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.