This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.2.0!spring-doc.cn

Coroutines

Kotlin Coroutines are Kotlin lightweight threads allowing to write non-blocking code in an imperative way. On language side, suspending functions provides an abstraction for asynchronous operations while on library side kotlinx.coroutines provides functions like async { } and types like Flow.spring-doc.cn

Spring Framework provides support for Coroutines on the following scope:spring-doc.cn

Dependencies

Coroutines support is enabled when kotlinx-coroutines-core and kotlinx-coroutines-reactor dependencies are in the classpath:spring-doc.cn

build.gradle.ktsspring-doc.cn

dependencies {

	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:${coroutinesVersion}")
	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:${coroutinesVersion}")
}

Version 1.4.0 and above are supported.spring-doc.cn

How Reactive translates to Coroutines?

For return values, the translation from Reactive to Coroutines APIs is the following:spring-doc.cn

  • fun handler(): Mono<Void> becomes suspend fun handler()spring-doc.cn

  • fun handler(): Mono<T> becomes suspend fun handler(): T or suspend fun handler(): T? depending on if the Mono can be empty or not (with the advantage of being more statically typed)spring-doc.cn

  • fun handler(): Flux<T> becomes fun handler(): Flow<T>spring-doc.cn

For input parameters:spring-doc.cn

  • If laziness is not needed, fun handler(mono: Mono<T>) becomes fun handler(value: T) since a suspending functions can be invoked to get the value parameter.spring-doc.cn

  • If laziness is needed, fun handler(mono: Mono<T>) becomes fun handler(supplier: suspend () → T) or fun handler(supplier: suspend () → T?)spring-doc.cn

Flow is Flux equivalent in Coroutines world, suitable for hot or cold stream, finite or infinite streams, with the following main differences:spring-doc.cn

Read this blog post about Going Reactive with Spring, Coroutines and Kotlin Flow for more details, including how to run code concurrently with Coroutines.spring-doc.cn

Controllers

Here is an example of a Coroutines @RestController.spring-doc.cn

@RestController
class CoroutinesRestController(client: WebClient, banner: Banner) {

	@GetMapping("/suspend")
	suspend fun suspendingEndpoint(): Banner {
		delay(10)
		return banner
	}

	@GetMapping("/flow")
	fun flowEndpoint() = flow {
		delay(10)
		emit(banner)
		delay(10)
		emit(banner)
	}

	@GetMapping("/deferred")
	fun deferredEndpoint() = GlobalScope.async {
		delay(10)
		banner
	}

	@GetMapping("/sequential")
	suspend fun sequential(): List<Banner> {
		val banner1 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		val banner2 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		return listOf(banner1, banner2)
	}

	@GetMapping("/parallel")
	suspend fun parallel(): List<Banner> = coroutineScope {
		val deferredBanner1: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		val deferredBanner2: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		listOf(deferredBanner1.await(), deferredBanner2.await())
	}

	@GetMapping("/error")
	suspend fun error() {
		throw IllegalStateException()
	}

	@GetMapping("/cancel")
	suspend fun cancel() {
		throw CancellationException()
	}

}

View rendering with a @Controller is also supported.spring-doc.cn

@Controller
class CoroutinesViewController(banner: Banner) {

	@GetMapping("/")
	suspend fun render(model: Model): String {
		delay(10)
		model["banner"] = banner
		return "index"
	}
}

WebFlux.fn

Here is an example of Coroutines router defined via the coRouter { } DSL and related handlers.spring-doc.cn

@Configuration
class RouterConfiguration {

	@Bean
	fun mainRouter(userHandler: UserHandler) = coRouter {
		GET("/", userHandler::listView)
		GET("/api/user", userHandler::listApi)
	}
}
class UserHandler(builder: WebClient.Builder) {

	private val client = builder.baseUrl("...").build()

	suspend fun listView(request: ServerRequest): ServerResponse =
			ServerResponse.ok().renderAndAwait("users", mapOf("users" to
			client.get().uri("...").awaitExchange().awaitBody<User>()))

	suspend fun listApi(request: ServerRequest): ServerResponse =
				ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).bodyAndAwait(
				client.get().uri("...").awaitExchange().awaitBody<User>())
}

Transactions

Transactions on Coroutines are supported via the programmatic variant of the Reactive transaction management provided as of Spring Framework 5.2.spring-doc.cn

For suspending functions, a TransactionalOperator.executeAndAwait extension is provided.spring-doc.cn

import org.springframework.transaction.reactive.executeAndAwait

class PersonRepository(private val operator: TransactionalOperator) {

    suspend fun initDatabase() = operator.executeAndAwait {
        insertPerson1()
        insertPerson2()
    }

    private suspend fun insertPerson1() {
        // INSERT SQL statement
    }

    private suspend fun insertPerson2() {
        // INSERT SQL statement
    }
}

For Kotlin Flow, a Flow<T>.transactional extension is provided.spring-doc.cn

import org.springframework.transaction.reactive.transactional

class PersonRepository(private val operator: TransactionalOperator) {

    fun updatePeople() = findPeople().map(::updatePerson).transactional(operator)

    private fun findPeople(): Flow<Person> {
        // SELECT SQL statement
    }

    private suspend fun updatePerson(person: Person): Person {
        // UPDATE SQL statement
    }
}