Future
public
interface
Future
java.util.concurrent.Future<V> |
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(String target) throws InterruptedException {
Callable<String> task = () -> searcher.search(target);
Future<String> future = executor.submit(task);
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<>(task);
executor.execute(future);
Memory consistency effects: Actions taken by the asynchronous computation
happen-before
actions following the corresponding Future.get()
in another thread.
See also:
Summary
Public methods | |
---|---|
abstract
boolean
|
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. |
abstract
V
|
get(long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. |
abstract
V
|
get()
Waits if necessary for the computation to complete, and then retrieves its result. |
abstract
boolean
|
isCancelled()
Returns |
abstract
boolean
|
isDone()
Returns |
Public methods
cancel
public abstract boolean cancel (boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. This method has no
effect if the task is already completed or cancelled, or could
not be cancelled for some other reason. Otherwise, if 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 (when known by the implementation)
is interrupted in an attempt to stop the task.
The return value from this method does not necessarily
indicate whether the task is now cancelled; use isCancelled()
.
Parameters | |
---|---|
mayInterruptIfRunning |
boolean : true if the thread
executing this task should be interrupted (if the thread is
known to the implementation); otherwise, in-progress tasks are
allowed to complete |
Returns | |
---|---|
boolean |
false if the task could not be cancelled,
typically because it has already completed; true
otherwise. If two or more threads cause a task to be cancelled,
then at least one of them returns true . Implementations
may provide stronger guarantees. |
get
public abstract V get (long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
Parameters | |
---|---|
timeout |
long : the maximum time to wait |
unit |
TimeUnit : the time unit of the timeout argument |
Returns | |
---|---|
V |
the computed result |
Throws | |
---|---|
CancellationException |
if the computation was cancelled |
ExecutionException |
if the computation threw an exception |
InterruptedException |
if the current thread was interrupted while waiting |
TimeoutException |
if the wait timed out |
get
public abstract V get ()
Waits if necessary for the computation to complete, and then retrieves its result.
Returns | |
---|---|
V |
the computed result |
Throws | |
---|---|
CancellationException |
if the computation was cancelled |
ExecutionException |
if the computation threw an exception |
InterruptedException |
if the current thread was interrupted while waiting |
isCancelled
public abstract boolean isCancelled ()
Returns true
if this task was cancelled before it completed
normally.
Returns | |
---|---|
boolean |
true if this task was cancelled before it completed |
isDone
public abstract 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
.
Returns | |
---|---|
boolean |
true if this task completed |