Spring WebFlux

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.

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.

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 Mono 와 Flux 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.

`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.

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.

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.

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 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.

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:

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

Reactor Netty:

Undertow:

Tomcat:

Jetty:

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:

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:

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.

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.

Same in Spring MVC

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.

Same in Spring MVC

The DispatcherHandler processes requests as follows:

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:

@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).

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:

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.

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.

Same in Spring MVC

Use the @EnableWebFlux annotation in your Java config:

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

Same in Spring MVC

In your Java config implement the WebFluxConfigurer interface:

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:

Same in Spring MVC

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:

Note that you can also register Validator's locally:

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:

Same in Spring MVC

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

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:

Same in Spring MVC

To configure view resolution:

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

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.

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;

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".

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:

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:

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.