jdk.util.concurrent.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 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.
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.cancelclj
(cancel this may-interrupt-if-running)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 isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true.
may-interrupt-if-running - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete - boolean
returns: false if the task could not be cancelled,
typically because it has already completed normally;
true otherwise - boolean
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 isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true. may-interrupt-if-running - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete - `boolean` returns: false if the task could not be cancelled, typically because it has already completed normally; true otherwise - `boolean`
cancelled?clj
(cancelled? this)Returns true if this task was cancelled before it completed normally.
returns: true if this task was cancelled before it completed - boolean
Returns true if this task was cancelled before it completed normally. returns: true if this task was cancelled before it completed - `boolean`
done?clj
(done? this)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.
returns: true if this task completed - boolean
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. returns: true if this task completed - `boolean`
getclj
(get this)(get this timeout unit)Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
timeout - the maximum time to wait - long
unit - the time unit of the timeout argument - java.util.concurrent.TimeUnit
returns: the computed result - V
throws: java.util.concurrent.CancellationException - if the computation was cancelled
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. timeout - the maximum time to wait - `long` unit - the time unit of the timeout argument - `java.util.concurrent.TimeUnit` returns: the computed result - `V` throws: java.util.concurrent.CancellationException - if the computation was cancelled
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