V
- The result type returned by this Future's get
methodRunnableFuture<V>
, RunnableScheduledFuture<V>
, ScheduledFuture<V>
CompletableFuture
, CountedCompleter
, ForkJoinTask
, FutureTask
, RecursiveAction
, RecursiveTask
, SwingWorker
public interface Future<V>
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.
Modifier and Type | Interface | Description |
---|---|---|
static enum |
Future.State |
Represents the computation state. |
Modifier and Type | Method | Description |
---|---|---|
boolean |
cancel |
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 |
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. |
boolean cancel(boolean mayInterruptIfRunning)
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 completefalse
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()
true
if this task was cancelled before it completed normally.true
if this task was cancelled before it completedboolean isDone()
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 completedV get() throws InterruptedException, ExecutionException
CancellationException
- if the computation was cancelledExecutionException
- if the computation threw an exceptionInterruptedException
- if the current thread was interrupted while waitingV get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
timeout
- the maximum time to waitunit
- the time unit of the timeout argumentCancellationException
- if the computation was cancelledExecutionException
- if the computation threw an exceptionInterruptedException
- if the current thread was interrupted while waitingTimeoutException
- if the wait timed outdefault V resultNow()
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 resultdefault Throwable exceptionNow()
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 cancelleddefault Future.State state()
isDone()
, isCancelled()
, and get()
to determine the state.
© 1993, 2023, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/Future.html