins to show its strengths and the differences can be dramatic.

Concurrency Model

Both Spring MVC and Spring WebFlux support annotated controllers, but there is a key difference in the concurrency model and default assumptions for blocking and threads.

In Spring MVC, and servlet applications in general, it is assumed that applications may block the current thread, e.g. for remote calls, and for this reason servlet containers use a large thread pool, to absorb potential blocking during request handling.

In Spring WebFlux, and non-blocking servers in general, it is assumed that applications will not block, and therefore non-blocking servers use a small, fixed-size thread pool (event loop workers) to handle requests.

Tip

To "scale" and "small number of threads" may sound contradictory but to never block the current thread, and rely on callbacks instead, means you don’t need extra threads as there are no blocking calls to absorb.

Invoking a Blocking API

What if you do need to use a blocking library? Both Reactor and RxJava provide the publishOn operator to continue processing on a different thread. That means there is an easy escape latch. Keep in mind however that blocking APIs are not a good fit for this concurrency model.

Mutable State

In Reactor and RxJava, logic is declared through operators, and at runtime, a reactive pipeline is formed where data is processed sequentially, in distinct stages. A key benefit of that is that it frees applications from having to protect mutable state because application code within that pipeline is never invoked concurrently.

Threading Model

What threads should you expect to see on a server running with Spring WebFlux?

  • On a "vanilla" Spring WebFlux server (e.g. no data access, nor other optional dependencies), you can expect one thread for the server, and several others for request processing (typically as many as the number of CPU cores). Servlet containers, however, may start with more threads (e.g. 10 on Tomcat), in support of both servlet, blocking I/O and servlet 3.1, non-blocking I/O usage.

  • The reactive WebClient operates in event loop style. So you’ll see a small, fixed number of processing threads related to that, e.g. "reactor-http-nio-" with the Reactor Netty connector. However if Reactor Netty is used for both client and server, the two will share event loop resources by default.

  • Reactor and RxJava provide thread pool abstractions, called Schedulers, to use with the publishOn operator that is used to switch processing to a different thread pool. The schedulers have names that suggest a specific concurrency strategy, e.g. "parallel" for CPU-bound work with a limited number of threads, or "elastic" for I/O-bound work with a large number of threads. If you see such threads it means some code is using a specific thread pool Scheduler strategy.

  • Data access libraries and other 3rd party dependencies may also create and use threads of their own.

Configuring

The Spring Framework does not provide support for starting and stopping servers. To configure the threading model for a server, you’ll need to use server-specific config APIs, or if using Spring Boot, check the Spring Boot configuration options for each server. The WebClient can be configured directly. For all other libraries, refer to their respective documentation.

Reactive Spring Web

The spring-web module provides low level infrastructure and HTTP abstractions — client and server, to build reactive web applications. All public APIs are build around Reactive Streams with Reactor as a backing implementation.

Server support is organized in two layers:

  • HttpHandler and server adapters — the most basic, common API for HTTP request handling with Reactive Streams back pressure.

  • WebHandler API — slightly higher level but still general purpose server web API with filter chain style processing.

HttpHandler

Every HTTP server has some API for HTTP request handling. {api-spring-framework}/http/server/reactive/HttpHandler.html[HttpHandler] is a simple contract with one method to handle a request and response. It is intentionally minimal. Its main purpose is to provide a common, Reactive Streams based API for HTTP request handling over different servers.

The spring-web module contains adapters for every supported server. The table below shows the server APIs are used and where Reactive Streams support comes from:

Server name Server API used Reactive Streams support

Netty

Netty API

Reactor Netty

Undertow

Undertow API

spring-web: Undertow to Reactive Streams bridge

Tomcat

Servlet 3.1 non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[]

spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge

Jetty

Servlet 3.1 non-blocking I/O; Jetty API to write ByteBuffers vs byte[]

spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge

Servlet 3.1 container

Servlet 3.1 non-blocking I/O

spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge

Here are required dependencies, supported versions, and code snippets for each server:

Server name Group id Artifact name

Reactor Netty

io.projectreactor.ipc

reactor-netty

Undertow

io.undertow

undertow-core

Tomcat

org.apache.tomcat.embed

tomcat-embed-core

Jetty

org.eclipse.jetty

jetty-server, jetty-servlet

Reactor Netty:

HttpHandler handler = ...
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
HttpServer.create(host, port).newHandler(adapter).block();

Undertow:

HttpHandler handler = ...
UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();
server.start();

Tomcat:

HttpHandler handler = ...
Servlet servlet = new TomcatHttpHandlerAdapter(handler);

Tomcat server = new Tomcat();
File base = new File(System.getProperty("java.io.tmpdir"));
Context rootContext = server.addContext("", base.getAbsolutePath());
Tomcat.addServlet(rootContext, "main", servlet);
rootContext.addServletMappingDecoded("/", "main");
server.setHost(host);
server.setPort(port);
server.start();

Jetty:

HttpHandler handler = ...
Servlet servlet = new JettyHttpHandlerAdapter(handler);

Server server = new Server();
ServletContextHandler contextHandler = new ServletContextHandler(server, "");
contextHandler.addServlet(new ServletHolder(servlet), "/");
contextHandler.start();

ServerConnector connector = new ServerConnector(server);
connector.setHost(host);
connector.setPort(port);
server.addConnector(connector);
server.start();
Note

To deploy as a WAR to a Servlet 3.1+ container, wrap HttpHandler with ServletHttpHandlerAdapter and register that as a Servlet. This can be automated through the use of {api-spring-framework}/web/server/adapter/AbstractReactiveWebInitializer.html[AbstractReactiveWebInitializer].

WebHandler API

The WebHandler API is a general purpose, server, web API for processing requests through a chain of {api-spring-framework}/web/server/WebExceptionHandler.html[WebExceptionHandler’s], {api-spring-framework}/web/server/WebFilter.html[WebFilter’s], and a target {api-spring-framework}/web/server/WebHandler.html[WebHandler]. The chain can be assembled with WebHttpHandlerBuilder either by adding components to the builder or by having them detected from a Spring ApplicationContext. The builder returns an HttpHandler that can then be used to run on any of the supported servers.

While HttpHandler aims to be the most minimal contract across HTTP servers, the WebHandler API provides essential features commonly used to build web applications. For example, the ServerWebExchange available to WebHandler API components provides access not only to the request and response, but also to request and session attributes, access to parsed form data, multipart data, and more.

Special bean types

The table below lists the components that WebHttpHandlerBuilder detects:

Bean name Bean type Count Description

<any>

WebExceptionHandler

0..N

Provide handling for exceptions from the chain of WebFilter's and the target WebHandler. For more details, see Exceptions.

<any>

WebFilter

0..N

Apply interception style logic to before and after the rest of the filter chain and the target WebHandler. For more details, see Filters.

"webHandler"

WebHandler

1

The handler for the request.

"webSessionManager"

WebSessionManager

0..1

The manager for WebSession's exposed through a method on ServerWebExchange. DefaultWebSessionManager by default.

"serverCodecConfigurer"

ServerCodecConfigurer

0..1

For access to HttpMessageReader's for parsing form data and multipart data that’s then exposed through methods on ServerWebExchange. ServerCodecConfigurer.create() by default.

"localeContextResolver"

LocaleContextResolver

0..1

The resolver for LocaleContext exposed through a method on ServerWebExchange. AcceptHeaderLocaleContextResolver by default.

Form data

ServerWebExchange exposes the following method for access to form data:

Mono<MultiValueMap<String, String>> getFormData();

The DefaultServerWebExchange uses the configured HttpMessageReader to parse form data ("application/x-www-form-urlencoded") into a MultiValueMap. By default FormHttpMessageReader is configured for use via the ServerCodecConfigurer bean (see Web Handler API).

Multipart data

ServerWebExchange exposes the following method for access to multipart data:

Mono<MultiValueMap<String, Part>> getMultipartData();

The DefaultServerWebExchange uses the configured HttpMessageReader<MultiValueMap<String, Part>> to parse "multipart/form-data" content into a MultiValueMap. At present Synchronoss NIO Multipart is the only 3rd party library supported, and the only library we know for non-blocking parsing of multipart requests. It is enabled through the ServerCodecConfigurer bean (see Web Handler API).

To parse multipart data in streaming fashion, use the Flux<Part> returned from an HttpMessageReader<Part> instead. For example in an annotated controller use of @RequestPart implies Map-like access to individual parts by name, and hence requires parsing multipart data in full. By contrast @RequestBody can be used to decode the content to Flux<Part> without collecting to a MultiValueMap.

Message Codecs

The spring-web module defines the {api-spring-framework}/http/codec/HttpMessageReader.html[HttpMessageReader] and {api-spring-framework}/http/codec/HttpMessageWriter.html[HttpMessageWriter] contracts for encoding and decoding the body of HTTP requests and responses via Rective Streams Publisher's. These contacts are used on the client side, e.g. in the WebClient, and on the server side, e.g. in annotated controllers and functional endpoints.

The spring-core module defines the {api-spring-framework}/core/codec/Encoder.html[Encoder] and {api-spring-framework}/core/codec/Decoder.html[Decoder] contracts that are independent of HTTP and rely on the {api-spring-framework}/core/io/buffer/DataBuffer.html[DataBuffer] contract that abstracts different byte buffer representations such as the Netty ByteBuf and java.nio.ByteBuffer (see Data Buffers and Codecs). An Encoder can be wrapped with EncoderHttpMessageWriter to be used as an HttpMessageWriter while a Decoder can be wrapped with DecoderHttpMessageReader to be used as an HttpMessageReader.

The spring-core module contains basic Encoder and Decoder implementations for byte[], ByteBuffer, DataBuffer, Resource, and String. The spring-web module adds Encoder's and Decoder's for Jackson JSON, Jackson Smile, and JAXB2. The spring-web module also contains some web-specific readers and writers for server-sent events, form data, and multipart requests.

To configure or customize the readers and writers to use applications will typically use ClientCodecConfigurer or ServerCodecConfigurer.

Jackson

The decoder relies on Jackson’s non-blocking, byte array parser to parse a stream of byte chunks into a TokenBuffer stream, which can then be turned into Objects with Jackson’s ObjectMapper. JSON and Smile (binary JSON) data formats are currently supported.

The encoder processes a Publisher<?> as follows:

  • if the Publisher is a Mono (i.e. single value), the value is encoded when available.

  • if media type is application/stream+json for JSON or application/stream+x-jackson-smile for Smile, each value produced by the Publisher is encoded individually (and followed by a new line in JSON).

  • otherwise all items from the Publisher are gathered in with Flux#collectToList() and the resulting collection is encoded as an array.

As a special case to the above rules the ServerSentEventHttpMessageWriter feeds items emitted from its input Publisher individually into the Jackson2JsonEncoder as a Mono<?>.

Note that both the Jackson JSON encoder and decoder explicitly back out of rendering elements of type String. Instead String's are treated as low level content, (i.e. serialized JSON) and are rendered as-is by the CharSequenceEncoder. If you want a Flux<String> rendered as a JSON array, you’ll have to use Flux#collectToList() and provide a Mono<List<String>> instead.

HTTP Streaming

When a multi-value, reactive type such as Flux is used for response rendering, it may be collected to a List and rendered as a whole (e.g. JSON array), or it may be treated as an infinite stream with each item flushed immediately. The determination for which is which is made based on content negotiation and the selected media type which may imply a streaming format (e.g. "text/event-stream", "application/stream+json"), or not (e.g. "application/json").

When streaming to the HTTP response, regardless of the media type (e.g. text/event-stream, application/stream+json), it is important to send data periodically, since the write would fail if the client has disconnected. The send could take the form of an empty (comment-only) SSE event, or any other data that the other side would have to interpret as a heartbeat and ignore.

Filters

In the WebHandler API, a WebFilter can be used to apply interception-style logic before and after the rest of the processing chain of filters and the target WebHandler. When using the WebFlux Config, registering a WebFilter is as simple as declaring it as a Spring bean, and optionally expressing precedence via @Order on the bean declaration or by implementing Ordered.

The following describe the available WebFilter implementations:

Forwarded Headers

As a request goes through proxies such as load balancers the host, port, and scheme may change presenting a challenge for applications that need to create links to resources since the links should reflect the host, port, and scheme of the original request as seen from a client perspective.

RFC 7239 defines the "Forwarded" HTTP header for proxies to use to provide information about the original request. There are also other non-standard headers in use such as "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".

ForwardedHeaderFilter detects, extracts, and uses information from the "Forwarded" header, or from "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto". It wraps the request in order to overlay its host, port, and scheme and also "hides" the forwarded headers for subsequent processing.

Note that there are security considerations when using forwarded headers as explained in Section 8 of RFC 7239. At the application level it is difficult to determine whether forwarded headers can be trusted or not. This is why the network upstream should be configured correctly to filter out untrusted forwarded headers from the outside.

Applications that don’t have a proxy and don’t need to use forwarded headers can configure the ForwardedHeaderFilter to remove and ignore such headers.

CORS

Spring WebFlux provides fine-grained support for CORS configuration through annotations on controllers. However when used with Spring Security it is advisable to rely on the built-in CorsFilter that must be ordered ahead of Spring Security’s chain of filters.

See the section on [webflux-cors] and the [webflux-cors-webfilter] for more details.

Exceptions

In the WebHandler API, a WebExceptionHandler can be used to to handle exceptions from the chain of WebFilter's and the target WebHandler. When using the WebFlux Config, registering a WebExceptionHandler is as simple as declaring it as a Spring bean, and optionally expressing precedence via @Order on the bean declaration or by implementing Ordered.

Below are the available WebExceptionHandler implementations:

Exception Handler Description

ResponseStatusExceptionHandler

Provides handling for exceptions of type {api-spring-framework}/web/server/ResponseStatusException.html[ResponseStatusException] by setting the response to the HTTP status code of the exception.

WebFluxResponseStatusExceptionHandler

Extension of ResponseStatusExceptionHandler that can also determine the HTTP status code an @ResponseStatus annotation on any exception.

This handler is declared in the WebFlux Config.

DispatcherHandler

Spring WebFlux, like Spring MVC, is designed around the front controller pattern where a central WebHandler, the DispatcherHandler, provides a shared algorithm for request processing while actual work is performed by configurable, delegate components. This model is flexible and supports diverse workflows.

DispatcherHandler discovers the delegate components it needs from Spring configuration. It is also designed to be a Spring bean itself and implements ApplicationContextAware for access to the context it runs in. If DispatcherHandler is declared with the bean name "webHandler" it is in turn discovered by {api-spring-framework}/web/server/adapter/WebHttpHandlerBuilder.html[WebHttpHandlerBuilder] which puts together a request processing chain as described in WebHandler API.

Spring configuration in a WebFlux application typically contains:

The configuration is given to WebHttpHandlerBuilder to build the processing chain:

ApplicationContext context = ...
HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context);

The resulting HttpHandler is ready for use with a server adapter.

Special bean types

The DispatcherHandler delegates to special beans to process requests and render the appropriate responses. By "special beans" we mean Spring-managed, Object instances that implement WebFlux framework contracts. Those usually come with built-in contracts but you can customize their properties, extend then, or replaced.

The table below lists the special beans detected by the DispatcherHandler. Note that there are also some other beans detected at a lower level, see Special bean types in the Web Handler API.

Bean type Explanation

HandlerMapping

Map a request to a handler. The mapping is based on some criteria the details of which vary by HandlerMapping implementation — annotated controllers, simple URL pattern mappings, etc.

The main HandlerMapping implementations are RequestMappingHandlerMapping for @RequestMapping annotated methods, RouterFunctionMapping for functional endpoint routes, and SimpleUrlHandlerMapping for explicit registrations of URI path patterns and WebHandler's.

HandlerAdapter

Help the DispatcherHandler to invoke a handler mapped to a request regardless of how the handler is actually invoked. For example invoking an annotated controller requires resolving annotations. The main purpose of a HandlerAdapter is to shield the DispatcherHandler from such details.

HandlerResultHandler

Process the result from the handler invocation and finalize the response. See Result Handling.

WebFlux Config

Applications can declare the infrastructure beans listed under Web Handler API and DispatcherHandler that are required to process requests. However in most cases the WebFlux Config is the best starting point. It declares the required beans and provides a higher level configuration callback API to customize it.

Note

Spring Boot relies on the WebFlux config to configure Spring WebFlux and also provides many extra convenient options.

Processing

The DispatcherHandler processes requests as follows:

  • Each HandlerMapping is asked to find a matching handler and the first match is used.

  • If a handler is found, it is executed through an appropriate HandlerAdapter which exposes the return value from the execution as HandlerResult.

  • The HandlerResult is given to an appropriate HandlerResultHandler to complete processing by writing to the response directly or using a view to render.

Result Handling

The return value from the invocation of a handler, through a HandlerAdapter, is wrapped as HandlerResult, along with some additional context, and passed to the first HandlerResultHandler that claims support for it. The table below shows the available HandlerResultHandler implementations all of which are declared in the WebFlux Config:

Result Handler Type Return Values Default Order

ResponseEntityResultHandler

ResponseEntity, typically from @Controller's.

0

ServerResponseResultHandler

ServerResponse, typically from functional endpoints.

0

ResponseBodyResultHandler

Handle return values from @ResponseBody methods or @RestController classes.

100

ViewResolutionResultHandler

CharSequence or {api-spring-framework}/web/reactive/result/view/View.html[View], {api-spring-framework}/ui/Model.html[Model] or Map, {api-spring-framework}/web/reactive/result/view/Rendering.html[Rendering], or any other Object is treated as a model attribute.

Also see View Resolution.

Integer.MAX_VALUE

Exceptions

The HandlerResult returned from a HandlerAdapter may expose a function for error handling based on some handler-specific mechanism. This error function is called if:

  • the handler (e.g. @Controller) invocation fails.

  • handling of the handler return value through a HandlerResultHandler fails.

The error function can change the response, e.g. to an error status, as long as an error signal occurs before the reactive type returned from the handler produces any data items.

This is how @ExceptionHandler methods in @Controller classes are supported. By contrast, support for the same in Spring MVC is built on a HandlerExceptionResolver. This generally shouldn’t matter, however, keep in mind that in WebFlux you cannot use a @ControllerAdvice to handle exceptions that occur before a handler is chosen.

See also Exceptions in the Annotated Controller section, or Exceptions in the WebHandler API section.

View Resolution

View resolution enables rendering to a browser with an HTML template and a model without tying you to a specific view technology. In Spring WebFlux, view resolution is supported through a dedicated HandlerResultHandler that uses ViewResolver's to map a String, representing a logical view name, to a View instance. The View is then used to render the response.

Handling

The HandlerResult passed into ViewResolutionResultHandler contains the return value from the handler, and also the model that contains attributes added during request handling. The return value is processed as one of the following:

  • String, CharSequence — a logical view name to be resolved to a View through the list of configured ViewResolver's.

  • void — select a default view name based on the request path minus the leading and trailing slash, and resolve it to a View. The same also happens when a view name was not provided, e.g. model attribute was returned, or an async return value, e.g. Mono completed empty.

  • {api-spring-framework}/web/reactive/result/view/Rendering.html[Rendering] — API for view resolution scenarios; explore the options in your IDE with code completion.

  • Model, Map — extra model attributes to be added to the model for the request.

  • Any other — any other return value (except for simple types, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty]) is treated as a model attribute to be added to the model. The attribute name is derived from the Class name, using {api-spring-framework}/core/Conventions.html[Conventions], unless a handler method @ModelAttribute annotation is present.

The model can contain asynchronous, reactive types (e.g. from Reactor, RxJava). Prior to rendering, AbstractView resolves such model attributes into concrete values and updates the model. Single-value reactive types are resolved to a single value, or no value (if empty) while multi-value reactive types, e.g. Flux<T> are collected and resolved to List<T>.

To configure view resolution is as simple as adding a ViewResolutionResultHandler bean to your Spring configuration. WebFlux Config provides a dedicated configuration API for view resolution.

See [webflux-view] for more on the view technologies integrated with Spring WebFlux.

Redirecting

The special redirect: prefix in a view name allows you to perform a redirect. The UrlBasedViewResolver (and sub-classes) recognize this as an instruction that a redirect is needed. The rest of the view name is the redirect URL.

The net effect is the same as if the controller had returned a RedirectView or Rendering.redirectTo("abc").build(), but now the controller itself can simply operate in terms of logical view names. A view name such as redirect:/some/resource is relative to the current application, while the view name redirect:http://example.com/arbitrary/path redirects to an absolute URL.

Content negotiation

ViewResolutionResultHandler supports content negotiation. It compares the request media type(s) with the media type(s) supported by each selected View. The first View that supports the requested media type(s) is used.

In order to support media types such as JSON and XML, Spring WebFlux provides HttpMessageWriterView which is a special View that renders through an HttpMessageWriter. Typically you would configure these as default views through the WebFlux Config. Default views are always selected and used if they match the requested media type.

Annotated Controllers

Spring WebFlux provides an annotation-based programming model where @Controller and @RestController components use annotations to express request mappings, request input, exception handling, and more. Annotated controllers have flexible method signatures and do not have to extend base classes nor implement specific interfaces.

Here is a basic example:

@RestController
public class HelloController {

	@GetMapping("/hello")
	public String handle() {
		return "Hello WebFlux";
	}
}

In this example the methods returns a String to be written to the response body.

@Controller

You can define controller beans using a standard Spring bean definition. The @Controller stereotype allows for auto-detection, aligned with Spring general support for detecting @Component classes in the classpath and auto-registering bean definitions for them. It also acts as a stereotype for the annotated class, indicating its role as a web component.

To enable auto-detection of such @Controller beans, you can add component scanning to your Java configuration:

@Configuration
@ComponentScan("org.example.web")
public class WebConfig {

	// ...
}

@RestController is a composed annotation that is itself meta-annotated with @Controller and @ResponseBody indicating a controller whose every method inherits the type-level @ResponseBody annotation and therefore writes directly to the response body vs view resolution and rendering with an HTML template.

Request Mapping

The @RequestMapping annotation is used to map requests to controllers methods. It has various attributes to match by URL, HTTP method, request parameters, headers, and media types. It can be used at the class-level to express shared mappings or at the method level to narrow down to a specific endpoint mapping.

There are also HTTP method specific shortcut variants of @RequestMapping:

  • @GetMapping

  • @PostMapping

  • @PutMapping

  • @DeleteMapping

  • @PatchMapping

The above are Custom Annotations that are provided out of the box because arguably most controller methods should be mapped to a specific HTTP method vs using @RequestMapping which by default matches to all HTTP methods. At the same an @RequestMapping is still needed at the class level to express shared mappings.

Below is an example with type and method level mappings:

@RestController
@RequestMapping("/persons")
class PersonController {

	@GetMapping("/{id}")
	public Person getPerson(@PathVariable Long id) {
		// ...
	}

	@PostMapping
	@ResponseStatus(HttpStatus.CREATED)
	public void add(@RequestBody Person person) {
		// ...
	}
}

URI Patterns

You can map requests using glob patterns and wildcards:

  • ? matches one character

  • * matches zero or more characters within a path segment

  • ** match zero or more path segments

You can also declare URI variables and access their values with @PathVariable:

@GetMapping("/owners/{ownerId}/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
	// ...
}

URI variables can be declared at the class and method level:

@Controller
@RequestMapping("/owners/{ownerId}")
public class OwnerController {

	@GetMapping("/pets/{petId}")
	public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
		// ...
	}
}

URI variables are automatically converted to the appropriate type or`TypeMismatchException` is raised. Simple types — int, long, Date, are supported by default and you can register support for any other data type. See Type Conversion and DataBinder.

URI variables can be named explicitly — e.g. @PathVariable("customId"), but you can leave that detail out if the names are the same and your code is compiled with debugging information or with the -parameters compiler flag on Java 8.

The syntax {*varName} declares a URI variable that matches zero or more remaining path segments. For example /resources/{*path} matches all files /resources/ and the "path" variable captures the complete relative path.

The syntax {varName:regex} declares a URI variable with a regular expressions with the syntax {varName:regex} — e.g. given URL "/spring-web-3.0.5 .jar", the below method extracts the name, version, and file extension:

@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
public void handle(@PathVariable String version, @PathVariable String ext) {
	// ...
}

URI path patterns can also have embedded ${…​} placeholders that are resolved on startup via PropertyPlaceHolderConfigurer against local, system, environment, and other property sources. This can be used for example to parameterize a base URL based on some external configuration.

Note

Spring WebFlux uses PathPattern and the PathPatternParser for URI path matching support both of which are located in spring-web and expressly designed for use with HTTP URL paths in web applications where a large number of URI path patterns are matched at runtime.

Spring WebFlux does not support suffix pattern matching — unlike Spring MVC, where a mapping such as /person also matches to /person.*. For URL based content negotiation, if needed, we recommend using a query parameter, which is simpler, more explicit, and less vulnerable to URL path based exploits.

Pattern Comparison

When multiple patterns match a URL, they must be compared to find the best match. This is done with PathPattern.SPECIFICITY_COMPARATOR which looks for patterns that more specific.

For every pattern, a score is computed based the number of URI variables and wildcards where a URI variable scores lower than a wildcard. A pattern with a lower total score wins. If two patterns have the same score, then the longer is chosen.

Catch-all patterns, e.g. **, {*varName}, are excluded from the scoring and are always sorted last instead. If two patterns are both catch-all, the longer is chosen.

Consumable Media Types

You can narrow the request mapping based on the Content-Type of the request:

@PostMapping(path = "/pets", consumes = "application/json")
public void addPet(@RequestBody Pet pet) {
	// ...
}

The consumes attribute also supports negation expressions — e.g. !text/plain means any content type other than "text/plain".

You can declare a shared consumes attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level consumes attribute overrides rather than extend the class level declaration.

Tip

MediaType provides constants for commonly used media types — e.g. APPLICATION_JSON_VALUE, APPLICATION_JSON_UTF8_VALUE.

Producible Media Types

You can narrow the request mapping based on the Accept request header and the list of content types that a controller method produces:

@GetMapping(path = "/pets/{petId}", produces = "application/json;charset=UTF-8")
@ResponseBody
public Pet getPet(@PathVariable String petId) {
	// ...
}

The media type can specify a character set. Negated expressions are supported — e.g. !text/plain means any content type other than "text/plain".

You can declare a shared produces attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level produces attribute overrides rather than extend the class level declaration.

Tip

MediaType provides constants for commonly used media types — e.g. APPLICATION_JSON_VALUE, APPLICATION_JSON_UTF8_VALUE.

Parameters and Headers

You can narrow request mappings based on query parameter conditions. You can test for the presence of a query parameter ("myParam"), for the absence ("!myParam"), or for a specific value ("myParam=myValue"):

@GetMapping(path = "/pets/{petId}", params = "myParam=myValue")
public void findPet(@PathVariable String petId) {
	// ...
}

You can also use the same with request header conditions:

@GetMapping(path = "/pets", headers = "myHeader=myValue")
public void findPet(@PathVariable String petId) {
	// ...
}

HTTP HEAD, OPTIONS

@GetMapping — and also @RequestMapping(method=HttpMethod.GET), support HTTP HEAD transparently for request mapping purposes. Controller methods don’t need to change. A response wrapper, applied in the HttpHandler server adapter, ensures a "Content-Length" header is set to the number of bytes written and without actually writing to the response.

By default HTTP OPTIONS is handled by setting the "Allow" response header to the list of HTTP methods listed in all @RequestMapping methods with matching URL patterns.

For a @RequestMapping without HTTP method declarations, the "Allow" header is set to "GET,HEAD,POST,PUT,PATCH,DELETE,OPTIONS". Controller methods should always declare the supported HTTP methods for example by using the HTTP method specific variants — @GetMapping, @PostMapping, etc.

@RequestMapping method can be explicitly mapped to HTTP HEAD and HTTP OPTIONS, but that is not necessary in the common case.

Custom Annotations

Spring WebFlux supports the use of composed annotations for request mapping. Those are annotations that are themselves meta-annotated with @RequestMapping and composed to redeclare a subset (or all) of the @RequestMapping attributes with a narrower, more specific purpose.

@GetMapping, @PostMapping, @PutMapping, @DeleteMapping, and @PatchMapping are examples of composed annotations. They’re provided out of the box because arguably most controller methods should be mapped to a specific HTTP method vs using @RequestMapping which by default matches to all HTTP methods. If you need an example of composed annotations, look at how those are declared.

Spring WebFlux also supports custom request mapping attributes with custom request matching logic. This is a more advanced option that requires sub-classing RequestMappingHandlerMapping and overriding the getCustomMethodCondition method where you can check the custom attribute and return your own RequestCondition.

Handler methods

@RequestMapping handler methods have a flexible signature and can choose from a range of supported controller method arguments and return values.

Method arguments

The table below shows supported controller method arguments.

Reactive types (Reactor, RxJava, or other) are supported on arguments that require blocking I/O, e.g. reading the request body, to be resolved. This is marked in the description column. Reactive types are not expected on arguments that don’t require blocking.

JDK 1.8’s java.util.Optional is supported as a method argument in combination with annotations that have a required attribute — e.g. @RequestParam, @RequestHeader, etc, and is equivalent to required=false.

Controller method argument Description

ServerWebExchange

Access to the full ServerWebExchange — container for the HTTP request and response, request and session attributes, checkNotModified methods, and others.

ServerHttpRequest, ServerHttpResponse

Access to the HTTP request or response.

WebSession

Access to the session; this does not force the start of a new session unless attributes are added. Supports reactive types.

java.security.Principal

Currently authenticated user; possibly a specific Principal implementation class if known. Supports reactive types.

org.springframework.http.HttpMethod

The HTTP method of the request.

java.util.Locale

The current request locale, determined by the most specific LocaleResolver available, in effect, the configured LocaleResolver/LocaleContextResolver.

java.util.TimeZone + java.time.ZoneId

The time zone associated with the current request, as determined by a LocaleContextResolver.

@PathVariable

For access to URI template variables. See URI Patterns.

@MatrixVariable

For access to name-value pairs in URI path segments. See Matrix variables.

@RequestParam

For access to Servlet request parameters. Parameter values are converted to the declared method argument type. See @RequestParam.

Note that use of @RequestParam is optional, e.g. to set its attributes. See "Any other argument" further below in this table.

@RequestHeader

For access to request headers. Header values are converted to the declared method argument type. See @RequestHeader.

@CookieValue

For access to cookies. Cookies values are converted to the declared method argument type. See @CookieValue.

@RequestBody

For access to the HTTP request body. Body content is converted to the declared method argument type using HttpMessageReader's. Supports reactive types. See @RequestBody.

HttpEntity<B>

For access to request headers and body. The body is converted with HttpMessageReader's. Supports reactive types. See HttpEntity.

@RequestPart

For access to a part in a "multipart/form-data" request. Supports reactive types. See Multipart and Multipart data.

java.util.Map, org.springframework.ui.Model, org.springframework.ui.ModelMap

For access to the model that is used in HTML controllers and exposed to templates as part of view rendering.

@ModelAttribute

For access to an existing attribute in the model (instantiated if not present) with data binding and validation applied. See @ModelAttribute as well as Model and DataBinder.

Note that use of @ModelAttribute is optional, e.g. to set its attributes. See "Any other argument" further below in this table.

Errors, BindingResult

For access to errors from validation and data binding for a command object (i.e. @ModelAttribute argument), or errors from the validation of an @RequestBody or @RequestPart arguments; an Errors, or BindingResult argument must be declared immediately after the validated method argument.

SessionStatus + class-level @SessionAttributes

For marking form processing complete which triggers cleanup of session attributes declared through a class-level @SessionAttributes annotation. See @SessionAttributes for more details.

UriComponentsBuilder

For preparing a URL relative to the current request’s host, port, scheme, context path, and the literal part of the servlet mapping also taking into account Forwarded and X-Forwarded-* headers. // TODO: See [webflux-uri-building].

@SessionAttribute

For access to any session attribute; in contrast to model attributes stored in the session as a result of a class-level @SessionAttributes declaration. See @SessionAttribute for more details.

@RequestAttribute

For access to request attributes. See @RequestAttribute for more details.

Any other argument

If a method argument is not matched to any of the above, by default it is resolved as an @RequestParam if it is a simple type, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty], or as an @ModelAttribute otherwise.

Return values

The table below shows supported controller method return values. Note that reactive types from libraries such as Reactor, RxJava, or other are generally supported for all return values.

Controller method return value Description

@ResponseBody

The return value is encoded through HttpMessageWriter's and written to the response. See @ResponseBody.

HttpEntity<B>, ResponseEntity<B>

The return value specifies the full response including HTTP headers and body be encoded through HttpMessageWriter's and written to the response. See ResponseEntity.

HttpHeaders

For returning a response with headers and no body.

String

A view name to be resolved with ViewResolver's and used together with the implicit model — determined through command objects and @ModelAttribute methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).

View

A View instance to use for rendering together with the implicit model — determined through command objects and @ModelAttribute methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).

java.util.Map, org.springframework.ui.Model

Attributes to be added to the implicit model with the view name implicitly determined based on the request path.

@ModelAttribute

An attribute to be added to the model with the view name implicitly determined based on the request path.

Note that @ModelAttribute is optional. See "Any other return value" further below in this table.

Rendering

An API for model and view rendering scenarios.

void

A method with a void, possibly async (e.g. Mono<Void>), return type (or a null return value) is considered to have fully handled the response if it also has a ServerHttpResponse, or a ServerWebExchange argument, or an @ResponseStatus annotation. The same is true also if the controller has made a positive ETag or lastModified timestamp check. // TODO: See [webflux-caching-etag-lastmodified] for details.

If none of the above is true, a void return type may also indicate "no response body" for REST controllers, or default view name selection for HTML controllers.

Flux<ServerSentEvent>, Observable<ServerSentEvent>, or other reactive type

Emit server-sent events; the SeverSentEvent wrapper can be omitted when only data needs to be written (however text/event-stream must be requested or declared in the mapping through the produces attribute).

Any other return value

If a return value is not matched to any of the above, by default it is treated as a view name, if it is String or void (default view name selection applies); or as a model attribute to be added to the model, unless it is a simple type, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty] in which case it remains unresolved.

Type Conversion

Some annotated controller method arguments that represent String-based request input — e.g. @RequestParam, @RequestHeader, @PathVariable, @MatrixVariable, and @CookieValue, may require type conversion if the argument is declared as something other than String.

For such cases type conversion is automatically applied based on the configured converters. By default simple types such as int, long, Date, etc. are supported. Type conversion can be customized through a WebDataBinder, see [mvc-ann-initbinder], or by registering Formatters with the FormattingConversionService, see Spring Field Formatting.

Matrix variables

RFC 3986 discusses name-value pairs in path segments. In Spring WebFlux we refer to those as "matrix variables" based on an "old post" by Tim Berners-Lee but they can be also be referred to as URI path parameters.

Matrix variables can appear in any path segment, each variable separated by semicolon and multiple values separated by comma, e.g. "/cars;color=red,green;year=2012". Multiple values can also be specified through repeated variable names, e.g. "color=red;color=green;color=blue".

Unlike Spring MVC, in WebFlux the presence or absence of matrix variables in a URL does not affect request mappings. In other words you’re not required to use a URI variable to mask variable content. That said if you want to access matrix variables from a controller method you need to add a URI variable to the path segment where matrix variables are expected. Below is an example:

// GET /pets/42;q=11;r=22

@GetMapping("/pets/{petId}")
public void findPet(@PathVariable String petId, @MatrixVariable int q) {

	// petId == 42
	// q == 11
}

Given that all path segments may contain matrix variables, sometimes you may need to disambiguate which path variable the matrix variable is expected to be in. For example:

// GET /owners/42;q=11/pets/21;q=22

@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
		@MatrixVariable(name="q", pathVar="ownerId") int q1,
		@MatrixVariable(name="q", pathVar="petId") int q2) {

	// q1 == 11
	// q2 == 22
}

A matrix variable may be defined as optional and a default value specified:

// GET /pets/42

@GetMapping("/pets/{petId}")
public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) {

	// q == 1
}

To get all matrix variables, use a MultiValueMap:

// GET /owners/42;q=11;r=12/pets/21;q=22;s=23

@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
		@MatrixVariable MultiValueMap<String, String> matrixVars,
		@MatrixVariable(pathVar="petId"") MultiValueMap<String, String> petMatrixVars) {

	// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
	// petMatrixVars: ["q" : 22, "s" : 23]
}

@RequestParam

Use the @RequestParam annotation to bind query parameters to a method argument in a controller. The following code snippet shows the usage:

@Controller
@RequestMapping("/pets")
public class EditPetForm {

	// ...

	@GetMapping
	public String setupForm(@RequestParam("petId") int petId, Model model) {
		Pet pet = this.clinic.loadPet(petId);
		model.addAttribute("pet", pet);
		return "petForm";
	}

	// ...

}
Tip

Unlike the Servlet API "request paramater" concept that conflate query parameters, form data, and multiparts into one, in WebFlux each is accessed individually through the ServerWebExchange. While @RequestParam binds to query parameters only, you can use data binding to apply query paramerters, form data, and multiparts to a command object.

Method parameters using using the @RequestParam annotation are required by default, but you can specify that a method parameter is optional by setting @RequestParam's required flag to false or by declaring the argument with an java.util.Optional wrapper.

Type conversion is applied automatically if the target method parameter type is not String. See [mvc-ann-typeconversion].

When an @RequestParam annotation is declared as Map<String, String> or MultiValueMap<String, String> argument, the map is populated with all query parameters.

Note that use of @RequestParam is optional, e.g. to set its attributes. By default any argument that is a simple value type, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty], and is not resolved by any other argument resolver, is treated as if it was annotated with @RequestParam.

@RequestHeader

Use the @RequestHeader annotation to bind a request header to a method argument in a controller.

Given request with headers:

Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300

The following gets the value of the Accept-Encoding and Keep-Alive headers:

@GetMapping("/demo")
public void handle(
		@RequestHeader("Accept-Encoding") String encoding,
		@RequestHeader("Keep-Alive") long keepAlive) {
	//...
}

Type conversion is applied automatically if the target method parameter type is not String. See [mvc-ann-typeconversion].

When an @RequestHeader annotation is used on a Map<String, String>, MultiValueMap<String, String>, or HttpHeaders argument, the map is populated with all header values.

Tip

Built-in support is available for converting a comma-separated string into an array/collection of strings or other types known to the type conversion system. For example a method parameter annotated with @RequestHeader("Accept") may be of type String but also String[] or List<String>.

@CookieValue

Use the @CookieValue annotation to bind the value of an HTTP cookie to a method argument in a controller.

Given request with the following cookie:

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

The following code sample demonstrates how to get the cookie value:

@GetMapping("/demo")
public void handle(@CookieValue("JSESSIONID") String cookie) {
	//...
}

Type conversion is applied automatically if the target method parameter type is not String. See [mvc-ann-typeconversion].

@ModelAttribute

Use the @ModelAttribute annotation on a method argument to access an attribute from the model, or have it instantiated if not present. The model attribute is also overlaid with values of query parameters and form fields whose names match to field names. This is referred to as data binding and it saves you from having to deal with parsing and converting individual query parameters and form fields. For example:

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute Pet pet) { }

The Pet instance above is resolved as follows:

  • From the model if already added via Model.

  • From the HTTP session via @SessionAttributes.

  • From the invocation of a default constructor.

  • From the invocation of a "primary constructor" with arguments matching to query parameters or form fields; argument names are determined via JavaBeans @ConstructorProperties or via runtime-retained parameter names in the bytecode.

After the model attribute instance is obtained, data binding is applied. The WebExchangeDataBinder class matches names of query parameters and form fields to field names on the target Object. Matching fields are populated after type conversion is applied where necessary. For more on data binding (and validation) see Validation. For more on customizing data binding see DataBinder.

Data binding may result in errors. By default a WebExchangeBindException is raised but to check for such errors in the controller method, add a BindingResult argument immediately next to the @ModelAttribute as shown below:

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) {
	if (result.hasErrors()) {
		return "petForm";
	}
	// ...
}

Validation can be applied automatically after data binding by adding the javax.validation.Valid annotation or Spring’s @Validated annotation (also see Bean validation and Spring validation). For example:

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) {
	if (result.hasErrors()) {
		return "petForm";
	}
	// ...
}

Spring WebFlux, unlike Spring MVC, supports reactive types in the model, e.g. Mono<Account> or io.reactivex.Single<Account>. An @ModelAttribute argument can be declared with or without a reactive type wrapper, and it will be resolved accordingly, to the actual value if necessary. Note however that in order to use a BindingResult argument, you must declare the @ModelAttribute argument before it without a reactive type wrapper, as shown earlier. Alternatively, you can handle any errors through the reactive type:

@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public Mono<String> processSubmit(@Valid @ModelAttribute("pet") Mono<Pet> petMono) {
	return petMono
		.flatMap(pet -> {
			// ...
		})
		.onErrorResume(ex -> {
			// ...
		});
}

Note that use of @ModelAttribute is optional, e.g. to set its attributes. By default any argument that is not a simple value type, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty], and is not resolved by any other argument resolver, is treated as if it was annotated with @ModelAttribute.

@SessionAttributes

@SessionAttributes is used to store model attributes in the WebSession between requests. It is a type-level annotation that declares session attributes used by a specific controller. This will typically list the names of model attributes or types of model attributes which should be transparently stored in the session for subsequent requests to access.

For example:

@Controller
@SessionAttributes("pet")
public class EditPetForm {
	// ...
}

On the first request when a model attribute with the name "pet" is added to the model, it is automatically promoted to and saved in the WebSession. It remains there until another controller method uses a SessionStatus method argument to clear the storage:

@Controller
@SessionAttributes("pet")
public class EditPetForm {

	// ...

	@PostMapping("/pets/{id}")
	public String handle(Pet pet, BindingResult errors, SessionStatus status) {
		if (errors.hasErrors) {
			// ...
		}
			status.setComplete();
			// ...
		}
	}
}

@SessionAttribute

If you need access to pre-existing session attributes that are managed globally, i.e. outside the controller (e.g. by a filter), and may or may not be present use the @SessionAttribute annotation on a method parameter:

@GetMapping("/")
public String handle(@SessionAttribute User user) {
	// ...
}

For use cases that require adding or removing session attributes consider injecting WebSession into the controller method.

For temporary storage of model attributes in the session as part of a controller workflow consider using SessionAttributes as described in @SessionAttributes.

@RequestAttribute

Similar to @SessionAttribute the @RequestAttribute annotation can be used to access pre-existing request attributes created earlier, e.g. by a WebFilter:

@GetMapping("/")
public String handle(@RequestAttribute Client client) {
	// ...
}

Multipart

As explained in Multipart data, ServerWebExchange provides access to multipart content. The best way to handle a file upload form (e.g. from a browser) in a controller is through data binding to a command object:

class MyForm {

	private String name;

	private MultipartFile file;

	// ...

}

@Controller
public class FileUploadController {

	@PostMapping("/form")
	public String handleFormUpload(MyForm form, BindingResult errors) {
		// ...
	}

}

Multipart requests can also be submitted from non-browser clients in a RESTful service scenario. For example a file along with JSON:

POST /someUrl
Content-Type: multipart/mixed

--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="meta-data"
Content-Type: application/json; charset=UTF-8
Content-Transfer-Encoding: 8bit

{
	"name": "value"
}
--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="file-data"; filename="file.properties"
Content-Type: text/xml
Content-Transfer-Encoding: 8bit
... File Data ...

You can access individual parts with @RequestPart:

@PostMapping("/")
public String handle(@RequestPart("meta-data") Part metadata,
		@RequestPart("file-data") FilePart file) {
	// ...
}

To deserialize the raw part content, for example to JSON (similar to @RequestBody), simply declare a concrete target Object, instead of Part:

@PostMapping("/")
public String handle(@RequestPart("meta-data") MetaData metadata) {
	// ...
}

@RequestPart can be used in combination with javax.validation.Valid, or Spring’s @Validated annotation, which causes Standard Bean Validation to be applied. By default validation errors cause a WebExchangeBindException which is turned into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally within the controller through an Errors or BindingResult argument:

@PostMapping("/")
public String handle(@Valid @RequestPart("meta-data") MetaData metadata,
		BindingResult result) {
	// ...
}

To access all multipart data in as a MultiValueMap use @RequestBody:

@PostMapping("/")
public String handle(@RequestBody Mono<MultiValueMap<String, Part>> parts) {
	// ...
}

To access multipart data sequentially, in streaming fashion, use @RequestBody with Flux<Part> instead. For example:

@PostMapping("/")
public String handle(@RequestBody Flux<Part> parts) {
	// ...
}

@RequestBody

Use the @RequestBody annotation to have the request body read and deserialized into an Object through an HttpMessageReader. Below is an example with an @RequestBody argument:

@PostMapping("/accounts")
public void handle(@RequestBody Account account) {
	// ...
}

Unlike Spring MVC, in WebFlux the @RequestBody method argument supports reactive types and fully non-blocking reading and (client-to-server) streaming:

@PostMapping("/accounts")
public void handle(@RequestBody Mono<Account> account) {
	// ...
}

You can use the HTTP message codecs option of the WebFlux Config to configure or customize message readers.

@RequestBody can be used in combination with javax.validation.Valid, or Spring’s @Validated annotation, which causes Standard Bean Validation to be applied. By default validation errors cause a WebExchangeBindException which is turned into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally within the controller through an Errors or BindingResult argument:

@PostMapping("/accounts")
public void handle(@Valid @RequestBody Account account, BindingResult result) {
	// ...
}

HttpEntity

HttpEntity is more or less identical to using @RequestBody but based on a container object that exposes request headers and body. Below is an example:

@PostMapping("/accounts")
public void handle(HttpEntity<Account> entity) {
	// ...
}

@ResponseBody

Use the @ResponseBody annotation on a method to have the return serialized to the response body through an HttpMessageWriter. For example:

@GetMapping("/accounts/{id}")
@ResponseBody
public Account handle() {
	// ...
}

@ResponseBody is also supported at the class level in which case it is inherited by all controller methods. This is the effect of @RestController which is nothing more than a meta-annotation marked with @Controller and @ResponseBody.

@ResponseBody supports reactive types which means you can return Reactor or RxJava types and have the asynchronous values they produce rendered to the response. For additional details, see HTTP Streaming and JSON rendering.

@ResponseBody methods can be combined with JSON serialization views. See Jackson JSON for details.

You can use the HTTP message codecs option of the WebFlux Config to configure or customize message writing.

ResponseEntity

ResponseEntity is more or less identical to using @ResponseBody but based on a container object that specifies request headers and body. Below is an example:

@PostMapping("/something")
public ResponseEntity<String> handle() {
	// ...
	URI location = ...
	return new ResponseEntity.created(location).build();
}

Jackson JSON

Jackson serialization views

Spring WebFlux provides built-in support for Jackson’s Serialization Views which allows rendering only a subset of all fields in an Object. To use it with @ResponseBody or ResponseEntity controller methods, use Jackson’s @JsonView annotation to activate a serialization view class:

@RestController
public class UserController {

	@GetMapping("/user")
	@JsonView(User.WithoutPasswordView.class)
	public User getUser() {
		return new User("eric", "7!jd#h23");
	}
}

public class User {

	public interface WithoutPasswordView {};
	public interface WithPasswordView extends WithoutPasswordView {};

	private String username;
	private String password;

	public User() {
	}

	public User(String username, String password) {
		this.username = username;
		this.password = password;
	}

	@JsonView(WithoutPasswordView.class)
	public String getUsername() {
		return this.username;
	}

	@JsonView(WithPasswordView.class)
	public String getPassword() {
		return this.password;
	}
}
Note

@JsonView allows an array of view classes but you can only specify only one per controller method. Use a composite interface if you need to activate multiple views.

Model

The @ModelAttribute annotation can be used:

  • On a method argument in @RequestMapping methods to create or access an Object from the model, and to bind it to the request through a WebDataBinder.

  • As a method-level annotation in @Controller or @ControllerAdvice classes helping to initialize the model prior to any @RequestMapping method invocation.

  • On a @RequestMapping method to mark its return value is a model attribute.

This section discusses @ModelAttribute methods, or the 2nd from the list above. A controller can have any number of @ModelAttribute methods. All such methods are invoked before @RequestMapping methods in the same controller. A @ModelAttribute method can also be shared across controllers via @ControllerAdvice. See the section on Controller Advice for more details.

@ModelAttribute methods have flexible method signatures. They support many of the same arguments as @RequestMapping methods except for @ModelAttribute itself nor anything related to the request body.

An example @ModelAttribute method:

@ModelAttribute
public void populateModel(@RequestParam String number, Model model) {
	model.addAttribute(accountRepository.findAccount(number));
	// add more ...
}

To add one attribute only:

@ModelAttribute
public Account addAccount(@RequestParam String number) {
	return accountRepository.findAccount(number);
}
Note

When a name is not explicitly specified, a default name is chosen based on the Object type as explained in the Javadoc for {api-spring-framework}/core/Conventions.html[Conventions]. You can always assign an explicit name by using the overloaded addAttribute method or through the name attribute on @ModelAttribute (for a return value).

Spring WebFlux, unlike Spring MVC, explicitly supports reactive types in the model, e.g. Mono<Account> or io.reactivex.Single<Account>. Such asynchronous model attributes may be transparently resolved (and the model updated) to their actual values at the time of @RequestMapping invocation, providing a @ModelAttribute argument is declared without a wrapper, for example:

@ModelAttribute
public void addAccount(@RequestParam String number) {
    Mono<Account> accountMono = accountRepository.findAccount(number);
    model.addAttribute("account", accountMono);
}

@PostMapping("/accounts")
public String handle(@ModelAttribute Account account, BindingResult errors) {
	// ...
}

In addition any model attributes that have a reactive type wrapper are resolved to their actual values (and the model updated) just prior to view rendering.

@ModelAttribute can also be used as a method-level annotation on @RequestMapping methods in which case the return value of the @RequestMapping method is interpreted as a model attribute. This is typically not required, as it is the default behavior in HTML controllers, unless the return value is a String which would otherwise be interpreted as a view name. @ModelAttribute can also help to customize the model attribute name:

@GetMapping("/accounts/{id}")
@ModelAttribute("myAccount")
public Account handle() {
	// ...
	return account;
}

DataBinder

@Controller or @ControllerAdvice classes can have @InitBinder methods in order to initialize instances of WebDataBinder, and those in turn are used to:

  • Bind request parameters (i.e. form data or query) to a model object.

  • Convert String-based request values such as request parameters, path variables, headers, cookies, and others, to the target type of controller method arguments.

  • Format model object values as String values when rendering HTML forms.

@InitBinder methods can register controller-specific java.bean.PropertyEditor, or Spring Converter and Formatter components. In addition, the WebFlux Java config can be used to register Converter and Formatter types in a globally shared FormattingConversionService.

@InitBinder methods support many of the same arguments that a @RequestMapping methods do, except for @ModelAttribute (command object) arguments. Typically they’re are declared with a WebDataBinder argument, for registrations, and a void return value. Below is an example:

@Controller
public class FormController {

	@InitBinder
	public void initBinder(WebDataBinder binder) {
		SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
		dateFormat.setLenient(false);
		binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
	}

	// ...
}

Alternatively when using a Formatter-based setup through a shared FormattingConversionService, you could re-use the same approach and register controller-specific Formatter's:

@Controller
public class FormController {

	@InitBinder
	protected void initBinder(WebDataBinder binder) {
		binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
	}

	// ...
}

Exceptions

@Controller and @ControllerAdvice classes can have @ExceptionHandler methods to handle exceptions from controller methods. For example:

@Controller
public class SimpleController {

	// ...

	@ExceptionHandler
	public ResponseEntity<String> handle(IOException ex) {
		// ...
	}

}

The annotation can list the exception types to match. Or simply declare the target exception as a method argument as shown above. When multiple exception methods match, a root exception match is generally preferred to a cause exception match. More formally the ExceptionDepthComparator is used to sort exceptions based on their depth from the thrown exception type.

In a multi-@ControllerAdvice arrangement, please declare your primary root exception mappings on a @ControllerAdvice prioritized with a corresponding order. While a root exception match is preferred to a cause, this is mainly among the methods of a given controller or @ControllerAdvice. That means a cause match on a higher-priority @ControllerAdvice is preferred to any match (e.g. root) on a lower-priority @ControllerAdvice.

Support for @ExceptionHandler methods in Spring WebFlux is provided by the HandlerAdapter for @RequestMapping methods. See Exceptions under the DispatcherHandler section for more details.

An @ExceptionHandler method in WebFlux supports the same method arguments and return values as an @RequestMapping method does with the exception of request body and @ModelAttribute related method arguments.

REST API exceptions

A common requirement for REST services is to include error details in the body of the response. The Spring Framework does not automatically do this because the representation of error details in the response body is application specific. However a @RestController may use @ExceptionHandler methods with a ResponseEntity return value to set the status and the body of the response. Such methods may also be declared in @ControllerAdvice classes to apply them globally.

Note

Note that Spring WebFlux does not have an equivalent for the Spring MVC ResponseEntityExceptionHandler because WebFlux only raises ResponseStatusException (or sub-classes of), which and those do not need to be translated translation to an HTTP status code.

Controller Advice

Typically @ExceptionHandler, @InitBinder, and @ModelAttribute methods apply within the @Controller class (or class hierarchy) they are declared in. If you want such methods to apply more globally, across controllers, you can declare them in a class marked with @ControllerAdvice or @RestControllerAdvice.

@ControllerAdvice is marked with @Component which means such classes can be registered as Spring beans via component scanning. @RestControllerAdvice is also a meta-annotation marked with both @ControllerAdvice and @ResponseBody which essentially means @ExceptionHandler methods are rendered to the response body via message conversion (vs view resolution/template rendering).

On startup, the infrastructure classes for @RequestMapping and @ExceptionHandler methods detect Spring beans of type @ControllerAdvice, and then apply their methods at runtime. Global @ExceptionHandler methods (from an @ControllerAdvice) are applied after local ones (from the @Controller). By contrast global @ModelAttribute and @InitBinder methods are applied before local ones.

By default @ControllerAdvice methods apply to every request, i.e. all controllers, but you can narrow that down to a subset of controllers via attributes on the annotation:

// Target all Controllers annotated with @RestController
@ControllerAdvice(annotations = RestController.class)
public class ExampleAdvice1 {}

// Target all Controllers within specific packages
@ControllerAdvice("org.example.controllers")
public class ExampleAdvice2 {}

// Target all Controllers assignable to specific classes
@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})
public class ExampleAdvice3 {}

Keep in mind the above selectors are evaluated at runtime and may negatively impact performance if used extensively. See the {api-spring-framework}/web/bind/annotation/ControllerAdvice.html[@ControllerAdvice] Javadoc for more details.

URI Links

This section describes various options available in the Spring Framework to prepare URIs.

Web Security

The Spring Security project provides support for protecting web applications from malicious exploits. Check out the Spring Security reference documentation including:

  • {doc-root}/spring-security/site/docs/current/reference/html5/#jc-webflux[WebFlux Security]

  • {doc-root}/spring-security/site/docs/current/reference/html5/#test-webflux["WebFlux Testing Support"]

  • {doc-root}/spring-security/site/docs/current/reference/html5/#csrf[CSRF Protection]

  • {doc-root}/spring-security/site/docs/current/reference/html5/#headers[Security Response Headers]

WebFlux Config

The WebFlux Java config declares components required to process requests with annotated controllers or functional endpoints, and it offers an API to customize the configuration. That means you do not need to understand the underlying beans created by the Java config but, if you want to, it’s very easy to see them in WebFluxConfigurationSupport or read more what they are in Special bean types.

For more advanced customizations, not available in the configuration API, it is also possible to gain full control over the configuration through the Advanced config mode.

Enable WebFlux config

Use the @EnableWebFlux annotation in your Java config:

@Configuration
@EnableWebFlux
public class WebConfig {
}

The above registers a number of Spring WebFlux infrastructure beans also adapting to dependencies available on the classpath — for JSON, XML, etc.

WebFlux config API

In your Java config implement the WebFluxConfigurer interface:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	// Implement configuration methods...

}

Conversion, formatting

By default formatters for Number and Date types are installed, including support for the @NumberFormat and @DateTimeFormat annotations. Full support for the Joda-Time formatting library is also installed if Joda-Time is present on the classpath.

To register custom formatters and converters:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void addFormatters(FormatterRegistry registry) {
		// ...
	}

}
Note

See FormatterRegistrar SPI and the FormattingConversionServiceFactoryBean for more information on when to use FormatterRegistrars.

Validation

By default if Bean Validation is present on the classpath — e.g. Hibernate Validator, the LocalValidatorFactoryBean is registered as a global Validator for use with @Valid and Validated on @Controller method arguments.

In your Java config, you can customize the global Validator instance:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public Validator getValidator(); {
		// ...
	}

}

Note that you can also register Validator's locally:

@Controller
public class MyController {

	@InitBinder
	protected void initBinder(WebDataBinder binder) {
		binder.addValidators(new FooValidator());
	}

}
Tip

If you need to have a LocalValidatorFactoryBean injected somewhere, create a bean and mark it with @Primary in order to avoid conflict with the one declared in the MVC config.

Content type resolvers

You can configure how Spring WebFlux determines the requested media types for @Controller's from the request. By default only the "Accept" header is checked but you can also enable a query parameter based strategy.

To customize the requested content type resolution:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configureContentTypeResolver(RequestedContentTypeResolverBuilder builder) {
		// ...
	}
}

HTTP message codecs

To customize how the request and response body are read and written:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
		// ...
	}
}

ServerCodecConfigurer provides a set of default readers and writers. You can use it to add more readers and writers, customize the default ones, or replace the default ones completely.

For Jackson JSON and XML, consider using the {api-spring-framework}/http/converter/json/Jackson2ObjectMapperBuilder.html[Jackson2ObjectMapperBuilder] which customizes Jackson’s default properties with the following ones:

It also automatically registers the following well-known modules if they are detected on the classpath:

  1. jackson-datatype-jdk7: support for Java 7 types like java.nio.file.Path.

  2. jackson-datatype-joda: support for Joda-Time types.

  3. jackson-datatype-jsr310: support for Java 8 Date & Time API types.

  4. jackson-datatype-jdk8: support for other Java 8 types like Optional.

View resolvers

To configure view resolution:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		// ...
	}
}

The ViewResolverRegistry has shortcuts for view technologies that the Spring Framework integrates with. Here is an example with FreeMarker which also requires configuring the underlying FreeMarker view technology:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {


	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		registry.freeMarker();
	}

	// Configure Freemarker...

	@Bean
	public FreeMarkerConfigurer freeMarkerConfigurer() {
		FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
		configurer.setTemplateLoaderPath("classpath:/templates");
		return configurer;
	}
}

You can also plug in any ViewResolver implementation:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {


	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		ViewResolver resolver = ... ;
		registry.viewResolver(resolver);
	}
}

To support Content negotiation and rendering other formats through view resolution, besides HTML, you can configure one or more default views based on the HttpMessageWriterView implementation which accepts any of the available Message Codecs from spring-web:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {


	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		registry.freeMarker();

		Jackson2JsonEncoder encoder = new Jackson2JsonEncoder();
		registry.defaultViews(new HttpMessageWriterView(encoder));
	}

	// ...
}

See [webflux-view] for more on the view technologies integrated with Spring WebFlux.

Static resources

This option provides a convenient way to serve static resources from a list of {api-spring-framework}/core/io/Resource.html[Resource]-based locations.

In the example below, given a request that starts with "/resources", the relative path is used to find and serve static resources relative to "/static" on the classpath. Resources will be served with a 1-year future expiration to ensure maximum use of the browser cache and a reduction in HTTP requests made by the browser. The Last-Modified header is also evaluated and if present a 304 status code is returned.

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void addResourceHandlers(ResourceHandlerRegistry registry) {
		registry.addResourceHandler("/resources/**")
			.addResourceLocations("/public", "classpath:/static/")
			.setCachePeriod(31556926);
	}

}

The resource handler also supports a chain of {api-spring-framework}/web/reactive/resource/ResourceResolver.html[ResourceResolver]'s and {api-spring-framework}/web/reactive/resource/ResourceTransformer.html[ResourceTransformer]'s. which can be used to create a toolchain for working with optimized resources.

The VersionResourceResolver can be used for versioned resource URLs based on an MD5 hash computed from the content, a fixed application version, or other. A ContentVersionStrategy (MD5 hash) is a good choice with some notable exceptions such as JavaScript resources used with a module loader.

For example in your Java config;

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void addResourceHandlers(ResourceHandlerRegistry registry) {
		registry.addResourceHandler("/resources/**")
				.addResourceLocations("/public/")
				.resourceChain(true)
				.addResolver(new VersionResourceResolver().addContentVersionStrategy("/**"));
	}

}

You can use ResourceUrlProvider to rewrite URLs and apply the full chain of resolvers and transformers — e.g. to insert versions. The WebFlux config provides a ResourceUrlProvider so it can be injected into others.

Unlike Spring MVC at present in WebFlux there is no way to transparently rewrite static resource URLs since there are no view technologies that can make use of a non-blocking chain of resolvers and transformers (e.g. resources on Amazon S3). When serving only local resources the workaround is to use ResourceUrlProvider directly (e.g. through a custom tag) and block for 0 seconds.

WebJars is also supported via WebJarsResourceResolver and automatically registered when "org.webjars:webjars-locator" is present on the classpath. The resolver can re-write URLs to include the version of the jar and can also match to incoming URLs without versions — e.g. "/jquery/jquery.min.js" to "/jquery/1.2.0/jquery.min.js".

Path Matching

Spring WebFlux uses parsed representation of path patterns — i.e. PathPattern, and also the incoming request path — i.e. RequestPath, which eliminates the need to indicate whether to decode the request path, or remove semicolon content, since PathPattern can now access decoded path segment values and match safely.

Spring WebFlux also does not support suffix pattern matching so effectively there are only two minor options to customize related to path matching — whether to match trailing slashes (true by default) and whether the match is case-sensitive (false).

To customize those options:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configurePathMatch(PathMatchConfigurer configurer) {
		// ...
	}

}

Advanced config mode

@EnableWebFlux imports DelegatingWebFluxConfiguration that (1) provides default Spring configuration for WebFlux applications and (2) detects and delegates to WebFluxConfigurer's to customize that configuration.

For advanced mode, remove @EnableWebFlux and extend directly from DelegatingWebFluxConfiguration instead of implementing WebFluxConfigurer:

@Configuration
public class WebConfig extends DelegatingWebFluxConfiguration {

	// ...

}

You can keep existing methods in WebConfig but you can now also override bean declarations from the base class and you can still have any number of other WebMvcConfigurer's on the classpath.

HTTP/2

Servlet 4 containers are required to support HTTP/2 and Spring Framework 5 is compatible with Servlet API 4. From a programming model perspective there is nothing specific that applications need to do. However there are considerations related to server configuration. For more details please check out the HTTP/2 wiki page.

Currently Spring WebFlux does not support HTTP/2 with Netty. There is also no support for pushing resources programmatically to the client.