2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain, as explained at
4 * http://creativecommons.org/licenses/publicdomain
7 package java.util.concurrent;
10 * A <tt>Future</tt> represents the result of an asynchronous
11 * computation. Methods are provided to check if the computation is
12 * complete, to wait for its completion, and to retrieve the result of
13 * the computation. The result can only be retrieved using method
14 * <tt>get</tt> when the computation has completed, blocking if
15 * necessary until it is ready. Cancellation is performed by the
16 * <tt>cancel</tt> method. Additional methods are provided to
17 * determine if the task completed normally or was cancelled. Once a
18 * computation has completed, the computation cannot be cancelled.
19 * If you would like to use a <tt>Future</tt> for the sake
20 * of cancellability but not provide a usable result, you can
21 * declare types of the form <tt>Future<?></tt> and
22 * return <tt>null</tt> as a result of the underlying task.
25 * <b>Sample Usage</b> (Note that the following classes are all
28 * interface ArchiveSearcher { String search(String target); }
30 * ExecutorService executor = ...
31 * ArchiveSearcher searcher = ...
32 * void showSearch(final String target)
33 * throws InterruptedException {
34 * Future<String> future
35 * = executor.submit(new Callable<String>() {
36 * public String call() {
37 * return searcher.search(target);
39 * displayOtherThings(); // do other things while searching
41 * displayText(future.get()); // use future
42 * } catch (ExecutionException ex) { cleanup(); return; }
47 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that
48 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
49 * For example, the above construction with <tt>submit</tt> could be replaced by:
51 * FutureTask<String> future =
52 * new FutureTask<String>(new Callable<String>() {
53 * public String call() {
54 * return searcher.search(target);
56 * executor.execute(future);
59 * <p>Memory consistency effects: Actions taken by the asynchronous computation
60 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
61 * actions following the corresponding {@code Future.get()} in another thread.
67 * @param <V> The result type returned by this Future's <tt>get</tt> method
69 public interface Future<V> {
72 * Attempts to cancel execution of this task. This attempt will
73 * fail if the task has already completed, has already been cancelled,
74 * or could not be cancelled for some other reason. If successful,
75 * and this task has not started when <tt>cancel</tt> is called,
76 * this task should never run. If the task has already started,
77 * then the <tt>mayInterruptIfRunning</tt> parameter determines
78 * whether the thread executing this task should be interrupted in
79 * an attempt to stop the task.
81 * <p>After this method returns, subsequent calls to {@link #isDone} will
82 * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
83 * will always return <tt>true</tt> if this method returned <tt>true</tt>.
85 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
86 * task should be interrupted; otherwise, in-progress tasks are allowed
88 * @return <tt>false</tt> if the task could not be cancelled,
89 * typically because it has already completed normally;
90 * <tt>true</tt> otherwise
92 boolean cancel(boolean mayInterruptIfRunning);
95 * Returns <tt>true</tt> if this task was cancelled before it completed
98 * @return <tt>true</tt> if this task was cancelled before it completed
100 boolean isCancelled();
103 * Returns <tt>true</tt> if this task completed.
105 * Completion may be due to normal termination, an exception, or
106 * cancellation -- in all of these cases, this method will return
109 * @return <tt>true</tt> if this task completed
114 * Waits if necessary for the computation to complete, and then
115 * retrieves its result.
117 * @return the computed result
118 * @throws CancellationException if the computation was cancelled
119 * @throws ExecutionException if the computation threw an
121 * @throws InterruptedException if the current thread was interrupted
124 V get() throws InterruptedException, ExecutionException;
127 * Waits if necessary for at most the given time for the computation
128 * to complete, and then retrieves its result, if available.
130 * @param timeout the maximum time to wait
131 * @param unit the time unit of the timeout argument
132 * @return the computed result
133 * @throws CancellationException if the computation was cancelled
134 * @throws ExecutionException if the computation threw an
136 * @throws InterruptedException if the current thread was interrupted
138 * @throws TimeoutException if the wait timed out
140 V get(long timeout, TimeUnit unit)
141 throws InterruptedException, ExecutionException, TimeoutException;