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Kotlin Support

The Framework also has been improved to support Kotlin lambdas for functions, so now you can use a combination of the Kotlin language and Spring Integration flow definitions:

@Bean
@Transformer(inputChannel = "functionServiceChannel")
fun kotlinFunction(): (String) -> String {
    return { it.toUpperCase() }
}

@Bean
@ServiceActivator(inputChannel = "messageConsumerServiceChannel")
fun kotlinConsumer(): (Message<Any>) -> Unit {
    return { print(it) }
}

@Bean
@InboundChannelAdapter(value = "counterChannel",
        poller = Poller(fixedRate = "10", maxMessagesPerPoll = "1"))
fun kotlinSupplier(): () -> String {
    return { "baz" }
}

Kotlin Coroutines

Starting with version 6.0, Spring Integration provides support for Kotlin Coroutines. Now suspend functions and kotlinx.coroutines.Deferred & kotlinx.coroutines.flow.Flow return types can be used for service methods:

@ServiceActivator(inputChannel = "suspendServiceChannel", outputChannel = "resultChannel")
suspend fun suspendServiceFunction(payload: String) = payload.uppercase()

@ServiceActivator(inputChannel = "flowServiceChannel", outputChannel = "resultChannel", async = "true")
fun flowServiceFunction(payload: String) =
    flow {
        for (i in 1..3) {
            emit("$payload #$i")
        }
    }

The framework treats them as Reactive Streams interactions and uses ReactiveAdapterRegistry to convert to respective Mono and Flux reactor types. Such a function reply is processed then in the reply channel, if it is a ReactiveStreamsSubscribableChannel, or as a result of CompletableFuture in the respective callback.

Note
 The functions with Flow result are not async by default on the @ServiceActivator, so Flow instance is produced as a reply message payload. It is the target application’s responsibility to process this object as a coroutine or convert it to Flux, respectively.

The @MessagingGateway interface methods also can be marked with a suspend modifier when declared in Kotlin. The framework utilizes a Mono internally to perform request-reply using the downstream flow. Such a Mono result is processed by the MonoKt.awaitSingleOrNull() API internally to fulfil a kotlin.coroutines.Continuation argument fo the called suspend function of the gateway:

@MessagingGateway(defaultRequestChannel = "suspendRequestChannel")
interface SuspendFunGateway {

    suspend fun suspendGateway(payload: String): String

}

This method has to be called as a coroutine according to Kotlin language requirements:

@Autowired
private lateinit var suspendFunGateway: SuspendFunGateway

fun someServiceMethod() {
    runBlocking {
        val reply = suspendFunGateway.suspendGateway("test suspend gateway")
    }
}