The state of Kotlin support in Spring Sbastien Deleuze @sdeleuze - PowerPoint PPT Presentation

The state of Kotlin support in Spring Sébastien Deleuze @sdeleuze Copenhagen Denmark

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Less flights 3 

Slower but more surprises ;-) 4 

Why Kotlin?

Improve signal to noise ratio 6 

Safety Improve the signal to noise ratio of the code 7 

Discoverability Improve the signal to noise ratio of the code 8 

Pleasure 9 

Android 1 0 

Spring Kotlin

How much?

Framework documentation in Kotlin! 13 

Status Reference documentation in Kotlin Spring Boot 2.2 Next year estimate Spring Framework Spring Boot Spring Data Spring Security 14 

Gradle Kotlin DSL on start.spring.io 15 

More DSLs

MockMvc DSL by Clint Checketts and JB Nizet 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() } 17 

Spring Cloud Contract DSL by Tim Ysewyn contract { request { url = url( "/foo" ) method = PUT headers { header( "foo" , "bar" ) } body = body( "foo" to "bar" ) } response { status = OK } } 18 

Spring Security DSL by Eleftheria Stein https://github.com/spring-projects-experimental/spring-security-kotlin-dsl http { formLogin { loginPage = "/log-in" } authorizeRequests { authorize( "/css/**" , permitAll ) authorize( "/user/**" , hasAuthority( "ROLE_USER" )) } } 19 

Spring MVC DSL and functional API

Handler API fun hello(request: ServerRequest): ServerResponse 21 

Handler implementation fun hello(request: ServerRequest) = ServerResponse.ok().body( "Hello world!" ) 22 

Router router { GET( "/hello" , ::hello) } fun hello(request: ServerRequest) = ServerResponse.ok().body( "Hello world!" ) 23 

Expose the router to Spring Boot @Configuration class RoutesConfiguration { @Bean fun routes(): RouterFunction<ServerResponse> = router { GET( "/hello" , ::hello) } fun hello(request: ServerRequest) = ServerResponse.ok().body( "Hello world!" ) } 24 

A more realistic handler @Component class PersonHandler( private val repository : PersonRepository) { fun listPeople(request: ServerRequest): ServerResponse { // ... } fun createPerson(request: ServerRequest): ServerResponse { // ... } fun getPerson(request: ServerRequest): ServerResponse { // ... } } 25 

A more realistic router @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) } } 26 

You can even create routes dynamically @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 !" ) } 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 28 

Coroutines

Coroutines are lightweight threads (you can create 100.000s of them) 30

The foundations are in Kotlin language 31

But most of the implementation lives on library side in kotlinx.coroutines 32

Coroutines allows to consume Spring Reactive stack with a nice balance between imperative and declarative style 33

Operations are sequential by default 34

Concurrency is explicit 35

3 main building blocks you need to know 36

Suspending function 37

Suspending functions suspend fun hello() { delay (1000) println ( "Hello world!" ) } 38 

Note: suspending functions color your API* * As explained by Bob Nystrom in its great blog post What Color is Your Function? 39

Structured concurrency 40

Structured concurrency suspend fun loadAndCombine(name1: String, name2: String) = coroutineScope { val deferred1: Deferred<Image> = async { loadImage(name1) } val deferred2: Deferred<Image> = async { loadImage(name2) } combineImages(deferred1.await(), deferred2.await()) } 41 

Flow<T> * * kotlinx.coroutines.flow.Flow not java.util.concurrent.Flow 42

Coroutines 1.3 introduces a new Reactive type: Flow<T> 43

Flow is the equivalent of Flux in Coroutines world 44

Flow is interoperable with Reactive Streams and supports backpressure 45

Flow API interface Flow<T> { suspend fun collect(collector: FlowCollector<T>) } interface FlowCollector<T> { suspend fun emit(value: T) } 46 

Operators as extensions easy to implement fun <T> Flow<T>.filter(predicate: suspend (T) -> Boolean): Flow<T> = transform { value -> if (predicate(value)) return @transform emit(value) } 47 

Create a Flow val flow = flow { for (i in 1..3) { delay (100) emit(i) } } 48 

Consume a Flow flow. filter { it < 2 } . map (::asyncOperation). collect () 49 

What about Spring support for Coroutines?

Spring provides official Coroutines support for Spring WebFlux, Data*, Vault, RSocket * Redis, MongoDB, Cassandra, R2DBC 51

No new types introduced 52

Seamless support with the annotation programming model 53

WebFlux suspending handler method @GetMapping( "/api/banner" ) suspend fun suspendingEndpoint(): Banner { delay (10) return Banner( "title" , "Lorem ipsum" ) } 54 

WebFlux suspending view rendering @GetMapping( "/banner" ) suspend fun render(model: Model): String { delay (10) model[ "banner" ] = Banner( "title" , "Lorem ipsum" ) return "index" } 55 

Reactive types extensions for Coroutines APIs 56

WebClient @GetMapping( "/banners" ) suspend fun flow(): Flow<Banner> = client .get() .uri( "/messages" ) .accept(MediaType. TEXT_EVENT_STREAM ) .retrieve() . bodyToFlow <String>() . map { Banner( "title" , it ) } 57 

WebFlux Router coRouter { GET( "/hello" , ::hello) } suspend fun hello(request: ServerRequest) = ServerResponse.ok(). bodyValueAndAwait ( "Hello world!" ) 58 

RSocket private val requester : RSocketRequester = ... @MessageMapping( "locate.radars.within" ) fun findRadars(request: MapRequest): Flow<Radar> = requester .route( "locate.radars.within" ) .data(request. viewBox ) . retrieveFlow <AirportLocation>() . take (request. maxRadars ) 59 

Spring Data R2DBC with transactions 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 <User>().table( "users" ).using(user). await () } } 60