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Java™ Platform Standard Ed. 6 |
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V
- The result type returned by this Future's get methodpublic 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(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; } } }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<String>(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.
FutureTask
,
Executor
Method Summary | |
---|---|
boolean |
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. |
V |
get()
Waits if necessary for the computation to complete, and then retrieves its result. |
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. |
boolean |
isCancelled()
Returns true if this task was cancelled before it completed normally. |
boolean |
isDone()
Returns true if this task completed. |
Method Detail |
---|
boolean cancel(boolean mayInterruptIfRunning)
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
boolean isCancelled()
boolean isDone()
V get() throws InterruptedException, ExecutionException
CancellationException
- if the computation was cancelled
ExecutionException
- if the computation threw an
exception
InterruptedException
- 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 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
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Java™ Platform Standard Ed. 6 |
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