java.util.concurrent.Future<V> |
Known Indirect Subclasses
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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.)
Theinterface ArchiveSearcher { String search(String target);
class App { ExecutorService executor = ... ArchiveSearcher searcher = ... void showSearch(final String target) throws InterruptedException { Futurefuture = executor.submit(new Callable () { 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 future =
new FutureTask(new Callable() {
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.
Public Methods | |||||||||||
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Attempts to cancel execution of this task.
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Waits if necessary for the computation to complete, and then
retrieves its result.
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Waits if necessary for at most the given time for the computation
to complete, and then retrieves its result, if available.
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Returns true if this task was cancelled before it completed
normally.
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Returns true if this task completed.
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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 |
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Waits if necessary for the computation to complete, and then retrieves its result.
CancellationException | if the computation was cancelled |
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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 |
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unit | the time unit of the timeout argument |
CancellationException | if the computation was cancelled |
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ExecutionException | if the computation threw an exception |
InterruptedException | if the current thread was interrupted while waiting |
TimeoutException | if the wait timed out |
Returns true if this task was cancelled before it completed normally.
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.