Slides of my Devoxx 2017 & SpringOne Platform talk, video available at https://www.youtube.com/watch?v=kbMXAjWEft0. Let's discovers Spring official Kotlin support with a journey from Spring Boot 1 with Java to Spring Boot 2 with Kotlin.
13 Expressive test function names with backticks class EmojTests { @Test fun `Why Spring ❤ Kotlin?`() { println("Because I can use emoj in function names \uD83D\uDE09") } } > Because I can use emoj in function names
14 @RestController public class UserController { private final UserRepository userRepository; public UserController(UserRepository userRepository) { this.userRepository = userRepository; } @GetMapping("/user/{login}") public User findOne(@PathVariable String login) { return userRepository.findOne(login); } @GetMapping("/user") public Iterable findAll() { return userRepository.findAll(); } @PostMapping("/user") public User save(@RequestBody User user) { return userRepository.save(user); } } Spring MVC controller written in Java
15 @RestController class UserController(private val repo: UserRepository) { @GetMapping("/user/{id}") fun findOne(@PathVariable id: String) = repo.findOne(id) @GetMapping("/user") fun findAll() = repo.findAll() @PostMapping("/user") fun save(@RequestBody user: User) = repo.save(user) } Spring MVC controller written in Kotlin
16 Inferred type hints in IDEA Settings Editor General Appearance Show parameter name hints Select Kotlin Check “Show function/property/local value return type hints”
kotlin-spring compiler plugin Automatically open Spring annotated classes and methods @SpringBootApplication open class Application { @Bean open fun foo() = Foo() @Bean open fun bar(foo: Foo) = Bar(foo) } @SpringBootApplication class Application { @Bean fun foo() = Foo() @Bean fun bar(foo: Foo) = Bar(foo) } Without kotlin-spring plugin With kotlin-spring plugin 17
Running Spring Boot 1 application with Kotlin 22 @SpringBootApplication class Application fun main(args: Array) { SpringApplication.run(Application::class.java, *args) }
Reified type parameters Kotlin extension inline fun RestOperations.exchange(url: String, method: HttpMethod, requestEntity: HttpEntity<*>? = null) = exchange(url, method, requestEntity, object : ParameterizedTypeReference() {}) List list = restTemplate .exchange("/api/article/", GET, null, new ParameterizedTypeReference>(){}) .getBody(); val list: List = restTemplate .exchange("/api/article/", GET) .getBody(); Goodbye type erasure, we are not going to miss you at all! 27
Null safety 28 Nothing enforced at type system No check at compile time NullPointerException at runtime Optional only usable for return values Default is non-null, ? suffix for nullable types Full check at compile time No NullPointerException !!! Functional constructs on nullable values
31 @Controller // Mandatory Optional class FooController(val foo: Foo, val bar: Bar?) { @GetMapping("/") // Equivalent to @RequestParam(required=false) fun foo(@RequestParam baz: String?) = ... } Leveraging Kotlin nullable information To determine @RequestParam or @Autowired required attribute
@ConfigurationProperties @ConfigurationProperties("foo") data class FooProperties( var baseUri: String? = null, var admin = Credentials()) { data class Credential( var username: String? = null, var password: String? = null) } One of the remaining pain points in Kotlin
34 JUnit 5 supports constructor based injection @ExtendWith(SpringExtension::class) @SpringBootTest(webEnvironment = RANDOM_PORT) class HtmlTests(@Autowired val restTemplate: TestRestTemplate) { @Test fun test1() { // ... } @Test fun test2() { // ... } } Allows to use val instead of lateinit var in tests
35 class SimpleTests { @Nested @DisplayName("a calculator") inner class Calculator { val calculator = SampleCalculator() @Test fun `should return the result of adding the first number to the second number`() { val sum = calculator.sum(2, 4) assertEquals(6, sum) } @Test fun `should return the result of subtracting the second number from the first number`() { val subtract = calculator.subtract(4, 2) assertEquals(2, subtract) } } } Specification-like tests with Kotlin and JUnit 5
44 @RestController class ReactiveUserController(val repository: ReactiveUserRepository) { @GetMapping("/user/{id}") fun findOne(@PathVariable id: String): Mono = repository.findOne(id) @GetMapping("/user") fun findAll(): Flux = repository.findAll() @PostMapping("/user") fun save(@RequestBody user: Mono): Mono = repository.save(user) } interface ReactiveUserRepository { fun findOne(id: String): Mono fun findAll(): Flux fun save(user: Mono): Mono } Spring WebFlux with annotations Spring Data Kay provides Reactive support for MongoDB, Redis, Cassandra and Couchbase
@Component class HtmlHandler(private val userRepository: UserRepository, private val converter: MarkdownConverter) { fun blog(req: ServerRequest) = ok().render("blog", mapOf( "title" to "Blog", "articles" to articleRepository.findAll() .flatMap { it.toDto(userRepository, converter) } )) } @Component class ArticleHandler(private val articleRepository: ArticleRepository, private val articleEventRepository: ArticleEventRepository) { fun findAll(req: ServerRequest) = ok().body(articleRepository.findAll()) fun notifications(req: ServerRequest) = ok().bodyToServerSentEvents(articleEventRepository.findWithTailableCursorBy()) } 50 Functional handlers within Boot
Immutable non-nullable @ConfigurationProperties @ConfigurationProperties("foo") data class FooProperties( val baseUri: String, val admin: Credential) { data class Credential( val username: String, val password: String) } Boot 2.1 ? See issue #8762 for more details @ConfigurationProperties("foo") data class FooProperties( lateinit var baseUri: String, var admin = Credentials()) { data class Credential( lateinit var username: String, lateinit var password: String) } Currently N ot yet available
WebTestClient Unusable in Kotlin due to a type inference issue, see KT-5464 otlin 1.3 For now, please use: - WebClient in Kotlin - WebTestClient in Java
Spring & Kotlin Coroutines 56 ● Coroutines are Kotlin lightweight threads ● Allows non-blocking imperative code ● Less powerful than Reactive API (backpressure, streaming, operators, etc.) ● kotlinx.coroutines: Reactive Streams and Reactor support ● spring-kotlin-coroutine: experimental Spring support (community driven) ● Warning ○ Coroutine are still experimental ○ No official Spring support yet, see SPR-15413 ○ Ongoing evaluation of performances and back-pressure interoperability Experim ental https://github.com/konrad-kaminski/spring-kotlin-coroutine
Reactive Coroutines interop 57 Operation Reactive Coroutines Async value fun foo(): Mono suspend fun foo(): T? Async collection fun foo(): Mono> suspend fun foo(): List Stream fun foo(): Flux suspend fun foo(): ReceiveChannel Async completion fun foo(): Mono suspend fun foo()
58 @RestController class CoroutineUserController(val repository: CoroutineUserRepository) { @GetMapping("/user/{id}") suspend fun findOne(@PathVariable id: String): User = repository.findOne(id) @GetMapping("/user") suspend fun findAll(): List = repository.findAll() @PostMapping("/user") suspend fun save(@RequestBody user: User) = repository.save(user) } interface CoroutineUserRepository { suspend fun findOne(id: String): User suspend fun findAll(): List suspend fun save(user: User) } Spring WebFlux with Coroutines Experim ental https://github.com/sdeleuze/spring-kotlin-deepdive/tree/step3-coroutine
Spring Framework 5 introduces Functional bean definition Very efficient, no reflection, no CGLIB proxy Lambdas instead of annotations Both declarative and programmatic
Functional bean definition with Spring Boot 64 @SpringBootApplication class Application fun main(args: Array) { runApplication(*args) { addInitializers(databaseContext, webContext) } } This nice syntax currently works only for running the application, not tests
Functional bean definition with Spring Boot 65 class ContextInitializer : ApplicationContextInitializer { override fun initialize(context: GenericApplicationContext) { databaseContext.initialize(context) webContext.initialize(context) } } context.initializer.classes=io.spring.deepdive.ContextInitializer This more verbose one works for running both application and tests
71 Kotlin to Javascript data class Article(val slug: String, val title: String) fun main(args: Array) { if (Notification.permission == NotificationPermission.GRANTED) { Notification.requestPermission().then { console.log(it) } } EventSource("/api/article/notifications").addEventListener("message", { val article = JSON.parse(it.data()); Notification(article.title).addEventListener("click", { window.location.href = "/${article.slug}" }) }) } fun Event.data() = (this as MessageEvent).data as String // See KT-20743 Type safety, null safety, only 10 Kbytes with Dead Code Elimination tool
72 Kotlin for frontend tomorrow with WebAssembly Read “An Abridged Cartoon Introduction To WebAssembly” by Lin Clark for more details https://goo.gl/I0kQsC A sandboxed native platform for the Web (W3C, no plugin involved)
73 Compiling Kotlin to WebAssembly + ➔ Kotlin supports WebAssembly via Kotlin/Native ➔ Better compilation target than JS: bytecode, performance, memory ➔ DOM API and GC are coming ... ➔ A Kotlin/Native Frontend ecosystem could arise Experim ental
Thanks! https://speakerdeck.com/sdeleuze/why-spring-loves-kotlin Follow me on @sdeleuze for fresh Spring + Kotlin news https://github.com/mixitconf/mixit https://github.com/sdeleuze/spring-kotlin-fullstack https://github.com/sdeleuze/spring-kotlin-deepdive https://github.com/sdeleuze/spring-kotlin-functional
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