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.
Nested Classes
static enum
Represents the computation state.
boolean
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
Returns the exception thrown by the task, without waiting.
Waits if necessary for the computation to complete, and then retrieves its result.
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
boolean
Returns true if this task was cancelled before it completed normally.
boolean
Returns true if this task completed.
Returns the computed result, without waiting.
Returns the computation state.
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().
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
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.
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
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();
isDone() to test if the task has completed. If done, it invokes get() to obtain the result.
IllegalStateException - if the task has not completed or the task did not complete with a result
This method is for cases where the caller knows that the task has already completed with an exception.
isDone() to test if the task has completed. If done and not cancelled, it invokes get() and catches the ExecutionException to obtain the exception.
IllegalStateException - if the task has not completed, the task completed normally, or the task was cancelled
Returns the computation state.
isDone(), isCancelled(), and get() to determine the state.
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