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/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
package kotlinx.coroutines.future
import kotlinx.coroutines.*
import kotlinx.coroutines.CancellationException
import java.util.concurrent.*
import java.util.function.*
import kotlin.coroutines.*
/**
* Starts a new coroutine and returns its result as an implementation of [CompletableFuture].
* The running coroutine is cancelled when the resulting future is cancelled or otherwise completed.
*
* The coroutine context is inherited from a [CoroutineScope], additional context elements can be specified with the [context] argument.
* If the context does not have any dispatcher nor any other [ContinuationInterceptor], then [Dispatchers.Default] is used.
* The parent job is inherited from a [CoroutineScope] as well, but it can also be overridden
* with corresponding [context] element.
*
* By default, the coroutine is immediately scheduled for execution.
* Other options can be specified via `start` parameter. See [CoroutineStart] for details.
* A value of [CoroutineStart.LAZY] is not supported
* (since `CompletableFuture` framework does not provide the corresponding capability) and
* produces [IllegalArgumentException].
*
* See [newCoroutineContext][CoroutineScope.newCoroutineContext] for a description of debugging facilities that are available for newly created coroutine.
*
* @param context additional to [CoroutineScope.coroutineContext] context of the coroutine.
* @param start coroutine start option. The default value is [CoroutineStart.DEFAULT].
* @param block the coroutine code.
*/
public fun <T> CoroutineScope.future(
context: CoroutineContext = EmptyCoroutineContext,
start: CoroutineStart = CoroutineStart.DEFAULT,
block: suspend CoroutineScope.() -> T
) : CompletableFuture<T> {
require(!start.isLazy) { "$start start is not supported" }
val newContext = this.newCoroutineContext(context)
val future = CompletableFuture<T>()
val coroutine = CompletableFutureCoroutine(newContext, future)
future.whenComplete(coroutine) // Cancel coroutine if future was completed externally
coroutine.start(start, coroutine, block)
return future
}
private class CompletableFutureCoroutine<T>(
context: CoroutineContext,
private val future: CompletableFuture<T>
) : AbstractCoroutine<T>(context), BiConsumer<T?, Throwable?> {
override fun accept(value: T?, exception: Throwable?) {
cancel()
}
override fun onCompleted(value: T) {
future.complete(value)
}
override fun onCancelled(cause: Throwable, handled: Boolean) {
if (!future.completeExceptionally(cause) && !handled) {
// prevents loss of exception that was not handled by parent & could not be set to CompletableFuture
handleCoroutineException(context, cause)
}
}
}
/**
* Converts this deferred value to the instance of [CompletableFuture].
* The deferred value is cancelled when the resulting future is cancelled or otherwise completed.
*/
public fun <T> Deferred<T>.asCompletableFuture(): CompletableFuture<T> {
val future = CompletableFuture<T>()
setupCancellation(future)
invokeOnCompletion {
try {
future.complete(getCompleted())
} catch (t: Throwable) {
future.completeExceptionally(t)
}
}
return future
}
/**
* Converts this job to the instance of [CompletableFuture].
* The job is cancelled when the resulting future is cancelled or otherwise completed.
*/
public fun Job.asCompletableFuture(): CompletableFuture<Unit> {
val future = CompletableFuture<Unit>()
setupCancellation(future)
invokeOnCompletion { cause ->
if (cause === null) future.complete(Unit)
else future.completeExceptionally(cause)
}
return future
}
private fun Job.setupCancellation(future: CompletableFuture<*>) {
future.whenComplete { _, exception ->
cancel(exception?.let {
it as? CancellationException ?: CancellationException("CompletableFuture was completed exceptionally", it)
})
}
}
/**
* Converts this [CompletionStage] to an instance of [Deferred].
*
* The [CompletableFuture] that corresponds to this [CompletionStage] (see [CompletionStage.toCompletableFuture])
* is cancelled when the resulting deferred is cancelled.
*/
@Suppress("DeferredIsResult")
public fun <T> CompletionStage<T>.asDeferred(): Deferred<T> {
val future = toCompletableFuture() // retrieve the future
// Fast path if already completed
if (future.isDone) {
return try {
@Suppress("UNCHECKED_CAST")
CompletableDeferred(future.get() as T)
} catch (e: Throwable) {
// unwrap original cause from ExecutionException
val original = (e as? ExecutionException)?.cause ?: e
CompletableDeferred<T>().also { it.completeExceptionally(original) }
}
}
val result = CompletableDeferred<T>()
whenComplete { value, exception ->
if (exception == null) {
// the future has completed normally
result.complete(value)
} else {
// the future has completed with an exception, unwrap it consistently with fast path
// Note: In the fast-path the implementation of CompletableFuture.get() does unwrapping
result.completeExceptionally((exception as? CompletionException)?.cause ?: exception)
}
}
result.cancelFutureOnCompletion(future)
return result
}
/**
* Awaits for completion of [CompletionStage] without blocking a thread.
*
* This suspending function is cancellable.
* If the [Job] of the current coroutine is cancelled or completed while this suspending function is waiting, this function
* stops waiting for the completion stage and immediately resumes with [CancellationException][kotlinx.coroutines.CancellationException].
*
* This method is intended to be used with one-shot futures, so on coroutine cancellation the [CompletableFuture] that
* corresponds to this [CompletionStage] (see [CompletionStage.toCompletableFuture])
* is cancelled. If cancelling the given stage is undesired, `stage.asDeferred().await()` should be used instead.
*/
public suspend fun <T> CompletionStage<T>.await(): T {
val future = toCompletableFuture() // retrieve the future
// fast path when CompletableFuture is already done (does not suspend)
if (future.isDone) {
try {
@Suppress("UNCHECKED_CAST", "BlockingMethodInNonBlockingContext")
return future.get() as T
} catch (e: ExecutionException) {
throw e.cause ?: e // unwrap original cause from ExecutionException
}
}
// slow path -- suspend
return suspendCancellableCoroutine { cont: CancellableContinuation<T> ->
val consumer = ContinuationConsumer(cont)
whenComplete(consumer)
cont.invokeOnCancellation {
future.cancel(false)
consumer.cont = null // shall clear reference to continuation to aid GC
}
}
}
private class ContinuationConsumer<T>(
@Volatile @JvmField var cont: Continuation<T>?
) : BiConsumer<T?, Throwable?> {
@Suppress("UNCHECKED_CAST")
override fun accept(result: T?, exception: Throwable?) {
val cont = this.cont ?: return // atomically read current value unless null
if (exception == null) {
// the future has completed normally
cont.resume(result as T)
} else {
// the future has completed with an exception, unwrap it to provide consistent view of .await() result and to propagate only original exception
cont.resumeWithException((exception as? CompletionException)?.cause ?: exception)
}
}
}
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