13.3 Micronaut for Kotlin - 13.3.5 Coroutines Support - 《Micronaut v2.3.1 Documentation》

13.3.5 Coroutines Support

13.3.5 Coroutines Support

Kotlin coroutines allow you to create asynchronous applications with imperative style code. A Micronaut’s controller action can be a suspend function:

Controller suspend function example

    @Get("/simple", produces = [MediaType.TEXT_PLAIN])
    suspend fun simple(): String { (1)
        return "Hello"
    }

1	The function is marked as `suspend`, though in reality it won’t be suspended.

Controller suspend function example

    @Get("/delayed", produces = [MediaType.TEXT_PLAIN])
    suspend fun delayed(): String { (1)
        delay(1) (2)
        return "Delayed"
    }

1	The function is marked as `suspend`.
2	The `delay` is called to make sure that a function is suspended and the response is returned from a different thread.

Controller suspend function example

    @Status(HttpStatus.CREATED) (1)
    @Get("/status")
    suspend fun status(): Unit {
    }

1	`suspend` function also works when all we want is to return a status.

Controller suspend function example

    @Status(HttpStatus.CREATED)
    @Get("/statusDelayed")
    suspend fun statusDelayed(): Unit {
        delay(1)
    }

You can also use Flow type for streaming server and client. A streaming controller can return Flow, for example:

Streaming JSON on the Server with Flow

    @Get(value = "/headlinesWithFlow", processes = [MediaType.APPLICATION_JSON_STREAM])
    internal fun streamHeadlinesWithFlow(): Flow<Headline> = (1)
        flow { (2)
            repeat(100) { (3)
                with (Headline()) {
                    text = "Latest Headline at " + ZonedDateTime.now()
                    emit(this) (4)
                    delay(1_000) (5)
                }
            }
        }

1	A method `streamHeadlinesWithFlow` is defined that produces `application/x-json-stream`
2	A `Flow` is created using `flow`
3	This `Flow` will emit 100 messages
4	Emitting will happen with `emit` `suspend` function
5	There will be a 1 second delay between messages

A streaming client can simply return a Flow, for example:

Streaming client with Flow

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Get
import io.micronaut.http.client.annotation.Client
import kotlinx.coroutines.flow.Flow
@Client("/streaming")
interface HeadlineFlowClient {
    @Get(value = "/headlinesWithFlow", processes = [MediaType.APPLICATION_JSON_STREAM]) (1)
    fun streamFlow(): Flow<Headline> (2)
}

1	The `@Get` method is defined as processing responses of type `APPLICATION_JSON_STREAM`
2	A `Flow` is used as the return type