Module java.base
Package java.util.concurrent

Class CompletableFuture<T>

java.lang.Object
java.util.concurrent.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