[[webflux]]
:chapter: webflux
= Spring WebFlux

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
https://www.reactive-streams.org/[Reactive Streams] back pressure, and runs on such servers as
Netty, Undertow, and Servlet containers.

Both web frameworks mirror the names of their source modules
({spring-framework-main-code}/spring-webmvc[spring-webmvc] and
{spring-framework-main-code}/spring-webflux[spring-webflux]) and co-exist side by side in the
Spring Framework. Each module is optional. Applications can use one or the other module or,
in some cases, both -- for example, Spring MVC controllers with the reactive `WebClient`.




[[webflux-new-framework]]
== Overview

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 fewer hardware resources. Servlet non-blocking I/O
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 as the addition of annotations
in Java 5 created opportunities (such as 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 https://reactivex.io/[ReactiveX]) that allow declarative
composition of asynchronous logic. At the programming-model level, Java 8 enabled Spring
WebFlux to offer functional web endpoints alongside annotated controllers.



[[webflux-why-reactive]]
=== 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 components reacting to I/O events, UI controllers reacting to mouse events, and others.
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
https://github.com/reactive-streams/reactive-streams-jvm/blob/master/README.md#specification[small spec]
(also https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Flow.html[adopted] in Java 9)
that defines the interaction between asynchronous components with back pressure.
For example a data repository (acting as
https://www.reactive-streams.org/reactive-streams-1.0.1-javadoc/org/reactivestreams/Publisher.html[Publisher])
can produce data that an HTTP server (acting as
https://www.reactive-streams.org/reactive-streams-1.0.1-javadoc/org/reactivestreams/Subscriber.html[Subscriber])
can then write to the response. The main purpose of Reactive Streams is to let the
  subscriber control how quickly or how slowly the publisher produces data.

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



[[webflux-reactive-api]]
=== 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. Applications need 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.

https://github.com/reactor/reactor[Reactor] is the reactive library of choice for
Spring WebFlux. It provides the
https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html[`Mono`] and
https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html[`Flux`] API types
to work on data sequences of 0..1 (`Mono`) and 0..N (`Flux`) through a rich set of operators aligned with the
ReactiveX https://reactivex.io/documentation/operators.html[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, a WebFlux API accepts a plain `Publisher`
as input, adapts it to a Reactor type internally, uses that, and returns either a
`Flux` or a `Mono` as output. So, you can pass any `Publisher` as input and you can apply
operations on the output, but you need to adapt the output for use with another reactive library.
Whenever feasible (for example, annotated controllers), WebFlux adapts transparently to the use
of RxJava or another reactive library. See <<webflux-reactive-libraries>> for more details.

NOTE: In addition to Reactive APIs, WebFlux can also be used with
<<languages.adoc#coroutines, Coroutines>> APIs in Kotlin which provides a more imperative style of programming.
The following Kotlin code samples will be provided with Coroutines APIs.



[[webflux-programming-models]]
=== Programming Models

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

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

* <<webflux-controller>>: Consistent with Spring MVC and based on the same annotations
from the `spring-web` module. Both Spring MVC and WebFlux controllers support reactive
(Reactor and 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, and functional programming model. You can 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 versus declaring intent through
annotations and being called back.



[[webflux-framework-choice]]
=== Applicability

Spring MVC or WebFlux?

A natural question to ask but one that sets up an unsound dichotomy. Actually, both
work 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 following diagram shows how the two relate, what they
have in common, and what each supports uniquely:

image::images/spring-mvc-and-webflux-venn.png[]

We suggest that you consider the following specific points:

* 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, and Servlet 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, you can 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, 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 would not 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, <<webflux-reactive-libraries, 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
(for example, concurrency on single-threaded Node.js) and its effects.



[[webflux-server-choice]]
=== Servers

Spring WebFlux is supported on Tomcat, Jetty, Servlet containers, as well as on
non-Servlet runtimes such as Netty and Undertow. All servers are adapted to a low-level,
<<webflux-httphandler, common API>> so that higher-level
<<webflux-programming-models, 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 <<webflux-web-handler-api, assemble>> an application from Spring configuration and
<<webflux-config, WebFlux infrastructure>> and <<webflux-httphandler, 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 by changing your
Maven or Gradle dependencies. Spring Boot defaults to Netty, because it is more widely
used in the asynchronous, non-blocking space and lets 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 are used is very different. Spring MVC relies on Servlet blocking I/O and
lets applications use the Servlet API directly if they need to. Spring WebFlux
relies on Servlet non-blocking I/O and uses the Servlet API behind a low-level
adapter. It is not exposed for direct use.

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



[[webflux-performance]]
=== Performance

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 run remote calls in parallel). On the whole, it requires more work to do
things the non-blocking way and that can slightly increase 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 is where the reactive stack begins to show its strengths, and the differences can be
dramatic.



[[webflux-concurrency-model]]
=== Concurrency Model

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

In Spring MVC (and servlet applications in general), it is assumed that applications can
block the current thread, (for example, for remote calls). 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
do not block. 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 that you do not 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 hatch. Keep in mind, however, that blocking APIs are not a good fit for
this concurrency model.

==== Mutable State

In Reactor and RxJava, you declare logic through operators. At runtime, a reactive
pipeline is formed where data is processed sequentially, in distinct stages. A key benefit
of this 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 (for example, 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 (for example, 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 can see a small, fixed
number of processing threads related to that (for example, `reactor-http-nio-` with the Reactor
Netty connector). However, if Reactor Netty is used for both client and server, the two
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 -- for example, "`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 third party dependencies can also create and use threads
of their own.

==== Configuring

The Spring Framework does not provide support for starting and stopping
<<webflux-server-choice, servers>>. To configure the threading model for a server,
you need to use server-specific configuration APIs, or, if you use Spring Boot,
check the Spring Boot configuration options for each server. You can
<<web-reactive.adoc#webflux-client-builder, configure>> the `WebClient` directly.
For all other libraries, see their respective documentation.




[[webflux-reactive-spring-web]]
== Reactive Core

The `spring-web` module contains the following foundational support for reactive web
applications:

* For server request processing there are two levels of support.
** <<webflux-httphandler, HttpHandler>>: Basic contract for HTTP request handling with
non-blocking I/O and Reactive Streams back pressure, along with adapters for Reactor Netty,
Undertow, Tomcat, Jetty, and any Servlet container.
** <<webflux-web-handler-api>>: Slightly higher level, general-purpose web API for
request handling, on top of which concrete programming models such as annotated
controllers and functional endpoints are built.
* For the client side, there is a basic `ClientHttpConnector` contract to perform HTTP
requests with non-blocking I/O and Reactive Streams back pressure, along with adapters for
https://github.com/reactor/reactor-netty[Reactor Netty], reactive
https://github.com/jetty-project/jetty-reactive-httpclient[Jetty HttpClient]
and https://hc.apache.org/[Apache HttpComponents].
The higher level <<web-reactive.adoc#webflux-client, WebClient>> used in applications
builds on this basic contract.
* For client and server, <<webflux-codecs, codecs>> for serialization and
deserialization of HTTP request and response content.



[[webflux-httphandler]]
=== `HttpHandler`

{api-spring-framework}/http/server/reactive/HttpHandler.html[HttpHandler]
is a simple contract with a single method to handle a request and a response. It is
intentionally minimal, and its main and only purpose is to be a minimal abstraction
over different HTTP server APIs.

The following table describes the supported server APIs:

[cols="1,2,2", options="header"]
|===
| Server name | Server API used | Reactive Streams support

| Netty
| Netty API
| https://github.com/reactor/reactor-netty[Reactor Netty]

| Undertow
| Undertow API
| spring-web: Undertow to Reactive Streams bridge

| Tomcat
| Servlet non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[]
| spring-web: Servlet non-blocking I/O to Reactive Streams bridge

| Jetty
| Servlet non-blocking I/O; Jetty API to write ByteBuffers vs byte[]
| spring-web: Servlet non-blocking I/O to Reactive Streams bridge

| Servlet container
| Servlet non-blocking I/O
| spring-web: Servlet non-blocking I/O to Reactive Streams bridge
|===

The following table describes server dependencies (also see
https://github.com/spring-projects/spring-framework/wiki/What%27s-New-in-the-Spring-Framework[supported versions]):

|===
|Server name|Group id|Artifact name

|Reactor Netty
|io.projectreactor.netty
|reactor-netty

|Undertow
|io.undertow
|undertow-core

|Tomcat
|org.apache.tomcat.embed
|tomcat-embed-core

|Jetty
|org.eclipse.jetty
|jetty-server, jetty-servlet
|===

The code snippets below show using the `HttpHandler` adapters with each server API:

*Reactor Netty*
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpHandler handler = ...
	ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
	HttpServer.create().host(host).port(port).handle(adapter).bindNow();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val handler: HttpHandler = ...
	val adapter = ReactorHttpHandlerAdapter(handler)
	HttpServer.create().host(host).port(port).handle(adapter).bindNow()
----

*Undertow*
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	HttpHandler handler = ...
	UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
	Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();
	server.start();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val handler: HttpHandler = ...
	val adapter = UndertowHttpHandlerAdapter(handler)
	val server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build()
	server.start()
----

*Tomcat*
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	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();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val handler: HttpHandler = ...
	val servlet = TomcatHttpHandlerAdapter(handler)

	val server = Tomcat()
	val base = File(System.getProperty("java.io.tmpdir"))
	val rootContext = server.addContext("", base.absolutePath)
	Tomcat.addServlet(rootContext, "main", servlet)
	rootContext.addServletMappingDecoded("/", "main")
	server.host = host
	server.setPort(port)
	server.start()
----

*Jetty*

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	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();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val handler: HttpHandler = ...
	val servlet = JettyHttpHandlerAdapter(handler)

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

	val connector = ServerConnector(server)
	connector.host = host
	connector.port = port
	server.addConnector(connector)
	server.start()
----

*Servlet Container*

To deploy as a WAR to any Servlet container, you can extend and include
{api-spring-framework}/web/server/adapter/AbstractReactiveWebInitializer.html[`AbstractReactiveWebInitializer`]
in the WAR. That class wraps an `HttpHandler` with `ServletHttpHandlerAdapter` and registers
that as a `Servlet`.



[[webflux-web-handler-api]]
=== `WebHandler` API

The `org.springframework.web.server` package builds on the <<webflux-httphandler>> contract
to provide a general-purpose web API for processing requests through a chain of multiple
{api-spring-framework}/web/server/WebExceptionHandler.html[`WebExceptionHandler`], multiple
{api-spring-framework}/web/server/WebFilter.html[`WebFilter`], and a single
{api-spring-framework}/web/server/WebHandler.html[`WebHandler`] component. The chain can
be put together with `WebHttpHandlerBuilder` by simply pointing to a Spring
`ApplicationContext` where components are
<<webflux-web-handler-api-special-beans, auto-detected>>, and/or by registering components
with the builder.

While `HttpHandler` has a simple goal to abstract the use of different HTTP servers, the
`WebHandler` API aims to provide a broader set of features commonly used in web applications
such as:

* User session with attributes.
* Request attributes.
* Resolved `Locale` or `Principal` for the request.
* Access to parsed and cached form data.
* Abstractions for multipart data.
* and more..

[[webflux-web-handler-api-special-beans]]
==== Special bean types

The table below lists the components that `WebHttpHandlerBuilder` can auto-detect in a
Spring ApplicationContext, or that can be registered directly with it:

[cols="2,2,1,3", options="header"]
|===
| Bean name | Bean type | Count | Description

| <any>
| `WebExceptionHandler`
| 0..N
| Provide handling for exceptions from the chain of `WebFilter` instances and the target
  `WebHandler`. For more details, see <<webflux-exception-handler>>.

| <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 <<webflux-filters>>.

| `webHandler`
| `WebHandler`
| 1
| The handler for the request.

| `webSessionManager`
| `WebSessionManager`
| 0..1
| The manager for `WebSession` instances exposed through a method on `ServerWebExchange`.
  `DefaultWebSessionManager` by default.

| `serverCodecConfigurer`
| `ServerCodecConfigurer`
| 0..1
| For access to `HttpMessageReader` instances for parsing form data and multipart data that is 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.

| `forwardedHeaderTransformer`
| `ForwardedHeaderTransformer`
| 0..1
| For processing forwarded type headers, either by extracting and removing them or by removing them only.
  Not used by default.
|===


[[webflux-form-data]]
==== Form Data

`ServerWebExchange` exposes the following method for accessing form data:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<MultiValueMap<String, String>> getFormData();
----
[source,Kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	suspend fun getFormData(): MultiValueMap<String, String>
----

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 by the `ServerCodecConfigurer` bean
(see the <<webflux-web-handler-api, Web Handler API>>).


[[webflux-multipart]]
==== Multipart Data
[.small]#<<web.adoc#mvc-multipart, See equivalent in the Servlet stack>>#

`ServerWebExchange` exposes the following method for accessing multipart data:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Mono<MultiValueMap<String, Part>> getMultipartData();
----
[source,Kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	suspend fun getMultipartData(): MultiValueMap<String, Part>
----

The `DefaultServerWebExchange` uses the configured
`HttpMessageReader<MultiValueMap<String, Part>>` to parse `multipart/form-data`,  
`multipart/mixed`, and `multipart/related` content into a `MultiValueMap`.
By default, this is the `DefaultPartHttpMessageReader`, which does not have any third-party
dependencies.
Alternatively, the `SynchronossPartHttpMessageReader` can be used, which is based on the
https://github.com/synchronoss/nio-multipart[Synchronoss NIO Multipart] library.
Both are configured through the `ServerCodecConfigurer` bean
(see the <<webflux-web-handler-api, Web Handler API>>).

To parse multipart data in streaming fashion, you can use the `Flux<PartEvent>` returned from the
`PartEventHttpMessageReader` instead of using `@RequestPart`, as that  implies `Map`-like access
to individual parts by name and, hence, requires parsing multipart data in full.
By contrast, you can use `@RequestBody` to decode the content to `Flux<PartEvent>` without
collecting to a `MultiValueMap`.


[[webflux-forwarded-headers]]
==== Forwarded Headers
[.small]#<<web.adoc#filters-forwarded-headers, See equivalent in the Servlet stack>>#

As a request goes through proxies (such as load balancers), the host, port, and
scheme may change. That makes it a challenge, from a client perspective, to create links that point to the correct
host, port, and scheme.

https://tools.ietf.org/html/rfc7239[RFC 7239] defines the `Forwarded` HTTP header
that proxies can use to provide information about the original request. There are other
non-standard headers, too, including `X-Forwarded-Host`, `X-Forwarded-Port`,
`X-Forwarded-Proto`, `X-Forwarded-Ssl`, and `X-Forwarded-Prefix`.

`ForwardedHeaderTransformer` is a component that modifies the host, port, and scheme of
the request, based on forwarded headers, and then removes those headers. If you declare
it as a bean with the name `forwardedHeaderTransformer`, it will be
<<webflux-web-handler-api-special-beans, detected>> and used.

There are security considerations for forwarded headers, since an application cannot know
if the headers were added by a proxy, as intended, or by a malicious client. This is why
a proxy at the boundary of trust should be configured to remove untrusted forwarded traffic coming
from the outside. You can also configure the `ForwardedHeaderTransformer` with
`removeOnly=true`, in which case it removes but does not use the headers.

NOTE: In 5.1 `ForwardedHeaderFilter` was deprecated and superseded by
`ForwardedHeaderTransformer` so forwarded headers can be processed earlier, before the
exchange is created. If the filter is configured anyway, it is taken out of the list of
filters, and `ForwardedHeaderTransformer` is used instead.



[[webflux-filters]]
=== Filters
[.small]#<<web.adoc#filters, See equivalent in the Servlet stack>>#

In the <<webflux-web-handler-api>>, you can use a `WebFilter` 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 by using `@Order` on
the bean declaration or by implementing `Ordered`.


[[webflux-filters-cors]]
==== CORS
[.small]#<<web.adoc#filters-cors, See equivalent in the Servlet stack>>#

Spring WebFlux provides fine-grained support for CORS configuration through annotations on
controllers. However, when you use it with Spring Security, we advise relying on the built-in
`CorsFilter`, which 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.


[[webflux-exception-handler]]
=== Exceptions
[.small]#<<web.adoc#mvc-ann-customer-servlet-container-error-page, See equivalent in the Servlet stack>>#

In the <<webflux-web-handler-api>>, you can use a `WebExceptionHandler` to handle
exceptions from the chain of `WebFilter` instances 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 by using `@Order` on the bean declaration or
by implementing `Ordered`.

The following table describes the available `WebExceptionHandler` implementations:

[cols="1,2", options="header"]
|===
| 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 of a `@ResponseStatus` annotation on any exception.

  This handler is declared in the <<webflux-config>>.

|===



[[webflux-codecs]]
=== Codecs
[.small]#<<integration.adoc#rest-message-conversion, See equivalent in the Servlet stack>>#

The `spring-web` and `spring-core` modules provide support for serializing and
deserializing byte content to and from higher level objects through non-blocking I/O with
Reactive Streams back pressure. The following describes this support:

* {api-spring-framework}/core/codec/Encoder.html[`Encoder`] and
{api-spring-framework}/core/codec/Decoder.html[`Decoder`] are low level contracts to
encode and decode content independent of HTTP.
* {api-spring-framework}/http/codec/HttpMessageReader.html[`HttpMessageReader`] and
{api-spring-framework}/http/codec/HttpMessageWriter.html[`HttpMessageWriter`] are contracts
to encode and decode HTTP message content.
* An `Encoder` can be wrapped with `EncoderHttpMessageWriter` to adapt it for use in a web
application, while a `Decoder` can be wrapped with `DecoderHttpMessageReader`.
* {api-spring-framework}/core/io/buffer/DataBuffer.html[`DataBuffer`] abstracts different
byte buffer representations (e.g. Netty `ByteBuf`, `java.nio.ByteBuffer`, etc.) and is
what all codecs work on. See <<core#databuffers,Data Buffers and Codecs>> in the
"Spring Core" section for more on this topic.

The `spring-core` module provides `byte[]`, `ByteBuffer`, `DataBuffer`, `Resource`, and
`String` encoder and decoder implementations. The `spring-web` module provides Jackson
JSON, Jackson Smile, JAXB2, Protocol Buffers and other encoders and decoders along with
web-only HTTP message reader and writer implementations for form data, multipart content,
server-sent events, and others.

`ClientCodecConfigurer` and `ServerCodecConfigurer` are typically used to configure and
customize the codecs to use in an application. See the section on configuring
<<webflux-config-message-codecs>>.

[[webflux-codecs-jackson]]
==== Jackson JSON

JSON and binary JSON (https://github.com/FasterXML/smile-format-specification[Smile]) are
both supported when the Jackson library is present.

The `Jackson2Decoder` works as follows:

* Jackson's asynchronous, non-blocking parser is used to aggregate a stream of byte chunks
into ``TokenBuffer``'s each representing a JSON object.
* Each `TokenBuffer` is passed to Jackson's `ObjectMapper` to create a higher level object.
* When decoding to a single-value publisher (e.g. `Mono`), there is one `TokenBuffer`.
* When decoding to a multi-value publisher (e.g. `Flux`), each `TokenBuffer` is passed to
the `ObjectMapper` as soon as enough bytes are received for a fully formed object. The
input content can be a JSON array, or any
https://en.wikipedia.org/wiki/JSON_streaming[line-delimited JSON] format such as NDJSON,
JSON Lines, or JSON Text Sequences.

The `Jackson2Encoder` works as follows:

* For a single value publisher (e.g. `Mono`), simply serialize it through the
`ObjectMapper`.
* For a multi-value publisher with `application/json`, by default collect the values with
`Flux#collectToList()` and then serialize the resulting collection.
* For a multi-value publisher with a streaming media type such as
`application/x-ndjson` or `application/stream+x-jackson-smile`, encode, write, and
flush each value individually using a
https://en.wikipedia.org/wiki/JSON_streaming[line-delimited JSON] format. Other
streaming media types may be registered with the encoder.
* For SSE the `Jackson2Encoder` is invoked per event and the output is flushed to ensure
delivery without delay.

[NOTE]
====
By default both `Jackson2Encoder` and `Jackson2Decoder` do not support elements of type
`String`. Instead the default assumption is that a string or a sequence of strings
represent serialized JSON content, to be rendered by the `CharSequenceEncoder`. If what
you need is to render a JSON array from `Flux<String>`, use `Flux#collectToList()` and
encode a `Mono<List<String>>`.
====

[[webflux-codecs-forms]]
==== Form Data

`FormHttpMessageReader` and `FormHttpMessageWriter` support decoding and encoding
`application/x-www-form-urlencoded` content.

On the server side where form content often needs to be accessed from multiple places,
`ServerWebExchange` provides a dedicated `getFormData()` method that parses the content
through `FormHttpMessageReader` and then caches the result for repeated access.
See <<webflux-form-data>> in the <<webflux-web-handler-api>> section.

Once `getFormData()` is used, the original raw content can no longer be read from the
request body. For this reason, applications are expected to go through `ServerWebExchange`
consistently for access to the cached form data versus reading from the raw request body.


[[webflux-codecs-multipart]]
==== Multipart

`MultipartHttpMessageReader` and `MultipartHttpMessageWriter` support decoding and
encoding "multipart/form-data", "multipart/mixed", and "multipart/related" content.
In turn `MultipartHttpMessageReader` delegates to another `HttpMessageReader` 
for the actual parsing to a `Flux<Part>` and then simply collects the parts into a `MultiValueMap`.
By default, the `DefaultPartHttpMessageReader` is used, but this can be changed through the
`ServerCodecConfigurer`.
For more information about the `DefaultPartHttpMessageReader`, refer to the
{api-spring-framework}/http/codec/multipart/DefaultPartHttpMessageReader.html[javadoc of `DefaultPartHttpMessageReader`].

On the server side where multipart form content may need to be accessed from multiple
places, `ServerWebExchange` provides a dedicated `getMultipartData()` method that parses
the content through `MultipartHttpMessageReader` and then caches the result for repeated access.
See <<webflux-multipart>> in the <<webflux-web-handler-api>> section.

Once `getMultipartData()` is used, the original raw content can no longer be read from the
request body. For this reason applications have to consistently use `getMultipartData()`
for repeated, map-like access to parts, or otherwise rely on the
`SynchronossPartHttpMessageReader` for a one-time access to `Flux<Part>`.


[[webflux-codecs-limits]]
==== Limits

`Decoder` and `HttpMessageReader` implementations that buffer some or all of the input
stream can be configured with a limit on the maximum number of bytes to buffer in memory.
In some cases buffering occurs because input is aggregated and represented as a single
object — for example, a controller method with `@RequestBody byte[]`,
`x-www-form-urlencoded` data, and so on. Buffering can also occur with streaming, when
splitting the input stream — for example, delimited text, a stream of JSON objects, and
so on. For those streaming cases, the limit applies to the number of bytes associated
with one object in the stream.

To configure buffer sizes, you can check if a given `Decoder` or `HttpMessageReader`
exposes a `maxInMemorySize` property and if so the Javadoc will have details about default
values. On the server side, `ServerCodecConfigurer` provides a single place from where to
set all codecs, see <<webflux-config-message-codecs>>. On the client side, the limit for
all codecs can be changed in
<<web-reactive.adoc#webflux-client-builder-maxinmemorysize, WebClient.Builder>>.

For <<webflux-codecs-multipart,Multipart parsing>> the `maxInMemorySize` property limits
the size of non-file parts. For file parts, it determines the threshold at which the part
is written to disk. For file parts written to disk, there is an additional
`maxDiskUsagePerPart` property to limit the amount of disk space per part. There is also
a `maxParts` property to limit the overall number of parts in a multipart request.
To configure all three in WebFlux, you'll need to supply a pre-configured instance of
`MultipartHttpMessageReader` to `ServerCodecConfigurer`.



[[webflux-codecs-streaming]]
==== Streaming
[.small]#<<web.adoc#mvc-ann-async-http-streaming, See equivalent in the Servlet stack>>#

When streaming to the HTTP response (for example, `text/event-stream`,
`application/x-ndjson`), it is important to send data periodically, in order to
reliably detect a disconnected client sooner rather than later. Such a send could be a
comment-only, empty SSE event or any other "no-op" data that would effectively serve as
a heartbeat.


[[webflux-codecs-buffers]]
==== `DataBuffer`

`DataBuffer` is the representation for a byte buffer in WebFlux. The Spring Core part of
this reference has more on that in the section on
<<core#databuffers, Data Buffers and Codecs>>. The key point to understand is that on some
servers like Netty, byte buffers are pooled and reference counted, and must be released
when consumed to avoid memory leaks.

WebFlux applications generally do not need to be concerned with such issues, unless they
consume or produce data buffers directly, as opposed to relying on codecs to convert to
and from higher level objects, or unless they choose to create custom codecs. For such
cases please review the information in <<core#databuffers, Data Buffers and Codecs>>,
especially the section on <<core#databuffers-using, Using DataBuffer>>.



[[webflux-logging]]
=== Logging
[.small]#<<web.adoc#mvc-logging, See equivalent in the Servlet stack>>#

`DEBUG` level logging in Spring WebFlux is designed to be compact, minimal, and
human-friendly. It focuses on high value bits of information that are useful over and
over again vs others that are useful only when debugging a specific issue.

`TRACE` level logging generally follows the same principles as `DEBUG` (and for example also
should not be a firehose) but can be used for debugging any issue. In addition, some log
messages may show a different level of detail at `TRACE` vs `DEBUG`.

Good logging comes from the experience of using the logs. If you spot anything that does
not meet the stated goals, please let us know.


[[webflux-logging-id]]
==== Log Id

In WebFlux, a single request can be run over multiple threads and the thread ID
is not useful for correlating log messages that belong to a specific request. This is why
WebFlux log messages are prefixed with a request-specific ID by default.

On the server side, the log ID is stored in the `ServerWebExchange` attribute
({api-spring-framework}/web/server/ServerWebExchange.html#LOG_ID_ATTRIBUTE[`LOG_ID_ATTRIBUTE`]),
while a fully formatted prefix based on that ID is available from
`ServerWebExchange#getLogPrefix()`. On the `WebClient` side, the log ID is stored in the
`ClientRequest` attribute
({api-spring-framework}/web/reactive/function/client/ClientRequest.html#LOG_ID_ATTRIBUTE[`LOG_ID_ATTRIBUTE`])
,while a fully formatted prefix is available from `ClientRequest#logPrefix()`.


[[webflux-logging-sensitive-data]]
==== Sensitive Data
[.small]#<<web.adoc#mvc-logging-sensitive-data, See equivalent in the Servlet stack>>#

`DEBUG` and `TRACE` logging can log sensitive information. This is why form parameters and
headers are masked by default and you must explicitly enable their logging in full.

The following example shows how to do so for server-side requests:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	class MyConfig implements WebFluxConfigurer {

		@Override
		public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
			configurer.defaultCodecs().enableLoggingRequestDetails(true);
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class MyConfig : WebFluxConfigurer {

		override fun configureHttpMessageCodecs(configurer: ServerCodecConfigurer) {
			configurer.defaultCodecs().enableLoggingRequestDetails(true)
		}
	}
----

The following example shows how to do so for client-side requests:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	Consumer<ClientCodecConfigurer> consumer = configurer ->
			configurer.defaultCodecs().enableLoggingRequestDetails(true);

	WebClient webClient = WebClient.builder()
			.exchangeStrategies(strategies -> strategies.codecs(consumer))
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val consumer: (ClientCodecConfigurer) -> Unit  = { configurer -> configurer.defaultCodecs().enableLoggingRequestDetails(true) }

	val webClient = WebClient.builder()
			.exchangeStrategies({ strategies -> strategies.codecs(consumer) })
			.build()
----


[[webflux-logging-appenders]]
==== Appenders

Logging libraries such as SLF4J and Log4J 2 provide asynchronous loggers that avoid
blocking. While those have their own drawbacks such as potentially dropping messages
that could not be queued for logging, they are the best available options currently
for use in a reactive, non-blocking application.



[[webflux-codecs-custom]]
==== Custom codecs

Applications can register custom codecs for supporting additional media types,
or specific behaviors that are not supported by the default codecs.

Some configuration options expressed by developers are enforced on default codecs.
Custom codecs might want to get a chance to align with those preferences,
like <<webflux-codecs-limits, enforcing buffering limits>>
or <<webflux-logging-sensitive-data, logging sensitive data>>.

The following example shows how to do so for client-side requests:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	WebClient webClient = WebClient.builder()
			.codecs(configurer -> {
					CustomDecoder decoder = new CustomDecoder();
                    configurer.customCodecs().registerWithDefaultConfig(decoder);
			})
			.build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val webClient = WebClient.builder()
			.codecs({ configurer ->
					val decoder = CustomDecoder()
            		configurer.customCodecs().registerWithDefaultConfig(decoder)
			 })
			.build()
----

[[webflux-dispatcher-handler]]
== `DispatcherHandler`
[.small]#<<web.adoc#mvc-servlet, See equivalent in the Servlet stack>>#

Spring WebFlux, similarly to 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 in which it runs. If `DispatcherHandler` is declared with a bean
name of `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 <<webflux-web-handler-api>>.

Spring configuration in a WebFlux application typically contains:

* `DispatcherHandler` with the bean name `webHandler`
* `WebFilter` and `WebExceptionHandler` beans
* <<webflux-special-bean-types,`DispatcherHandler` special beans>>
* Others

The configuration is given to `WebHttpHandlerBuilder` to build the processing chain,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	ApplicationContext context = ...
	HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	val context: ApplicationContext = ...
	val handler = WebHttpHandlerBuilder.applicationContext(context).build()
----

The resulting `HttpHandler` is ready for use with a <<webflux-httphandler, server adapter>>.



[[webflux-special-bean-types]]
=== Special Bean Types
[.small]#<<web.adoc#mvc-servlet-special-bean-types, See equivalent in the Servlet stack>>#

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 them, or replace them.

The following table lists the special beans detected by the `DispatcherHandler`. Note that
there are also some other beans detected at a lower level (see
<<webflux-web-handler-api-special-beans>> in the Web Handler API).

[[webflux-special-beans-table]]
[cols="1,2", options="header"]
|===
| 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, and others.

  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` instances.

| `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 <<webflux-resulthandling>>.

|===



[[webflux-framework-config]]
=== WebFlux Config
[.small]#<<web.adoc#mvc-servlet-config, See equivalent in the Servlet stack>>#

Applications can declare the infrastructure beans (listed under
<<webflux-web-handler-api-special-beans, Web Handler API>> and
<<webflux-special-bean-types, `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.



[[webflux-dispatcher-handler-sequence]]
=== Processing
[.small]#<<web.adoc#mvc-servlet-sequence, See equivalent in the Servlet stack>>#

`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 run 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 by using a view to render.



[[webflux-resulthandling]]
=== Result Handling

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

[cols="1,2,1", options="header"]
|===
| Result Handler Type | Return Values | Default Order

| `ResponseEntityResultHandler`
| `ResponseEntity`, typically from `@Controller` instances.
| 0

| `ServerResponseResultHandler`
| `ServerResponse`, typically from functional endpoints.
| 0

| `ResponseBodyResultHandler`
| Handle return values from `@ResponseBody` methods or `@RestController` classes.
| 100

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

  See also <<webflux-viewresolution>>.
| `Integer.MAX_VALUE`

|===



[[webflux-dispatcher-exceptions]]
=== Exceptions
[.small]#<<web.adoc#mvc-exceptionhandlers, See equivalent in the Servlet stack>>#

`HandlerAdapter` implementations can handle internally exceptions from invoking a request
handler, such as a controller method. However, an exception may be deferred if the request
handler returns an asynchronous value.

A `HandlerAdapter` may expose its exception handling mechanism as a
`DispatchExceptionHandler` set on the `HandlerResult` it returns. When that's set,
`DispatcherHandler` will also apply it to the handling of the result.

A `HandlerAdapter` may also choose to implement `DispatchExceptionHandler`. In that case
`DispatcherHandler` will apply it to exceptions that arise before a handler is mapped,
e.g. during handler mapping, or earlier, e.g. in a `WebFilter`.

See also <<webflux-ann-controller-exceptions>> in the "`Annotated Controller`" section or
<<webflux-exception-handler>> in the WebHandler API section.



[[webflux-viewresolution]]
=== View Resolution
[.small]#<<web.adoc#mvc-viewresolver, See equivalent in the Servlet stack>>#

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 <<webflux-resulthandling, HandlerResultHandler>> that uses
  `ViewResolver` instances to map a String (representing a logical view name) to a `View`
instance. The `View` is then used to render the response.


[[webflux-viewresolution-handling]]
==== Handling
[.small]#<<web.adoc#mvc-viewresolver-handling, See equivalent in the Servlet stack>>#

The `HandlerResult` passed into `ViewResolutionResultHandler` contains the return value
from the handler and 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` implementations.
* `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 (for example, model attribute was returned) or an async return value
(for example, `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 by using {api-spring-framework}/core/Conventions.html[conventions],
unless a handler method `@ModelAttribute` annotation is present.

The model can contain asynchronous, reactive types (for example, from Reactor or 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 (for example, `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-view-resolvers, WebFlux Config>> provides a
dedicated configuration API for view resolution.

See <<webflux-view>> for more on the view technologies integrated with Spring WebFlux.


[[webflux-redirecting-redirect-prefix]]
==== Redirecting
[.small]#<<web.adoc#mvc-redirecting-redirect-prefix, See equivalent in the Servlet stack>>#

The special `redirect:` prefix in a view name lets you 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
operate in terms of logical view names. A view name such as
`redirect:/some/resource` is relative to the current application, while a view name such as
`redirect:https://example.com/arbitrary/path` redirects to an absolute URL.


[[webflux-multiple-representations]]
==== Content Negotiation
[.small]#<<web.adoc#mvc-multiple-representations, See equivalent in the Servlet stack>>#

`ViewResolutionResultHandler` supports content negotiation. It compares the request
media types with the media types 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
<<webflux-codecs, HttpMessageWriter>>. Typically, you would configure these as default
views through the <<webflux-config-view-resolvers, WebFlux Configuration>>. Default views are
always selected and used if they match the requested media type.




[[webflux-controller]]
== Annotated Controllers
[.small]#<<web.adoc#mvc-controller, See equivalent in the Servlet stack>>#

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

The following listing shows a basic example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	public class HelloController {

		@GetMapping("/hello")
		public String handle() {
			return "Hello WebFlux";
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	class HelloController {

		@GetMapping("/hello")
		fun handle() = "Hello WebFlux"
	}
----

In the preceding example, the method returns a `String` to be written to the response body.



[[webflux-ann-controller]]
=== `@Controller`
[.small]#<<web.adoc#mvc-ann-controller, See equivalent in the Servlet stack>>#

You can define controller beans by using a standard Spring bean definition.
The `@Controller` stereotype allows for auto-detection and is 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, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@ComponentScan("org.example.web") // <1>
	public class WebConfig {

		// ...
	}
----
<1> Scan the `org.example.web` package.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@ComponentScan("org.example.web") // <1>
	class WebConfig {

		// ...
	}
----
<1> Scan the `org.example.web` package.

`@RestController` is a <<core.adoc#beans-meta-annotations, 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 versus view resolution and rendering with an HTML template.



[[webflux-ann-requestmapping-proxying]]
==== AOP Proxies
[.small]#<<web.adoc#mvc-ann-requestmapping-proxying, See equivalent in the Servlet stack>>#

In some cases, you may need to decorate a controller with an AOP proxy at runtime.
One example is if you choose to have `@Transactional` annotations directly on the
controller. When this is the case, for controllers specifically, we recommend
using class-based proxying. This is automatically the case with such annotations
directly on the controller.

If the controller implements an interface, and needs AOP proxying, you may need to
explicitly configure class-based proxying. For example, with `@EnableTransactionManagement`
you can change to `@EnableTransactionManagement(proxyTargetClass = true)`, and with
`<tx:annotation-driven/>` you can change to `<tx:annotation-driven proxy-target-class="true"/>`.

NOTE: Keep in mind that as of 6.0, with interface proxying, Spring WebFlux no longer detects
controllers based solely on a type-level `@RequestMapping` annotation on the interface.
Please, enable class based proxying, or otherwise the interface must also have an
`@Controller` annotation.




[[webflux-ann-requestmapping]]
=== Request Mapping
[.small]#<<web.adoc#mvc-ann-requestmapping, See equivalent in the Servlet stack>>#

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. You can use it 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 preceding annotations are <<webflux-ann-requestmapping-composed>> that are provided
because, arguably, most controller methods should be mapped to a specific HTTP method versus
using `@RequestMapping`, which, by default, matches to all HTTP methods. At the same time, a
`@RequestMapping` is still needed at the class level to express shared mappings.

The following example uses type and method level mappings:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RestController
	@RequestMapping("/persons")
	class PersonController {

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

		@PostMapping
		@ResponseStatus(HttpStatus.CREATED)
		public void add(@RequestBody Person person) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	@RequestMapping("/persons")
	class PersonController {

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

		@PostMapping
		@ResponseStatus(HttpStatus.CREATED)
		fun add(@RequestBody person: Person) {
			// ...
		}
	}
----


[[webflux-ann-requestmapping-uri-templates]]
==== URI Patterns
[.small]#<<web.adoc#mvc-ann-requestmapping-uri-templates, See equivalent in the Servlet stack>>#

You can map requests by using glob patterns and wildcards:

[cols="2,3,5"]
|===
|Pattern |Description |Example

| `+?+`
| Matches one character
| `+"/pages/t?st.html"+` matches `+"/pages/test.html"+` and `+"/pages/t3st.html"+`

| `+*+`
| Matches zero or more characters within a path segment
| `+"/resources/*.png"+` matches `+"/resources/file.png"+`

`+"/projects/*/versions"+` matches `+"/projects/spring/versions"+` but does not match `+"/projects/spring/boot/versions"+`

| `+**+`
| Matches zero or more path segments until the end of the path
| `+"/resources/**"+` matches `+"/resources/file.png"+` and `+"/resources/images/file.png"+`

`+"/resources/**/file.png"+` is invalid as `+**+` is only allowed at the end of the path.

| `+{name}+`
| Matches a path segment and captures it as a variable named "name"
| `+"/projects/{project}/versions"+` matches `+"/projects/spring/versions"+` and captures `+project=spring+`

| `+{name:[a-z]+}+`
| Matches the regexp `+"[a-z]+"+` as a path variable named "name"
| `+"/projects/{project:[a-z]+}/versions"+` matches `+"/projects/spring/versions"+` but not `+"/projects/spring1/versions"+`

| `+{*path}+`
| Matches zero or more path segments until the end of the path and captures it as a variable named "path"
| `+"/resources/{*file}"+` matches `+"/resources/images/file.png"+` and captures `+file=/images/file.png+`

|===

Captured URI variables can be accessed with `@PathVariable`, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	fun findPet(@PathVariable ownerId: Long, @PathVariable petId: Long): Pet {
		// ...
	}
----
--

You can declare URI variables at the class and method levels, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@RequestMapping("/owners/{ownerId}") // <1>
	public class OwnerController {

		@GetMapping("/pets/{petId}") // <2>
		public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
			// ...
		}
	}
----
<1> Class-level URI mapping.
<2> Method-level URI mapping.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@RequestMapping("/owners/{ownerId}") // <1>
	class OwnerController {

		@GetMapping("/pets/{petId}") // <2>
		fun findPet(@PathVariable ownerId: Long, @PathVariable petId: Long): Pet {
			// ...
		}
	}
----
<1> Class-level URI mapping.
<2> Method-level URI mapping.
--


URI variables are automatically converted to the appropriate type or a `TypeMismatchException`
is raised. Simple types (`int`, `long`, `Date`, and so on) are supported by default and you can
register support for any other data type.
See <<webflux-ann-typeconversion>> and <<webflux-ann-initbinder>>.

URI variables can be named explicitly (for example, `@PathVariable("customId")`), but you can
leave that detail out if the names are the same and you compile your code with the `-parameters`
compiler flag.

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

The syntax `{varName:regex}` declares a URI variable with a regular expression that has the
syntax: `{varName:regex}`. For example, given a URL of `/spring-web-3.0.5.jar`, the following method
extracts the name, version, and file extension:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
	public void handle(@PathVariable String version, @PathVariable String ext) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
	fun handle(@PathVariable version: String, @PathVariable ext: String) {
		// ...
	}
----
--

URI path patterns can also have embedded `${...}` placeholders that are resolved on startup
through `PropertySourcesPlaceholderConfigurer` against local, system, environment, and
other property sources. You can use this to, for example, parameterize a base URL based on
some external configuration.

NOTE: Spring WebFlux uses `PathPattern` and the `PathPatternParser` for URI path matching support.
Both classes are located in `spring-web` and are 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.{asterisk}`. 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.


[[webflux-ann-requestmapping-pattern-comparison]]
==== Pattern Comparison
[.small]#<<web.adoc#mvc-ann-requestmapping-pattern-comparison, See equivalent in the Servlet stack>>#

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 are more specific.

For every pattern, a score is computed, based on 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, the longer is chosen.

Catch-all patterns (for example, `**`, `{*varName}`) are excluded from the scoring and are always
sorted last instead. If two patterns are both catch-all, the longer is chosen.


[[webflux-ann-requestmapping-consumes]]
==== Consumable Media Types
[.small]#<<web.adoc#mvc-ann-requestmapping-consumes, See equivalent in the Servlet stack>>#

You can narrow the request mapping based on the `Content-Type` of the request,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping(path = "/pets", consumes = "application/json")
	public void addPet(@RequestBody Pet pet) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/pets", consumes = ["application/json"])
	fun addPet(@RequestBody pet: Pet) {
		// ...
	}
----

The consumes attribute also supports negation expressions -- for example, `!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 extends the class-level declaration.

TIP: `MediaType` provides constants for commonly used media types -- for example,
`APPLICATION_JSON_VALUE` and `APPLICATION_XML_VALUE`.


[[webflux-ann-requestmapping-produces]]
==== Producible Media Types
[.small]#<<web.adoc#mvc-ann-requestmapping-produces, See equivalent in the Servlet stack>>#

You can narrow the request mapping based on the `Accept` request header and the list of
content types that a controller method produces, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets/{petId}", produces = "application/json")
	@ResponseBody
	public Pet getPet(@PathVariable String petId) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets/{petId}", produces = ["application/json"])
	@ResponseBody
	fun getPet(@PathVariable petId: String): Pet {
		// ...
	}
----

The media type can specify a character set. Negated expressions are supported -- for example,
`!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_XML_VALUE`.


[[webflux-ann-requestmapping-params-and-headers]]
==== Parameters and Headers
[.small]#<<web.adoc#mvc-ann-requestmapping-params-and-headers, See equivalent in the Servlet stack>>#

You can narrow request mappings based on query parameter conditions. You can test for the
presence of a query parameter (`myParam`), for its absence (`!myParam`), or for a
specific value (`myParam=myValue`). The following examples tests for a parameter with a value:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets/{petId}", params = "myParam=myValue") // <1>
	public void findPet(@PathVariable String petId) {
		// ...
	}
----
<1> Check that `myParam` equals `myValue`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets/{petId}", params = ["myParam=myValue"]) // <1>
	fun findPet(@PathVariable petId: String) {
		// ...
	}
----
<1> Check that `myParam` equals `myValue`.

You can also use the same with request header conditions, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping(path = "/pets/{petId}", headers = "myHeader=myValue") // <1>
	public void findPet(@PathVariable String petId) {
		// ...
	}
----
<1> Check that `myHeader` equals `myValue`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/pets/{petId}", headers = ["myHeader=myValue"]) // <1>
	fun findPet(@PathVariable petId: String) {
		// ...
	}
----
<1> Check that `myHeader` equals `myValue`.



[[webflux-ann-requestmapping-head-options]]
==== HTTP HEAD, OPTIONS
[.small]#<<web.adoc#mvc-ann-requestmapping-head-options, See equivalent in the Servlet stack>>#

`@GetMapping` and `@RequestMapping(method=HttpMethod.GET)` support HTTP HEAD
transparently for request mapping purposes. Controller methods need not change.
A response wrapper, applied in the `HttpHandler` server adapter, ensures a `Content-Length`
header is set to the number of bytes written 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`, and others).

You can explicitly map a `@RequestMapping` method to HTTP HEAD and HTTP OPTIONS, but that
is not necessary in the common case.


[[webflux-ann-requestmapping-composed]]
==== Custom Annotations
[.small]#<<web.adoc#mvc-ann-requestmapping-composed, See equivalent in the Servlet stack>>#

Spring WebFlux supports the use of <<core.adoc#beans-meta-annotations, 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 are provided, because, arguably, most
controller methods should be mapped to a specific HTTP method versus 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`.


[[webflux-ann-requestmapping-registration]]
==== Explicit Registrations
[.small]#<<web.adoc#mvc-ann-requestmapping-registration, See equivalent in the Servlet stack>>#

You can programmatically register Handler methods, which can be used for dynamic
registrations or for advanced cases, such as different instances of the same handler
under different URLs. The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	public class MyConfig {

		@Autowired
		public void setHandlerMapping(RequestMappingHandlerMapping mapping, UserHandler handler) // <1>
				throws NoSuchMethodException {

			RequestMappingInfo info = RequestMappingInfo
					.paths("/user/{id}").methods(RequestMethod.GET).build(); // <2>

			Method method = UserHandler.class.getMethod("getUser", Long.class); // <3>

			mapping.registerMapping(info, handler, method); // <4>
		}

	}
----
<1> Inject target handlers and the handler mapping for controllers.
<2> Prepare the request mapping metadata.
<3> Get the handler method.
<4> Add the registration.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	class MyConfig {

		@Autowired
		fun setHandlerMapping(mapping: RequestMappingHandlerMapping, handler: UserHandler) { // <1>

			val info = RequestMappingInfo.paths("/user/{id}").methods(RequestMethod.GET).build() // <2>

			val method = UserHandler::class.java.getMethod("getUser", Long::class.java) // <3>

			mapping.registerMapping(info, handler, method) // <4>
		}
	}
----
<1> Inject target handlers and the handler mapping for controllers.
<2> Prepare the request mapping metadata.
<3> Get the handler method.
<4> Add the registration.



[[webflux-ann-methods]]
=== Handler Methods
[.small]#<<web.adoc#mvc-ann-methods, See equivalent in the Servlet stack>>#

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


[[webflux-ann-arguments]]
==== Method Arguments
[.small]#<<web.adoc#mvc-ann-arguments, See equivalent in the Servlet stack>>#

The following table shows the supported controller method arguments.

Reactive types (Reactor, RxJava, <<webflux-reactive-libraries, or other>>) are
supported on arguments that require blocking I/O (for example, reading the request body) to
be resolved. This is marked in the Description column. Reactive types are not expected
on arguments that do not require blocking.

JDK 1.8's `java.util.Optional` is supported as a method argument in combination with
annotations that have a `required` attribute (for example, `@RequestParam`, `@RequestHeader`,
and others) and is equivalent to `required=false`.

[cols="1,2", options="header"]
|===
| 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`
| The 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 <<webflux-ann-requestmapping-uri-templates>>.

| `@MatrixVariable`
| For access to name-value pairs in URI path segments. See <<webflux-ann-matrix-variables>>.

| `@RequestParam`
| For access to query parameters. Parameter values are converted to the declared method argument
  type. See <<webflux-ann-requestparam>>.

  Note that use of `@RequestParam` is optional -- for example, to set its attributes.
  See "`Any other argument`" later in this table.

| `@RequestHeader`
| For access to request headers. Header values are converted to the declared method argument
  type. See <<webflux-ann-requestheader>>.

| `@CookieValue`
| For access to cookies. Cookie values are converted to the declared method argument type.
  See <<webflux-ann-cookievalue>>.

| `@RequestBody`
| For access to the HTTP request body. Body content is converted to the declared method
  argument type by using `HttpMessageReader` instances. Supports reactive types.
  See <<webflux-ann-requestbody>>.

| `HttpEntity<B>`
| For access to request headers and body. The body is converted with `HttpMessageReader` instances.
  Supports reactive types. See <<webflux-ann-httpentity>>.

| `@RequestPart`
| For access to a part in  a `multipart/form-data` request. Supports reactive types.
  See <<webflux-multipart-forms>> and <<webflux-multipart>>.

| `java.util.Map`, `org.springframework.ui.Model`, and `org.springframework.ui.ModelMap`.
| For access to the model that is used in HTML controllers and is 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 <<webflux-ann-modelattrib-method-args>> as well
  as <<webflux-ann-modelattrib-methods>> and <<webflux-ann-initbinder>>.

  Note that use of `@ModelAttribute` is optional -- for example, to set its attributes.
  See "`Any other argument`" later in this table.

| `Errors`, `BindingResult`
| For access to errors from validation and data binding for a command object, i.e. a
  `@ModelAttribute` argument. 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 <<webflux-ann-sessionattributes>> for more details.

| `UriComponentsBuilder`
| For preparing a URL relative to the current request's host, port, scheme, and
  context path. 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
  <<webflux-ann-sessionattribute>> for more details.

| `@RequestAttribute`
| For access to request attributes. See <<webflux-ann-requestattrib>> for more details.

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


[[webflux-ann-return-types]]
==== Return Values
[.small]#<<web.adoc#mvc-ann-return-types, See equivalent in the Servlet stack>>#

The following table shows the supported controller method return values. Note that reactive
types from libraries such as Reactor, RxJava, <<webflux-reactive-libraries, or other>> are
generally supported for all return values.

[cols="1,2", options="header"]
|===
| Controller method return value | Description

| `@ResponseBody`
| The return value is encoded through `HttpMessageWriter` instances and written to the response.
  See <<webflux-ann-responsebody>>.

| `HttpEntity<B>`, `ResponseEntity<B>`
| The return value specifies the full response, including HTTP headers, and the body is encoded
  through `HttpMessageWriter` instances and written to the response.
  See <<webflux-ann-responseentity>>.

| `HttpHeaders`
| For returning a response with headers and no body.

| `ErrorResponse`
| To render an RFC 7807 error response with details in the body,
  see <<webflux-ann-rest-exceptions>>

| `ProblemDetail`
| To render an RFC 7807 error response with details in the body,
  see <<webflux-ann-rest-exceptions>>

| `String`
| A view name to be resolved with `ViewResolver` instances and used together with the implicit
  model -- determined through command objects and `@ModelAttribute` methods. The handler
  method can also programmatically enrich the model by declaring a `Model` argument
  (described <<webflux-viewresolution-handling, earlier>>).

| `View`
| A `View` instance to use for rendering together with the implicit model -- determined
  through command objects and `@ModelAttribute` methods. The handler method can also
  programmatically enrich the model by declaring a `Model` argument
  (described <<webflux-viewresolution-handling, earlier>>).

| `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`" later in
  this table.

| `Rendering`
| An API for model and view rendering scenarios.

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

  If none of the above is true, a `void` return type can 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 `ServerSentEvent` 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).

| Other return values
| If a return value remains unresolved in any other way, it is treated as a model
  attribute, 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.
|===


[[webflux-ann-typeconversion]]
==== Type Conversion
[.small]#<<web.adoc#mvc-ann-typeconversion, See equivalent in the Servlet stack>>#

Some annotated controller method arguments that represent String-based request input (for example,
`@RequestParam`, `@RequestHeader`, `@PathVariable`, `@MatrixVariable`, and `@CookieValue`)
can 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`, and others) are supported. Type conversion
can be customized through a `WebDataBinder` (see <<webflux-ann-initbinder>>) or by registering
`Formatters` with the `FormattingConversionService` (see <<core.adoc#format, Spring Field Formatting>>).

A practical issue in type conversion is the treatment of an empty String source value.
Such a value is treated as missing if it becomes `null` as a result of type conversion.
This can be the case for `Long`, `UUID`, and other target types. If you want to allow `null`
to be injected, either use the `required` flag on the argument annotation, or declare the
argument as `@Nullable`.


[[webflux-ann-matrix-variables]]
==== Matrix Variables
[.small]#<<web.adoc#mvc-ann-matrix-variables, See equivalent in the Servlet stack>>#

https://tools.ietf.org/html/rfc3986#section-3.3[RFC 3986] discusses name-value pairs in
path segments. In Spring WebFlux, we refer to those as "`matrix variables`" based on an
https://www.w3.org/DesignIssues/MatrixURIs.html["`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, with each variable separated by a semicolon and
multiple values separated by commas -- for example, `"/cars;color=red,green;year=2012"`. Multiple
values can also be specified through repeated variable names -- for example,
`"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 are 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. The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /pets/42;q=11;r=22

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

		// petId == 42
		// q == 11
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /pets/42;q=11;r=22

	@GetMapping("/pets/{petId}")
	fun findPet(@PathVariable petId: String, @MatrixVariable q: Int) {

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


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

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// 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
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	fun findPet(
			@MatrixVariable(name = "q", pathVar = "ownerId") q1: Int,
			@MatrixVariable(name = "q", pathVar = "petId") q2: Int) {

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

You can define a matrix variable may be defined as optional and specify a default value
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// GET /pets/42

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

		// q == 1
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /pets/42

	@GetMapping("/pets/{petId}")
	fun findPet(@MatrixVariable(required = false, defaultValue = "1") q: Int) {

		// q == 1
	}
----

To get all matrix variables, use a `MultiValueMap`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// 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]
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// GET /owners/42;q=11;r=12/pets/21;q=22;s=23

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

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


[[webflux-ann-requestparam]]
==== `@RequestParam`
[.small]#<<web.adoc#mvc-ann-requestparam, See equivalent in the Servlet stack>>#

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

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@RequestMapping("/pets")
	public class EditPetForm {

		// ...

		@GetMapping
		public String setupForm(@RequestParam("petId") int petId, Model model) { <1>
			Pet pet = this.clinic.loadPet(petId);
			model.addAttribute("pet", pet);
			return "petForm";
		}

		// ...
	}
----
<1> Using `@RequestParam`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.ui.set

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

		// ...

		@GetMapping
		fun setupForm(@RequestParam("petId") petId: Int, model: Model): String { // <1>
			val pet = clinic.loadPet(petId)
			model["pet"] = pet
			return "petForm"
		}

		// ...
	}
----
<1> Using `@RequestParam`.

TIP: The Servlet API "`request parameter`" concept conflates query parameters, form
data, and multiparts into one. However, in WebFlux, each is accessed individually through
`ServerWebExchange`. While `@RequestParam` binds to query parameters only, you can use
data binding to apply query parameters, form data, and multiparts to a
<<webflux-ann-modelattrib-method-args, command object>>.

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

Type conversion is applied automatically if the target method parameter type is not
`String`. See <<webflux-ann-typeconversion>>.

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

Note that use of `@RequestParam` is optional -- for example, 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 were annotated
with `@RequestParam`.


[[webflux-ann-requestheader]]
==== `@RequestHeader`
[.small]#<<web.adoc#mvc-ann-requestheader, See equivalent in the Servlet stack>>#

You can use the `@RequestHeader` annotation to bind a request header to a method argument in a
controller.

The following example shows a request with headers:

[literal]
[subs="verbatim,quotes"]
----
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 example gets the value of the `Accept-Encoding` and `Keep-Alive` headers:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/demo")
	public void handle(
			@RequestHeader("Accept-Encoding") String encoding, // <1>
			@RequestHeader("Keep-Alive") long keepAlive) { // <2>
		//...
	}
----
<1> Get the value of the `Accept-Encoding` header.
<2> Get the value of the `Keep-Alive` header.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/demo")
	fun handle(
			@RequestHeader("Accept-Encoding") encoding: String, // <1>
			@RequestHeader("Keep-Alive") keepAlive: Long) { // <2>
		//...
	}
----
<1> Get the value of the `Accept-Encoding` header.
<2> Get the value of the `Keep-Alive` header.

Type conversion is applied automatically if the target method parameter type is not
`String`. See <<webflux-ann-typeconversion>>.

When a `@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 or 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 of `String[]` or `List<String>`.


[[webflux-ann-cookievalue]]
==== `@CookieValue`
[.small]#<<web.adoc#mvc-ann-cookievalue, See equivalent in the Servlet stack>>#

You can use the `@CookieValue` annotation to bind the value of an HTTP cookie to a method argument
in a controller.

The following example shows a request with a cookie:

[literal,subs="verbatim,quotes"]
----
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
----

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

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/demo")
	public void handle(@CookieValue("JSESSIONID") String cookie) { // <1>
		//...
	}
----
<1> Get the cookie value.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/demo")
	fun handle(@CookieValue("JSESSIONID") cookie: String) { // <1>
		//...
	}
----
<1> Get the cookie value.


Type conversion is applied automatically if the target method parameter type is not
`String`. See <<webflux-ann-typeconversion>>.


[[webflux-ann-modelattrib-method-args]]
==== `@ModelAttribute`
[.small]#<<web.adoc#mvc-ann-modelattrib-method-args, See equivalent in the Servlet stack>>#

You can 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
the 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. The following example binds an instance of `Pet`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public String processSubmit(@ModelAttribute Pet pet) { } // <1>
----
<1> Bind an instance of `Pet`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@ModelAttribute pet: Pet): String { } // <1>
----
<1> Bind an instance of `Pet`.

The `Pet` instance in the preceding example is resolved as follows:

* From the model if already added through <<webflux-ann-modelattrib-methods>>.
* From the HTTP session through <<webflux-ann-sessionattributes>>.
* From the invocation of a default constructor.
* From the invocation of a "`primary constructor`" with arguments that match query
parameters or form fields. Argument names are determined through JavaBeans
`@ConstructorProperties` or through 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
<<core.adoc#validation, Validation>>. For more on customizing data binding, see
<<webflux-ann-initbinder>>.

Data binding can result in errors. By default, a `WebExchangeBindException` is raised, but,
to check for such errors in the controller method, you can add a `BindingResult` argument
immediately next to the `@ModelAttribute`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) { <1>
		if (result.hasErrors()) {
			return "petForm";
		}
		// ...
	}
----
<1> Adding a `BindingResult`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@ModelAttribute("pet") pet: Pet, result: BindingResult): String { // <1>
		if (result.hasErrors()) {
			return "petForm"
		}
		// ...
	}
----
<1> Adding a `BindingResult`.

You can automatically apply validation after data binding by adding the
`jakarta.validation.Valid` annotation or Spring's `@Validated` annotation (see also
<<core.adoc#validation-beanvalidation, Bean Validation>> and
<<core.adoc#validation, Spring validation>>). The following example uses the `@Valid` annotation:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) { // <1>
		if (result.hasErrors()) {
			return "petForm";
		}
		// ...
	}
----
<1> Using `@Valid` on a model attribute argument.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@Valid @ModelAttribute("pet") pet: Pet, result: BindingResult): String { // <1>
		if (result.hasErrors()) {
			return "petForm"
		}
		// ...
	}
----
<1> Using `@Valid` on a model attribute argument.

Spring WebFlux, unlike Spring MVC, supports reactive types in the model -- for example,
`Mono<Account>` or `io.reactivex.Single<Account>`. You can declare a `@ModelAttribute` argument
with or without a reactive type wrapper, and it will be resolved accordingly,
to the actual value if necessary. However, note that, 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, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	public Mono<String> processSubmit(@Valid @ModelAttribute("pet") Mono<Pet> petMono) {
		return petMono
			.flatMap(pet -> {
				// ...
			})
			.onErrorResume(ex -> {
				// ...
			});
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
	fun processSubmit(@Valid @ModelAttribute("pet") petMono: Mono<Pet>): Mono<String> {
		return petMono
				.flatMap { pet ->
					// ...
				}
				.onErrorResume{ ex ->
					// ...
				}
	}
----

Note that use of `@ModelAttribute` is optional -- for example, 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 were annotated
with `@ModelAttribute`.


[[webflux-ann-sessionattributes]]
==== `@SessionAttributes`
[.small]#<<web.adoc#mvc-ann-sessionattributes, See equivalent in the Servlet stack>>#

`@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 typically lists the names of model attributes or types of
model attributes that should be transparently stored in the session for subsequent
requests to access.

Consider the following example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@SessionAttributes("pet") <1>
	public class EditPetForm {
		// ...
	}
----
<1> Using the `@SessionAttributes` annotation.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@SessionAttributes("pet") // <1>
	class EditPetForm {
		// ...
	}
----
<1> Using the `@SessionAttributes` annotation.

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,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	@SessionAttributes("pet") // <1>
	public class EditPetForm {

		// ...

		@PostMapping("/pets/{id}")
		public String handle(Pet pet, BindingResult errors, SessionStatus status) { // <2>
			if (errors.hasErrors()) {
				// ...
			}
				status.setComplete();
				// ...
			}
		}
	}
----
<1> Using the `@SessionAttributes` annotation.
<2> Using a `SessionStatus` variable.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	@SessionAttributes("pet") // <1>
	class EditPetForm {

		// ...

		@PostMapping("/pets/{id}")
		fun handle(pet: Pet, errors: BindingResult, status: SessionStatus): String { // <2>
			if (errors.hasErrors()) {
				// ...
			}
			status.setComplete()
			// ...
		}
	}
----
<1> Using the `@SessionAttributes` annotation.
<2> Using a `SessionStatus` variable.


[[webflux-ann-sessionattribute]]
==== `@SessionAttribute`
[.small]#<<web.adoc#mvc-ann-sessionattribute, See equivalent in the Servlet stack>>#

If you need access to pre-existing session attributes that are managed globally
(that is, outside the controller -- for example, by a filter) and may or may not be present,
you can use the `@SessionAttribute` annotation on a method parameter, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/")
	public String handle(@SessionAttribute User user) { // <1>
		// ...
	}
----
<1> Using `@SessionAttribute`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/")
	fun handle(@SessionAttribute user: User): String { // <1>
		// ...
	}
----
<1> Using `@SessionAttribute`.

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
<<webflux-ann-sessionattributes>>.


[[webflux-ann-requestattrib]]
==== `@RequestAttribute`
[.small]#<<web.adoc#mvc-ann-requestattrib, See equivalent in the Servlet stack>>#

Similarly to `@SessionAttribute`, you can use the `@RequestAttribute` annotation to
access pre-existing request attributes created earlier (for example, by a `WebFilter`),
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/")
	public String handle(@RequestAttribute Client client) { <1>
		// ...
	}
----
<1> Using `@RequestAttribute`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/")
	fun handle(@RequestAttribute client: Client): String { // <1>
		// ...
	}
----
<1> Using `@RequestAttribute`.


[[webflux-multipart-forms]]
==== Multipart Content
[.small]#<<web.adoc#mvc-multipart-forms, See equivalent in the Servlet stack>>#

As explained in <<webflux-multipart>>, `ServerWebExchange` provides access to multipart
content. The best way to handle a file upload form (for example, from a browser) in a controller
is through data binding to a <<webflux-ann-modelattrib-method-args, command object>>,
as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	class MyForm {

		private String name;

		private MultipartFile file;

		// ...

	}

	@Controller
	public class FileUploadController {

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

	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	class MyForm(
			val name: String,
			val file: MultipartFile)

	@Controller
	class FileUploadController {

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

	}
----
--

You can also submit multipart requests from non-browser clients in a RESTful service
scenario. The following example uses a file along with JSON:

[literal,subs="verbatim,quotes"]
----
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`, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@RequestPart("meta-data") Part metadata, // <1>
			@RequestPart("file-data") FilePart file) { // <2>
		// ...
	}
----
<1> Using `@RequestPart` to get the metadata.
<2> Using `@RequestPart` to get the file.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@RequestPart("meta-data") Part metadata, // <1>
			@RequestPart("file-data") FilePart file): String { // <2>
		// ...
	}
----
<1> Using `@RequestPart` to get the metadata.
<2> Using `@RequestPart` to get the file.
--


To deserialize the raw part content (for example, to JSON -- similar to `@RequestBody`),
you can declare a concrete target `Object`, instead of `Part`, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@RequestPart("meta-data") MetaData metadata) { // <1>
		// ...
	}
----
<1> Using `@RequestPart` to get the metadata.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@RequestPart("meta-data") metadata: MetaData): String { // <1>
		// ...
	}
----
<1> Using `@RequestPart` to get the metadata.
--

You can use `@RequestPart` in combination with `jakarta.validation.Valid` or Spring's
`@Validated` annotation, which causes Standard Bean Validation to be applied. Validation
errors lead to a `WebExchangeBindException` that results in a 400 (BAD_REQUEST) response.
The exception contains a `BindingResult` with the error details and can also be handled
in the controller method by declaring the argument with an async wrapper and then using
error related operators:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@Valid @RequestPart("meta-data") Mono<MetaData> metadata) {
		// use one of the onError* operators...
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@Valid @RequestPart("meta-data") metadata: MetaData): String {
		// ...
	}
----
--

To access all multipart data as a `MultiValueMap`, you can use `@RequestBody`,
as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/")
	public String handle(@RequestBody Mono<MultiValueMap<String, Part>> parts) { // <1>
		// ...
	}
----
<1> Using `@RequestBody`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@RequestBody parts: MultiValueMap<String, Part>): String { // <1>
		// ...
	}
----
<1> Using `@RequestBody`.
--

===== `PartEvent`

To access multipart data sequentially, in a streaming fashion, you can use `@RequestBody` with
`Flux<PartEvent>` (or `Flow<PartEvent>` in Kotlin).
Each part in a multipart HTTP message will produce at
least one `PartEvent` containing both headers and a buffer with the contents of the part.

- Form fields will produce a *single* `FormPartEvent`, containing the value of the field.
- File uploads will produce *one or more* `FilePartEvent` objects, containing the filename used
when uploading. If the file is large enough to be split across multiple buffers, the first
`FilePartEvent` will be followed by subsequent events.


For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
  @PostMapping("/")
  public void handle(@RequestBody Flux<PartEvent> allPartsEvents) { <1>
      allPartsEvents.windowUntil(PartEvent::isLast) <2>
              .concatMap(p -> p.switchOnFirst((signal, partEvents) -> { <3>
                  if (signal.hasValue()) {
                      PartEvent event = signal.get();
                      if (event instanceof FormPartEvent formEvent) { <4>
                          String value = formEvent.value();
                          // handle form field
                      }
                      else if (event instanceof FilePartEvent fileEvent) { <5>
                          String filename = fileEvent.filename();
                          Flux<DataBuffer> contents = partEvents.map(PartEvent::content); <6>
                          // handle file upload
                      }
                      else {
                          return Mono.error(new RuntimeException("Unexpected event: " + event));
                      }
                  }
                  else {
                      return partEvents; // either complete or error signal
                  }
              }));
  }
----
<1> Using `@RequestBody`.
<2> The final `PartEvent` for a particular part will have `isLast()` set to `true`, and can be
followed by additional events belonging to subsequent parts.
This makes the `isLast` property suitable as a predicate for the `Flux::windowUntil` operator, to
split events from all parts into windows that each belong to a single part.
<3> The `Flux::switchOnFirst` operator allows you to see whether you are handling a form field or
file upload.
<4> Handling the form field.
<5> Handling the file upload.
<6> The body contents must be completely consumed, relayed, or released to avoid memory leaks.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/")
	fun handle(@RequestBody allPartsEvents: Flux<PartEvent>) = { // <1>
      allPartsEvents.windowUntil(PartEvent::isLast) <2>
          .concatMap {
              it.switchOnFirst { signal, partEvents -> <3>
                  if (signal.hasValue()) {
                      val event = signal.get()
                      if (event is FormPartEvent) { <4>
                          val value: String = event.value();
                          // handle form field
                      } else if (event is FilePartEvent) { <5>
                          val filename: String = event.filename();
                          val contents: Flux<DataBuffer> = partEvents.map(PartEvent::content); <6>
                          // handle file upload
                      } else {
                          return Mono.error(RuntimeException("Unexpected event: " + event));
                      }
                  } else {
                      return partEvents; // either complete or error signal
                  }
              }
          }
}
----
<1> Using `@RequestBody`.
<2> The final `PartEvent` for a particular part will have `isLast()` set to `true`, and can be
followed by additional events belonging to subsequent parts.
This makes the `isLast` property suitable as a predicate for the `Flux::windowUntil` operator, to
split events from all parts into windows that each belong to a single part.
<3> The `Flux::switchOnFirst` operator allows you to see whether you are handling a form field or
file upload.
<4> Handling the form field.
<5> Handling the file upload.
<6> The body contents must be completely consumed, relayed, or released to avoid memory leaks.

Received part events can also be relayed to another service by using the `WebClient`.
See <<webflux-client-body-multipart>>.


[[webflux-ann-requestbody]]
==== `@RequestBody`
[.small]#<<web.adoc#mvc-ann-requestbody, See equivalent in the Servlet stack>>#

You can use the `@RequestBody` annotation to have the request body read and deserialized into an
`Object` through an <<webflux-codecs,HttpMessageReader>>.
The following example uses a `@RequestBody` argument:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(@RequestBody Account account) {
		// ...
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun 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.

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(@RequestBody Mono<Account> account) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(@RequestBody accounts: Flow<Account>) {
		// ...
	}
----

You can use the <<webflux-config-message-codecs>> option of the <<webflux-config>> to
configure or customize message readers.

You can use `@RequestBody` in combination with `jakarta.validation.Valid` or Spring's
`@Validated` annotation, which causes Standard Bean Validation to be applied. Validation
errors cause a `WebExchangeBindException`, which results in a 400 (BAD_REQUEST) response.
The exception contains a `BindingResult` with error details and can be handled in the
controller method by declaring the argument with an async wrapper and then using error
related operators:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(@Valid @RequestBody Mono<Account> account) {
		// use one of the onError* operators...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(@Valid @RequestBody account: Mono<Account>) {
		// ...
	}
----


[[webflux-ann-httpentity]]
==== `HttpEntity`
[.small]#<<web.adoc#mvc-ann-httpentity, See equivalent in the Servlet stack>>#

`HttpEntity` is more or less identical to using <<webflux-ann-requestbody>> but is based on a
container object that exposes request headers and the body. The following example uses an
`HttpEntity`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@PostMapping("/accounts")
	public void handle(HttpEntity<Account> entity) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@PostMapping("/accounts")
	fun handle(entity: HttpEntity<Account>) {
		// ...
	}
----


[[webflux-ann-responsebody]]
==== `@ResponseBody`
[.small]#<<web.adoc#mvc-ann-responsebody, See equivalent in the Servlet stack>>#

You can use the `@ResponseBody` annotation on a method to have the return serialized
to the response body through an <<webflux-codecs, HttpMessageWriter>>. The following
example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/accounts/{id}")
	@ResponseBody
	public Account handle() {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/accounts/{id}")
	@ResponseBody
	fun handle(): Account {
		// ...
	}
----

`@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 <<webflux-codecs-streaming>> and
<<webflux-codecs-jackson,JSON rendering>>.

You can combine `@ResponseBody` methods with JSON serialization views.
See <<webflux-ann-jackson>> for details.

You can use the <<webflux-config-message-codecs>> option of the <<webflux-config>> to
configure or customize message writing.


[[webflux-ann-responseentity]]
==== `ResponseEntity`
[.small]#<<web.adoc#mvc-ann-responseentity, See equivalent in the Servlet stack>>#

`ResponseEntity` is like <<webflux-ann-responsebody>> but with status and headers. For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/something")
	public ResponseEntity<String> handle() {
		String body = ... ;
		String etag = ... ;
		return ResponseEntity.ok().eTag(etag).body(body);
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/something")
	fun handle(): ResponseEntity<String> {
		val body: String = ...
		val etag: String = ...
		return ResponseEntity.ok().eTag(etag).build(body)
	}
----

WebFlux supports using a single value <<webflux-reactive-libraries, reactive type>> to
produce the `ResponseEntity` asynchronously, and/or single and multi-value reactive types
for the body. This allows a variety of async responses with `ResponseEntity` as follows:

* `ResponseEntity<Mono<T>>` or `ResponseEntity<Flux<T>>` make the response status and
  headers known immediately while the body is provided asynchronously at a later point.
  Use `Mono` if the body consists of 0..1 values or `Flux` if it can produce multiple values.
* `Mono<ResponseEntity<T>>` provides all three -- response status, headers, and body,
  asynchronously at a later point. This allows the response status and headers to vary
  depending on the outcome of asynchronous request handling.
* `Mono<ResponseEntity<Mono<T>>>` or `Mono<ResponseEntity<Flux<T>>>` are yet another
  possible, albeit less common alternative. They provide the response status and headers
  asynchronously first and then the response body, also asynchronously, second.


[[webflux-ann-jackson]]
==== Jackson JSON

Spring offers support for the Jackson JSON library.

[[webflux-ann-jsonview]]
===== JSON Views
[.small]#<<web.adoc#mvc-ann-jackson, See equivalent in the Servlet stack>>#

Spring WebFlux provides built-in support for
https://www.baeldung.com/jackson-json-view-annotation[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, you can use Jackson's
`@JsonView` annotation to activate a serialization view class, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@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;
		}
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RestController
	class UserController {

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

	class User(
			@JsonView(WithoutPasswordView::class) val username: String,
			@JsonView(WithPasswordView::class) val password: String
	) {
		interface WithoutPasswordView
		interface WithPasswordView : WithoutPasswordView
	}
----

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.



[[webflux-ann-modelattrib-methods]]
=== `Model`
[.small]#<<web.adoc#mvc-ann-modelattrib-methods, See equivalent in the Servlet stack>>#

You can use the `@ModelAttribute` annotation:

* On a <<webflux-ann-modelattrib-method-args, 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 as a model attribute.

This section discusses `@ModelAttribute` methods, or the second item from the preceding list.
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 through `@ControllerAdvice`. See the section on
<<webflux-ann-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 and anything
related to the request body).

The following example uses a `@ModelAttribute` method:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ModelAttribute
	public void populateModel(@RequestParam String number, Model model) {
		model.addAttribute(accountRepository.findAccount(number));
		// add more ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ModelAttribute
	fun populateModel(@RequestParam number: String, model: Model) {
		model.addAttribute(accountRepository.findAccount(number))
		// add more ...
	}
----

The following example adds one attribute only:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@ModelAttribute
	public Account addAccount(@RequestParam String number) {
		return accountRepository.findAccount(number);
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@ModelAttribute
	fun addAccount(@RequestParam number: String): Account {
		return accountRepository.findAccount(number);
	}
----

NOTE: When a name is not explicitly specified, a default name is chosen based on the 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
(for example, `Mono<Account>` or `io.reactivex.Single<Account>`). Such asynchronous model
attributes can be transparently resolved (and the model updated) to their actual values
at the time of `@RequestMapping` invocation, provided a `@ModelAttribute` argument is
declared without a wrapper, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@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) {
		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	import org.springframework.ui.set

	@ModelAttribute
	fun addAccount(@RequestParam number: String) {
		val accountMono: Mono<Account> = accountRepository.findAccount(number)
		model["account"] = accountMono
	}

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


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.

You can also use `@ModelAttribute` 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` that would otherwise be interpreted
as a view name. `@ModelAttribute` can also help to customize the model attribute name,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/accounts/{id}")
	@ModelAttribute("myAccount")
	public Account handle() {
		// ...
		return account;
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/accounts/{id}")
	@ModelAttribute("myAccount")
	fun handle(): Account {
		// ...
		return account
	}
----



[[webflux-ann-initbinder]]
=== `DataBinder`
[.small]#<<web.adoc#mvc-ann-initbinder, See equivalent in the Servlet stack>>#

`@Controller` or `@ControllerAdvice` classes can have `@InitBinder` methods, to
initialize instances of `WebDataBinder`. Those, in turn, are used to:

* Bind request parameters (that is, 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.beans.PropertyEditor` or
Spring `Converter` and `Formatter` components. In addition, you can use the
<<webflux-config-conversion, WebFlux Java configuration>> to register `Converter` and
`Formatter` types in a globally shared `FormattingConversionService`.

`@InitBinder` methods support many of the same arguments that `@RequestMapping` methods
do, except for `@ModelAttribute` (command object) arguments. Typically, they are declared
with a `WebDataBinder` argument, for registrations, and a `void` return value.
The following example uses the `@InitBinder` annotation:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class FormController {

		@InitBinder // <1>
		public void initBinder(WebDataBinder binder) {
			SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
			dateFormat.setLenient(false);
			binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
		}

		// ...
	}
----
<1> Using the `@InitBinder` annotation.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class FormController {

		@InitBinder // <1>
		fun initBinder(binder: WebDataBinder) {
			val dateFormat = SimpleDateFormat("yyyy-MM-dd")
			dateFormat.isLenient = false
			binder.registerCustomEditor(Date::class.java, CustomDateEditor(dateFormat, false))
		}

		// ...
	}
----
<1> Using the `@InitBinder` annotation.
--

Alternatively, when using a `Formatter`-based setup through a shared
`FormattingConversionService`, you could re-use the same approach and register
controller-specific `Formatter` instances, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class FormController {

		@InitBinder
		protected void initBinder(WebDataBinder binder) {
			binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd")); <1>
		}

		// ...
	}
----
<1> Adding a custom formatter (a `DateFormatter`, in this case).

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class FormController {

		@InitBinder
		fun initBinder(binder: WebDataBinder) {
			binder.addCustomFormatter(DateFormatter("yyyy-MM-dd")) // <1>
		}

		// ...
	}
----
<1> Adding a custom formatter (a `DateFormatter`, in this case).
--


[[webflux-ann-initbinder-model-design]]
==== Model Design
[.small]#<<web.adoc#mvc-ann-initbinder-model-design, See equivalent in the Servlet stack>>#

include::web-data-binding-model-design.adoc[]


[[webflux-ann-controller-exceptions]]
=== Exceptions
[.small]#<<web.adoc#mvc-ann-exceptionhandler, See equivalent in the Servlet stack>>#

`@Controller` and <<webflux-ann-controller-advice, @ControllerAdvice>> classes can have
`@ExceptionHandler` methods to handle exceptions from controller methods. The following
example includes such a handler method:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class SimpleController {

		// ...

		@ExceptionHandler // <1>
		public ResponseEntity<String> handle(IOException ex) {
			// ...
		}
	}
----
<1> Declaring an `@ExceptionHandler`.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class SimpleController {

		// ...

		@ExceptionHandler // <1>
		fun handle(ex: IOException): ResponseEntity<String> {
			// ...
		}
	}
----
<1> Declaring an `@ExceptionHandler`.


The exception can match against a top-level exception being propagated (that is, a direct
`IOException` being thrown) or against the immediate cause within a top-level wrapper
exception (for example, an `IOException` wrapped inside an `IllegalStateException`).

For matching exception types, preferably declare the target exception as a method argument,
as shown in the preceding example. Alternatively, the annotation declaration can narrow the
exception types to match. We generally recommend being as specific as possible in the
argument signature and to declare your primary root exception mappings on a
`@ControllerAdvice` prioritized with a corresponding order.
See <<web.adoc#mvc-ann-exceptionhandler, the MVC section>> for details.

NOTE: An `@ExceptionHandler` method in WebFlux supports the same method arguments and
return values as a `@RequestMapping` method, with the exception of request body-
and `@ModelAttribute`-related method arguments.

Support for `@ExceptionHandler` methods in Spring WebFlux is provided by the
`HandlerAdapter` for `@RequestMapping` methods. See <<webflux-dispatcher-handler>>
for more detail.



[[webflux-ann-exceptionhandler-args]]
==== Method Arguments
[.small]#<<web.adoc#mvc-ann-exceptionhandler-args, See equivalent in the Servlet stack>>#

`@ExceptionHandler` methods support the same <<webflux-ann-arguments,method arguments>>
as `@RequestMapping` methods, except the request body might have been consumed already.



[[webflux-ann-exceptionhandler-return-values]]
==== Return Values
[.small]#<<web.adoc#mvc-ann-exceptionhandler-return-values, See equivalent in the Servlet stack>>#

`@ExceptionHandler` methods support the same <<webflux-ann-return-types,return values>>
as `@RequestMapping` methods.



[[webflux-ann-controller-advice]]
=== Controller Advice
[.small]#<<web.adoc#mvc-ann-controller-advice, See equivalent in the Servlet stack>>#

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

`@ControllerAdvice` is annotated with `@Component`, which means that such classes can be
registered as Spring beans through <<core.adoc#beans-java-instantiating-container-scan,
component scanning>>. `@RestControllerAdvice` is a composed annotation that is annotated
with both `@ControllerAdvice` and `@ResponseBody`, which essentially means
`@ExceptionHandler` methods are rendered to the response body through message conversion
(versus view resolution or template rendering).

On startup, the infrastructure classes for `@RequestMapping` and `@ExceptionHandler`
methods detect Spring beans annotated with `@ControllerAdvice` and then apply their
methods at runtime. Global `@ExceptionHandler` methods (from a `@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 (that is, all controllers),
but you can narrow that down to a subset of controllers by using attributes on the
annotation, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// 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 {}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// 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 {}
----

The selectors in the preceding example 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.

include::webflux-functional.adoc[leveloffset=+1]




[[webflux-uri-building]]
== URI Links
[.small]#<<web.adoc#mvc-uri-building, See equivalent in the Servlet stack>>#

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

include::web-uris.adoc[leveloffset=+2]

include::webflux-cors.adoc[leveloffset=+1]


[[webflux-ann-rest-exceptions]]
== Error Responses
[.small]#<<webmvc.adoc#mvc-ann-rest-exceptions, See equivalent in the Servlet stack>>#

A common requirement for REST services is to include details in the body of error
responses. The Spring Framework supports the "Problem Details for HTTP APIs"
specification, https://www.rfc-editor.org/rfc/rfc7807.html[RFC 7807].

The following are the main abstractions for this support:

- `ProblemDetail` -- representation for an RFC 7807 problem detail; a simple container
for both standard fields defined in the spec, and for non-standard ones.
- `ErrorResponse` -- contract to expose HTTP error response details including HTTP
status, response headers, and a body in the format of RFC 7807; this allows exceptions to
encapsulate and expose the details of how they map to an HTTP response. All Spring WebFlux
exceptions implement this.
- `ErrorResponseException` -- basic `ErrorResponse` implementation that others
can use as a convenient base class.
- `ResponseEntityExceptionHandler` -- convenient base class for an
<<webflux-ann-controller-advice,@ControllerAdvice>> that handles all Spring WebFlux exceptions,
and any `ErrorResponseException`, and renders an error response with a body.



[[webflux-ann-rest-exceptions-render]]
=== Render
[.small]#<<webmvc.adoc#mvc-ann-rest-exceptions-render, See equivalent in the Servlet stack>>#

You can return `ProblemDetail` or `ErrorResponse` from any `@ExceptionHandler` or from
any `@RequestMapping` method to render an RFC 7807 response. This is processed as follows:

- The `status` property of `ProblemDetail` determines the HTTP status.
- The `instance` property of `ProblemDetail` is set from the current URL path, if not
already set.
- For content negotiation, the Jackson `HttpMessageConverter` prefers
"application/problem+json" over "application/json" when rendering a `ProblemDetail`,
and also falls back on it if no compatible media type is found.

To enable RFC 7807 responses for Spring WebFlux exceptions and for any
`ErrorResponseException`, extend `ResponseEntityExceptionHandler` and declare it as an
<<webflux-ann-controller-advice,@ControllerAdvice>> in Spring configuration. The handler
has an `@ExceptionHandler` method that handles any `ErrorResponse` exception, which
includes all built-in web exceptions. You can add more exception handling methods, and
use a protected method to map any exception to a `ProblemDetail`.



[[webflux-ann-rest-exceptions-non-standard]]
=== Non-Standard Fields
[.small]#<<webmvc.adoc#mvc-ann-rest-exceptions-non-standard, See equivalent in the Servlet stack>>#

You can extend an RFC 7807 response with non-standard fields in one of two ways.

One, insert into the "properties" `Map` of `ProblemDetail`. When using the Jackson
library, the Spring Framework registers `ProblemDetailJacksonMixin` that ensures this
"properties" `Map` is unwrapped and rendered as top level JSON properties in the
response, and likewise any unknown property during deserialization is inserted into
this `Map`.

You can also extend `ProblemDetail` to add dedicated non-standard properties.
The copy constructor in `ProblemDetail` allows a subclass to make it easy to be created
from an existing `ProblemDetail`. This could be done centrally, e.g. from an
`@ControllerAdvice` such as `ResponseEntityExceptionHandler` that re-creates the
`ProblemDetail` of an exception into a subclass with the additional non-standard fields.



[[webflux-ann-rest-exceptions-i18n]]
=== Internationalization
[.small]#<<webmvc.adoc#mvc-ann-rest-exceptions-i18n, See equivalent in the Servlet stack>>#

It is a common requirement to internationalize error response details, and good practice
to customize the problem details for Spring WebFlux exceptions. This is supported as follows:

- Each `ErrorResponse` exposes a message code and arguments to resolve the "detail" field
through a <<core.adoc#context-functionality-messagesource,MessageSource>>.
The actual message code value is parameterized with placeholders, e.g.
`"HTTP method {0} not supported"` to be expanded from the arguments.
- Each `ErrorResponse` also exposes a message code to resolve the "title" field.
- `ResponseEntityExceptionHandler` uses the message code and arguments to resolve the
"detail" and the "title" fields.

By default, the message code for the "detail" field is "problemDetail." + the fully
qualified exception class name. Some exceptions may expose additional message codes in
which case a suffix is added to the default message code. The table below lists message
arguments and codes for Spring WebFlux exceptions:

[[webflux-ann-rest-exceptions-codes]]
[cols="1,1,2", options="header"]
|===
| Exception | Message Code | Message Code Arguments

| `UnsupportedMediaTypeStatusException`
| (default)
| `{0}` the media type that is not supported, `{1}` list of supported media types

| `UnsupportedMediaTypeStatusException`
| (default) + ".parseError"
|

| `MissingRequestValueException`
| (default)
| `{0}` a label for the value (e.g. "request header", "cookie value", ...), `{1}` the value name

| `UnsatisfiedRequestParameterException`
| (default)
| `{0}` the list of parameter conditions

| `WebExchangeBindException`
| (default)
| `{0}` the list of global errors, `{1}` the list of field errors.
Message codes and arguments for each error within the `BindingResult` are also resolved
via `MessageSource`.

| `NotAcceptableStatusException`
| (default)
| `{0}` list of supported media types

| `NotAcceptableStatusException`
| (default) + ".parseError"
|

| `ServerErrorException`
| (default)
| `{0}` the failure reason provided to the class constructor

| `MethodNotAllowedException`
| (default)
| `{0}` the current HTTP method, `{1}` the list of supported HTTP methods

|===

By default, the message code for the "title" field is "problemDetail.title." + the fully
qualified exception class name.




[[webflux-ann-rest-exceptions-client]]
=== Client Handling
[.small]#<<webmvc.adoc#mvc-ann-rest-exceptions-client, See equivalent in the Servlet stack>>#

A client application can catch `WebClientResponseException`, when using the `WebClient`,
or `RestClientResponseException` when using the `RestTemplate`, and use their
`getResponseBodyAs` methods to decode the error response body to any target type such as
`ProblemDetail`, or a subclass of `ProblemDetail`.




[[webflux-web-security]]
== Web Security
[.small]#<<web.adoc#mvc-web-security, See equivalent in the Servlet stack>>#

The https://spring.io/projects/spring-security[Spring Security] project provides support
for protecting web applications from malicious exploits. See the Spring Security
reference documentation, including:

* {docs-spring-security}/reactive/configuration/webflux.html[WebFlux Security]
* {docs-spring-security}/reactive/test/index.html[WebFlux Testing Support]
* {docs-spring-security}/features/exploits/csrf.html#csrf-protection[CSRF protection]
* {docs-spring-security}/features/exploits/headers.html[Security Response Headers]




[[webflux-caching]]
== HTTP Caching
[.small]#<<web.adoc#mvc-caching, See equivalent in the Servlet stack>>#

HTTP caching can significantly improve the performance of a web application. HTTP caching
revolves around the `Cache-Control` response header and subsequent conditional request
headers, such as `Last-Modified` and `ETag`. `Cache-Control` advises private (for example, browser)
and public (for example, proxy) caches how to cache and re-use responses. An `ETag` header is used
to make a conditional request that may result in a 304 (NOT_MODIFIED) without a body,
if the content has not changed. `ETag` can be seen as a more sophisticated successor to
the `Last-Modified` header.

This section describes the HTTP caching related options available in Spring WebFlux.



[[webflux-caching-cachecontrol]]
=== `CacheControl`
[.small]#<<web.adoc#mvc-caching-cachecontrol, See equivalent in the Servlet stack>>#

{api-spring-framework}/http/CacheControl.html[`CacheControl`] provides support for
configuring settings related to the `Cache-Control` header and is accepted as an argument
in a number of places:

* <<webflux-caching-etag-lastmodified>>
* <<webflux-caching-static-resources>>

While https://tools.ietf.org/html/rfc7234#section-5.2.2[RFC 7234] describes all possible
directives for the `Cache-Control` response header, the `CacheControl` type takes a
use case-oriented approach that focuses on the common scenarios, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);

	// Prevent caching - "Cache-Control: no-store"
	CacheControl ccNoStore = CacheControl.noStore();

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic();
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	// Cache for an hour - "Cache-Control: max-age=3600"
	val ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS)

	// Prevent caching - "Cache-Control: no-store"
	val ccNoStore = CacheControl.noStore()

	// Cache for ten days in public and private caches,
	// public caches should not transform the response
	// "Cache-Control: max-age=864000, public, no-transform"
	val ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic()

----



[[webflux-caching-etag-lastmodified]]
=== Controllers
[.small]#<<web.adoc#mvc-caching-etag-lastmodified, See equivalent in the Servlet stack>>#

Controllers can add explicit support for HTTP caching. We recommend doing so, since the
`lastModified` or `ETag` value for a resource needs to be calculated before it can be compared
against conditional request headers. A controller can add an `ETag` and `Cache-Control`
settings to a `ResponseEntity`, as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@GetMapping("/book/{id}")
	public ResponseEntity<Book> showBook(@PathVariable Long id) {

		Book book = findBook(id);
		String version = book.getVersion();

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book);
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@GetMapping("/book/{id}")
	fun showBook(@PathVariable id: Long): ResponseEntity<Book> {

		val book = findBook(id)
		val version = book.getVersion()

		return ResponseEntity
				.ok()
				.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
				.eTag(version) // lastModified is also available
				.body(book)
	}
----
--

The preceding example sends a 304 (NOT_MODIFIED) response with an empty body if the comparison
to the conditional request headers indicates the content has not changed. Otherwise, the
`ETag` and `Cache-Control` headers are added to the response.

You can also make the check against conditional request headers in the controller,
as the following example shows:

--
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@RequestMapping
	public String myHandleMethod(ServerWebExchange exchange, Model model) {

		long eTag = ... // <1>

		if (exchange.checkNotModified(eTag)) {
			return null; // <2>
		}

		model.addAttribute(...); // <3>
		return "myViewName";
	}
----
<1> Application-specific calculation.
<2> Response has been set to 304 (NOT_MODIFIED). No further processing.
<3> Continue with request processing.

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@RequestMapping
	fun myHandleMethod(exchange: ServerWebExchange, model: Model): String? {

		val eTag: Long = ... // <1>

		if (exchange.checkNotModified(eTag)) {
			return null// <2>
		}

		model.addAttribute(...) // <3>
		return "myViewName"
	}
----
<1> Application-specific calculation.
<2> Response has been set to 304 (NOT_MODIFIED). No further processing.
<3> Continue with request processing.
--

There are three variants for checking conditional requests against `eTag` values, `lastModified`
values, or both. For conditional `GET` and `HEAD` requests, you can set the response to
304 (NOT_MODIFIED). For conditional `POST`, `PUT`, and `DELETE`, you can instead set the response
to 412 (PRECONDITION_FAILED) to prevent concurrent modification.



[[webflux-caching-static-resources]]
=== Static Resources
[.small]#<<web.adoc#mvc-caching-static-resources, See equivalent in the Servlet stack>>#

You should serve static resources with a `Cache-Control` and conditional response headers
for optimal performance. See the section on configuring <<webflux-config-static-resources>>.


include::webflux-view.adoc[leveloffset=+1]


[[webflux-config]]
== WebFlux Config
[.small]#<<web.adoc#mvc-config, See equivalent in the Servlet stack>>#

The WebFlux Java configuration declares the components that are 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 configuration. However, if you want to understand them,
you can see them in `WebFluxConfigurationSupport` or read more about what they are
in <<webflux-special-bean-types>>.

For more advanced customizations, not available in the configuration API, you can
gain full control over the configuration through the
<<webflux-config-advanced-java>>.



[[webflux-config-enable]]
=== Enabling WebFlux Config
[.small]#<<web.adoc#mvc-config-enable, See equivalent in the Servlet stack>>#

You can use the `@EnableWebFlux` annotation in your Java config, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig {
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig
----

The preceding example registers a number of Spring WebFlux
<<webflux-special-bean-types, infrastructure beans>> and adapts to dependencies
available on the classpath -- for JSON, XML, and others.



[[webflux-config-customize]]
=== WebFlux config API
[.small]#<<web.adoc#mvc-config-customize, See equivalent in the Servlet stack>>#

In your Java configuration, you can implement the `WebFluxConfigurer` interface,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		// Implement configuration methods...
	}
----

[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	// Implement configuration methods...
}
----



[[webflux-config-conversion]]
=== Conversion, formatting
[.small]#<<web.adoc#mvc-config-conversion, See equivalent in the Servlet stack>>#

By default, formatters for various number and date types are installed, along with support
for customization via `@NumberFormat` and `@DateTimeFormat` on fields.

To register custom formatters and converters in Java config, use the following:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

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

	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun addFormatters(registry: FormatterRegistry) {
			// ...
		}
	}
----

By default Spring WebFlux considers the request Locale when parsing and formatting date
values. This works for forms where dates are represented as Strings with "input" form
fields. For "date" and "time" form fields, however, browsers use a fixed format defined
in the HTML spec. For such cases date and time formatting can be customized as follows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void addFormatters(FormatterRegistry registry) {
			DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
			registrar.setUseIsoFormat(true);
			registrar.registerFormatters(registry);
      	}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun addFormatters(registry: FormatterRegistry) {
			val registrar = DateTimeFormatterRegistrar()
			registrar.setUseIsoFormat(true)
			registrar.registerFormatters(registry)
		}
	}
----

NOTE: See <<core.adoc#format-FormatterRegistrar-SPI, `FormatterRegistrar` SPI>>
and the `FormattingConversionServiceFactoryBean` for more information on when to
use `FormatterRegistrar` implementations.



[[webflux-config-validation]]
=== Validation
[.small]#<<web.adoc#mvc-config-validation, See equivalent in the Servlet stack>>#

By default, if <<core.adoc#validation-beanvalidation-overview, Bean Validation>> is present
on the classpath (for example, the Hibernate Validator), the `LocalValidatorFactoryBean`
is registered as a global <<core.adoc#validator,validator>> for use with `@Valid` and
`@Validated` on `@Controller` method arguments.

In your Java configuration, you can customize the global `Validator` instance,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

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

	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun getValidator(): Validator {
			// ...
		}

	}
----

Note that you can also register `Validator` implementations locally,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Controller
	public class MyController {

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

	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Controller
	class MyController {

		@InitBinder
		protected fun initBinder(binder: WebDataBinder) {
			binder.addValidators(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.



[[webflux-config-content-negotiation]]
=== Content Type Resolvers
[.small]#<<web.adoc#mvc-config-content-negotiation, See equivalent in the Servlet stack>>#

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

The following example shows how to customize the requested content type resolution:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void configureContentTypeResolver(RequestedContentTypeResolverBuilder builder) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun configureContentTypeResolver(builder: RequestedContentTypeResolverBuilder) {
			// ...
		}
	}
----



[[webflux-config-message-codecs]]
=== HTTP message codecs
[.small]#<<web.adoc#mvc-config-message-converters, See equivalent in the Servlet stack>>#

The following example shows how to customize how the request and response body are read and written:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
			configurer.defaultCodecs().maxInMemorySize(512 * 1024);
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun configureHttpMessageCodecs(configurer: ServerCodecConfigurer) {
			// ...
		}
	}
----

`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
{api-spring-framework}/http/converter/json/Jackson2ObjectMapperBuilder.html[`Jackson2ObjectMapperBuilder`],
which customizes Jackson's default properties with the following ones:

* https://fasterxml.github.io/jackson-databind/javadoc/2.6/com/fasterxml/jackson/databind/DeserializationFeature.html#FAIL_ON_UNKNOWN_PROPERTIES[`DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES`] is disabled.
* https://fasterxml.github.io/jackson-databind/javadoc/2.6/com/fasterxml/jackson/databind/MapperFeature.html#DEFAULT_VIEW_INCLUSION[`MapperFeature.DEFAULT_VIEW_INCLUSION`] is disabled.

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

* https://github.com/FasterXML/jackson-datatype-joda[`jackson-datatype-joda`]: Support for Joda-Time types.
* https://github.com/FasterXML/jackson-datatype-jsr310[`jackson-datatype-jsr310`]: Support for Java 8 Date and Time API types.
* https://github.com/FasterXML/jackson-datatype-jdk8[`jackson-datatype-jdk8`]: Support for other Java 8 types, such as `Optional`.
* https://github.com/FasterXML/jackson-module-kotlin[`jackson-module-kotlin`]: Support for Kotlin classes and data classes.



[[webflux-config-view-resolvers]]
=== View Resolvers
[.small]#<<web.adoc#mvc-config-view-resolvers, See equivalent in the Servlet stack>>#

The following example shows how to configure view resolution:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			// ...
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			// ...
		}
	}
----

The `ViewResolverRegistry` has shortcuts for view technologies with which the Spring Framework
integrates. The following example uses FreeMarker (which also requires configuring the
underlying FreeMarker view technology):

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@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;
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			registry.freeMarker()
		}

		// Configure Freemarker...

		@Bean
		fun freeMarkerConfigurer() = FreeMarkerConfigurer().apply {
			setTemplateLoaderPath("classpath:/templates")
		}
	}
----

You can also plug in any `ViewResolver` implementation, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {


		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			ViewResolver resolver = ... ;
			registry.viewResolver(resolver);
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			val resolver: ViewResolver = ...
			registry.viewResolver(resolver
		}
	}
----

To support <<webflux-multiple-representations>> 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
<<webflux-codecs>> from `spring-web`. The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {


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

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

		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {


		override fun configureViewResolvers(registry: ViewResolverRegistry) {
			registry.freeMarker()

			val encoder = Jackson2JsonEncoder()
			registry.defaultViews(HttpMessageWriterView(encoder))
		}

		// ...
	}
----

See <<webflux-view>> for more on the view technologies that are integrated with Spring WebFlux.



[[webflux-config-static-resources]]
=== Static Resources
[.small]#<<web.adoc#mvc-config-static-resources, See equivalent in the Servlet stack>>#

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 next example, 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
are served with a one-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. The following listing shows
the example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public", "classpath:/static/")
					.setCacheControl(CacheControl.maxAge(365, TimeUnit.DAYS));
		}

	}
----
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun addResourceHandlers(registry: ResourceHandlerRegistry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public", "classpath:/static/")
					.setCacheControl(CacheControl.maxAge(365, TimeUnit.DAYS))
		}
	}
----

See also <<webflux-caching-static-resources, HTTP caching support for static resources>>.

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

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

The following example shows how to use `VersionResourceResolver` in your Java configuration:

[source,java,indent=0,subs="verbatim",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

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

	}
----
[source,kotlin,indent=0,subs="verbatim",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		override fun addResourceHandlers(registry: ResourceHandlerRegistry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public/")
					.resourceChain(true)
					.addResolver(VersionResourceResolver().addContentVersionStrategy("/**"))
		}

	}
----

You can use `ResourceUrlProvider` to rewrite URLs and apply the full chain of resolvers and
transformers (for example, to insert versions). The WebFlux configuration provides a `ResourceUrlProvider`
so that 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. When serving only local resources, the workaround is to use
`ResourceUrlProvider` directly (for example, through a custom element) and block.

Note that, when using both `EncodedResourceResolver` (for example, Gzip, Brotli encoded) and
`VersionedResourceResolver`, they must be registered in that order, to ensure content-based
versions are always computed reliably based on the unencoded file.

For https://www.webjars.org/documentation[WebJars], versioned URLs like
`/webjars/jquery/1.2.0/jquery.min.js` are the recommended and most efficient way to use them.
The related resource location is configured out of the box with Spring Boot (or can be configured
manually via `ResourceHandlerRegistry`) and does not require to add the
`org.webjars:webjars-locator-core` dependency.

Version-less URLs like `/webjars/jquery/jquery.min.js` are supported through the
`WebJarsResourceResolver` which is automatically registered when the
`org.webjars:webjars-locator-core` library is present on the classpath, at the cost of a
classpath scanning that could slow down application startup. The resolver can re-write URLs to
include the version of the jar and can also match against incoming URLs without versions
-- for example, from `/webjars/jquery/jquery.min.js` to `/webjars/jquery/1.2.0/jquery.min.js`.

TIP: The Java configuration based on `ResourceHandlerRegistry` provides further options
for fine-grained control, e.g. last-modified behavior and optimized resource resolution.



[[webflux-config-path-matching]]
=== Path Matching
[.small]#<<web.adoc#mvc-config-path-matching, See equivalent in the Servlet stack>>#

You can customize options related to path matching. For details on the individual options, see the
{api-spring-framework}/web/reactive/config/PathMatchConfigurer.html[`PathMatchConfigurer`] javadoc.
The following example shows how to use `PathMatchConfigurer`:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public void configurePathMatch(PathMatchConfigurer configurer) {
			configurer
				.setUseCaseSensitiveMatch(true)
				.addPathPrefix("/api", HandlerTypePredicate.forAnnotation(RestController.class));
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		@Override
		fun configurePathMatch(configurer: PathMatchConfigurer) {
			configurer
				.setUseCaseSensitiveMatch(true)
				.addPathPrefix("/api", HandlerTypePredicate.forAnnotation(RestController::class.java))
		}
	}
----

[TIP]
====
Spring WebFlux relies on a parsed representation of the request path called
`RequestPath` for access to decoded path segment values, with semicolon content removed
(that is, path or matrix variables). That means, unlike in Spring MVC, you need not indicate
whether to decode the request path nor whether to remove semicolon content for
path matching purposes.

Spring WebFlux also does not support suffix pattern matching, unlike in Spring MVC, where we
are also <<web.adoc#mvc-ann-requestmapping-suffix-pattern-match, recommend>> moving away from
reliance on it.
====



[[webflux-config-websocket-service]]
=== WebSocketService

The WebFlux Java config declares of a `WebSocketHandlerAdapter` bean which provides
support for the invocation of WebSocket handlers. That means all that remains to do in
order to handle a WebSocket handshake request is to map a `WebSocketHandler` to a URL
via `SimpleUrlHandlerMapping`.

In some cases it may be necessary to create the `WebSocketHandlerAdapter` bean with a
provided `WebSocketService` service which allows configuring WebSocket server properties.
For example:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	@EnableWebFlux
	public class WebConfig implements WebFluxConfigurer {

		@Override
		public WebSocketService getWebSocketService() {
			TomcatRequestUpgradeStrategy strategy = new TomcatRequestUpgradeStrategy();
			strategy.setMaxSessionIdleTimeout(0L);
			return new HandshakeWebSocketService(strategy);
		}
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	@EnableWebFlux
	class WebConfig : WebFluxConfigurer {

		@Override
		fun webSocketService(): WebSocketService {
			val strategy = TomcatRequestUpgradeStrategy().apply {
				setMaxSessionIdleTimeout(0L)
			}
			return HandshakeWebSocketService(strategy)
		}
	}
----




[[webflux-config-advanced-java]]
=== Advanced Configuration Mode
[.small]#<<web.adoc#mvc-config-advanced-java, See equivalent in the Servlet stack>>#

`@EnableWebFlux` imports `DelegatingWebFluxConfiguration` that:

*  Provides default Spring configuration for WebFlux applications

* detects and delegates to `WebFluxConfigurer` implementations to customize that configuration.

For advanced mode, you can remove `@EnableWebFlux` and extend directly from
`DelegatingWebFluxConfiguration` instead of implementing `WebFluxConfigurer`,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes",role="primary"]
.Java
----
	@Configuration
	public class WebConfig extends DelegatingWebFluxConfiguration {

		// ...
	}
----
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
.Kotlin
----
	@Configuration
	class WebConfig : DelegatingWebFluxConfiguration {

		// ...
	}
----

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




[[webflux-http2]]
== HTTP/2
[.small]#<<web.adoc#mvc-http2, See equivalent in the Servlet stack>>#

HTTP/2 is supported with Reactor Netty, Tomcat, Jetty, and Undertow. However, there are
considerations related to server configuration. For more details, see the
https://github.com/spring-projects/spring-framework/wiki/HTTP-2-support[HTTP/2 wiki page].
