util/concurrent/ExecutorService.java

/*
 * 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;
import java.util.List;
import java.util.Collection;

/**
 * An {@link Executor} that provides methods to manage termination and
 * methods that can produce a {@link Future} for tracking progress of
 * one or more asynchronous tasks.
 *
 * <p> An <tt>ExecutorService</tt> can be shut down, which will cause
 * it to reject new tasks.  Two different methods are provided for
 * shutting down an <tt>ExecutorService</tt>. The {@link #shutdown}
 * method will allow previously submitted tasks to execute before
 * terminating, while the {@link #shutdownNow} method prevents waiting
 * tasks from starting and attempts to stop currently executing tasks.
 * Upon termination, an executor has no tasks actively executing, no
 * tasks awaiting execution, and no new tasks can be submitted.  An
 * unused <tt>ExecutorService</tt> should be shut down to allow
 * reclamation of its resources.
 *
 * <p> Method <tt>submit</tt> extends base method {@link
 * Executor#execute} by creating and returning a {@link Future} that
 * can be used to cancel execution and/or wait for completion.
 * Methods <tt>invokeAny</tt> and <tt>invokeAll</tt> perform the most
 * commonly useful forms of bulk execution, executing a collection of
 * tasks and then waiting for at least one, or all, to
 * complete. (Class {@link ExecutorCompletionService} can be used to
 * write customized variants of these methods.)
 *
 * <p>The {@link Executors} class provides factory methods for the
 * executor services provided in this package.
 *
 * <h3>Usage Examples</h3>
 *
 * Here is a sketch of a network service in which threads in a thread
 * pool service incoming requests. It uses the preconfigured {@link
 * Executors#newFixedThreadPool} factory method:
 *
 * <pre>
 * class NetworkService implements Runnable {
 *   private final ServerSocket serverSocket;
 *   private final ExecutorService pool;
 *
 *   public NetworkService(int port, int poolSize)
 *       throws IOException {
 *     serverSocket = new ServerSocket(port);
 *     pool = Executors.newFixedThreadPool(poolSize);
 *   }
 *
 *   public void run() { // run the service
 *     try {
 *       for (;;) {
 *         pool.execute(new Handler(serverSocket.accept()));
 *       }
 *     } catch (IOException ex) {
 *       pool.shutdown();
 *     }
 *   }
 * }
 *
 * class Handler implements Runnable {
 *   private final Socket socket;
 *   Handler(Socket socket) { this.socket = socket; }
 *   public void run() {
 *     // read and service request on socket
 *   }
 * }
 * </pre>
 *
 * The following method shuts down an <tt>ExecutorService</tt> in two phases,
 * first by calling <tt>shutdown</tt> to reject incoming tasks, and then
 * calling <tt>shutdownNow</tt>, if necessary, to cancel any lingering tasks:
 *
 * <pre>
 * void shutdownAndAwaitTermination(ExecutorService pool) {
 *   pool.shutdown(); // Disable new tasks from being submitted
 *   try {
 *     // Wait a while for existing tasks to terminate
 *     if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
 *       pool.shutdownNow(); // Cancel currently executing tasks
 *       // Wait a while for tasks to respond to being cancelled
 *       if (!pool.awaitTermination(60, TimeUnit.SECONDS))
 *           System.err.println("Pool did not terminate");
 *     }
 *   } catch (InterruptedException ie) {
 *     // (Re-)Cancel if current thread also interrupted
 *     pool.shutdownNow();
 *     // Preserve interrupt status
 *     Thread.currentThread().interrupt();
 *   }
 * }
 * </pre>
 *
 * <p>Memory consistency effects: Actions in a thread prior to the
 * submission of a {@code Runnable} or {@code Callable} task to an
 * {@code ExecutorService}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * any actions taken by that task, which in turn <i>happen-before</i> the
 * result is retrieved via {@code Future.get()}.
 *
 * @since 1.5
 * @author Doug Lea
 */
public interface ExecutorService extends Executor {

    /**
     * Initiates an orderly shutdown in which previously submitted
     * tasks are executed, but no new tasks will be accepted.
     * Invocation has no additional effect if already shut down.
     *
     * <p>This method does not wait for previously submitted tasks to
     * complete execution.  Use {@link #awaitTermination awaitTermination}
     * to do that.
     *
     * @throws SecurityException if a security manager exists and
     *         shutting down this ExecutorService may manipulate
     *         threads that the caller is not permitted to modify
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
     *         or the security manager's <tt>checkAccess</tt> method
     *         denies access.
     */
    void shutdown();

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution.
     *
     * <p>This method does not wait for actively executing tasks to
     * terminate.  Use {@link #awaitTermination awaitTermination} to
     * do that.
     *
     * <p>There are no guarantees beyond best-effort attempts to stop
     * processing actively executing tasks.  For example, typical
     * implementations will cancel via {@link Thread#interrupt}, so any
     * task that fails to respond to interrupts may never terminate.
     *
     * @return list of tasks that never commenced execution
     * @throws SecurityException if a security manager exists and
     *         shutting down this ExecutorService may manipulate
     *         threads that the caller is not permitted to modify
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
     *         or the security manager's <tt>checkAccess</tt> method
     *         denies access.
     */
    List<Runnable> shutdownNow();

    /**
     * Returns <tt>true</tt> if this executor has been shut down.
     *
     * @return <tt>true</tt> if this executor has been shut down
     */
    boolean isShutdown();

    /**
     * Returns <tt>true</tt> if all tasks have completed following shut down.
     * Note that <tt>isTerminated</tt> is never <tt>true</tt> unless
     * either <tt>shutdown</tt> or <tt>shutdownNow</tt> was called first.
     *
     * @return <tt>true</tt> if all tasks have completed following shut down
     */
    boolean isTerminated();

    /**
     * Blocks until all tasks have completed execution after a shutdown
     * request, or the timeout occurs, or the current thread is
     * interrupted, whichever happens first.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return <tt>true</tt> if this executor terminated and
     *         <tt>false</tt> if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException;


    /**
     * Submits a value-returning task for execution and returns a
     * Future representing the pending results of the task. The
     * Future's <tt>get</tt> method will return the task's result upon
     * successful completion.
     *
     * <p>
     * If you would like to immediately block waiting
     * for a task, you can use constructions of the form
     * <tt>result = exec.submit(aCallable).get();</tt>
     *
     * <p> Note: The {@link Executors} class includes a set of methods
     * that can convert some other common closure-like objects,
     * for example, {@link java.security.PrivilegedAction} to
     * {@link Callable} form so they can be submitted.
     *
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Callable<T> task);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's <tt>get</tt> method will
     * return the given result upon successful completion.
     *
     * @param task the task to submit
     * @param result the result to return
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Runnable task, T result);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's <tt>get</tt> method will
     * return <tt>null</tt> upon <em>successful</em> completion.
     *
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future<?> submit(Runnable task);

    /**
     * Executes the given tasks, returning a list of Futures holding
     * their status and results when all complete.
     * {@link Future#isDone} is <tt>true</tt> for each
     * element of the returned list.
     * Note that a <em>completed</em> task could have
     * terminated either normally or by throwing an exception.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @return A list of Futures representing the tasks, in the same
     *         sequential order as produced by the iterator for the
     *         given task list, each of which has completed.
     * @throws InterruptedException if interrupted while waiting, in
     *         which case unfinished tasks are cancelled.
     * @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
     * @throws RejectedExecutionException if any task cannot be
     *         scheduled for execution
     */

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException;

    /**
     * Executes the given tasks, returning a list of Futures holding
     * their status and results
     * when all complete or the timeout expires, whichever happens first.
     * {@link Future#isDone} is <tt>true</tt> for each
     * element of the returned list.
     * Upon return, tasks that have not completed are cancelled.
     * Note that a <em>completed</em> task could have
     * terminated either normally or by throwing an exception.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return a list of Futures representing the tasks, in the same
     *         sequential order as produced by the iterator for the
     *         given task list. If the operation did not time out,
     *         each task will have completed. If it did time out, some
     *         of these tasks will not have completed.
     * @throws InterruptedException if interrupted while waiting, in
     *         which case unfinished tasks are cancelled
     * @throws NullPointerException if tasks, any of its elements, or
     *         unit are <tt>null</tt>
     * @throws RejectedExecutionException if any task cannot be scheduled
     *         for execution
     */
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                  long timeout, TimeUnit unit)
        throws InterruptedException;

    /**
     * Executes the given tasks, returning the result
     * of one that has completed successfully (i.e., without throwing
     * an exception), if any do. Upon normal or exceptional return,
     * tasks that have not completed are cancelled.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @return the result returned by one of the tasks
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks or any element task
     *         subject to execution is <tt>null</tt>
     * @throws IllegalArgumentException if tasks is empty
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException;

    /**
     * Executes the given tasks, returning the result
     * of one that has completed successfully (i.e., without throwing
     * an exception), if any do before the given timeout elapses.
     * Upon normal or exceptional return, tasks that have not
     * completed are cancelled.
     * The results of this method are undefined if the given
     * collection is modified while this operation is in progress.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the result returned by one of the tasks.
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks, or unit, or any element
     *         task subject to execution is <tt>null</tt>
     * @throws TimeoutException if the given timeout elapses before
     *         any task successfully completes
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}
Revision:	1.52
Committed:	Wed Jun 8 00:50:35 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.51:	+0 -2 lines
Log Message:	clean up imports
#	Content
1	/*
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/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import java.util.List;
9	import java.util.Collection;
10
11	/**
12	* An {@link Executor} that provides methods to manage termination and
13	* methods that can produce a {@link Future} for tracking progress of
14	* one or more asynchronous tasks.
15	*
16	* <p> An <tt>ExecutorService</tt> can be shut down, which will cause
17	* it to reject new tasks. Two different methods are provided for
18	* shutting down an <tt>ExecutorService</tt>. The {@link #shutdown}
19	* method will allow previously submitted tasks to execute before
20	* terminating, while the {@link #shutdownNow} method prevents waiting
21	* tasks from starting and attempts to stop currently executing tasks.
22	* Upon termination, an executor has no tasks actively executing, no
23	* tasks awaiting execution, and no new tasks can be submitted. An
24	* unused <tt>ExecutorService</tt> should be shut down to allow
25	* reclamation of its resources.
26	*
27	* <p> Method <tt>submit</tt> extends base method {@link
28	* Executor#execute} by creating and returning a {@link Future} that
29	* can be used to cancel execution and/or wait for completion.
30	* Methods <tt>invokeAny</tt> and <tt>invokeAll</tt> perform the most
31	* commonly useful forms of bulk execution, executing a collection of
32	* tasks and then waiting for at least one, or all, to
33	* complete. (Class {@link ExecutorCompletionService} can be used to
34	* write customized variants of these methods.)
35	*
36	* <p>The {@link Executors} class provides factory methods for the
37	* executor services provided in this package.
38	*
39	* <h3>Usage Examples</h3>
40	*
41	* Here is a sketch of a network service in which threads in a thread
42	* pool service incoming requests. It uses the preconfigured {@link
43	* Executors#newFixedThreadPool} factory method:
44	*
45	* <pre>
46	* class NetworkService implements Runnable {
47	* private final ServerSocket serverSocket;
48	* private final ExecutorService pool;
49	*
50	* public NetworkService(int port, int poolSize)
51	* throws IOException {
52	* serverSocket = new ServerSocket(port);
53	* pool = Executors.newFixedThreadPool(poolSize);
54	* }
55	*
56	* public void run() { // run the service
57	* try {
58	* for (;;) {
59	* pool.execute(new Handler(serverSocket.accept()));
60	* }
61	* } catch (IOException ex) {
62	* pool.shutdown();
63	* }
64	* }
65	* }
66	*
67	* class Handler implements Runnable {
68	* private final Socket socket;
69	* Handler(Socket socket) { this.socket = socket; }
70	* public void run() {
71	* // read and service request on socket
72	* }
73	* }
74	* </pre>
75	*
76	* The following method shuts down an <tt>ExecutorService</tt> in two phases,
77	* first by calling <tt>shutdown</tt> to reject incoming tasks, and then
78	* calling <tt>shutdownNow</tt>, if necessary, to cancel any lingering tasks:
79	*
80	* <pre>
81	* void shutdownAndAwaitTermination(ExecutorService pool) {
82	* pool.shutdown(); // Disable new tasks from being submitted
83	* try {
84	* // Wait a while for existing tasks to terminate
85	* if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
86	* pool.shutdownNow(); // Cancel currently executing tasks
87	* // Wait a while for tasks to respond to being cancelled
88	* if (!pool.awaitTermination(60, TimeUnit.SECONDS))
89	* System.err.println("Pool did not terminate");
90	* }
91	* } catch (InterruptedException ie) {
92	* // (Re-)Cancel if current thread also interrupted
93	* pool.shutdownNow();
94	* // Preserve interrupt status
95	* Thread.currentThread().interrupt();
96	* }
97	* }
98	* </pre>
99	*
100	* <p>Memory consistency effects: Actions in a thread prior to the
101	* submission of a {@code Runnable} or {@code Callable} task to an
102	* {@code ExecutorService}
103	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
104	* any actions taken by that task, which in turn <i>happen-before</i> the
105	* result is retrieved via {@code Future.get()}.
106	*
107	* @since 1.5
108	* @author Doug Lea
109	*/
110	public interface ExecutorService extends Executor {
111
112	/**
113	* Initiates an orderly shutdown in which previously submitted
114	* tasks are executed, but no new tasks will be accepted.
115	* Invocation has no additional effect if already shut down.
116	*
117	* <p>This method does not wait for previously submitted tasks to
118	* complete execution. Use {@link #awaitTermination awaitTermination}
119	* to do that.
120	*
121	* @throws SecurityException if a security manager exists and
122	* shutting down this ExecutorService may manipulate
123	* threads that the caller is not permitted to modify
124	* because it does not hold {@link
125	* java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
126	* or the security manager's <tt>checkAccess</tt> method
127	* denies access.
128	*/
129	void shutdown();
130
131	/**
132	* Attempts to stop all actively executing tasks, halts the
133	* processing of waiting tasks, and returns a list of the tasks
134	* that were awaiting execution.
135	*
136	* <p>This method does not wait for actively executing tasks to
137	* terminate. Use {@link #awaitTermination awaitTermination} to
138	* do that.
139	*
140	* <p>There are no guarantees beyond best-effort attempts to stop
141	* processing actively executing tasks. For example, typical
142	* implementations will cancel via {@link Thread#interrupt}, so any
143	* task that fails to respond to interrupts may never terminate.
144	*
145	* @return list of tasks that never commenced execution
146	* @throws SecurityException if a security manager exists and
147	* shutting down this ExecutorService may manipulate
148	* threads that the caller is not permitted to modify
149	* because it does not hold {@link
150	* java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
151	* or the security manager's <tt>checkAccess</tt> method
152	* denies access.
153	*/
154	List<Runnable> shutdownNow();
155
156	/**
157	* Returns <tt>true</tt> if this executor has been shut down.
158	*
159	* @return <tt>true</tt> if this executor has been shut down
160	*/
161	boolean isShutdown();
162
163	/**
164	* Returns <tt>true</tt> if all tasks have completed following shut down.
165	* Note that <tt>isTerminated</tt> is never <tt>true</tt> unless
166	* either <tt>shutdown</tt> or <tt>shutdownNow</tt> was called first.
167	*
168	* @return <tt>true</tt> if all tasks have completed following shut down
169	*/
170	boolean isTerminated();
171
172	/**
173	* Blocks until all tasks have completed execution after a shutdown
174	* request, or the timeout occurs, or the current thread is
175	* interrupted, whichever happens first.
176	*
177	* @param timeout the maximum time to wait
178	* @param unit the time unit of the timeout argument
179	* @return <tt>true</tt> if this executor terminated and
180	* <tt>false</tt> if the timeout elapsed before termination
181	* @throws InterruptedException if interrupted while waiting
182	*/
183	boolean awaitTermination(long timeout, TimeUnit unit)
184	throws InterruptedException;
185
186
187	/**
188	* Submits a value-returning task for execution and returns a
189	* Future representing the pending results of the task. The
190	* Future's <tt>get</tt> method will return the task's result upon
191	* successful completion.
192	*
193	* <p>
194	* If you would like to immediately block waiting
195	* for a task, you can use constructions of the form
196	* <tt>result = exec.submit(aCallable).get();</tt>
197	*
198	* <p> Note: The {@link Executors} class includes a set of methods
199	* that can convert some other common closure-like objects,
200	* for example, {@link java.security.PrivilegedAction} to
201	* {@link Callable} form so they can be submitted.
202	*
203	* @param task the task to submit
204	* @return a Future representing pending completion of the task
205	* @throws RejectedExecutionException if the task cannot be
206	* scheduled for execution
207	* @throws NullPointerException if the task is null
208	*/
209	<T> Future<T> submit(Callable<T> task);
210
211	/**
212	* Submits a Runnable task for execution and returns a Future
213	* representing that task. The Future's <tt>get</tt> method will
214	* return the given result upon successful completion.
215	*
216	* @param task the task to submit
217	* @param result the result to return
218	* @return a Future representing pending completion of the task
219	* @throws RejectedExecutionException if the task cannot be
220	* scheduled for execution
221	* @throws NullPointerException if the task is null
222	*/
223	<T> Future<T> submit(Runnable task, T result);
224
225	/**
226	* Submits a Runnable task for execution and returns a Future
227	* representing that task. The Future's <tt>get</tt> method will
228	* return <tt>null</tt> upon <em>successful</em> completion.
229	*
230	* @param task the task to submit
231	* @return a Future representing pending completion of the task
232	* @throws RejectedExecutionException if the task cannot be
233	* scheduled for execution
234	* @throws NullPointerException if the task is null
235	*/
236	Future<?> submit(Runnable task);
237
238	/**
239	* Executes the given tasks, returning a list of Futures holding
240	* their status and results when all complete.
241	* {@link Future#isDone} is <tt>true</tt> for each
242	* element of the returned list.
243	* Note that a <em>completed</em> task could have
244	* terminated either normally or by throwing an exception.
245	* The results of this method are undefined if the given
246	* collection is modified while this operation is in progress.
247	*
248	* @param tasks the collection of tasks
249	* @return A list of Futures representing the tasks, in the same
250	* sequential order as produced by the iterator for the
251	* given task list, each of which has completed.
252	* @throws InterruptedException if interrupted while waiting, in
253	* which case unfinished tasks are cancelled.
254	* @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
255	* @throws RejectedExecutionException if any task cannot be
256	* scheduled for execution
257	*/
258
259	<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
260	throws InterruptedException;
261
262	/**
263	* Executes the given tasks, returning a list of Futures holding
264	* their status and results
265	* when all complete or the timeout expires, whichever happens first.
266	* {@link Future#isDone} is <tt>true</tt> for each
267	* element of the returned list.
268	* Upon return, tasks that have not completed are cancelled.
269	* Note that a <em>completed</em> task could have
270	* terminated either normally or by throwing an exception.
271	* The results of this method are undefined if the given
272	* collection is modified while this operation is in progress.
273	*
274	* @param tasks the collection of tasks
275	* @param timeout the maximum time to wait
276	* @param unit the time unit of the timeout argument
277	* @return a list of Futures representing the tasks, in the same
278	* sequential order as produced by the iterator for the
279	* given task list. If the operation did not time out,
280	* each task will have completed. If it did time out, some
281	* of these tasks will not have completed.
282	* @throws InterruptedException if interrupted while waiting, in
283	* which case unfinished tasks are cancelled
284	* @throws NullPointerException if tasks, any of its elements, or
285	* unit are <tt>null</tt>
286	* @throws RejectedExecutionException if any task cannot be scheduled
287	* for execution
288	*/
289	<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
290	long timeout, TimeUnit unit)
291	throws InterruptedException;
292
293	/**
294	* Executes the given tasks, returning the result
295	* of one that has completed successfully (i.e., without throwing
296	* an exception), if any do. Upon normal or exceptional return,
297	* tasks that have not completed are cancelled.
298	* The results of this method are undefined if the given
299	* collection is modified while this operation is in progress.
300	*
301	* @param tasks the collection of tasks
302	* @return the result returned by one of the tasks
303	* @throws InterruptedException if interrupted while waiting
304	* @throws NullPointerException if tasks or any element task
305	* subject to execution is <tt>null</tt>
306	* @throws IllegalArgumentException if tasks is empty
307	* @throws ExecutionException if no task successfully completes
308	* @throws RejectedExecutionException if tasks cannot be scheduled
309	* for execution
310	*/
311	<T> T invokeAny(Collection<? extends Callable<T>> tasks)
312	throws InterruptedException, ExecutionException;
313
314	/**
315	* Executes the given tasks, returning the result
316	* of one that has completed successfully (i.e., without throwing
317	* an exception), if any do before the given timeout elapses.
318	* Upon normal or exceptional return, tasks that have not
319	* completed are cancelled.
320	* The results of this method are undefined if the given
321	* collection is modified while this operation is in progress.
322	*
323	* @param tasks the collection of tasks
324	* @param timeout the maximum time to wait
325	* @param unit the time unit of the timeout argument
326	* @return the result returned by one of the tasks.
327	* @throws InterruptedException if interrupted while waiting
328	* @throws NullPointerException if tasks, or unit, or any element
329	* task subject to execution is <tt>null</tt>
330	* @throws TimeoutException if the given timeout elapses before
331	* any task successfully completes
332	* @throws ExecutionException if no task successfully completes
333	* @throws RejectedExecutionException if tasks cannot be scheduled
334	* for execution
335	*/
336	<T> T invokeAny(Collection<? extends Callable<T>> tasks,
337	long timeout, TimeUnit unit)
338	throws InterruptedException, ExecutionException, TimeoutException;
339	}