Known Indirect Subclasses
CompletableFuture<T>,
CountedCompleter<T>,
ForkJoinTask<V>,
FutureTask<V>,
RecursiveAction,
RecursiveTask<V>,
RunnableFuture<V>,
RunnableScheduledFuture<V>,
ScheduledFuture<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(final String target)
throws InterruptedException {
Future<String> future
= executor.submit(new Callable<String>() {
public String call() {
return searcher.search(target);
}});
displayOtherThings(); // do other things while searching
try {
displayText(future.get()); // use future
} catch (ExecutionException ex) { cleanup(); return; }
}
}
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<>(new Callable<String>() {
public String call() {
return searcher.search(target);
}});
executor.execute(future);
Memory consistency effects: Actions taken by the asynchronous computation
happen-before
actions following the corresponding Future.get()
in another thread.
See Also
Public Method Summary
abstract boolean |
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
|
abstract V | |
abstract V |
get()
Waits if necessary for the computation to complete, and then
retrieves its result.
|
abstract boolean |
isCancelled()
Returns
true if this task was cancelled before it completed
normally. |
abstract boolean |
isDone()
Returns
true if this task completed. |
Public Methods
public abstract 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
.
Parameters
mayInterruptIfRunning | true if the thread executing this
task should be interrupted; otherwise, in-progress tasks are allowed
to complete |
---|
Returns
false
if the task could not be cancelled, typically because it has already completed normally;true
otherwise
public abstract 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.
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 |
public abstract V get ()
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 |
public abstract boolean isCancelled ()
Returns true
if this task was cancelled before it completed
normally.
Returns
true
if this task was cancelled before it completed
public abstract 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