A Future represents a value which may or may not be currently available, but will be available at some point, or an exception if that value could not be made available.
Asynchronous computations are created by calling Future.apply, which yields instances of Future. Computations are executed using an ExecutionContext, which is usually supplied implicitly, and which is commonly backed by a thread pool.
import ExecutionContext.Implicits.global
val s = "Hello"
val f: Future[String] = Future {
s + " future!"
}
f foreach {
msg => println(msg)
}
Note that the global context is convenient but restricted: "fatal" exceptions are reported only by printing a stack trace, and the underlying thread pool may be shared by a mix of jobs. For any nontrivial application, see the caveats explained at ExecutionContext and also the overview linked below, which explains exception handling in depth.
Attributes
- See also
- Companion
- object
- Source
- Future.scala
- Graph
-
- Supertypes
- Known subtypes
-
object never
Members list
Grouped members
Callbacks
Applies the side-effecting function to the result of this future, and returns a new future with the result of this future.
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 parameters
- U
-
only used to accept any return type of the given
PartialFunction
Value parameters
- pf
-
a
PartialFunctionwhich will be conditionally applied to the outcome of thisFuture
Attributes
- Returns
-
a
Futurewhich will be completed with the exact same outcome as thisFuturebut after thePartialFunctionhas been executed. - Source
- Future.scala
Asynchronously processes the value in the future once the value becomes available.
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 parameters
- U
-
only used to accept any return type of the given callback function
Value parameters
- f
-
the function which will be executed if this
Futurecompletes with a result, the return value offwill be discarded.
Attributes
- Source
- Future.scala
When this future is completed, either through an exception, or a value, apply the provided function.
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 parameters
- U
-
only used to accept any return type of the given callback function
Value parameters
- f
-
the function to be executed when this
Futurecompletes
Attributes
- Source
- Future.scala
Polling
Returns whether the future had already been completed with a value or an exception.
Returns whether the future had already been completed with a value or an exception.
Note: using this method yields nondeterministic dataflow programs.
Attributes
- Returns
-
trueif the future was completed,falseotherwise - Source
- Future.scala
The current value of this Future.
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.
Attributes
- Returns
-
Noneif theFuturewasn't completed,Someif it was. - Source
- Future.scala
Transformations
Creates a new future by mapping the value of the current future, if the given partial function is defined at that value.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- pf
-
the
PartialFunctionto apply to the successful result of thisFuture
Attributes
- Returns
-
a
Futureholding the result of application of thePartialFunctionor aNoSuchElementException - Source
- Future.scala
The returned Future will be successfully completed with the Throwable of the original Future if the original Future fails.
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 an ExecutionException that has one of the exceptions above as its cause. If a future is failed with a scala.runtime.NonLocalReturnControl, it is completed with a value from that throwable instead.
Attributes
- Returns
-
a failed projection of this
Future. - Source
- Future.scala
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.
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 parameters
- U
-
the type of the other
Futureand the resultingFuture
Value parameters
- that
-
the
Futurewhose result we want to use if thisFuturefails.
Attributes
- Returns
-
a
Futurewith the successful result of this or thatFutureor the failure of thisFutureif both fail - Source
- Future.scala
Creates a new future by filtering the value of the current future with a predicate.
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 parameters
- p
-
the predicate to apply to the successful result of this
Future
Attributes
- Returns
-
a
Futurewhich will hold the successful result of thisFutureif it matches the predicate or aNoSuchElementException - Source
- Future.scala
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.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- f
-
the function which will be applied to the successful result of this
Future
Attributes
- Returns
-
a
Futurewhich will be completed with the result of the application of the function - Source
- Future.scala
Creates a new future with one level of nesting flattened, this method is equivalent to flatMap(identity).
Creates a new future with one level of nesting flattened, this method is equivalent to flatMap(identity).
Type parameters
- S
-
the type of the returned
Future
Attributes
- Source
- Future.scala
Creates a new future by applying a function to the successful result of this future.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- f
-
the function which will be applied to the successful result of this
Future
Attributes
- Returns
-
a
Futurewhich will be completed with the result of the application of the function - Source
- Future.scala
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.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- tag
-
the
ClassTagwhich will be used to cast the result of thisFuture
Attributes
- Returns
-
a
Futureholding the casted result of thisFutureor aClassCastExceptionotherwise - Source
- Future.scala
Creates a new future that will handle any matching throwable that this future might contain.
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 parameters
- U
-
the type of the returned
Future
Value parameters
- pf
-
the
PartialFunctionto apply if thisFuturefails
Attributes
- Returns
-
a
Futurewith the successful value of thisFutureor the result of thePartialFunction - Source
- Future.scala
Creates a new future that will handle any matching throwable that this future might contain by assigning it a value of another future.
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 parameters
- U
-
the type of the returned
Future
Value parameters
- pf
-
the
PartialFunctionto apply if thisFuturefails
Attributes
- Returns
-
a
Futurewith the successful value of thisFutureor the outcome of theFuturereturned by thePartialFunction - Source
- Future.scala
Creates a new future by applying the 's' function to the successful result of this future, or the 'f' function to the failed result.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- 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
Attributes
- Returns
-
a
Futurethat will be completed with the transformed value - Source
- Future.scala
Creates a new Future by applying the specified function to the result of this Future.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- f
-
function that transforms the result of this future
Attributes
- Returns
-
a
Futurethat will be completed with the transformed value - Source
- Future.scala
Creates a new Future by applying the specified function, which produces a Future, to the result of this Future.
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 parameters
- S
-
the type of the returned
Future
Value parameters
- f
-
function that transforms the result of this future
Attributes
- Returns
-
a
Futurethat will be completed with the transformed value - Source
- Future.scala
Used by for-comprehensions.
Zips the values of this and that future, and creates a new future holding the tuple of their results.
Zips the values of this and that future, and creates a new future holding the tuple of their results.
If either input future fails, the resulting future is failed with the same throwable, without waiting for the other input future to complete.
If the application of f throws a non-fatal throwable, the resulting future is failed with that throwable.
Type parameters
- U
-
the type of the other
Future
Value parameters
- that
-
the other
Future
Attributes
- Returns
-
a
Futurewith the results of both futures or the failure of the first of them that failed - Source
- Future.scala
Zips the values of this and that future using a function f, and creates a new future holding the result.
Zips the values of this and that future using a function f, and creates a new future holding the result.
If either input future fails, the resulting future is failed with the same throwable, without waiting for the other input future to complete.
If the application of f throws a non-fatal throwable, the resulting future is failed with that throwable.
Type parameters
- R
-
the type of the resulting
Future - U
-
the type of the other
Future
Value parameters
- f
-
the function to apply to the results of
thisandthat - that
-
the other
Future
Attributes
- Returns
-
a
Futurewith the result of the application offto the results ofthisandthat - Source
- Future.scala
Value members
Inherited and Abstract methods
Await the "completed" state of this Awaitable.
Await the "completed" state of this Awaitable.
This method should not be called directly; use Await.ready instead.
Value parameters
- atMost
-
maximum wait time, which may be negative (no waiting is done), Duration.Inf for unbounded waiting, or a finite positive duration
Attributes
- Returns
-
this
Awaitable - Throws
-
IllegalArgumentException if
atMostis Duration.UndefinedInterruptedException if the current thread is interrupted while waiting
TimeoutException if after waiting for the specified time this
Awaitableis still not ready - Inherited from:
- Awaitable
- Source
- Awaitable.scala
Await and return the result (of type T) of this Awaitable.
Await and return the result (of type T) of this Awaitable.
This method should not be called directly; use Await.result instead.
Value parameters
- atMost
-
maximum wait time, which may be negative (no waiting is done), Duration.Inf for unbounded waiting, or a finite positive duration
Attributes
- Returns
-
the result value if the
Awaitableis completed within the specific maximum wait time - Throws
-
IllegalArgumentException if
atMostis Duration.UndefinedInterruptedException if the current thread is interrupted while waiting
TimeoutException if after waiting for the specified time this
Awaitableis still not ready - Inherited from:
- Awaitable
- Source
- Awaitable.scala