scala.concurrent
This package object contains primitives for concurrent and parallel programming.
Guide
A more detailed guide to Futures and Promises, including discussion and examples can be found at https://docs.scala-lang.org/overviews/core/futures.html.
Common Imports
When working with Futures, you will often find that importing the whole concurrent package is convenient:
import scala.concurrent._
When using things like Future
s, it is often required to have an implicit ExecutionContext
in scope. The general advice for these implicits are as follows.
If the code in question is a class or method definition, and no ExecutionContext
is available, request one from the caller by adding an implicit parameter list:
def myMethod(myParam: MyType)(implicit ec: ExecutionContext) = …
//Or
class MyClass(myParam: MyType)(implicit ec: ExecutionContext) { … }
This allows the caller of the method, or creator of the instance of the class, to decide which ExecutionContext
should be used.
For typical REPL usage and experimentation, importing the global ExecutionContext
is often desired.
import scala.concurrent.ExcutionContext.Implicits.global
Specifying Durations
Operations often require a duration to be specified. A duration DSL is available to make defining these easier:
import scala.concurrent.duration._
val d: Duration = 10.seconds
Using Futures For Non-blocking Computation
Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current thread. In order to create the Future you will need either an implicit or explicit ExecutionContext to be provided:
import scala.concurrent._
import ExecutionContext.Implicits.global // implicit execution context
val firstZebra: Future[Int] = Future {
val words = Files.readAllLines("/etc/dictionaries-common/words").asScala
words.indexOfSlice("zebra")
}
Avoid Blocking
Although blocking is possible in order to await results (with a mandatory timeout duration):
import scala.concurrent.duration._
Await.result(firstZebra, 10.seconds)
and although this is sometimes necessary to do, in particular for testing purposes, blocking in general is discouraged when working with Futures and concurrency in order to avoid potential deadlocks and improve performance. Instead, use callbacks or combinators to remain in the future domain:
val animalRange: Future[Int] = for {
aardvark <- firstAardvark
zebra <- firstZebra
} yield zebra - aardvark
animalRange.onSuccess {
case x if x > 500000 => println("It's a long way from Aardvark to Zebra")
}
Attributes
Members list
Packages
Type members
Classlikes
Await
is what is used to ensure proper handling of blocking for Awaitable
instances.
Await
is what is used to ensure proper handling of blocking for Awaitable
instances.
While occasionally useful, e.g. for testing, it is recommended that you avoid Await whenever possible— instead favoring combinators and/or callbacks. Await's result
and ready
methods will block the calling thread's execution until they return, which will cause performance degradation, and possibly, deadlock issues.
Attributes
- Source
- package.scala
- Supertypes
- Self type
-
Await.type
An object that may eventually be completed with a result value of type T
which may be awaited using blocking methods.
An object that may eventually be completed with a result value of type T
which may be awaited using blocking methods.
The Await object provides methods that allow accessing the result of an Awaitable
by blocking the current thread until the Awaitable
has been completed or a timeout has occurred.
Attributes
- Source
- Awaitable.scala
- Supertypes
- Known subtypes
Marker trait to indicate that a Runnable is Batchable by BatchingExecutors
Marker trait to indicate that a Runnable is Batchable by BatchingExecutors
Attributes
- Source
- BatchingExecutor.scala
- Supertypes
- Known subtypes
-
trait OnCompleteRunnable
- Self type
A context to be notified by scala.concurrent.blocking when a thread is about to block.
A context to be notified by scala.concurrent.blocking when a thread is about to block. In effect this trait provides the implementation for scala.concurrent.Await. scala.concurrent.Await.result and scala.concurrent.Await.ready locates an instance of BlockContext
by first looking for one provided through BlockContext.withBlockContext and failing that, checking whether Thread.currentThread
is an instance of BlockContext
. So a thread pool can have its java.lang.Thread
instances implement BlockContext
. There's a default BlockContext
used if the thread doesn't implement BlockContext
.
Typically, you'll want to chain to the previous BlockContext
, like this:
val oldContext = BlockContext.current
val myContext = new BlockContext {
override def blockOn[T](thunk: => T)(implicit permission: CanAwait): T = {
// you'd have code here doing whatever you need to do
// when the thread is about to block.
// Then you'd chain to the previous context:
oldContext.blockOn(thunk)
}
}
BlockContext.withBlockContext(myContext) {
// then this block runs with myContext as the handler
// for scala.concurrent.blocking
}
Attributes
- Companion
- object
- Source
- BlockContext.scala
- Supertypes
Attributes
- Companion
- trait
- Source
- BlockContext.scala
- Supertypes
- Self type
-
BlockContext.type
This marker trait is used by Await to ensure that Awaitable.ready and Awaitable.result are not directly called by user code.
This marker trait is used by Await to ensure that Awaitable.ready and Awaitable.result are not directly called by user code. An implicit instance of this trait is only available when user code is currently calling the methods on Await.
Attributes
- Source
- package.scala
- Supertypes
An ExecutionContext
can execute program logic asynchronously, typically but not necessarily on a thread pool.
An ExecutionContext
can execute program logic asynchronously, typically but not necessarily on a thread pool.
A general purpose ExecutionContext
must be asynchronous in executing any Runnable
that is passed into its execute
-method. A special purpose ExecutionContext
may be synchronous but must only be passed to code that is explicitly safe to be run using a synchronously executing ExecutionContext
.
APIs such as Future.onComplete
require you to provide a callback and an implicit ExecutionContext
. The implicit ExecutionContext
will be used to execute the callback.
While it is possible to simply import scala.concurrent.ExecutionContext.Implicits.global
to obtain an implicit ExecutionContext
, application developers should carefully consider where they want to define the execution policy; ideally, one place per application — or per logically related section of code — will make a decision about which ExecutionContext
to use. That is, you will mostly want to avoid hardcoding, especially via an import, scala.concurrent.ExecutionContext.Implicits.global
. The recommended approach is to add (implicit ec: ExecutionContext)
to methods, or class constructor parameters, which need an ExecutionContext
.
Then locally import a specific ExecutionContext
in one place for the entire application or module, passing it implicitly to individual methods. Alternatively define a local implicit val with the required ExecutionContext
.
A custom ExecutionContext
may be appropriate to execute code which blocks on IO or performs long-running computations. ExecutionContext.fromExecutorService
and ExecutionContext.fromExecutor
are good ways to create a custom ExecutionContext
.
The intent of ExecutionContext
is to lexically scope code execution. That is, each method, class, file, package, or application determines how to run its own code. This avoids issues such as running application callbacks on a thread pool belonging to a networking library. The size of a networking library's thread pool can be safely configured, knowing that only that library's network operations will be affected. Application callback execution can be configured separately.
Attributes
- Companion
- object
- Source
- ExecutionContext.scala
- Supertypes
- Known subtypes
Contains factory methods for creating execution contexts.
Contains factory methods for creating execution contexts.
Attributes
- Companion
- trait
- Source
- ExecutionContext.scala
- Supertypes
- Self type
-
ExecutionContext.type
An ExecutionContext that is also a Java Executor.
An ExecutionContext that is also a Java Executor.
Attributes
- Source
- ExecutionContext.scala
- Supertypes
- Known subtypes
-
object parasitic
An ExecutionContext that is also a Java ExecutorService.
An ExecutionContext that is also a Java ExecutorService.
Attributes
- Source
- ExecutionContext.scala
- Supertypes
-
trait ExecutorServicetrait ExecutionContextExecutortrait Executortrait ExecutionContextclass Objecttrait Matchableclass AnyShow all
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.
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
- Supertypes
- Known subtypes
-
object never
Promise is an object which can be completed with a value or failed with an exception.
Promise is an object which can be completed with a value or failed with an exception.
A promise should always eventually be completed, whether for success or failure, in order to avoid unintended resource retention for any associated Futures' callbacks or transformations.
Attributes
- Companion
- object
- Source
- Promise.scala
- Supertypes
Deprecated classlikes
This class provides a simple FIFO queue of data objects, which are read by one or more reader threads.
This class provides a simple FIFO queue of data objects, which are read by one or more reader threads.
Type parameters
- A
-
type of data exchanged
Attributes
- Deprecated
-
[Since version 2.13.0]
- Source
- Channel.scala
- Supertypes
A DelayedLazyVal
is a wrapper for lengthy computations which have a valid partially computed result.
A DelayedLazyVal
is a wrapper for lengthy computations which have a valid partially computed result.
The first argument is a function for obtaining the result at any given point in time, and the second is the lengthy computation. Once the computation is complete, the apply
method will stop recalculating it and return a fixed value from that point forward.
Value parameters
- body
-
the computation to run to completion in another thread
- f
-
the function to obtain the current value at any point in time
Attributes
- Deprecated
-
[Since version 2.13.0]
- Source
- DelayedLazyVal.scala
- Supertypes
The JavaConversions
object provides implicit conversions supporting interoperability between Scala and Java concurrency classes.
The JavaConversions
object provides implicit conversions supporting interoperability between Scala and Java concurrency classes.
Attributes
- Deprecated
- true
- Source
- JavaConversions.scala
- Supertypes
- Self type
-
JavaConversions.type
A SyncChannel
allows one to exchange data synchronously between a reader and a writer thread.
A SyncChannel
allows one to exchange data synchronously between a reader and a writer thread. The writer thread is blocked until the data to be written has been read by a corresponding reader thread.
Attributes
- Deprecated
-
[Since version 2.13.0]
- Source
- SyncChannel.scala
- Supertypes
A class to provide safe concurrent access to a mutable cell.
A class to provide safe concurrent access to a mutable cell. All methods are synchronized.
Type parameters
- A
-
type of the contained value
Attributes
- Deprecated
-
[Since version 2.13.0]
- Source
- SyncVar.scala
- Supertypes
Types
Attributes
- Source
- package.scala
Attributes
- Source
- package.scala
Attributes
- Source
- package.scala
Value members
Concrete methods
Used to designate a piece of code which potentially blocks, allowing the current BlockContext to adjust the runtime's behavior.
Used to designate a piece of code which potentially blocks, allowing the current BlockContext to adjust the runtime's behavior. Properly marking blocking code may improve performance or avoid deadlocks.
Blocking on an Awaitable should be done using Await.result instead of blocking
.
Value parameters
- body
-
A piece of code which contains potentially blocking or long running calls.
Attributes
- Throws
-
CancellationException if the computation was cancelled
InterruptedException in the case that a wait within the blocking
body
was interrupted - Source
- package.scala