Future
interface Future<V : Any!>
java.util.concurrent.Future |
A 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 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.)
<code>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; } } }</code>
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:
<code>FutureTask<String> future = new FutureTask<>(task); executor.execute(future);</code>
Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get()
in another thread.
Summary
Public methods | |
---|---|
abstract Boolean |
Attempts to cancel execution of this task. |
abstract V |
get() Waits if necessary for the computation to complete, and then retrieves its result. |
abstract V |
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. |
abstract Boolean |
Returns |
abstract Boolean |
isDone() Returns |
Public methods
cancel
abstract fun cancel(mayInterruptIfRunning: Boolean): Boolean
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 |
Boolean: 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 |
Return | |
---|---|
Boolean |
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. |
get
abstract fun get(): V
Waits if necessary for the computation to complete, and then retrieves its result.
Return | |
---|---|
V |
the computed result |
Exceptions | |
---|---|
java.util.concurrent.CancellationException |
if the computation was cancelled |
java.util.concurrent.ExecutionException |
if the computation threw an exception |
java.lang.InterruptedException |
if the current thread was interrupted while waiting |
get
abstract fun get(
timeout: Long,
unit: TimeUnit!
): V
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
Parameters | |
---|---|
timeout |
Long: the maximum time to wait |
unit |
TimeUnit!: the time unit of the timeout argument |
Return | |
---|---|
V |
the computed result |
Exceptions | |
---|---|
java.util.concurrent.CancellationException |
if the computation was cancelled |
java.util.concurrent.ExecutionException |
if the computation threw an exception |
java.lang.InterruptedException |
if the current thread was interrupted while waiting |
java.util.concurrent.TimeoutException |
if the wait timed out |
isCancelled
abstract fun isCancelled(): Boolean
Returns true
if this task was cancelled before it completed normally.
Return | |
---|---|
Boolean |
true if this task was cancelled before it completed |
isDone
abstract fun isDone(): Boolean
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
.
Return | |
---|---|
Boolean |
true if this task completed |