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/* * @(#)Future.java 1.11 06/03/08 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util.concurrent; /** * A <tt>Future</tt> 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 * <tt>get</tt> when the computation has completed, blocking if * necessary until it is ready. Cancellation is performed by the * <tt>cancel</tt> 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 <tt>Future</tt> for the sake * of cancellability but not provide a usable result, you can * declare types of the form <tt>Future<?></tt> and * return <tt>null</tt> as a result of the underlying task. * * <p> * <b>Sample Usage</b> (Note that the following classes are all * made-up.) <p> * <pre> * 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; } * } * } * </pre> * * The {@link FutureTask} class is an implementation of <tt>Future</tt> that * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. * For example, the above construction with <tt>submit</tt> could be replaced by: * <pre> * FutureTask<String> future = * new FutureTask<String>(new Callable<String>() { * public String call() { * return searcher.search(target); * }}); * executor.execute(future); * </pre> * * <p>Memory consistency effects: Actions taken by the asynchronous computation * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> * actions following the corresponding {@code Future.get()} in another thread. * * @see FutureTask * @see Executor * @since 1.5 * @author Doug Lea * @param <V> The result type returned by this Future's <tt>get</tt> method */ public interface Future<V> { /** * 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 <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task. * * <p>After this method returns, subsequent calls to {@link #isDone} will * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled} * will always return <tt>true</tt> if this method returned <tt>true</tt>. * * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete * @return <tt>false</tt> if the task could not be cancelled, * typically because it has already completed normally; * <tt>true</tt> otherwise */ boolean cancel(boolean mayInterruptIfRunning); /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. * * @return <tt>true</tt> if this task was cancelled before it completed */ boolean isCancelled(); /** * Returns <tt>true</tt> if this task completed. * * Completion may be due to normal termination, an exception, or * cancellation -- in all of these cases, this method will return * <tt>true</tt>. * * @return <tt>true</tt> 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; }