The state of Kotlin support in Spring Sébastien Deleuze October 19, 2019

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3  Less flights

Why Kotlin?

5  Improve signal to noise ratio

6  Improve the signal to noise ratio of the code Safety

7  Improve the signal to noise ratio of the code Discoverability

8  Pleasure

9  Android

Spring Kotlin

How much?

Framework documentation in Kotlin! 12 

13  Status Reference documentation in Kotlin Spring Boot 2.2 Next year estimate Spring Framework Spring Boot Spring Data Spring Security

Gradle Kotlin DSL on start.spring.io 14 

More DSLs

16  mockMvc.request(HttpMethod.GET, "/person/{name}", "Lee") { secure = true accept = APPLICATION_JSON headers { contentLanguage = Locale.FRANCE } principal = Principal { "foo" } }.andExpect { status { isOk } content { contentType(APPLICATION_JSON) } jsonPath("$.name") { value("Lee") } content { json("""{"someBoolean": false}""", false) } }.andDo { print() } MockMvc DSL by Clint Checketts and JB Nizet

17 contract { request { url = url("/foo") method = PUT headers { header("foo", "bar") } body = body("foo" to "bar") } response { status = OK } } Spring Cloud Contract DSL by Tim Ysewyn

18  http { formLogin { loginPage = "/log-in" } authorizeRequests { authorize("/css/**", permitAll) authorize("/user/**", hasAuthority("ROLE_USER")) } } Spring Security DSL by Eleftheria Stein https://github.com/spring-projects-experimental/spring-security-kotlin-dsl

Spring MVC DSL and functional API

20  fun hello(request: ServerRequest): ServerResponse Handler API

21  fun hello(request: ServerRequest) = ServerResponse.ok().body("Hello world!") Handler implementation

22  router { GET("/hello", ::hello) } fun hello(request: ServerRequest) = ServerResponse.ok().body("Hello world!") Router

23  @Configuration class RoutesConfiguration { @Bean fun routes(): RouterFunction = router { GET("/hello", ::hello) } fun hello(request: ServerRequest) = ServerResponse.ok().body("Hello world!") } Expose the router to Spring Boot

24  @Component class PersonHandler(private val repository: PersonRepository) { fun listPeople(request: ServerRequest): ServerResponse { // ... } fun createPerson(request: ServerRequest): ServerResponse { // ... } fun getPerson(request: ServerRequest): ServerResponse { // ... } } A more realistic handler

25  @Configuration class RouteConfiguration { @Bean fun routes(handler: PersonHandler) = router { accept(APPLICATION_JSON).nest { GET("/person/{id}", handler::getPerson) GET("/person", handler::listPeople) } POST("/person", handler::createPerson) } } A more realistic router

26  @Configuration class RouteConfiguration { @Bean fun routes(routeRepository: RouteRepository) = router { for (route in routeRepository.listRoutes()) { GET("/$route") { request -> hello(request, route) } } } fun hello(request: ServerRequest, message: String) = ServerResponse.ok().body("Hello $message!") } You can even create routes dynamically

27  Status Kotlin DSLs Spring Boot 2.1 Spring Boot 2.2 Next year estimate Beans DSL WebFlux router DSL WebFlux Coroutines router DSL WebMvc router DSL MockMvc DSL Spring Security DSL

Coroutines

Coroutines are lightweight threads (you can create 100.000s of them) 29

The foundations are in Kotlin language 30

But most of the implementation lives on library side in kotlinx.coroutines 31

Coroutines allows to consume Spring Reactive stack with a nice balance between imperative and declarative style 32

Operations are sequential by default 33

Concurrency is explicit 34

3 main building blocks you need to know 35

Suspending function 36

37  suspend fun hello() { delay(1000) println("Hello world!") } Suspending functions

Note: suspending functions color your API* 38 * As explained by Bob Nystrom in its great blog post What Color is Your Function?

Structured concurrency 39

40  suspend fun loadAndCombine(name1: String, name2: String) = coroutineScope { val deferred1: Deferred = async { loadImage(name1) } val deferred2: Deferred = async { loadImage(name2) } combineImages(deferred1.await(), deferred2.await()) } Structured concurrency

Flow* 41 * kotlinx.coroutines.flow.Flow not java.util.concurrent.Flow

Coroutines 1.3 introduces a new Reactive type: Flow 42

Flow is the equivalent of Flux in Coroutines world 43

Flow is interoperable with Reactive Streams and supports backpressure 44

45  interface Flow { suspend fun collect(collector: FlowCollector) } interface FlowCollector { suspend fun emit(value: T) } Flow API

46  fun Flow.filter(predicate: suspend (T) -> Boolean): Flow = transform { value -> if (predicate(value)) return@transform emit(value) } Operators as extensions easy to implement

47  val flow = flow { for (i in 1..3) { delay(100) emit(i) } } Create a Flow

48  flow.filter { it < 2 }.map(::asyncOperation).collect() Consume a Flow

What about Spring support for Coroutines?

Spring provides official Coroutines support for Spring WebFlux, Data*, Vault, RSocket 50 * Redis, MongoDB, Cassandra, R2DBC

No new types introduced 51

Seamless support with the annotation programming model 52

53  @GetMapping("/api/banner") suspend fun suspendingEndpoint(): Banner { delay(10) return Banner("title", "Lorem ipsum") } WebFlux suspending handler method

54  @GetMapping("/banner") suspend fun render(model: Model): String { delay(10) model["banner"] = Banner("title", "Lorem ipsum") return "index" } WebFlux suspending view rendering

Reactive types extensions for Coroutines APIs 55

56  @GetMapping("/banners") suspend fun flow(): Flow = client.get() .uri("/messages") .accept(MediaType.TEXT_EVENT_STREAM) .retrieve() .bodyToFlow() .map { Banner("title", it) } WebClient

57  coRouter { GET("/hello", ::hello) } suspend fun hello(request: ServerRequest) = ServerResponse.ok().bodyValueAndAwait("Hello world!") WebFlux Router

58  private val requester: RSocketRequester = ... @MessageMapping("locate.radars.within") fun findRadars(request: MapRequest): Flow = requester .route("locate.radars.within") .data(request.viewBox) .retrieveFlow() .take(request.maxRadars) RSocket

59  class PersonRepository(private val client: DatabaseClient, private val operator: TransactionalOperator) { suspend fun initDatabase() = operator.executeAndAwait { save(User("smaldini", "Stéphane", "Maldini")) save(User("sdeleuze", "Sébastien", "Deleuze")) save(User("bclozel", "Brian", "Clozel")) } suspend fun save(user: User) { client.insert().into().table("users").using(user).await() } } Spring Data R2DBC with transactions

60  class UserRepository(private val mongo: ReactiveFluentMongoOperations) { fun findAll(): Flow = mongo.query().flow() suspend fun findOne(id: String): User = mongo.query() .matching(query(where("id").isEqualTo(id))).awaitOne() suspend fun insert(user: User): User = mongo.insert().oneAndAwait(user) suspend fun update(user: User): User = mongo.update().replaceWith(user) .asType().findReplaceAndAwait() } Spring Data MongoDB

61  Coroutines are now the default way to go Reactive in Kotlin Coroutines dependency added by default on start.spring.io WebFlux & RSocket Kotlin code samples provided with Coroutines API

62  Status Coroutines support Spring Boot 2.2 Next year estimate Spring WebFlux functional APIs Spring WebFlux WebClient Spring WebFlux @RequestMapping RSocket @MessageMapping Spring Data Reactive APIs Functional transactions Spring MVC @RequestMapping Spring Data repositories (DATACMNS-1508) @Transactional

https://github.com/sdeleuze/spring-boot-coroutines-demo 63

Spring Boot

65  @ConfigurationProperties("blog") class BlogProperties { lateinit var title: String val banner = Banner() class Banner { var title: String? = null lateinit var content: String } } @ConfigurationProperties

66  @ConstructorBinding @ConfigurationProperties("blog") data class BlogProperties(val title: String, val banner: Banner) { data class Banner(val title: String?, val content: String) } @ConfigurationProperties + @ConstructorBinding

67  start.spring.io helps to share Kotlin projects

Kofu: the mother of all DSLs

application { } 69

Explicit configuration for Spring Boot using Kotlin DSLs 70

Experimental 71

Leverage existing DSLs : beans, router, security 7 2

73  val app = application(WebApplicationType.SERVLET) { beans { bean() bean() } webMvc { port = if (profiles.contains("test")) 8181 else 8080 router { val handler = ref() GET("/", handler::hello) GET("/api", handler::json) } converters { string() jackson() } } } Spring Boot configured with Kofu DSL

Composable configurations 74

Discoverability via auto-complete instead of conventions 75

Faster startup, less memory consumption 76

77  Faster startup

Demo

https://github.com/spring-projects-experimental/spring-fu 79

My hope is to make Kofu the Spring Boot Kotlin DSL 80

GraalVM Native

https://github.com/spring-projects-experimental/spring-graal-native Kotlin with Spring MVC or Spring WebFlux works on GraalVM native Coroutines support is broken due to graal#366 82

Thanks Twitter: @sdeleuze Credits to ibrandify and flaticon for some icons used