Introduction

스프링 프레임워크, 스프링 Web MVC에 있는 웹프레임워크는 Servlet API와 Servlet 컨테이너를 만들기위한 용도였습니다. 반응형스택, 웹프레임워크, 스프링 웹플럭스는 5.0 버전 이후에 추가되었습니다. 완전한 논블럭킹, Reactive Streams 지원, 역압(back pressure) 그리고 Netty, Undertow, Servlet 3.1 이상 서버에서 동작합니다.

두 웹프레임워크들 spring-webmvc, spring-webflux 는 스프링 프레임워크에서 나란히 공존하며 모듈들은 옵션입니다. 어플리케이션은 하나 또는 둘 다 사용할 수 있습니다. e.g 스프링 MVC 컨트롤러와 리액티브 웹클라이언트

새로운 웹프레임워크가 왜 필요할까?

첫째, 적은 스레드 갯수로 비동기를 다루고 논블럭킹 웹스택과 적은 하드웨어 자원으로 확장이 필요하다는 것이다.서블렛 3.1은 논블럭킹 I/O API를 지원했다. 하지만, 동기(Filter, Servlet), 블럭킹(getParameter, getPart)은 Servlet API로 부터 멀어지게합니다. using it leads away from the rest of the Servlet API where contracts are synchronous (Filter, Servlet) or blocking (getParameter,getPart). 이것이 모든 논블럭킹 런타임에 대해 기반 역할을하는 새로운 공통 API를 설계하게된 동기였습니다. 공통 API는 네티처럼 비동기, 논블럭 기반의 잘만들어진 서버들 때문에 중요합니다.

둘째, 함수형 프로그래밍입니다. Java 5때 어노테이션 추가처럼 Java 8에서도 기회가 생겼습니다. — e.g. 어노테이션이 붙은 REST 컨트롤러나 단위 테스트. 자바8에 람다 표현식이 추가됨은 자바에서 함수형 API를 이용할 기회가 생겼습니다. 이건 논블럭킹 어플리케이션과 continuation 스타일 API에 도움이 됩니다. — `CompletableFuture`나 ReactiveX는 비동기 로직의 선언적 프로그래밍을 지원합니다. 자바8은 스프링 웹플럭스가 어노테이션이 붙은 컨트롤러와 함수형 웹 엔드포인트 두가지를 제공할 수 있게하였습니다.

리액티브: 무엇이고 왜 쓰는거야?

왜 리액티브이며 우리에게 어떤 의미일까?

"리액티브"란 용어는 변화에 반응하는 프로그램 모델입니다. — 네트워크 컨퍼넌트는 I/O 이벤트에 반응하고, UI 컨트롤러는 마우스 이벤트에 반응하고 기타. 그런 의미에서 논블럭킹은 완료 명령(시그널)이나 데이터가 사용할 수 있을 때 반응합니다. 논블럭킹 역압 또한 중요한 메카니즘입니다. 동기, 명령형 코드, 블럭킹 호출에서 클라이언트를 기다리게 강제했습니다.(역주 - request가 처리량 이상 오는경우 응답이 급격히 느려지는 것을 의미하는것으로 보임.) 하지만 논블럭킹 코드는 처리하는 놈의 처리량을 넘치지 않게 이벤트 속도를 조절하는 것이 중요합니다.

reactive streams(역주 - reactive streams 리액티브 라이브러리 표준 인터페이스)는 small spec 와 자바 9의 adopted 이며 이들은 역압과 비동기 구성요소들간 상호작용을 정의합니다. 예를들어 데이터 저장소 — Publisher는 HTTP 서버처럼 데이터를 생성할 수 있습니다. Subscriber는 응답을 쓸수 있습니다. 리액티브 스트림의 주요 목적은 구독자가 퍼블리셔로부터 발생하는 데이터를 조절하는데 있습니다.

리액티브 API

reactive streams는 인터페이스로 중요한 역할을 합니다만 reactive stremas는 너무 로우레벨이라 어플리케이션 API에 유용하지 않습니다. 어플리케이션이 원하는건 비동기 로직을 구성하는데 필요로하는 더 추상화되고 더 풍부한 표현력을 가진 (컬렉션만 적용된 자바 8 Stream API와 유사한) 함수형 API입니다. 이게 리액티브 라이브러리가 수행하는 역할입니다.

Reactor은 Spring WebFlux를 위해 선택한 리액티브 라이브러리입니다. 이건 MonoFlux API 타입들은 ReactiveX의 vocabulary of operators와 비슷한 풍부한 오퍼레잍터들을 통해 0..1(Mono)과 0..N(Flux)의 데이터 시퀀스에서 작업할 수 있습니다. 리액터는 리액티브 스트림스 라이브러리이며 오퍼레이터는 논블록킹 역압을 지원합니다. 리액터는 서버사이드쪽의 자바에 특화되어있습니다. 스프링과 함께 가깝게 협업하며 개발되고 있습니다.

웹플럭스는 핵심 디펜던시로 리액터를 필요로합니다만 리액티브 스트림스를 통해 다른 리액티브 라이브러리랑 호환됩니다.(역 - Reactive streams는 interface, reactor와 rxjava는 구현체) 일반적으로 웹플럭스 API들은 `Publisher`를 입력으로 받고 내부적으로 Reactor 타입으로 적용시킨다. 그리고 `Flux`나 `Mono`를 결과로 반환합니다. 그럼 당신은 `Publisher`를 입력값으로 전달하고 결과값에 오퍼레이션들을 적용킬수 있다. — e.g. 어노테이션이 붙은 컨트롤러, 웹플럭스는 RxJava나 다른 리액티브 라이브러리를 적용할 수 있습니다. [웹플럭스-리액티브-라이브러리]에 더 상세하게 있습니다.

프로그래밍 모델

`spring-web`모듈은 스프링 웹플럭스의 기초가 되는 리액티브 기반을 포함합니다. — HTTP 추상화, 리액티브 스트림 서버 어댑터, 리액티브 코덱, 핵심 Web API

스프링 웹플럭스는 두가지 프로그래밍 모델 선택할 수 있게 제공합니다.

  • [웹플럭스 컨트롤러] — 스프링 MVC와 일치. spring-web 모듈과 동일한 어노테이션 기반 컨트롤러. 스프링 MVC와 WebFlux 컨트롤러는 반환 타입으로 리액티브(Reactor, RxJava)를 지원합니다. 그래서 스프링 MVC와 WebFlux를 구별하기란 쉽지 않습니다.

  • [웹플럭스-fn] — 가볍고, 람다 기반의 함수형 프로그래밍 모델입니다. 라우팅과 리퀘스트 처리를 할수 있는 작은 라이브러리 또는 유틸리티의 셋으로 생각하세요.

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, 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 blog post is a good resource.

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.

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.

Reactive Spring Web

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

Server support is organized in two layers:

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

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

HttpHandler

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

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

Server name Server API used Reactive Streams support

Netty

Netty API

Reactor Netty

Undertow

Undertow API

spring-web: Undertow to Reactive Streams bridge

Tomcat

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

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

Jetty

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

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

Servlet 3.1 container

Servlet 3.1 non-blocking I/O

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

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

Server name Group id Artifact name

Reactor Netty

io.projectreactor.ipc

reactor-netty

Undertow

io.undertow

undertow-core

Tomcat

org.apache.tomcat.embed

tomcat-embed-core

Jetty

org.eclipse.jetty

jetty-server, jetty-servlet

Reactor Netty:

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

Undertow:

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

Tomcat:

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

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

Jetty:

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

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

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

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

WebHandler API

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 server adapter. To use the builder either add components individually or point to an ApplicationContext to have the following detected:

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

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.

DispatcherHandler

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

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

Spring configuration in a WebFlux application typically contains:

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

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

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

Special bean types

The DispatcherHandler delegates to special beans to process requests and render the appropriate responses. By "special beans" we mean Spring-managed Object instances that implement 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.

Bean type Explanation

HandlerMapping

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

The main HandlerMapping implementations are RequestMappingHandlerMapping based on @RequestMapping annotated methods, RouterFunctionMapping based on functional endpoint routes, and SimpleUrlHandlerMapping based on explicit registrations of URI path patterns to handlers.

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.

The built-in HandlerResultHandler implementations are ResponseEntityResultHandler supporting ResponseEntity return values, ResponseBodyResultHandler supporting @ResponseBody methods, ServerResponseResultHandler supporting the ServerResponse returned from functional endpoints, and ViewResolutionResultHandler supporting rendering with a view and a model.

Framework Config

The DispatcherHandler detects the special beans it needs in the ApplicationContext. Applications can declare the special beans they wish to have. However most applications will find a better starting point in the WebFlux Java config which provide a higher level configuration API that in turn make the necessary bean declarations. See WebFlux Java Config for more details.

Processing

The DispatcherHandler processes requests as follows:

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

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

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

Annotated Controllers

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

Here is a basic example:

@RestController
public class HelloController {

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

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

@Controller

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

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

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

	// ...
}

@RestController is a composed annotation that is itself 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).

Request Mapping

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

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

  • @GetMapping

  • @PostMapping

  • @PutMapping

  • @DeleteMapping

  • @PatchMapping

The shortcut variants are 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.

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

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

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

URI Patterns

You can map requests using glob patterns and wildcards:

  • ? matches one character

  • * matches zero or more characters within a path segment

  • ** match zero or more path segments

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

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

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

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

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

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

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

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

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

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

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

Note

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

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

Pattern Comparison

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

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

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

Consumable Media Types

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

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

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

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

Producible Media Types

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

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

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

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

Parameters and Headers

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

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

You can also use the same with request header conditions:

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

HTTP HEAD, OPTIONS

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

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

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

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

Handler methods

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

Method arguments

The table below shows supported controller method arguments.

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

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

Controller method argument Description

ServerWebExchange

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

ServerHttpRequest, ServerHttpResponse

Access to the HTTP request or response.

WebSession

Access to the session; this does not 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.

@MatrixVariable

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

@RequestParam

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

@RequestHeader

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

@RequestBody

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

HttpEntity<B>

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

@RequestPart

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

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.

Return values

The table below shows supported controller method return values. Reactive types — Reactor, RxJava, or other are supported for all return values.

Controller method return value Description

@ResponseBody

The return value is encoded through HttpMessageWriters and written to the response.

HttpEntity<B>, ResponseEntity<B>

The return value specifies the full response including HTTP headers and body be encoded through HttpMessageWriters and written to the response.

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

A method with a void, possibly async (e.g. Mono<Void>), return type (or a null return value) is considered to have fully handled the response if it also has a ServerHttpResponse, or a ServerWebExchange argument, or an @ResponseStatus annotation. The same is true also if the controller has made a positive ETag or lastModified timestamp check.

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

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

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

Any other return 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.

Unresolved directive in <stdin> - include::webflux-functional.adoc[leveloffset=+1]

WebFlux Java Config

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 Advanced config mode.

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 Special bean types.

Enable WebFlux config

Use the @EnableWebFlux annotation in your Java config:

@Configuration
@EnableWebFlux
public class WebConfig {
}

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

WebFlux config API

In your Java config implement the WebFluxConfigurer interface:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	// Implement configuration methods...

}

Conversion, formatting

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

To register custom formatters and converters:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

}
Note

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

Validation

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

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

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

}

Note that you can also register Validator's locally:

@Controller
public class MyController {

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

}
Tip

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

Content type resolvers

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

To customize the requested content type resolution:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

HTTP message codecs

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

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

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

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

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

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

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

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

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

View resolvers

To configure view resolution:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

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

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	// ...

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

}

Static resources

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

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

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

}

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

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

For example in your Java config;

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

}

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

Unlike Spring MVC at present in WebFlux there is no way to 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.

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

Path Matching

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

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

To customize those options:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

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

}

Advanced config mode

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

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

@Configuration
public class WebConfig extends DelegatingWebFluxConfiguration {

	// ...

}

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

HTTP/2

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

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