Futurepublic 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. |
| Methods Summary |
|---|
| public 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 {@link #isDone} will
always return true. Subsequent calls to {@link #isCancelled}
will always return true if this method returned true.
| | public V | get()Waits if necessary for the computation to complete, and then
retrieves its result.
| | public V | get(long timeout, java.util.concurrent.TimeUnit unit)Waits if necessary for at most the given time for the computation
to complete, and then retrieves its result, if available.
| | public boolean | isCancelled()Returns true if this task was cancelled before it completed
normally.
| | public 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.
|
|
