Module java.base

Package java.util.concurrent

Interface Future<V>

Type Parameters:

V - The result type returned by this Future's get method

All Known Subinterfaces:

RunnableFuture<V>, RunnableScheduledFuture<V>, ScheduledFuture<V>

All Known Implementing Classes:

CompletableFuture, CountedCompleter, ForkJoinTask, FutureTask, RecursiveAction, RecursiveTask, SwingWorker

public interface Future<V>

A Future represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future<?> and return null as a result of the underlying task.

Sample Usage (Note that the following classes are all made-up.)

 
 interface ArchiveSearcher { String search(String target); }
 class App {
   ExecutorService executor = ...;
   ArchiveSearcher searcher = ...;
   void showSearch(String target) throws InterruptedException {
     Callable<String> task = () -> searcher.search(target);
     Future<String> future = executor.submit(task);
     displayOtherThings(); // do other things while searching
     try {
       displayText(future.get()); // use future
     } catch (ExecutionException ex) { cleanup(); return; }
   }
 }

The FutureTask class is an implementation of Future that implements Runnable, and so may be executed by an Executor. For example, the above construction with submit could be replaced by:

 
 FutureTask<String> future = new FutureTask<>(task);
 executor.execute(future);

Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.

Since:

1.5

See Also:

• FutureTask
• Executor

Nested Class Summary

Nested Classes

Modifier and Type	Interface	Description
static enum	Future.State	Represents the computation state.

Method Summary

Modifier and Type	Method	Description
boolean	cancel(boolean mayInterruptIfRunning)	Attempts to cancel execution of this task.
default Throwable	exceptionNow()	Returns the exception thrown by the task, without waiting.
V	get()	Waits if necessary for the computation to complete, and then retrieves its result.
V	get(long timeout, TimeUnit unit)	Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
boolean	isCancelled()	Returns true if this task was cancelled before it completed normally.
boolean	isDone()	Returns true if this task completed.
default V	resultNow()	Returns the computed result, without waiting.
default Future.State	state()	Returns the computation state.

Method Details

cancel

boolean cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This method has no effect if the task is already completed or cancelled, or could not be cancelled for some other reason. Otherwise, if this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task (when known by the implementation) is interrupted in an attempt to stop the task.

The return value from this method does not necessarily indicate whether the task is now cancelled; use isCancelled().

Parameters:

mayInterruptIfRunning - true if the thread executing this task should be interrupted (if the thread is known to the implementation); otherwise, in-progress tasks are allowed to complete

Returns:

false if the task could not be cancelled, typically because it has already completed; true otherwise. If two or more threads cause a task to be cancelled, then at least one of them returns true. Implementations may provide stronger guarantees.

isCancelled

boolean isCancelled()

Returns true if this task was cancelled before it completed normally.

Returns:

true if this task was cancelled before it completed

isDone

boolean isDone()

Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

Returns:

true if this task completed

get

V get()
throws InterruptedException,
ExecutionException

Waits if necessary for the computation to complete, and then retrieves its result.

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

ExecutionException - if the computation threw an exception

InterruptedException - if the current thread was interrupted while waiting

get

V get(long timeout,
 TimeUnit unit)
throws InterruptedException,
ExecutionException,
TimeoutException

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Parameters:

timeout - the maximum time to wait

unit - the time unit of the timeout argument

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

ExecutionException - if the computation threw an exception

InterruptedException - if the current thread was interrupted while waiting

TimeoutException - if the wait timed out

resultNow

default V resultNow()

Returns the computed result, without waiting.

This method is for cases where the caller knows that the task has already completed successfully, for example when filtering a stream of Future objects for the successful tasks and using a mapping operation to obtain a stream of results.

    results = futures.stream()
               .filter(f -> f.state() == Future.State.SUCCESS)
               .map(Future::resultNow)
               .toList();

Implementation Requirements:

The default implementation invokes isDone() to test if the task has completed. If done, it invokes get() to obtain the result.

Returns:

the computed result

Throws:

IllegalStateException - if the task has not completed or the task did not complete with a result

Since:

19

exceptionNow

default Throwable exceptionNow()

Returns the exception thrown by the task, without waiting.

This method is for cases where the caller knows that the task has already completed with an exception.

Implementation Requirements:

The default implementation invokes isDone() to test if the task has completed. If done and not cancelled, it invokes get() and catches the ExecutionException to obtain the exception.

Returns:

the exception thrown by the task

Throws:

IllegalStateException - if the task has not completed, the task completed normally, or the task was cancelled

Since:

19

state

default Future.State state()

Returns the computation state.

Implementation Requirements:

The default implementation uses isDone(), isCancelled(), and get() to determine the state.

Returns:

the computation state

Since:

19