Module java.base
Package java.util.concurrent

Class CompletableFuture<T>

  • Type Parameters:
    T - The result type returned by this future's join and get methods
    All Implemented Interfaces:
    CompletionStage<T>, Future<T>
    public class CompletableFuture<T>
extends Object
implements Future<T>, CompletionStage<T>
    A Future that may be explicitly completed (setting its value and status), and may be used as a CompletionStage, supporting dependent functions and actions that trigger upon its completion.

    When two or more threads attempt to complete, completeExceptionally, or cancel a CompletableFuture, only one of them succeeds.

    In addition to these and related methods for directly manipulating status and results, CompletableFuture implements interface CompletionStage with the following policies:

    • Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.
    • All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task). This may be overridden for non-static methods in subclasses by defining method defaultExecutor(). To simplify monitoring, debugging, and tracking, all generated asynchronous tasks are instances of the marker interface CompletableFuture.AsynchronousCompletionTask. Operations with time-delays can use adapter methods defined in this class, for example: supplyAsync(supplier, delayedExecutor(timeout, timeUnit)). To support methods with delays and timeouts, this class maintains at most one daemon thread for triggering and cancelling actions, not for running them.
    • All CompletionStage methods are implemented independently of other public methods, so the behavior of one method is not impacted by overrides of others in subclasses.
    • All CompletionStage methods return CompletableFutures. To restrict usages to only those methods defined in interface CompletionStage, use method minimalCompletionStage(). Or to ensure only that clients do not themselves modify a future, use method copy().

    CompletableFuture also implements Future with the following policies:

    • Since (unlike FutureTask) this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion. Method cancel has the same effect as completeExceptionally(new CancellationException()). Method isCompletedExceptionally() can be used to determine if a CompletableFuture completed in any exceptional fashion.
    • In case of exceptional completion with a CompletionException, methods get() and get(long, TimeUnit) throw an ExecutionException with the same cause as held in the corresponding CompletionException. To simplify usage in most contexts, this class also defines methods join() and getNow(T) that instead throw the CompletionException directly in these cases.

    Arguments used to pass a completion result (that is, for parameters of type T) for methods accepting them may be null, but passing a null value for any other parameter will result in a NullPointerException being thrown.

    Subclasses of this class should normally override the "virtual constructor" method newIncompleteFuture(), which establishes the concrete type returned by CompletionStage methods. For example, here is a class that substitutes a different default Executor and disables the obtrude methods: 

     
 class MyCompletableFuture<T> extends CompletableFuture<T> {
   static final Executor myExecutor = ...;
   public MyCompletableFuture() { }
   public <U> CompletableFuture<U> newIncompleteFuture() {
     return new MyCompletableFuture<U>(); }
   public Executor defaultExecutor() {
     return myExecutor; }
   public void obtrudeValue(T value) {
     throw new UnsupportedOperationException(); }
   public void obtrudeException(Throwable ex) {
     throw new UnsupportedOperationException(); }
 }
    Since:
    1.8

    • Constructor Detail

      • CompletableFuture

        public CompletableFuture()
        Creates a new incomplete CompletableFuture.

    • Method Detail

      • supplyAsync

        public static <U> CompletableFuture<U> supplyAsync​(Supplier<U> supplier)
        Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() with the value obtained by calling the given Supplier.
        Type Parameters:
        U - the function's return type
        Parameters:
        supplier - a function returning the value to be used to complete the returned CompletableFuture
        Returns:
        the new CompletableFuture

      • supplyAsync

        public static <U> CompletableFuture<U> supplyAsync​(Supplier<U> supplier,
                                                   Executor executor)
        Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor with the value obtained by calling the given Supplier.
        Type Parameters:
        U - the function's return type
        Parameters:
        supplier - a function returning the value to be used to complete the returned CompletableFuture
        executor - the executor to use for asynchronous execution
        Returns:
        the new CompletableFuture

      • runAsync

        public static CompletableFuture<Void> runAsync​(Runnable runnable)
        Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() after it runs the given action.
        Parameters:
        runnable - the action to run before completing the returned CompletableFuture
        Returns:
        the new CompletableFuture

      • runAsync

        public static CompletableFuture<Void> runAsync​(Runnable runnable,
                                               Executor executor)
        Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor after it runs the given action.
        Parameters:
        runnable - the action to run before completing the returned CompletableFuture
        executor - the executor to use for asynchronous execution
        Returns:
        the new CompletableFuture

      • completedFuture

        public static <U> CompletableFuture<U> completedFuture​(U value)
        Returns a new CompletableFuture that is already completed with the given value.
        Type Parameters:
        U - the type of the value
        Parameters:
        value - the value
        Returns:
        the completed CompletableFuture

      • isDone

        public boolean isDone()
        Returns true if completed in any fashion: normally, exceptionally, or via cancellation.
        Specified by:
        isDone in interface Future<T>
        Returns:
        true if completed

      • join

        public T join()
        Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally. To better conform with the use of common functional forms, if a computation involved in the completion of this CompletableFuture threw an exception, this method throws an (unchecked) CompletionException with the underlying exception as its cause.
        Returns:
        the result value
        Throws:
        CancellationException - if the computation was cancelled
        CompletionException - if this future completed exceptionally or a completion computation threw an exception

      • getNow

        public T getNow​(T valueIfAbsent)
        Returns the result value (or throws any encountered exception) if completed, else returns the given valueIfAbsent.
        Parameters:
        valueIfAbsent - the value to return if not completed
        Returns:
        the result value, if completed, else the given valueIfAbsent
        Throws:
        CancellationException - if the computation was cancelled
        CompletionException - if this future completed exceptionally or a completion computation threw an exception

      • complete

        public boolean complete​(T value)
        If not already completed, sets the value returned by get() and related methods to the given value.
        Parameters:
        value - the result value
        Returns:
        true if this invocation caused this CompletableFuture to transition to a completed state, else false

      • completeExceptionally

        public boolean completeExceptionally​(Throwable ex)
        If not already completed, causes invocations of get() and related methods to throw the given exception.
        Parameters:
        ex - the exception
        Returns:
        true if this invocation caused this CompletableFuture to transition to a completed state, else false

      • exceptionally

        public CompletableFuture<T> exceptionally​(Function<Throwable,​? extends T> fn)
        Returns a new CompletableFuture that is completed when this CompletableFuture completes, with the result of the given function of the exception triggering this CompletableFuture's completion when it completes exceptionally; otherwise, if this CompletableFuture completes normally, then the returned CompletableFuture also completes normally with the same value. Note: More flexible versions of this functionality are available using methods whenComplete and handle.
        Specified by:
        exceptionally in interface CompletionStage<T>
        Parameters:
        fn - the function to use to compute the value of the returned CompletableFuture if this CompletableFuture completed exceptionally
        Returns:
        the new CompletableFuture

      • allOf

        public static CompletableFuture<Void> allOf​(CompletableFuture<?>... cfs)
        Returns a new CompletableFuture that is completed when all of the given CompletableFutures complete. If any of the given CompletableFutures complete exceptionally, then the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. Otherwise, the results, if any, of the given CompletableFutures are not reflected in the returned CompletableFuture, but may be obtained by inspecting them individually. If no CompletableFutures are provided, returns a CompletableFuture completed with the value null.

        Among the applications of this method is to await completion of a set of independent CompletableFutures before continuing a program, as in: CompletableFuture.allOf(c1, c2, c3).join();.

        Parameters:
        cfs - the CompletableFutures
        Returns:
        a new CompletableFuture that is completed when all of the given CompletableFutures complete
        Throws:
        NullPointerException - if the array or any of its elements are null

      • anyOf

        public static CompletableFuture<Object> anyOf​(CompletableFuture<?>... cfs)
        Returns a new CompletableFuture that is completed when any of the given CompletableFutures complete, with the same result. Otherwise, if it completed exceptionally, the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. If no CompletableFutures are provided, returns an incomplete CompletableFuture.
        Parameters:
        cfs - the CompletableFutures
        Returns:
        a new CompletableFuture that is completed with the result or exception of any of the given CompletableFutures when one completes
        Throws:
        NullPointerException - if the array or any of its elements are null

      • cancel

        public boolean cancel​(boolean mayInterruptIfRunning)
        If not already completed, completes this CompletableFuture with a CancellationException. Dependent CompletableFutures that have not already completed will also complete exceptionally, with a CompletionException caused by this CancellationException.
        Specified by:
        cancel in interface Future<T>
        Parameters:
        mayInterruptIfRunning - this value has no effect in this implementation because interrupts are not used to control processing.
        Returns:
        true if this task is now cancelled

      • isCancelled

        public boolean isCancelled()
        Returns true if this CompletableFuture was cancelled before it completed normally.
        Specified by:
        isCancelled in interface Future<T>
        Returns:
        true if this CompletableFuture was cancelled before it completed normally

      • isCompletedExceptionally

        public boolean isCompletedExceptionally()
        Returns true if this CompletableFuture completed exceptionally, in any way. Possible causes include cancellation, explicit invocation of completeExceptionally, and abrupt termination of a CompletionStage action.
        Returns:
        true if this CompletableFuture completed exceptionally

      • obtrudeValue

        public void obtrudeValue​(T value)
        Forcibly sets or resets the value subsequently returned by method get() and related methods, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes.
        Parameters:
        value - the completion value

      • obtrudeException

        public void obtrudeException​(Throwable ex)
        Forcibly causes subsequent invocations of method get() and related methods to throw the given exception, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes.
        Parameters:
        ex - the exception
        Throws:
        NullPointerException - if the exception is null

      • getNumberOfDependents

        public int getNumberOfDependents()
        Returns the estimated number of CompletableFutures whose completions are awaiting completion of this CompletableFuture. This method is designed for use in monitoring system state, not for synchronization control.
        Returns:
        the number of dependent CompletableFutures

      • toString

        public String toString()
        Returns a string identifying this CompletableFuture, as well as its completion state. The state, in brackets, contains the String "Completed Normally" or the String "Completed Exceptionally", or the String "Not completed" followed by the number of CompletableFutures dependent upon its completion, if any.
        Overrides:
        toString in class Object
        Returns:
        a string identifying this CompletableFuture, as well as its state

      • newIncompleteFuture

        public <U> CompletableFuture<U> newIncompleteFuture()
        Returns a new incomplete CompletableFuture of the type to be returned by a CompletionStage method. Subclasses should normally override this method to return an instance of the same class as this CompletableFuture. The default implementation returns an instance of class CompletableFuture.
        Type Parameters:
        U - the type of the value
        Returns:
        a new CompletableFuture
        Since:
        9

      • defaultExecutor

        public Executor defaultExecutor()
        Returns the default Executor used for async methods that do not specify an Executor. This class uses the ForkJoinPool.commonPool() if it supports more than one parallel thread, or else an Executor using one thread per async task. This method may be overridden in subclasses to return an Executor that provides at least one independent thread.
        Returns:
        the executor
        Since:
        9

      • copy

        public CompletableFuture<T> copy()
        Returns a new CompletableFuture that is completed normally with the same value as this CompletableFuture when it completes normally. If this CompletableFuture completes exceptionally, then the returned CompletableFuture completes exceptionally with a CompletionException with this exception as cause. The behavior is equivalent to thenApply(x -> x). This method may be useful as a form of "defensive copying", to prevent clients from completing, while still being able to arrange dependent actions.
        Returns:
        the new CompletableFuture
        Since:
        9

      • minimalCompletionStage

        public CompletionStage<T> minimalCompletionStage()
        Returns a new CompletionStage that is completed normally with the same value as this CompletableFuture when it completes normally, and cannot be independently completed or otherwise used in ways not defined by the methods of interface CompletionStage. If this CompletableFuture completes exceptionally, then the returned CompletionStage completes exceptionally with a CompletionException with this exception as cause.

        Unless overridden by a subclass, a new non-minimal CompletableFuture with all methods available can be obtained from a minimal CompletionStage via toCompletableFuture(). For example, completion of a minimal stage can be awaited by 

         minimalStage.toCompletableFuture().join();
        Returns:
        the new CompletionStage
        Since:
        9

      • completeAsync

        public CompletableFuture<T> completeAsync​(Supplier<? extends T> supplier,
                                          Executor executor)
        Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the given executor.
        Parameters:
        supplier - a function returning the value to be used to complete this CompletableFuture
        executor - the executor to use for asynchronous execution
        Returns:
        this CompletableFuture
        Since:
        9

      • completeAsync

        public CompletableFuture<T> completeAsync​(Supplier<? extends T> supplier)
        Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the default executor.
        Parameters:
        supplier - a function returning the value to be used to complete this CompletableFuture
        Returns:
        this CompletableFuture
        Since:
        9

      • orTimeout

        public CompletableFuture<T> orTimeout​(long timeout,
                                      TimeUnit unit)
        Exceptionally completes this CompletableFuture with a TimeoutException if not otherwise completed before the given timeout.
        Parameters:
        timeout - how long to wait before completing exceptionally with a TimeoutException, in units of unit
        unit - a TimeUnit determining how to interpret the timeout parameter
        Returns:
        this CompletableFuture
        Since:
        9

      • completeOnTimeout

        public CompletableFuture<T> completeOnTimeout​(T value,
                                              long timeout,
                                              TimeUnit unit)
        Completes this CompletableFuture with the given value if not otherwise completed before the given timeout.
        Parameters:
        value - the value to use upon timeout
        timeout - how long to wait before completing normally with the given value, in units of unit
        unit - a TimeUnit determining how to interpret the timeout parameter
        Returns:
        this CompletableFuture
        Since:
        9

      • delayedExecutor

        public static Executor delayedExecutor​(long delay,
                                       TimeUnit unit,
                                       Executor executor)
        Returns a new Executor that submits a task to the given base executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's execute method.
        Parameters:
        delay - how long to delay, in units of unit
        unit - a TimeUnit determining how to interpret the delay parameter
        executor - the base executor
        Returns:
        the new delayed executor
        Since:
        9

      • delayedExecutor

        public static Executor delayedExecutor​(long delay,
                                       TimeUnit unit)
        Returns a new Executor that submits a task to the default executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's execute method.
        Parameters:
        delay - how long to delay, in units of unit
        unit - a TimeUnit determining how to interpret the delay parameter
        Returns:
        the new delayed executor
        Since:
        9

      • completedStage

        public static <U> CompletionStage<U> completedStage​(U value)
        Returns a new CompletionStage that is already completed with the given value and supports only those methods in interface CompletionStage.
        Type Parameters:
        U - the type of the value
        Parameters:
        value - the value
        Returns:
        the completed CompletionStage
        Since:
        9

      • failedFuture

        public static <U> CompletableFuture<U> failedFuture​(Throwable ex)
        Returns a new CompletableFuture that is already completed exceptionally with the given exception.
        Type Parameters:
        U - the type of the value
        Parameters:
        ex - the exception
        Returns:
        the exceptionally completed CompletableFuture
        Since:
        9

      • failedStage

        public static <U> CompletionStage<U> failedStage​(Throwable ex)
        Returns a new CompletionStage that is already completed exceptionally with the given exception and supports only those methods in interface CompletionStage.
        Type Parameters:
        U - the type of the value
        Parameters:
        ex - the exception
        Returns:
        the exceptionally completed CompletionStage
        Since:
        9