[[webflux]]
= Spring WebFlux

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

Both web frameworks mirror the names of their source modules
https://github.com/spring-projects/spring-framework/tree/master/spring-webmvc[spring-webmvc] and
https://github.com/spring-projects/spring-framework/tree/master/spring-webflux[spring-webflux]
and co-exist side by side in the Spring Framework. Each module is optional.
Applications may use one or the other module, or in some cases both --
e.g. Spring MVC controllers with the reactive `WebClient`.


[[webflux-new-framework]]
=== Why a new web framework?

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

The other part of the answer is functional programming. Much like the addition of annotations
in Java 5 created opportunities -- e.g. annotated REST controllers or unit tests, the addition
of lambda expressions in Java 8 created opportunities for functional APIs in Java.
This is a boon for non-blocking applications and continuation style APIs -- as popularized
by `CompletableFuture` and http://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 with annotated controllers.


[[webflux-why-reactive]]
=== Reactive: what and why?

We touched on non-blocking and functional but why reactive and what do we mean?

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

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

Reactive Streams is a
https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.1/README.md#specification[small spec],
also http://download.java.net/java/jdk9/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
http://www.reactive-streams.org/reactive-streams-1.0.1-javadoc/org/reactivestreams/Publisher.html[Publisher],
can produce data that an HTTP server -- acting as
http://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 allow the
subscriber to control how fast or how slow the publisher will produce data.

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


[[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. What applications need is a higher level and richer, functional API to
compose async logic -- similar to the Java 8 `Stream` API but not only for collections.
This is the role that reactive libraries play.

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 and 0..N through a rich set of operators aligned with the
ReactiveX http://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 WebFlux APIs accept a plain `Publisher`
as input, adapt it to Reactor types internally, use those, and then return either
`Flux` or `Mono` as output. So you can pass any `Publisher` as input and you can apply
operations on the output, but you'll need to adapt the output for use with another reactive library.
Whenever feasible -- e.g. annotated controllers, WebFlux adapts transparently to the use
of RxJava or other reactive library. See <<webflux-reactive-libraries>> for more details.


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

The `spring-web` module contains the reactive foundation that underlies Spring WebFlux --
HTTP abstractions, Reactive Streams server adapters, reactive codecs, and a
core Web API whose role is comparable to the Servlet API but with non-blocking semantics.

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, RxJava) return types and as a result it is not easy to tell them apart. One notable
difference is that WebFlux also supports reactive `@RequestBody` arguments.
- <<webflux-fn>> -- lambda-based, lightweight, functional programming model. Think of
this as a small library or a set of utilities that an application can use to route and
handle requests. The big difference with annotated controllers is that the application
is in charge of request handling from start to finish vs declaring intent through
annotations and being called back.


[[webflux-framework-choice]]
=== Choosing a web framework

Should you use Spring MVC or WebFlux? Let's cover a few different perspectives.

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

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

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

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

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

If you have a Spring MVC application with calls to remote services, try the reactive `WebClient`.
You can return reactive types (Reactor, RxJava, <<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 (e.g. concurrency on single-threaded Node.js is not an oxymoron) and its effects.
The tag line is "scale with less hardware" but that effect is not guaranteed, not without
some network I/O that can be slow or unpredictable. This Netflix
https://medium.com/netflix-techblog/zuul-2-the-netflix-journey-to-asynchronous-non-blocking-systems-45947377fb5c[blog post]
is a good resource.


[[webflux-server-choice]]
=== Choosing a server

Spring WebFlux is supported on Netty, Undertow, Tomcat, Jetty, and Servlet 3.1+ containers.
Each server is adapted to a common Reactive Streams API. The Spring WebFlux programming
models are built on that common API.

[NOTE]
====
*Common question: how can Tomcat and Jetty be used in both stacks?* +
Tomcat and Jetty are non-blocking at their core. It's the Servlet API that adds a
blocking facade. Starting in version 3.1 the Servlet API adds a choice for non-blocking I/O.
However its use requires care to avoid other synchronous and blocking parts. For this
reason Spring's reactive web stack has a low-level Servlet adapter to bridge to Reactive
Streams but the Servlet API is otherwise not exposed for direct use.
====

Spring Boot 2 uses Netty by default with WebFlux because Netty is more widely used in the
async, non-blocking space and also provides both client and server that can share resources.
By comparison Servlet 3.1 non-blocking I/O hasn't seen much use because the bar to use it
is so high. Spring WebFlux opens one practical path to adoption.

The default server choice in Spring Boot is mainly about the out-of-the-box experience.
Applications can still choose any of the other supported servers which are also highly
optimized for performance, fully non-blocking, and adapted to Reactive Streams back
pressure. In Spring Boot it is trivial to make the switch.


[[webflux-performance]]
=== Performance vs scale

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

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



[[webflux-reactive-spring-web]]
== Reactive Spring Web

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

Server support is organized in two layers:

* <<webflux-httphandler,HttpHandler>> and server adapters -- the most basic, common API
for HTTP request handling with Reactive Streams back pressure.
* <<webflux-web-handler-api,WebHandler API>> -- slightly higher level but still general
purpose server web API with filter chain style processing.


[[webflux-httphandler]]
=== HttpHandler

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

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

[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 3.1 non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[]
|spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge

|Jetty
|Servlet 3.1 non-blocking I/O; Jetty API to write ByteBuffers vs byte[]
|spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge

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

Here are required dependencies,
https://github.com/spring-projects/spring-framework/wiki/What%27s-New-in-the-Spring-Framework[supported versions],
and code snippets for each server:

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

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

|Undertow
|io.undertow
|undertow-core

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

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

Reactor Netty:
[source,java,indent=0]
[subs="verbatim,quotes"]
----
HttpHandler handler = ...
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
HttpServer.create(host, port).newHandler(adapter).block();
----

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

Tomcat:
[source,java,indent=0]
[subs="verbatim,quotes"]
----
HttpHandler handler = ...
Servlet servlet = new TomcatHttpHandlerAdapter(handler);

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

Jetty:
[source,java,indent=0]
[subs="verbatim,quotes"]
----
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();
----

You can also deploy as a WAR to any Servlet 3.1 container by wrapping the handler with
`ServletHttpHandlerAdapter` as a `Servlet`.



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

`HttpHandler` is the basis for running on different servers. On that base the WebHandler
API provides a slightly higher level processing chain of
exception handlers
({api-spring-framework}/web/server/WebExceptionHandler.html[WebExceptionHandler]), filters
({api-spring-framework}/web/server/WebFilter.html[WebFilter]), and a target handler
({api-spring-framework}/web/server/WebHandler.html[WebHandler]).

All components work on `ServerWebExchange` -- a container for the HTTP request and
response that also adds request attributes, session attributes, access to form data,
multipart data, and more.

The processing chain can be put together with `WebHttpHandlerBuilder` which builds an
`HttpHandler` that in turn can be run with a <<webflux-httphandler,server adapter>>.
To use the builder either add components individually or point to an `ApplicationContext`
to have the following detected:

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

|"webHandler"
|WebHandler
|1
|Target handler after filters

|<any>
|WebFilter
|0..N
|Filters

|<any>
|WebExceptionHandler
|0..N
|Exception handlers after filter chain

|"webSessionManager"
|WebSessionManager
|0..1
|Custom session manager; `DefaultWebSessionManager` by default

|"serverCodecConfigurer"
|ServerCodecConfigurer
|0..1
|Custom form and multipart data decoders; `ServerCodecConfigurer.create()` by default

|"localeContextResolver"
|LocaleContextResolver
|0..1
|Custom resolver for `LocaleContext`; `AcceptHeaderLocaleContextResolver` by default
|===


[[webflux-codecs]]
=== Codecs

The `spring-web` module provides
{api-spring-framework}/http/codec/HttpMessageReader.html[HttpMessageReader] and
{api-spring-framework}/http/codec/HttpMessageWriter.html[HttpMessageWriter]
for encoding and decoding the HTTP request and response body with Reactive Streams.
It builds on lower level contracts from `spring-core`:

* {api-spring-framework}/core/io/buffer/DataBuffer.html[DataBuffer] -- abstraction for
byte buffers -- e.g. Netty `ByteBuf`, `java.nio.ByteBuffer`
* {api-spring-framework}/core/codec/Encoder.html[Encoder] -- serialize a stream of Objects
to a stream of data buffers
* {api-spring-framework}/core/codec/Decoder.html[Decoder] -- deserialize a stream of data
buffers into a stream of Objects

Basic `Encoder` and `Decoder` implementations exist in `spring-core` but `spring-web` adds
more for JSON, XML, and other formats. You can wrap any `Encoder` and `Decoder` as a reader
or writer with `EncoderHttpMessageWriter` and `DecoderHttpMessageReader`. There are some
additional, web-only reader and writer implementations for server-sent events, form data,
and more.

Finally, `ClientCodecConfigurer` and `ServerCodecConfigurer` can be used to initialize
a list of readers and writers. They include support for classpath detection and a
of defaults along with the ability to override or replace those defaults.



[[webflux-dispatcher-handler]]
== The DispatcherHandler
[.small]#<<web.adoc#mvc-servlet,Same in Spring MVC>>#

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

`DispatcherHandler` discovers the delegate components it needs from Spring configuration.
It is also designed to be a Spring bean itself and implements `ApplicationContextAware`
for access to the context it runs in. If `DispatcherHandler` is declared with the bean
name "webHandler" it is in turn discovered by
{api-spring-framework}/web/server/adapter/WebHttpHandlerBuilder.html[WebHttpHandlerBuilder]
which puts together a request processing chain as described in
<<webflux-web-handler-api,WebHandler 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:
[source,java,indent=0]
[subs="verbatim,quotes"]
----
ApplicationContext context = ...
HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context);
----

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,Same in Spring MVC>>#

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 one of the framework contracts listed in the table below.
Spring WebFlux provides built-in implementations of these contracts but you can also
customize, extend, or replace them.

.Special bean types in the ApplicationContext
|===
| Bean type| Explanation

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

| HandlerAdapter
| Helps 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 various annotations. The main purpose of a `HandlerAdapter` is
  to shield the `DispatcherHandler` from such details.

| HandlerResultHandler
| Process the `HandlerResult` returned from a `HandlerAdapter`.
|===


[[webflux-dispatcher-handler-sequence]]
=== Processing sequence
[.small]#<<web.adoc#mvc-servlet-sequence,Same in Spring MVC>>#

The `DispatcherHandler` processes requests as follows:

* Each `HandlerMapping` is asked to find a matching handler and the first match is used.
* If a handler is found, it is executed through an appropriate `HandlerAdapter` which
exposes the return value from the execution as `HandlerResult`.
* The `HandlerResult` is given to an appropriate `HandlerResultHandler` to complete
processing by writing to the response directly or using a view to render.



[[webflux-controller]]
== Annotated Controllers
[.small]#<<web.adoc#mvc-controller,Same in Spring MVC>>#

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

Here is a basic example:

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

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

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



[[webflux-ann-controller]]
=== @Controller declaration
[.small]#<<web.adoc#mvc-ann-controller,Same in Spring MVC>>#

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

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

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@ComponentScan("org.example.web")
	public class WebConfig {

		// ...
	}
----

[NOTE]
====
`@RestController` is a composed annotation that is itself annotated with `@Controller` and
`@ResponseBody` indicating a controller whose every method inherits the type-level
`@ResponseBody` annotation and therefore writes to the response body (vs model-and-vew
rendering).
====


[[webflux-ann-requestmapping]]
=== Mapping Requests
[.small]#<<web.adoc#mvc-ann-requestmapping,Same in Spring MVC>>#

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

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

- `@GetMapping`
- `@PostMapping`
- `@PutMapping`
- `@DeleteMapping`
- `@PatchMapping`

The shortcut variants are
https://github.com/spring-projects/spring-framework/wiki/Spring-Annotation-Programming-Model#composed-annotations[composed annotations]
-- themselves annotated with `@RequestMapping`. They are commonly used at the method level.
At the class level an `@RequestMapping` is more useful for expressing shared mappings.

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

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

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


[[webflux-ann-requestmapping-uri-templates]]
==== URI Patterns
[.small]#<<web.adoc#mvc-ann-requestmapping-uri-templates,Same in Spring MVC>>#

You can map requests using glob patterns and wildcards:

* `?` matches one character
* `*` matches zero or more characters within a path segment
* `**` match zero or more path segments

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

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@GetMapping("/owners/{ownerId}/pets/{petId}")
	public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
		// ...
	}
----

URI variables can be declared at the class and method level:
[source,java,intent=0]
[subs="verbatim,quotes"]
----
@Controller
@RequestMapping("/owners/{ownerId}")
public class OwnerController {

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

URI variables are automatically converted to the appropriate type or`TypeMismatchException`
is raised. Simple types -- `int`, `long`, `Date`, are supported by default and you can
register support for any other data type.
// TODO: see <<webflux-ann-typeconversion>> and <<webflux-ann-webdatabinder>>.

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

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

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

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

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

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

Spring WebFlux does not support suffix pattern matching -- unlike Spring MVC, where a
mapping such as `/person` also matches to `/person.{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,Same in Spring MVC>>#

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

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

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


[[webflux-ann-requestmapping-consumes]]
==== Consumable Media Types
[.small]#<<web.adoc#mvc-ann-requestmapping-consumes,Same in Spring MVC>>#

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

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@PostMapping(path = "/pets", **consumes = "application/json"**)
	public void addPet(@RequestBody Pet pet) {
		// ...
	}
----

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

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

[TIP]
====
`MediaType` provides constants for commonly used media types -- e.g.
`APPLICATION_JSON_VALUE`, `APPLICATION_JSON_UTF8_VALUE`.
====


[[webflux-ann-requestmapping-produces]]
==== Producible Media Types
[.small]#<<web.adoc#mvc-ann-requestmapping-produces,Same in Spring MVC>>#

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

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@GetMapping(path = "/pets/{petId}", **produces = "application/json;charset=UTF-8"**)
	@ResponseBody
	public Pet getPet(@PathVariable String petId) {
		// ...
	}
----

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

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

[TIP]
====
`MediaType` provides constants for commonly used media types -- e.g.
`APPLICATION_JSON_VALUE`, `APPLICATION_JSON_UTF8_VALUE`.
====


[[webflux-ann-requestmapping-params-and-headers]]
==== Parameters and Headers
[.small]#<<web.adoc#mvc-ann-requestmapping-params-and-headers,Same in Spring MVC>>#

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

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@GetMapping(path = "/pets/{petId}", **params = "myParam=myValue"**)
	public void findPet(@PathVariable String petId) {
		// ...
	}
----

You can also use the same with request header conditions:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@GetMapping(path = "/pets", **headers = "myHeader=myValue"**)
	public void findPet(@PathVariable String petId) {
		// ...
	}
----


[[webflux-ann-requestmapping-head-options]]
==== HTTP HEAD, OPTIONS
[.small]#<<web.adoc#mvc-ann-requestmapping-head-options,Same in Spring MVC>>#

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

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

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

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



[[webflux-ann-methods]]
=== Handler methods
[.small]#<<web.adoc#mvc-ann-methods,Same in Spring MVC>>#

`@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,Same in Spring MVC>>#

The table below shows supported controller method arguments.

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

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

[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 forcing the start of a new session unless attributes
are added. Supports reactive types.

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

|`org.springframework.http.HttpMethod`
|The HTTP method of the request.

 |`java.util.Locale`
 |The current request locale, determined by the most specific `LocaleResolver` available, in
 effect, the configured `LocaleResolver`/`LocaleContextResolver`.

|Java 6+: `java.util.TimeZone` +
Java 8+: `java.time.ZoneId`
|The time zone associated with the current request, as determined by a `LocaleContextResolver`.

|`@PathVariable`
|For access to URI template variables.
// TODO: See <<webflux-ann-requestmapping-uri-templates>>.

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

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

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

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

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

|`java.util.Map`, `org.springframework.ui.Model`, `org.springframework.ui.ModelMap`
|For access and updates of the implicit model that is exposed to the web view.

|Command or form object (with optional `@ModelAttribute`)
|Command object whose properties to bind to request parameters -- via setters or directly to
fields, with customizable type conversion, depending on `@InitBinder` methods and/or the
HandlerAdapter configuration (see the `webBindingInitializer` property on
`RequestMappingHandlerAdapter`).

Command objects along with their validation results are exposed as model attributes, by
default using the command class name - e.g. model attribute "orderAddress" for a command
object of type "some.package.OrderAddress". `@ModelAttribute` can be used to customize the
model attribute name.

Supports reactive types.

|`Errors`, `BindingResult`
|Validation results for the command/form object data binding; this argument must be
declared immediately after the command/form object in the controller method signature.

|`SessionStatus`
|For marking form processing complete which triggers cleanup of session attributes
declared through a class-level `@SessionAttributes` annotation.

|`UriComponentsBuilder`
|For preparing a URL relative to the current request's host, port, scheme, context path, and
the literal  part of the servlet mapping also taking into account `Forwarded` and
`X-Forwarded-*` headers.

|`@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.

|`@RequestAttribute`
|For access to request attributes.
|===

[[webflux-ann-return-types]]
==== Return values
[.small]#<<web.adoc#mvc-ann-return-types,Same in Spring MVC>>#

The table below shows supported controller method return values. Reactive types --
Reactor, RxJava, <<webflux-reactive-libraries,or other>> are supported for all return
values.

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

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

|`HttpEntity<B>`, `ResponseEntity<B>`
|The return value specifies the full response including HTTP headers and body be encoded
through ``HttpMessageWriter``s and written to the response.
// TODO: See <<webflux-ann-httpentity>>.

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

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

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

|`java.util.Map`, `org.springframework.ui.Model`
|Attributes to be added to the implicit model with the view name implicitly determined
from the request path.

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

|`void`
|For use in method that don't write the response body; or methods where the view name is
supposed to be determined implicitly from the request path.

|`Flux<ServerSentEvent>`, `Observable<ServerSentEvent>`, or other reactive type
|Emit server-sent events; the `SeverSentEvent` wrapper can be omitted when only data needs
to be written (however `text/event-stream` must be requested or declared in the mapping
through the produces attribute).

|Any other return type
|A single model attribute to be added to the implicit model with the view name implicitly
determined through a `RequestToViewNameTranslator`; the attribute name may be specified
through a method-level `@ModelAttribute` or otherwise a name is selected based on the
class name of the return type.
|===



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



[[webflux-config]]
== WebFlux Java Config
[.small]#<<web.adoc#mvc-config,Same in Spring MVC>>#

The WebFlux Java config provides default configuration suitable for most applications along
with a configuration API to customize it. For more advanced customizations, not available in
the configuration API, see <<webflux-config-advanced-java>>.

You do not need to understand the underlying beans created by the Java config, but it's
easy to seem them in `WebFluxConfigurationSupport`, and if you want to learn more, see
<<webflux-special-bean-types>>.


[[webflux-config-enable]]
=== Enable the configuration
[.small]#<<web.adoc#mvc-config-enable,Same in Spring MVC>>#

Use the `@EnableWebFlux` annotation in your Java config:

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

The above registers a number of Spring WebFlux
<<mvc-webflux-special-bean-types,infrastructure beans>> also adapting to dependencies
available on the classpath -- for JSON, XML, etc.


[[webflux-config-customize]]
=== Configuration API
[.small]#<<web.adoc#mvc-config-customize,Same in Spring MVC>>#

In your Java config implement the `WebFluxConfigurer` interface:

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

		// Implement configuration methods...

	}
----



[[webflux-config-conversion]]
=== Conversion, formatting
[.small]#<<web.adoc#mvc-config-conversion,Same in Spring MVC>>#

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

To register custom formatters and converters:

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

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

	}
----

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


[[webflux-config-validation]]
=== Validation
[.small]#<<web.adoc#mvc-config-validation,Same in Spring MVC>>#

By default if <<core.adoc#validation-beanvalidation-overview,Bean Validation>> is present
on the classpath -- e.g. 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 config, you can customize the global `Validator` instance:

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

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

	}
----

Note that you can also register ``Validator``'s locally:

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

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

	}
----

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


[[webflux-config-content-negotiation]]
=== Content type resolvers
[.small]#<<web.adoc#mvc-config-content-negotiation,Same in Spring MVC>>#

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

To customize the requested content type resolution:

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

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

[[webflux-config-message-codecs]]
=== HTTP message codecs
[.small]#<<web.adoc#mvc-config-message-converters,Same in Spring MVC>>#

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

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

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

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

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

. http://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.
. http://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-jdk7[jackson-datatype-jdk7]: support for Java 7 types like `java.nio.file.Path`.
. 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 & Time API types.
. https://github.com/FasterXML/jackson-datatype-jdk8[jackson-datatype-jdk8]: support for other Java 8 types like `Optional`.


[[webflux-config-view-resolvers]]
=== View resolvers
[.small]#<<web.adoc#mvc-config-view-resolvers,Same in Spring MVC>>#

To configure view resolution:

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

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

Note that FreeMarker also requires configuration of the underlying view technology:

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

		// ...

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

	}
----


[[webflux-config-static-resources]]
=== Static resources
[.small]#<<web.adoc#mvc-config-static-resources,Same in Spring MVC>>#

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

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

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

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

	}
----

// TODO:
// 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]'s and
{api-spring-framework}/web/reactive/resource/ResourceTransformer.html[ResourceTransformer]'s.
which can be used to create a toolchain for working with optimized resources.

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

For example in your Java config;

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

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

	}
----

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

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

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


[[webflux-config-path-matching]]
=== Path Matching
[.small]#<<web.adoc#mvc-config-path-matching,Same in Spring MVC>>#

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

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

To customize those options:

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

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

	}
----


[[webflux-config-advanced-java]]
=== Advanced config mode
[.small]#<<web.adoc#mvc-config-advanced-java,Same in Spring MVC>>#

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

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

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

		// ...

	}
----

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


