Scala 3/API/scala.concurrent/Future

Future

trait Future[+T] extends Awaitable[T]
A Future represents a value which may or may not *currently* be available, but will be available at some point, or an exception if that value could not be made available.
Asynchronous computations that yield futures are created with the Future.apply call and are computed using a supplied ExecutionContext, which can be backed by a Thread pool.
import ExecutionContext.Implicits.global
val s = "Hello"
val f: Future[String] = Future {
  s + " future!"
}
f foreach {
  msg => println(msg)
}
See also
Companion
object
Source
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trait Awaitable[T]
class Object
trait Matchable
class Any
object never

Callbacks

def onComplete[U](f: Try[T] => U)(executor: ExecutionContext): Unit
When this future is completed, either through an exception, or a value, apply the provided function.
If the future has already been completed, this will either be applied immediately or be scheduled asynchronously.
Note that the returned value of f will be discarded.
Since this method executes asynchronously and does not produce a return value, any non-fatal exceptions thrown will be reported to the ExecutionContext.
Multiple callbacks may be registered; there is no guarantee that they will be executed in a particular order.
The provided callback always runs in the provided implicit ExecutionContext, though there is no guarantee that the execute() method on the ExecutionContext will be called once per callback or that execute() will be called in the current thread. That is, the implementation may run multiple callbacks in a batch within a single execute() and it may run execute() either immediately or asynchronously. Completion of the Future must *happen-before* the invocation of the callback.
Type Params
U
only used to accept any return type of the given callback function
Value Params
f
the function to be executed when this Future completes
Source
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def foreach[U](f: T => U)(executor: ExecutionContext): Unit
Asynchronously processes the value in the future once the value becomes available.
WARNING: Will not be called if this future is never completed or if it is completed with a failure.
Since this method executes asynchronously and does not produce a return value, any non-fatal exceptions thrown will be reported to the ExecutionContext.
Type Params
U
only used to accept any return type of the given callback function
Value Params
f
the function which will be executed if this Future completes with a result, the return value of f will be discarded.
Source
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def andThen[U](pf: PartialFunction[Try[T], U])(executor: ExecutionContext): Future[T]
Applies the side-effecting function to the result of this future, and returns a new future with the result of this future.
This method allows one to enforce that the callbacks are executed in a specified order.
Note that if one of the chained andThen callbacks throws an exception, that exception is not propagated to the subsequent andThen callbacks. Instead, the subsequent andThen callbacks are given the original value of this future.
The following example prints out 5:
val f = Future { 5 }
f andThen {
  case r => throw new RuntimeException("runtime exception")
} andThen {
  case Failure(t) => println(t)
  case Success(v) => println(v)
}
Since this method executes asynchronously and does not produce a return value, any non-fatal exceptions thrown will be reported to the ExecutionContext.
Type Params
U
only used to accept any return type of the given PartialFunction
Value Params
pf
a PartialFunction which will be conditionally applied to the outcome of this Future
Returns
a Future which will be completed with the exact same outcome as this Future but after the PartialFunction has been executed.
Source
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Polling

def isCompleted: Boolean
Returns whether the future had already been completed with a value or an exception.
Note: using this method yields nondeterministic dataflow programs.
Returns
true if the future was completed, false otherwise
Source
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def value: Option[Try[T]]
The current value of this Future.
Note: using this method yields nondeterministic dataflow programs.
If the future was not completed the returned value will be None. If the future was completed the value will be Some(Success(t)) if it contained a valid result, or Some(Failure(error)) if it contained an exception.
Returns
None if the Future wasn't completed, Some if it was.
Source
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Transformations

def failed: Future[Throwable]
The returned Future will be successfully completed with the Throwable of the original Future if the original Future fails.
If the original Future is successful, the returned Future is failed with a NoSuchElementException.
This future may contain a throwable object and this means that the future failed. Futures obtained through combinators have the same exception as the future they were obtained from. The following throwable objects are not contained in the future: - Error - fatal errors are not contained within futures - InterruptedException - not contained within futures - all scala.util.control.ControlThrowable except NonLocalReturnControl - not contained within futures
Instead, the future is completed with a ExecutionException with one of the exceptions above as the cause. If a future is failed with a scala.runtime.NonLocalReturnControl, it is completed with a value from that throwable instead.
Returns
a failed projection of this Future.
Source
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def transform[S](s: T => S, f: Throwable => Throwable)(executor: ExecutionContext): Future[S]
Creates a new future by applying the 's' function to the successful result of this future, or the 'f' function to the failed result. If there is any non-fatal exception thrown when 's' or 'f' is applied, that exception will be propagated to the resulting future.
Type Params
S
the type of the returned Future
Value Params
f
function that transforms a failure of the receiver into a failure of the returned future
s
function that transforms a successful result of the receiver into a successful result of the returned future
Returns
a Future that will be completed with the transformed value
Source
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def transform[S](f: Try[T] => Try[S])(executor: ExecutionContext): Future[S]
Creates a new Future by applying the specified function to the result of this Future. If there is any non-fatal exception thrown when 'f' is applied then that exception will be propagated to the resulting future.
Type Params
S
the type of the returned Future
Value Params
f
function that transforms the result of this future
Returns
a Future that will be completed with the transformed value
Source
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def transformWith[S](f: Try[T] => Future[S])(executor: ExecutionContext): Future[S]
Creates a new Future by applying the specified function, which produces a Future, to the result of this Future. If there is any non-fatal exception thrown when 'f' is applied then that exception will be propagated to the resulting future.
Type Params
S
the type of the returned Future
Value Params
f
function that transforms the result of this future
Returns
a Future that will be completed with the transformed value
Source
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def map[S](f: T => S)(executor: ExecutionContext): Future[S]
Creates a new future by applying a function to the successful result of this future. If this future is completed with an exception then the new future will also contain this exception.
Example:
val f = Future { "The future" }
val g = f map { x: String => x + " is now!" }
Note that a for comprehension involving a Future may expand to include a call to map and or flatMap and withFilter. See scala.concurrent.Future#flatMap for an example of such a comprehension.
Type Params
S
the type of the returned Future
Value Params
f
the function which will be applied to the successful result of this Future
Returns
a Future which will be completed with the result of the application of the function
Source
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def flatMap[S](f: T => Future[S])(executor: ExecutionContext): Future[S]
Creates a new future by applying a function to the successful result of this future, and returns the result of the function as the new future. If this future is completed with an exception then the new future will also contain this exception.
Example:
val f = Future { 5 }
val g = Future { 3 }
val h = for {
  x: Int <- f // returns Future(5)
  y: Int <- g // returns Future(3)
} yield x + y
is translated to:
f flatMap { (x: Int) => g map { (y: Int) => x + y } }
Type Params
S
the type of the returned Future
Value Params
f
the function which will be applied to the successful result of this Future
Returns
a Future which will be completed with the result of the application of the function
Source
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def flatten[S](ev: T <:< Future[S]): Future[S]
Creates a new future with one level of nesting flattened, this method is equivalent to flatMap(identity).
Type Params
S
the type of the returned Future
Source
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def filter(p: T => Boolean)(executor: ExecutionContext): Future[T]
Creates a new future by filtering the value of the current future with a predicate.
If the current future contains a value which satisfies the predicate, the new future will also hold that value. Otherwise, the resulting future will fail with a NoSuchElementException.
If the current future fails, then the resulting future also fails.
Example:
val f = Future { 5 }
val g = f filter { _ % 2 == 1 }
val h = f filter { _ % 2 == 0 }
g foreach println // Eventually prints 5
Await.result(h, Duration.Zero) // throw a NoSuchElementException
Value Params
p
the predicate to apply to the successful result of this Future
Returns
a Future which will hold the successful result of this Future if it matches the predicate or a NoSuchElementException
Source
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final def withFilter(p: T => Boolean)(executor: ExecutionContext): Future[T]
Used by for-comprehensions.
Source
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def collect[S](pf: PartialFunction[T, S])(executor: ExecutionContext): Future[S]
Creates a new future by mapping the value of the current future, if the given partial function is defined at that value.
If the current future contains a value for which the partial function is defined, the new future will also hold that value. Otherwise, the resulting future will fail with a NoSuchElementException.
If the current future fails, then the resulting future also fails.
Example:
val f = Future { -5 }
val g = f collect {
  case x if x < 0 => -x
}
val h = f collect {
  case x if x > 0 => x * 2
}
g foreach println // Eventually prints 5
Await.result(h, Duration.Zero) // throw a NoSuchElementException
Type Params
S
the type of the returned Future
Value Params
pf
the PartialFunction to apply to the successful result of this Future
Returns
a Future holding the result of application of the PartialFunction or a NoSuchElementException
Source
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def recover[U >: T](pf: PartialFunction[Throwable, U])(executor: ExecutionContext): Future[U]
Creates a new future that will handle any matching throwable that this future might contain. If there is no match, or if this future contains a valid result then the new future will contain the same.
Example:
Future (6 / 0) recover { case e: ArithmeticException => 0 } // result: 0
Future (6 / 0) recover { case e: NotFoundException   => 0 } // result: exception
Future (6 / 2) recover { case e: ArithmeticException => 0 } // result: 3
Type Params
U
the type of the returned Future
Value Params
pf
the PartialFunction to apply if this Future fails
Returns
a Future with the successful value of this Future or the result of the PartialFunction
Source
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def recoverWith[U >: T](pf: PartialFunction[Throwable, Future[U]])(executor: ExecutionContext): Future[U]
Creates a new future that will handle any matching throwable that this future might contain by assigning it a value of another future.
If there is no match, or if this future contains a valid result then the new future will contain the same result.
Example:
val f = Future { Int.MaxValue }
Future (6 / 0) recoverWith { case e: ArithmeticException => f } // result: Int.MaxValue
Type Params
U
the type of the returned Future
Value Params
pf
the PartialFunction to apply if this Future fails
Returns
a Future with the successful value of this Future or the outcome of the Future returned by the PartialFunction
Source
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def zip[U](that: Future[U]): Future[(T, U)]
Zips the values of this and that future, and creates a new future holding the tuple of their results.
If this future fails, the resulting future is failed with the throwable stored in this. Otherwise, if that future fails, the resulting future is failed with the throwable stored in that.
Type Params
U
the type of the other Future
Value Params
that
the other Future
Returns
a Future with the results of both futures or the failure of the first of them that failed
Source
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def zipWith[U, R](that: Future[U])(f: (T, U) => R)(executor: ExecutionContext): Future[R]
Zips the values of this and that future using a function f, and creates a new future holding the result.
If this future fails, the resulting future is failed with the throwable stored in this. Otherwise, if that future fails, the resulting future is failed with the throwable stored in that. If the application of f throws a throwable, the resulting future is failed with that throwable if it is non-fatal.
Type Params
R
the type of the resulting Future
U
the type of the other Future
Value Params
f
the function to apply to the results of this and that
that
the other Future
Returns
a Future with the result of the application of f to the results of this and that
Source
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def fallbackTo[U >: T](that: Future[U]): Future[U]
Creates a new future which holds the result of this future if it was completed successfully, or, if not, the result of the that future if that is completed successfully. If both futures are failed, the resulting future holds the throwable object of the first future.
Using this method will not cause concurrent programs to become nondeterministic.
Example:
val f = Future { throw new RuntimeException("failed") }
val g = Future { 5 }
val h = f fallbackTo g
h foreach println // Eventually prints 5
Type Params
U
the type of the other Future and the resulting Future
Value Params
that
the Future whose result we want to use if this Future fails.
Returns
a Future with the successful result of this or that Future or the failure of this Future if both fail
Source
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def mapTo[S](tag: ClassTag[S]): Future[S]
Creates a new Future[S] which is completed with this Future's result if that conforms to S's erased type or a ClassCastException otherwise.
Type Params
S
the type of the returned Future
Value Params
tag
the ClassTag which will be used to cast the result of this Future
Returns
a Future holding the casted result of this Future or a ClassCastException otherwise
Source
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Value members

Inherited methods

@throws(scala.Predef.classOf[scala.concurrent.TimeoutException]) @throws(scala.Predef.classOf[scala.InterruptedException])
def ready(atMost: Duration)(permit: CanAwait): Future[T]
Await the "completed" state of this Awaitable.
This method should not be called directly; use Await.ready instead.
Value Params
atMost
maximum wait time, which may be negative (no waiting is done), Duration.Inf for unbounded waiting, or a finite positive duration
Returns
this Awaitable
Throws
IllegalArgumentException
IllegalArgumentException
InterruptedException
InterruptedException
TimeoutException
TimeoutException
Inhertied from
Awaitable
Source
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@throws(scala.Predef.classOf[scala.concurrent.TimeoutException]) @throws(scala.Predef.classOf[scala.InterruptedException])
def result(atMost: Duration)(permit: CanAwait): T
Await and return the result (of type T) of this Awaitable.
This method should not be called directly; use Await.result instead.
Value Params
atMost
maximum wait time, which may be negative (no waiting is done), Duration.Inf for unbounded waiting, or a finite positive duration
Returns
the result value if the Awaitable is completed within the specific maximum wait time
Throws
IllegalArgumentException
IllegalArgumentException
InterruptedException
InterruptedException
TimeoutException
TimeoutException
Inhertied from
Awaitable
Source
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