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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.List;
import java.util.Collection;
import java.security.PrivilegedAction;
import java.security.PrivilegedExceptionAction;

/**
 * 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:	1.50
Committed:	Wed Aug 5 01:42:33 2009 UTC (14 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.49:	+4 -4 lines
Log Message:	weaken invokeAny NPE specs
#	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/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.List;
9	import java.util.Collection;
10	import java.security.PrivilegedAction;
11	import java.security.PrivilegedExceptionAction;
12
13	/**
14	* An {@link Executor} that provides methods to manage termination and
15	* methods that can produce a {@link Future} for tracking progress of
16	* one or more asynchronous tasks.
17	*
18	* <p> An <tt>ExecutorService</tt> can be shut down, which will cause
19	* it to reject new tasks. Two different methods are provided for
20	* shutting down an <tt>ExecutorService</tt>. The {@link #shutdown}
21	* method will allow previously submitted tasks to execute before
22	* terminating, while the {@link #shutdownNow} method prevents waiting
23	* tasks from starting and attempts to stop currently executing tasks.
24	* Upon termination, an executor has no tasks actively executing, no
25	* tasks awaiting execution, and no new tasks can be submitted. An
26	* unused <tt>ExecutorService</tt> should be shut down to allow
27	* reclamation of its resources.
28	*
29	* <p> Method <tt>submit</tt> extends base method {@link
30	* Executor#execute} by creating and returning a {@link Future} that
31	* can be used to cancel execution and/or wait for completion.
32	* Methods <tt>invokeAny</tt> and <tt>invokeAll</tt> perform the most
33	* commonly useful forms of bulk execution, executing a collection of
34	* tasks and then waiting for at least one, or all, to
35	* complete. (Class {@link ExecutorCompletionService} can be used to
36	* write customized variants of these methods.)
37	*
38	* <p>The {@link Executors} class provides factory methods for the
39	* executor services provided in this package.
40	*
41	* <h3>Usage Examples</h3>
42	*
43	* Here is a sketch of a network service in which threads in a thread
44	* pool service incoming requests. It uses the preconfigured {@link
45	* Executors#newFixedThreadPool} factory method:
46	*
47	* <pre>
48	* class NetworkService implements Runnable {
49	* private final ServerSocket serverSocket;
50	* private final ExecutorService pool;
51	*
52	* public NetworkService(int port, int poolSize)
53	* throws IOException {
54	* serverSocket = new ServerSocket(port);
55	* pool = Executors.newFixedThreadPool(poolSize);
56	* }
57	*
58	* public void run() { // run the service
59	* try {
60	* for (;;) {
61	* pool.execute(new Handler(serverSocket.accept()));
62	* }
63	* } catch (IOException ex) {
64	* pool.shutdown();
65	* }
66	* }
67	* }
68	*
69	* class Handler implements Runnable {
70	* private final Socket socket;
71	* Handler(Socket socket) { this.socket = socket; }
72	* public void run() {
73	* // read and service request on socket
74	* }
75	* }
76	* </pre>
77	*
78	* The following method shuts down an <tt>ExecutorService</tt> in two phases,
79	* first by calling <tt>shutdown</tt> to reject incoming tasks, and then
80	* calling <tt>shutdownNow</tt>, if necessary, to cancel any lingering tasks:
81	*
82	* <pre>
83	* void shutdownAndAwaitTermination(ExecutorService pool) {
84	* pool.shutdown(); // Disable new tasks from being submitted
85	* try {
86	* // Wait a while for existing tasks to terminate
87	* if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
88	* pool.shutdownNow(); // Cancel currently executing tasks
89	* // Wait a while for tasks to respond to being cancelled
90	* if (!pool.awaitTermination(60, TimeUnit.SECONDS))
91	* System.err.println("Pool did not terminate");
92	* }
93	* } catch (InterruptedException ie) {
94	* // (Re-)Cancel if current thread also interrupted
95	* pool.shutdownNow();
96	* // Preserve interrupt status
97	* Thread.currentThread().interrupt();
98	* }
99	* }
100	* </pre>
101	*
102	* <p>Memory consistency effects: Actions in a thread prior to the
103	* submission of a {@code Runnable} or {@code Callable} task to an
104	* {@code ExecutorService}
105	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
106	* any actions taken by that task, which in turn <i>happen-before</i> the
107	* result is retrieved via {@code Future.get()}.
108	*
109	* @since 1.5
110	* @author Doug Lea
111	*/
112	public interface ExecutorService extends Executor {
113
114	/**
115	* Initiates an orderly shutdown in which previously submitted
116	* tasks are executed, but no new tasks will be accepted.
117	* Invocation has no additional effect if already shut down.
118	*
119	* <p>This method does not wait for previously submitted tasks to
120	* complete execution. Use {@link #awaitTermination awaitTermination}
121	* to do that.
122	*
123	* @throws SecurityException if a security manager exists and
124	* shutting down this ExecutorService may manipulate
125	* threads that the caller is not permitted to modify
126	* because it does not hold {@link
127	* java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
128	* or the security manager's <tt>checkAccess</tt> method
129	* denies access.
130	*/
131	void shutdown();
132
133	/**
134	* Attempts to stop all actively executing tasks, halts the
135	* processing of waiting tasks, and returns a list of the tasks
136	* that were awaiting execution.
137	*
138	* <p>This method does not wait for actively executing tasks to
139	* terminate. Use {@link #awaitTermination awaitTermination} to
140	* do that.
141	*
142	* <p>There are no guarantees beyond best-effort attempts to stop
143	* processing actively executing tasks. For example, typical
144	* implementations will cancel via {@link Thread#interrupt}, so any
145	* task that fails to respond to interrupts may never terminate.
146	*
147	* @return list of tasks that never commenced execution
148	* @throws SecurityException if a security manager exists and
149	* shutting down this ExecutorService may manipulate
150	* threads that the caller is not permitted to modify
151	* because it does not hold {@link
152	* java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
153	* or the security manager's <tt>checkAccess</tt> method
154	* denies access.
155	*/
156	List<Runnable> shutdownNow();
157
158	/**
159	* Returns <tt>true</tt> if this executor has been shut down.
160	*
161	* @return <tt>true</tt> if this executor has been shut down
162	*/
163	boolean isShutdown();
164
165	/**
166	* Returns <tt>true</tt> if all tasks have completed following shut down.
167	* Note that <tt>isTerminated</tt> is never <tt>true</tt> unless
168	* either <tt>shutdown</tt> or <tt>shutdownNow</tt> was called first.
169	*
170	* @return <tt>true</tt> if all tasks have completed following shut down
171	*/
172	boolean isTerminated();
173
174	/**
175	* Blocks until all tasks have completed execution after a shutdown
176	* request, or the timeout occurs, or the current thread is
177	* interrupted, whichever happens first.
178	*
179	* @param timeout the maximum time to wait
180	* @param unit the time unit of the timeout argument
181	* @return <tt>true</tt> if this executor terminated and
182	* <tt>false</tt> if the timeout elapsed before termination
183	* @throws InterruptedException if interrupted while waiting
184	*/
185	boolean awaitTermination(long timeout, TimeUnit unit)
186	throws InterruptedException;
187
188
189	/**
190	* Submits a value-returning task for execution and returns a
191	* Future representing the pending results of the task. The
192	* Future's <tt>get</tt> method will return the task's result upon
193	* successful completion.
194	*
195	* <p>
196	* If you would like to immediately block waiting
197	* for a task, you can use constructions of the form
198	* <tt>result = exec.submit(aCallable).get();</tt>
199	*
200	* <p> Note: The {@link Executors} class includes a set of methods
201	* that can convert some other common closure-like objects,
202	* for example, {@link java.security.PrivilegedAction} to
203	* {@link Callable} form so they can be submitted.
204	*
205	* @param task the task to submit
206	* @return a Future representing pending completion of the task
207	* @throws RejectedExecutionException if the task cannot be
208	* scheduled for execution
209	* @throws NullPointerException if the task is null
210	*/
211	<T> Future<T> submit(Callable<T> task);
212
213	/**
214	* Submits a Runnable task for execution and returns a Future
215	* representing that task. The Future's <tt>get</tt> method will
216	* return the given result upon successful completion.
217	*
218	* @param task the task to submit
219	* @param result the result to return
220	* @return a Future representing pending completion of the task
221	* @throws RejectedExecutionException if the task cannot be
222	* scheduled for execution
223	* @throws NullPointerException if the task is null
224	*/
225	<T> Future<T> submit(Runnable task, T result);
226
227	/**
228	* Submits a Runnable task for execution and returns a Future
229	* representing that task. The Future's <tt>get</tt> method will
230	* return <tt>null</tt> upon <em>successful</em> completion.
231	*
232	* @param task the task to submit
233	* @return a Future representing pending completion of the task
234	* @throws RejectedExecutionException if the task cannot be
235	* scheduled for execution
236	* @throws NullPointerException if the task is null
237	*/
238	Future<?> submit(Runnable task);
239
240	/**
241	* Executes the given tasks, returning a list of Futures holding
242	* their status and results when all complete.
243	* {@link Future#isDone} is <tt>true</tt> for each
244	* element of the returned list.
245	* Note that a <em>completed</em> task could have
246	* terminated either normally or by throwing an exception.
247	* The results of this method are undefined if the given
248	* collection is modified while this operation is in progress.
249	*
250	* @param tasks the collection of tasks
251	* @return A list of Futures representing the tasks, in the same
252	* sequential order as produced by the iterator for the
253	* given task list, each of which has completed.
254	* @throws InterruptedException if interrupted while waiting, in
255	* which case unfinished tasks are cancelled.
256	* @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
257	* @throws RejectedExecutionException if any task cannot be
258	* scheduled for execution
259	*/
260
261	<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
262	throws InterruptedException;
263
264	/**
265	* Executes the given tasks, returning a list of Futures holding
266	* their status and results
267	* when all complete or the timeout expires, whichever happens first.
268	* {@link Future#isDone} is <tt>true</tt> for each
269	* element of the returned list.
270	* Upon return, tasks that have not completed are cancelled.
271	* Note that a <em>completed</em> task could have
272	* terminated either normally or by throwing an exception.
273	* The results of this method are undefined if the given
274	* collection is modified while this operation is in progress.
275	*
276	* @param tasks the collection of tasks
277	* @param timeout the maximum time to wait
278	* @param unit the time unit of the timeout argument
279	* @return a list of Futures representing the tasks, in the same
280	* sequential order as produced by the iterator for the
281	* given task list. If the operation did not time out,
282	* each task will have completed. If it did time out, some
283	* of these tasks will not have completed.
284	* @throws InterruptedException if interrupted while waiting, in
285	* which case unfinished tasks are cancelled
286	* @throws NullPointerException if tasks, any of its elements, or
287	* unit are <tt>null</tt>
288	* @throws RejectedExecutionException if any task cannot be scheduled
289	* for execution
290	*/
291	<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
292	long timeout, TimeUnit unit)
293	throws InterruptedException;
294
295	/**
296	* Executes the given tasks, returning the result
297	* of one that has completed successfully (i.e., without throwing
298	* an exception), if any do. Upon normal or exceptional return,
299	* tasks that have not completed are cancelled.
300	* The results of this method are undefined if the given
301	* collection is modified while this operation is in progress.
302	*
303	* @param tasks the collection of tasks
304	* @return the result returned by one of the tasks
305	* @throws InterruptedException if interrupted while waiting
306	* @throws NullPointerException if tasks or any element task
307	* subject to execution is <tt>null</tt>
308	* @throws IllegalArgumentException if tasks is empty
309	* @throws ExecutionException if no task successfully completes
310	* @throws RejectedExecutionException if tasks cannot be scheduled
311	* for execution
312	*/
313	<T> T invokeAny(Collection<? extends Callable<T>> tasks)
314	throws InterruptedException, ExecutionException;
315
316	/**
317	* Executes the given tasks, returning the result
318	* of one that has completed successfully (i.e., without throwing
319	* an exception), if any do before the given timeout elapses.
320	* Upon normal or exceptional return, tasks that have not
321	* completed are cancelled.
322	* The results of this method are undefined if the given
323	* collection is modified while this operation is in progress.
324	*
325	* @param tasks the collection of tasks
326	* @param timeout the maximum time to wait
327	* @param unit the time unit of the timeout argument
328	* @return the result returned by one of the tasks.
329	* @throws InterruptedException if interrupted while waiting
330	* @throws NullPointerException if tasks, or unit, or any element
331	* task subject to execution is <tt>null</tt>
332	* @throws TimeoutException if the given timeout elapses before
333	* any task successfully completes
334	* @throws ExecutionException if no task successfully completes
335	* @throws RejectedExecutionException if tasks cannot be scheduled
336	* for execution
337	*/
338	<T> T invokeAny(Collection<? extends Callable<T>> tasks,
339	long timeout, TimeUnit unit)
340	throws InterruptedException, ExecutionException, TimeoutException;
341	}