V - The result type returned by this Future's get method
RunnableFuture<V>, RunnableScheduledFuture<V>, ScheduledFuture<V>
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.
FutureTask, Executor
boolean cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
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.
boolean cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and 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 should be interrupted in an attempt to stop the task.
After this method returns, subsequent calls to isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true.
mayInterruptIfRunning - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete
false if the task could not be cancelled, typically because it has already completed normally; true otherwise
boolean isCancelled()
Returns true if this task was cancelled before it completed normally.
true if this task was cancelled before it completed
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.
true if this task completed
Waits if necessary for the computation to complete, and then retrieves its result.
CancellationException - if the computation was cancelled
ExecutionException - if the computation threw an exception
InterruptedException - if the current thread was interrupted while waiting
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
timeout - the maximum time to wait
unit - the time unit of the timeout argument
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
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