Future
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)
}
Callbacks
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
- (source)
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 off
will be discarded.
- Source
- (source)
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 thisFuture
- Returns
-
a
Future
which will be completed with the exact same outcome as thisFuture
but after thePartialFunction
has been executed. - Source
- (source)
Polling
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
- (source)
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 theFuture
wasn't completed,Some
if it was. - Source
- (source)
Transformations
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 futuresInstead, 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
- (source)
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
- (source)
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
- (source)
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
- (source)
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
- (source)
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
- (source)
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
- (source)
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 thisFuture
if it matches the predicate or aNoSuchElementException
- Source
- (source)
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 thisFuture
- Returns
-
a
Future
holding the result of application of thePartialFunction
or aNoSuchElementException
- Source
- (source)
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 thisFuture
fails
- Returns
-
a
Future
with the successful value of thisFuture
or the result of thePartialFunction
- Source
- (source)
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 thisFuture
fails
- Returns
-
a
Future
with the successful value of thisFuture
or the outcome of theFuture
returned by thePartialFunction
- Source
- (source)
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
- (source)
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
andthat
- that
-
the other
Future
- Returns
-
a
Future
with the result of the application off
to the results ofthis
andthat
- Source
- (source)
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 resultingFuture
- Value Params
- that
-
the
Future
whose result we want to use if thisFuture
fails.
- Returns
-
a
Future
with the successful result of this or thatFuture
or the failure of thisFuture
if both fail - Source
- (source)
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 thisFuture
- Returns
-
a
Future
holding the casted result of thisFuture
or aClassCastException
otherwise - Source
- (source)
Value members
Inherited methods
@throws(scala.Predef.classOf[scala.concurrent.TimeoutException]) @throws(scala.Predef.classOf[scala.InterruptedException])
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
- (source)
@throws(scala.Predef.classOf[scala.concurrent.TimeoutException]) @throws(scala.Predef.classOf[scala.InterruptedException])
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
- (source)