util/concurrent/CyclicBarrier.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.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A synchronization aid that allows a set of threads to all wait for
 * each other to reach a common barrier point.  CyclicBarriers are
 * useful in programs involving a fixed sized party of threads that
 * must occasionally wait for each other. The barrier is called
 * <em>cyclic</em> because it can be re-used after the waiting threads
 * are released.
 *
 * <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
 * that is run once per barrier point, after the last thread in the party
 * arrives, but before any threads are released.
 * This <em>barrier action</em> is useful
 * for updating shared-state before any of the parties continue.
 *
 * <p><b>Sample usage:</b> Here is an example of using a barrier in a
 * parallel decomposition design:
 *
 * <pre> {@code
 * class Solver {
 *   final int N;
 *   final float[][] data;
 *   final CyclicBarrier barrier;
 *
 *   class Worker implements Runnable {
 *     int myRow;
 *     Worker(int row) { myRow = row; }
 *     public void run() {
 *       while (!done()) {
 *         processRow(myRow);
 *
 *         try {
 *           barrier.await();
 *         } catch (InterruptedException ex) {
 *           return;
 *         } catch (BrokenBarrierException ex) {
 *           return;
 *         }
 *       }
 *     }
 *   }
 *
 *   public Solver(float[][] matrix) {
 *     data = matrix;
 *     N = matrix.length;
 *     Runnable barrierAction = () -> mergeRows(...);
 *     barrier = new CyclicBarrier(N, barrierAction);
 *
 *     List<Thread> threads = new ArrayList<>(N);
 *     for (int i = 0; i < N; i++) {
 *       Thread thread = new Thread(new Worker(i));
 *       threads.add(thread);
 *       thread.start();
 *     }
 *
 *     // wait until done
 *     for (Thread thread : threads)
 *       thread.join();
 *   }
 * }}</pre>
 *
 * Here, each worker thread processes a row of the matrix, then waits at the
 * barrier until all rows have been processed. When all rows are processed the
 * supplied {@link Runnable} barrier action is executed and merges the rows.
 * If the merger determines that a solution has been found then {@code done()}
 * will return {@code true} and each worker will terminate.
 *
 * <p>If the barrier action does not rely on the parties being suspended when
 * it is executed, then any of the threads in the party could execute that
 * action when it is released. To facilitate this, each invocation of
 * {@link #await} returns the arrival index of that thread at the barrier.
 * You can then choose which thread should execute the barrier action, for
 * example:
 * <pre> {@code
 * if (barrier.await() == 0) {
 *   // log the completion of this iteration
 * }}</pre>
 *
 * <p>The {@code CyclicBarrier} uses an all-or-none breakage model
 * for failed synchronization attempts: If a thread leaves a barrier
 * point prematurely because of interruption, failure, or timeout, all
 * other threads waiting at that barrier point will also leave
 * abnormally via {@link BrokenBarrierException} (or
 * {@link InterruptedException} if they too were interrupted at about
 * the same time).
 *
 * <p>Memory consistency effects: Actions in a thread prior to calling
 * {@code await()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions that are part of the barrier action, which in turn
 * <i>happen-before</i> actions following a successful return from the
 * corresponding {@code await()} in other threads.
 *
 * @see CountDownLatch
 *
 * @author Doug Lea
 * @since 1.5
 */
public class CyclicBarrier {
    /**
     * Each use of the barrier is represented as a generation instance.
     * The generation changes whenever the barrier is tripped, or
     * is reset. There can be many generations associated with threads
     * using the barrier - due to the non-deterministic way the lock
     * may be allocated to waiting threads - but only one of these
     * can be active at a time (the one to which {@code count} applies)
     * and all the rest are either broken or tripped.
     * There need not be an active generation if there has been a break
     * but no subsequent reset.
     */
    private static class Generation {
        Generation() {}                 // prevent access constructor creation
        boolean broken;                 // initially false
    }

    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    private final int parties;
    /** The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    /**
     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            int index = --count;
            if (index == 0) {  // tripped
                Runnable command = barrierCommand;
                if (command != null) {
                    try {
                        command.run();
                    } catch (Throwable ex) {
                        breakBarrier();
                        throw ex;
                    }
                }
                nextGeneration();
                return 0;
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        Thread.currentThread().interrupt();
                    }
                }

                if (g.broken)
                    throw new BrokenBarrierException();

                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @param barrierAction the command to execute when the barrier is
     *        tripped, or {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

    /**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
    public int getParties() {
        return parties;
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} on this barrier.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * {@code await} is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @return the arrival index of the current thread, where index
     *         {@code getParties() - 1} indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was
     *         broken when {@code await} was called, or the barrier
     *         action (if present) failed due to an exception
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * {@code await} on this barrier, or the specified waiting time elapses.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>The specified timeout elapses; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * one of the other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown. If the time is less than or equal to zero, the
     * method will not wait at all.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #isBroken is broken} when
     * {@code await} is invoked, or while any thread is waiting, then
     * {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while
     * waiting, then all other waiting threads will throw {@link
     * BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @param timeout the time to wait for the barrier
     * @param unit the time unit of the timeout parameter
     * @return the arrival index of the current thread, where index
     *         {@code getParties() - 1} indicates the first
     *         to arrive and zero indicates the last to arrive
     * @throws InterruptedException if the current thread was interrupted
     *         while waiting
     * @throws TimeoutException if the specified timeout elapses.
     *         In this case the barrier will be broken.
     * @throws BrokenBarrierException if <em>another</em> thread was
     *         interrupted or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was broken
     *         when {@code await} was called, or the barrier action (if
     *         present) failed due to an exception
     */
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * Queries if this barrier is in a broken state.
     *
     * @return {@code true} if one or more parties broke out of this
     *         barrier due to interruption or timeout since
     *         construction or the last reset, or a barrier action
     *         failed due to an exception; {@code false} otherwise.
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Resets the barrier to its initial state.  If any parties are
     * currently waiting at the barrier, they will return with a
     * {@link BrokenBarrierException}. Note that resets <em>after</em>
     * a breakage has occurred for other reasons can be complicated to
     * carry out; threads need to re-synchronize in some other way,
     * and choose one to perform the reset.  It may be preferable to
     * instead create a new barrier for subsequent use.
     */
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns the number of parties currently waiting at the barrier.
     * This method is primarily useful for debugging and assertions.
     *
     * @return the number of parties currently blocked in {@link #await}
     */
    public int getNumberWaiting() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }
}
Revision:	1.60
Committed:	Sat Feb 2 04:02:49 2019 UTC (5 years, 4 months ago) by jsr166
Branch:	MAIN
Changes since 1.59:	+5 -6 lines
Log Message:	add a comma for clarity
#	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
9	import java.util.concurrent.locks.Condition;
10	import java.util.concurrent.locks.ReentrantLock;
11
12	/**
13	* A synchronization aid that allows a set of threads to all wait for
14	* each other to reach a common barrier point. CyclicBarriers are
15	* useful in programs involving a fixed sized party of threads that
16	* must occasionally wait for each other. The barrier is called
17	* <em>cyclic</em> because it can be re-used after the waiting threads
18	* are released.
19	*
20	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
21	* that is run once per barrier point, after the last thread in the party
22	* arrives, but before any threads are released.
23	* This <em>barrier action</em> is useful
24	* for updating shared-state before any of the parties continue.
25	*
26	* <p><b>Sample usage:</b> Here is an example of using a barrier in a
27	* parallel decomposition design:
28	*
29	* <pre> {@code
30	* class Solver {
31	* final int N;
32	* final float[][] data;
33	* final CyclicBarrier barrier;
34	*
35	* class Worker implements Runnable {
36	* int myRow;
37	* Worker(int row) { myRow = row; }
38	* public void run() {
39	* while (!done()) {
40	* processRow(myRow);
41	*
42	* try {
43	* barrier.await();
44	* } catch (InterruptedException ex) {
45	* return;
46	* } catch (BrokenBarrierException ex) {
47	* return;
48	* }
49	* }
50	* }
51	* }
52	*
53	* public Solver(float[][] matrix) {
54	* data = matrix;
55	* N = matrix.length;
56	* Runnable barrierAction = () -> mergeRows(...);
57	* barrier = new CyclicBarrier(N, barrierAction);
58	*
59	* List<Thread> threads = new ArrayList<>(N);
60	* for (int i = 0; i < N; i++) {
61	* Thread thread = new Thread(new Worker(i));
62	* threads.add(thread);
63	* thread.start();
64	* }
65	*
66	* // wait until done
67	* for (Thread thread : threads)
68	* thread.join();
69	* }
70	* }}</pre>
71	*
72	* Here, each worker thread processes a row of the matrix, then waits at the
73	* barrier until all rows have been processed. When all rows are processed the
74	* supplied {@link Runnable} barrier action is executed and merges the rows.
75	* If the merger determines that a solution has been found then {@code done()}
76	* will return {@code true} and each worker will terminate.
77	*
78	* <p>If the barrier action does not rely on the parties being suspended when
79	* it is executed, then any of the threads in the party could execute that
80	* action when it is released. To facilitate this, each invocation of
81	* {@link #await} returns the arrival index of that thread at the barrier.
82	* You can then choose which thread should execute the barrier action, for
83	* example:
84	* <pre> {@code
85	* if (barrier.await() == 0) {
86	* // log the completion of this iteration
87	* }}</pre>
88	*
89	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
90	* for failed synchronization attempts: If a thread leaves a barrier
91	* point prematurely because of interruption, failure, or timeout, all
92	* other threads waiting at that barrier point will also leave
93	* abnormally via {@link BrokenBarrierException} (or
94	* {@link InterruptedException} if they too were interrupted at about
95	* the same time).
96	*
97	* <p>Memory consistency effects: Actions in a thread prior to calling
98	* {@code await()}
99	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
100	* actions that are part of the barrier action, which in turn
101	* <i>happen-before</i> actions following a successful return from the
102	* corresponding {@code await()} in other threads.
103	*
104	* @see CountDownLatch
105	*
106	* @author Doug Lea
107	* @since 1.5
108	*/
109	public class CyclicBarrier {
110	/**
111	* Each use of the barrier is represented as a generation instance.
112	* The generation changes whenever the barrier is tripped, or
113	* is reset. There can be many generations associated with threads
114	* using the barrier - due to the non-deterministic way the lock
115	* may be allocated to waiting threads - but only one of these
116	* can be active at a time (the one to which {@code count} applies)
117	* and all the rest are either broken or tripped.
118	* There need not be an active generation if there has been a break
119	* but no subsequent reset.
120	*/
121	private static class Generation {
122	Generation() {} // prevent access constructor creation
123	boolean broken; // initially false
124	}
125
126	/** The lock for guarding barrier entry */
127	private final ReentrantLock lock = new ReentrantLock();
128	/** Condition to wait on until tripped */
129	private final Condition trip = lock.newCondition();
130	/** The number of parties */
131	private final int parties;
132	/** The command to run when tripped */
133	private final Runnable barrierCommand;
134	/** The current generation */
135	private Generation generation = new Generation();
136
137	/**
138	* Number of parties still waiting. Counts down from parties to 0
139	* on each generation. It is reset to parties on each new
140	* generation or when broken.
141	*/
142	private int count;
143
144	/**
145	* Updates state on barrier trip and wakes up everyone.
146	* Called only while holding lock.
147	*/
148	private void nextGeneration() {
149	// signal completion of last generation
150	trip.signalAll();
151	// set up next generation
152	count = parties;
153	generation = new Generation();
154	}
155
156	/**
157	* Sets current barrier generation as broken and wakes up everyone.
158	* Called only while holding lock.
159	*/
160	private void breakBarrier() {
161	generation.broken = true;
162	count = parties;
163	trip.signalAll();
164	}
165
166	/**
167	* Main barrier code, covering the various policies.
168	*/
169	private int dowait(boolean timed, long nanos)
170	throws InterruptedException, BrokenBarrierException,
171	TimeoutException {
172	final ReentrantLock lock = this.lock;
173	lock.lock();
174	try {
175	final Generation g = generation;
176
177	if (g.broken)
178	throw new BrokenBarrierException();
179
180	if (Thread.interrupted()) {
181	breakBarrier();
182	throw new InterruptedException();
183	}
184
185	int index = --count;
186	if (index == 0) { // tripped
187	Runnable command = barrierCommand;
188	if (command != null) {
189	try {
190	command.run();
191	} catch (Throwable ex) {
192	breakBarrier();
193	throw ex;
194	}
195	}
196	nextGeneration();
197	return 0;
198	}
199
200	// loop until tripped, broken, interrupted, or timed out
201	for (;;) {
202	try {
203	if (!timed)
204	trip.await();
205	else if (nanos > 0L)
206	nanos = trip.awaitNanos(nanos);
207	} catch (InterruptedException ie) {
208	if (g == generation && ! g.broken) {
209	breakBarrier();
210	throw ie;
211	} else {
212	// We're about to finish waiting even if we had not
213	// been interrupted, so this interrupt is deemed to
214	// "belong" to subsequent execution.
215	Thread.currentThread().interrupt();
216	}
217	}
218
219	if (g.broken)
220	throw new BrokenBarrierException();
221
222	if (g != generation)
223	return index;
224
225	if (timed && nanos <= 0L) {
226	breakBarrier();
227	throw new TimeoutException();
228	}
229	}
230	} finally {
231	lock.unlock();
232	}
233	}
234
235	/**
236	* Creates a new {@code CyclicBarrier} that will trip when the
237	* given number of parties (threads) are waiting upon it, and which
238	* will execute the given barrier action when the barrier is tripped,
239	* performed by the last thread entering the barrier.
240	*
241	* @param parties the number of threads that must invoke {@link #await}
242	* before the barrier is tripped
243	* @param barrierAction the command to execute when the barrier is
244	* tripped, or {@code null} if there is no action
245	* @throws IllegalArgumentException if {@code parties} is less than 1
246	*/
247	public CyclicBarrier(int parties, Runnable barrierAction) {
248	if (parties <= 0) throw new IllegalArgumentException();
249	this.parties = parties;
250	this.count = parties;
251	this.barrierCommand = barrierAction;
252	}
253
254	/**
255	* Creates a new {@code CyclicBarrier} that will trip when the
256	* given number of parties (threads) are waiting upon it, and
257	* does not perform a predefined action when the barrier is tripped.
258	*
259	* @param parties the number of threads that must invoke {@link #await}
260	* before the barrier is tripped
261	* @throws IllegalArgumentException if {@code parties} is less than 1
262	*/
263	public CyclicBarrier(int parties) {
264	this(parties, null);
265	}
266
267	/**
268	* Returns the number of parties required to trip this barrier.
269	*
270	* @return the number of parties required to trip this barrier
271	*/
272	public int getParties() {
273	return parties;
274	}
275
276	/**
277	* Waits until all {@linkplain #getParties parties} have invoked
278	* {@code await} on this barrier.
279	*
280	* <p>If the current thread is not the last to arrive then it is
281	* disabled for thread scheduling purposes and lies dormant until
282	* one of the following things happens:
283	* <ul>
284	* <li>The last thread arrives; or
285	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
286	* the current thread; or
287	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
288	* one of the other waiting threads; or
289	* <li>Some other thread times out while waiting for barrier; or
290	* <li>Some other thread invokes {@link #reset} on this barrier.
291	* </ul>
292	*
293	* <p>If the current thread:
294	* <ul>
295	* <li>has its interrupted status set on entry to this method; or
296	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
297	* </ul>
298	* then {@link InterruptedException} is thrown and the current thread's
299	* interrupted status is cleared.
300	*
301	* <p>If the barrier is {@link #reset} while any thread is waiting,
302	* or if the barrier {@linkplain #isBroken is broken} when
303	* {@code await} is invoked, or while any thread is waiting, then
304	* {@link BrokenBarrierException} is thrown.
305	*
306	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
307	* then all other waiting threads will throw
308	* {@link BrokenBarrierException} and the barrier is placed in the broken
309	* state.
310	*
311	* <p>If the current thread is the last thread to arrive, and a
312	* non-null barrier action was supplied in the constructor, then the
313	* current thread runs the action before allowing the other threads to
314	* continue.
315	* If an exception occurs during the barrier action then that exception
316	* will be propagated in the current thread and the barrier is placed in
317	* the broken state.
318	*
319	* @return the arrival index of the current thread, where index
320	* {@code getParties() - 1} indicates the first
321	* to arrive and zero indicates the last to arrive
322	* @throws InterruptedException if the current thread was interrupted
323	* while waiting
324	* @throws BrokenBarrierException if <em>another</em> thread was
325	* interrupted or timed out while the current thread was
326	* waiting, or the barrier was reset, or the barrier was
327	* broken when {@code await} was called, or the barrier
328	* action (if present) failed due to an exception
329	*/
330	public int await() throws InterruptedException, BrokenBarrierException {
331	try {
332	return dowait(false, 0L);
333	} catch (TimeoutException toe) {
334	throw new Error(toe); // cannot happen
335	}
336	}
337
338	/**
339	* Waits until all {@linkplain #getParties parties} have invoked
340	* {@code await} on this barrier, or the specified waiting time elapses.
341	*
342	* <p>If the current thread is not the last to arrive then it is
343	* disabled for thread scheduling purposes and lies dormant until
344	* one of the following things happens:
345	* <ul>
346	* <li>The last thread arrives; or
347	* <li>The specified timeout elapses; or
348	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
349	* the current thread; or
350	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
351	* one of the other waiting threads; or
352	* <li>Some other thread times out while waiting for barrier; or
353	* <li>Some other thread invokes {@link #reset} on this barrier.
354	* </ul>
355	*
356	* <p>If the current thread:
357	* <ul>
358	* <li>has its interrupted status set on entry to this method; or
359	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
360	* </ul>
361	* then {@link InterruptedException} is thrown and the current thread's
362	* interrupted status is cleared.
363	*
364	* <p>If the specified waiting time elapses then {@link TimeoutException}
365	* is thrown. If the time is less than or equal to zero, the
366	* method will not wait at all.
367	*
368	* <p>If the barrier is {@link #reset} while any thread is waiting,
369	* or if the barrier {@linkplain #isBroken is broken} when
370	* {@code await} is invoked, or while any thread is waiting, then
371	* {@link BrokenBarrierException} is thrown.
372	*
373	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
374	* waiting, then all other waiting threads will throw {@link
375	* BrokenBarrierException} and the barrier is placed in the broken
376	* state.
377	*
378	* <p>If the current thread is the last thread to arrive, and a
379	* non-null barrier action was supplied in the constructor, then the
380	* current thread runs the action before allowing the other threads to
381	* continue.
382	* If an exception occurs during the barrier action then that exception
383	* will be propagated in the current thread and the barrier is placed in
384	* the broken state.
385	*
386	* @param timeout the time to wait for the barrier
387	* @param unit the time unit of the timeout parameter
388	* @return the arrival index of the current thread, where index
389	* {@code getParties() - 1} indicates the first
390	* to arrive and zero indicates the last to arrive
391	* @throws InterruptedException if the current thread was interrupted
392	* while waiting
393	* @throws TimeoutException if the specified timeout elapses.
394	* In this case the barrier will be broken.
395	* @throws BrokenBarrierException if <em>another</em> thread was
396	* interrupted or timed out while the current thread was
397	* waiting, or the barrier was reset, or the barrier was broken
398	* when {@code await} was called, or the barrier action (if
399	* present) failed due to an exception
400	*/
401	public int await(long timeout, TimeUnit unit)
402	throws InterruptedException,
403	BrokenBarrierException,
404	TimeoutException {
405	return dowait(true, unit.toNanos(timeout));
406	}
407
408	/**
409	* Queries if this barrier is in a broken state.
410	*
411	* @return {@code true} if one or more parties broke out of this
412	* barrier due to interruption or timeout since
413	* construction or the last reset, or a barrier action
414	* failed due to an exception; {@code false} otherwise.
415	*/
416	public boolean isBroken() {
417	final ReentrantLock lock = this.lock;
418	lock.lock();
419	try {
420	return generation.broken;
421	} finally {
422	lock.unlock();
423	}
424	}
425
426	/**
427	* Resets the barrier to its initial state. If any parties are
428	* currently waiting at the barrier, they will return with a
429	* {@link BrokenBarrierException}. Note that resets <em>after</em>
430	* a breakage has occurred for other reasons can be complicated to
431	* carry out; threads need to re-synchronize in some other way,
432	* and choose one to perform the reset. It may be preferable to
433	* instead create a new barrier for subsequent use.
434	*/
435	public void reset() {
436	final ReentrantLock lock = this.lock;
437	lock.lock();
438	try {
439	breakBarrier(); // break the current generation
440	nextGeneration(); // start a new generation
441	} finally {
442	lock.unlock();
443	}
444	}
445
446	/**
447	* Returns the number of parties currently waiting at the barrier.
448	* This method is primarily useful for debugging and assertions.
449	*
450	* @return the number of parties currently blocked in {@link #await}
451	*/
452	public int getNumberWaiting() {
453	final ReentrantLock lock = this.lock;
454	lock.lock();
455	try {
456	return parties - count;
457	} finally {
458	lock.unlock();
459	}
460	}
461	}