Future (Java Platform SE 7 ) (original) (raw)

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.