Interface Future<V>
- Type Parameters:
V- The result type returned by this Future'sgetmethod
- All Known Subinterfaces:
RunnableFuture<V>,RunnableScheduledFuture<V>,ScheduledFuture<V>
- All Known Implementing Classes:
CompletableFuture,CountedCompleter,ForkJoinTask,FutureTask,RecursiveAction,RecursiveTask,SwingWorker
public interface 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.
- Since:
- 1.5
- See Also:
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Nested Class Summary
Nested Classes -
Method Summary
Modifier and TypeMethodDescriptionbooleancancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.default ThrowableReturns the exception thrown by the task, without waiting.get()Waits if necessary for the computation to complete, and then retrieves its result.Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.booleanReturnstrueif this task was cancelled before it completed normally.booleanisDone()Returnstrueif this task completed.default VReturns the computed result, without waiting.default Future.Statestate()Returns the computation state.
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Method Details
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cancel
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 whencancelis called, this task should never run. If the task has already started, then themayInterruptIfRunningparameter 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-trueif 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:
falseif the task could not be cancelled, typically because it has already completed;trueotherwise. If two or more threads cause a task to be cancelled, then at least one of them returnstrue. Implementations may provide stronger guarantees.
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isCancelled
boolean isCancelled()Returnstrueif this task was cancelled before it completed normally.- Returns:
trueif this task was cancelled before it completed
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isDone
boolean isDone()Returnstrueif this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will returntrue.- Returns:
trueif this task completed
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get
Waits if necessary for the computation to complete, and then retrieves its result.- Returns:
- the computed result
- Throws:
CancellationException- if the computation was cancelledExecutionException- if the computation threw an exceptionInterruptedException- if the current thread was interrupted while waiting
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get
V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.- Parameters:
timeout- the maximum time to waitunit- the time unit of the timeout argument- Returns:
- the computed result
- Throws:
CancellationException- if the computation was cancelledExecutionException- if the computation threw an exceptionInterruptedException- if the current thread was interrupted while waitingTimeoutException- if the wait timed out
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resultNow
Returns the computed result, without waiting.This method is for cases where the caller knows that the task has already completed successfully, for example when filtering a stream of Future objects for the successful tasks and using a mapping operation to obtain a stream of results.
results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList();- Implementation Requirements:
- The default implementation invokes
isDone()to test if the task has completed. If done, it invokesget()to obtain the result. - Returns:
- the computed result
- Throws:
IllegalStateException- if the task has not completed or the task did not complete with a result- Since:
- 19
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exceptionNow
Returns the exception thrown by the task, without waiting.This method is for cases where the caller knows that the task has already completed with an exception.
- Implementation Requirements:
- The default implementation invokes
isDone()to test if the task has completed. If done and not cancelled, it invokesget()and catches theExecutionExceptionto obtain the exception. - Returns:
- the exception thrown by the task
- Throws:
IllegalStateException- if the task has not completed, the task completed normally, or the task was cancelled- Since:
- 19
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state
Returns the computation state.- Implementation Requirements:
- The default implementation uses
isDone(),isCancelled(), andget()to determine the state. - Returns:
- the computation state
- Since:
- 19
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