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public interface Future
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 {@link 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 {@code Future.get()} in another thread.
Method Summary | |
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boolean |
cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task. |
Object |
get() Waits if necessary for the computation to complete, and then retrieves its result. |
Object |
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. |
boolean |
Returns true if this task was cancelled before it completed normally. |
boolean |
isDone() Returns true if this task completed. |
Method Detail |
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public boolean cancel(boolean mayInterruptIfRunning)
After this method returns, subsequent calls to {@link #isDone} will always return true. Subsequent calls to {@link #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 completepublic Object get() throws InterruptedException, ExecutionException
InterruptedException
- if the current thread was interrupted
while waitingExecutionException
- if the computation threw an
exceptionpublic Object get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
timeout
- the maximum time to waitunit
- the time unit of the timeout argumentInterruptedException
- if the current thread was interrupted
while waitingExecutionException
- if the computation threw an
exceptionTimeoutException
- if the wait timed outpublic boolean isCancelled()
public boolean isDone()
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