java.util.concurrent.Future Maven / Gradle / Ivy
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* published by the Free Software Foundation. Oracle designates this
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*
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*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
/**
* 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 {@code Future} and
* return null as a result of the underlying task.
*
*
* Sample Usage (Note that the following classes are all
* made-up.)
*
{@code
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(final String target)
* throws InterruptedException {
* Future future
* = executor.submit(new Callable() {
* 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 {@link 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:
* {@code
* FutureTask future =
* new FutureTask(new Callable() {
* 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 {@code Future.get()} in another thread.
*
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param The result type returned by this Future's get method
*/
public interface Future {
/**
* 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 {@link #isDone} will
* always return true. Subsequent calls to {@link #isCancelled}
* will always return true if this method returned true.
*
* @param mayInterruptIfRunning true if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return false if the task could not be cancelled,
* typically because it has already completed normally;
* true otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns true if this task was cancelled before it completed
* normally.
*
* @return true if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* 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.
*
* @return true if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}