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26 * This file is available under and governed by the GNU General Public
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31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
36 package java.util.concurrent;
39 * A <tt>Future</tt> represents the result of an asynchronous
40 * computation. Methods are provided to check if the computation is
41 * complete, to wait for its completion, and to retrieve the result of
42 * the computation. The result can only be retrieved using method
43 * <tt>get</tt> when the computation has completed, blocking if
44 * necessary until it is ready. Cancellation is performed by the
45 * <tt>cancel</tt> method. Additional methods are provided to
46 * determine if the task completed normally or was cancelled. Once a
47 * computation has completed, the computation cannot be cancelled.
48 * If you would like to use a <tt>Future</tt> for the sake
49 * of cancellability but not provide a usable result, you can
50 * declare types of the form {@code Future<?>} and
51 * return <tt>null</tt> as a result of the underlying task.
54 * <b>Sample Usage</b> (Note that the following classes are all
57 * interface ArchiveSearcher { String search(String target); }
59 * ExecutorService executor = ...
60 * ArchiveSearcher searcher = ...
61 * void showSearch(final String target)
62 * throws InterruptedException {
63 * Future<String> future
64 * = executor.submit(new Callable<String>() {
65 * public String call() {
66 * return searcher.search(target);
68 * displayOtherThings(); // do other things while searching
70 * displayText(future.get()); // use future
71 * } catch (ExecutionException ex) { cleanup(); return; }
75 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that
76 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
77 * For example, the above construction with <tt>submit</tt> could be replaced by:
79 * FutureTask<String> future =
80 * new FutureTask<String>(new Callable<String>() {
81 * public String call() {
82 * return searcher.search(target);
84 * executor.execute(future);}</pre>
86 * <p>Memory consistency effects: Actions taken by the asynchronous computation
87 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
88 * actions following the corresponding {@code Future.get()} in another thread.
94 * @param <V> The result type returned by this Future's <tt>get</tt> method
96 public interface Future<V> {
99 * Attempts to cancel execution of this task. This attempt will
100 * fail if the task has already completed, has already been cancelled,
101 * or could not be cancelled for some other reason. If successful,
102 * and this task has not started when <tt>cancel</tt> is called,
103 * this task should never run. If the task has already started,
104 * then the <tt>mayInterruptIfRunning</tt> parameter determines
105 * whether the thread executing this task should be interrupted in
106 * an attempt to stop the task.
108 * <p>After this method returns, subsequent calls to {@link #isDone} will
109 * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
110 * will always return <tt>true</tt> if this method returned <tt>true</tt>.
112 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
113 * task should be interrupted; otherwise, in-progress tasks are allowed
115 * @return <tt>false</tt> if the task could not be cancelled,
116 * typically because it has already completed normally;
117 * <tt>true</tt> otherwise
119 boolean cancel(boolean mayInterruptIfRunning);
122 * Returns <tt>true</tt> if this task was cancelled before it completed
125 * @return <tt>true</tt> if this task was cancelled before it completed
127 boolean isCancelled();
130 * Returns <tt>true</tt> if this task completed.
132 * Completion may be due to normal termination, an exception, or
133 * cancellation -- in all of these cases, this method will return
136 * @return <tt>true</tt> if this task completed
141 * Waits if necessary for the computation to complete, and then
142 * retrieves its result.
144 * @return the computed result
145 * @throws CancellationException if the computation was cancelled
146 * @throws ExecutionException if the computation threw an
148 * @throws InterruptedException if the current thread was interrupted
151 V get() throws InterruptedException, ExecutionException;
154 * Waits if necessary for at most the given time for the computation
155 * to complete, and then retrieves its result, if available.
157 * @param timeout the maximum time to wait
158 * @param unit the time unit of the timeout argument
159 * @return the computed result
160 * @throws CancellationException if the computation was cancelled
161 * @throws ExecutionException if the computation threw an
163 * @throws InterruptedException if the current thread was interrupted
165 * @throws TimeoutException if the wait timed out
167 V get(long timeout, TimeUnit unit)
168 throws InterruptedException, ExecutionException, TimeoutException;