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;
 *     barrier = new CyclicBarrier(N,
 *                                 new Runnable() {
 *                                   public void run() {
 *                                     mergeRows(...);
 *                                   }
 *                                 });
 *     for (int i = 0; i < N; ++i)
 *       new Thread(new Worker(i)).start();
 *
 *     waitUntilDone();
 *   }
 * }}</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.
 *
 * @since 1.5
 * @see CountDownLatch
 *
 * @author Doug Lea
 */
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 {
        boolean broken = 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
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // 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
     *         <tt>{@link #getParties()} - 1</tt> 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
     *         <tt>{@link #getParties()} - 1</tt> 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
     * @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:	1.3
Committed:	Tue Feb 5 19:00:18 2013 UTC (11 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.2:	+2 -2 lines
Log Message:	typo
#	User	Rev	Content
1	dl	1.1	/*
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.concurrent.locks.Condition;
9			import java.util.concurrent.locks.ReentrantLock;
10
11			/**
12			* A synchronization aid that allows a set of threads to all wait for
13			* each other to reach a common barrier point. CyclicBarriers are
14			* useful in programs involving a fixed sized party of threads that
15			* must occasionally wait for each other. The barrier is called
16			* <em>cyclic</em> because it can be re-used after the waiting threads
17			* are released.
18			*
19	jsr166	1.2	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
20	dl	1.1	* that is run once per barrier point, after the last thread in the party
21			* arrives, but before any threads are released.
22			* This <em>barrier action</em> is useful
23			* for updating shared-state before any of the parties continue.
24			*
25			* <p><b>Sample usage:</b> Here is an example of
26			* using a barrier in a parallel decomposition design:
27			*
28			* <pre> {@code
29			* class Solver {
30			* final int N;
31			* final float[][] data;
32			* final CyclicBarrier barrier;
33			*
34			* class Worker implements Runnable {
35			* int myRow;
36			* Worker(int row) { myRow = row; }
37			* public void run() {
38			* while (!done()) {
39			* processRow(myRow);
40			*
41			* try {
42			* barrier.await();
43			* } catch (InterruptedException ex) {
44			* return;
45			* } catch (BrokenBarrierException ex) {
46			* return;
47			* }
48			* }
49			* }
50			* }
51			*
52			* public Solver(float[][] matrix) {
53			* data = matrix;
54			* N = matrix.length;
55			* barrier = new CyclicBarrier(N,
56			* new Runnable() {
57			* public void run() {
58			* mergeRows(...);
59			* }
60			* });
61			* for (int i = 0; i < N; ++i)
62			* new Thread(new Worker(i)).start();
63			*
64			* waitUntilDone();
65			* }
66			* }}</pre>
67			*
68			* Here, each worker thread processes a row of the matrix then waits at the
69			* barrier until all rows have been processed. When all rows are processed
70			* the supplied {@link Runnable} barrier action is executed and merges the
71			* rows. If the merger
72	jsr166	1.2	* determines that a solution has been found then {@code done()} will return
73			* {@code true} and each worker will terminate.
74	dl	1.1	*
75			* <p>If the barrier action does not rely on the parties being suspended when
76			* it is executed, then any of the threads in the party could execute that
77			* action when it is released. To facilitate this, each invocation of
78			* {@link #await} returns the arrival index of that thread at the barrier.
79			* You can then choose which thread should execute the barrier action, for
80			* example:
81			* <pre> {@code
82			* if (barrier.await() == 0) {
83			* // log the completion of this iteration
84			* }}</pre>
85			*
86	jsr166	1.2	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
87	dl	1.1	* for failed synchronization attempts: If a thread leaves a barrier
88			* point prematurely because of interruption, failure, or timeout, all
89			* other threads waiting at that barrier point will also leave
90			* abnormally via {@link BrokenBarrierException} (or
91			* {@link InterruptedException} if they too were interrupted at about
92			* the same time).
93			*
94			* <p>Memory consistency effects: Actions in a thread prior to calling
95			* {@code await()}
96			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
97			* actions that are part of the barrier action, which in turn
98			* <i>happen-before</i> actions following a successful return from the
99			* corresponding {@code await()} in other threads.
100			*
101			* @since 1.5
102			* @see CountDownLatch
103			*
104			* @author Doug Lea
105			*/
106			public class CyclicBarrier {
107			/**
108			* Each use of the barrier is represented as a generation instance.
109			* The generation changes whenever the barrier is tripped, or
110			* is reset. There can be many generations associated with threads
111			* using the barrier - due to the non-deterministic way the lock
112			* may be allocated to waiting threads - but only one of these
113	jsr166	1.2	* can be active at a time (the one to which {@code count} applies)
114	dl	1.1	* and all the rest are either broken or tripped.
115			* There need not be an active generation if there has been a break
116			* but no subsequent reset.
117			*/
118			private static class Generation {
119			boolean broken = false;
120			}
121
122			/** The lock for guarding barrier entry */
123			private final ReentrantLock lock = new ReentrantLock();
124			/** Condition to wait on until tripped */
125			private final Condition trip = lock.newCondition();
126			/** The number of parties */
127			private final int parties;
128			/* The command to run when tripped */
129			private final Runnable barrierCommand;
130			/** The current generation */
131			private Generation generation = new Generation();
132
133			/**
134			* Number of parties still waiting. Counts down from parties to 0
135			* on each generation. It is reset to parties on each new
136			* generation or when broken.
137			*/
138			private int count;
139
140			/**
141			* Updates state on barrier trip and wakes up everyone.
142			* Called only while holding lock.
143			*/
144			private void nextGeneration() {
145			// signal completion of last generation
146			trip.signalAll();
147			// set up next generation
148			count = parties;
149			generation = new Generation();
150			}
151
152			/**
153			* Sets current barrier generation as broken and wakes up everyone.
154			* Called only while holding lock.
155			*/
156			private void breakBarrier() {
157			generation.broken = true;
158			count = parties;
159			trip.signalAll();
160			}
161
162			/**
163			* Main barrier code, covering the various policies.
164			*/
165			private int dowait(boolean timed, long nanos)
166			throws InterruptedException, BrokenBarrierException,
167			TimeoutException {
168			final ReentrantLock lock = this.lock;
169			lock.lock();
170			try {
171			final Generation g = generation;
172
173			if (g.broken)
174			throw new BrokenBarrierException();
175
176			if (Thread.interrupted()) {
177			breakBarrier();
178			throw new InterruptedException();
179			}
180
181			int index = --count;
182			if (index == 0) { // tripped
183			boolean ranAction = false;
184			try {
185			final Runnable command = barrierCommand;
186			if (command != null)
187			command.run();
188			ranAction = true;
189			nextGeneration();
190			return 0;
191			} finally {
192			if (!ranAction)
193			breakBarrier();
194			}
195			}
196
197			// loop until tripped, broken, interrupted, or timed out
198			for (;;) {
199			try {
200			if (!timed)
201			trip.await();
202			else if (nanos > 0L)
203			nanos = trip.awaitNanos(nanos);
204			} catch (InterruptedException ie) {
205			if (g == generation && ! g.broken) {
206			breakBarrier();
207			throw ie;
208			} else {
209			// We're about to finish waiting even if we had not
210			// been interrupted, so this interrupt is deemed to
211			// "belong" to subsequent execution.
212			Thread.currentThread().interrupt();
213			}
214			}
215
216			if (g.broken)
217			throw new BrokenBarrierException();
218
219			if (g != generation)
220			return index;
221
222			if (timed && nanos <= 0L) {
223			breakBarrier();
224			throw new TimeoutException();
225			}
226			}
227			} finally {
228			lock.unlock();
229			}
230			}
231
232			/**
233	jsr166	1.2	* Creates a new {@code CyclicBarrier} that will trip when the
234	dl	1.1	* given number of parties (threads) are waiting upon it, and which
235			* will execute the given barrier action when the barrier is tripped,
236			* performed by the last thread entering the barrier.
237			*
238			* @param parties the number of threads that must invoke {@link #await}
239			* before the barrier is tripped
240			* @param barrierAction the command to execute when the barrier is
241			* tripped, or {@code null} if there is no action
242			* @throws IllegalArgumentException if {@code parties} is less than 1
243			*/
244			public CyclicBarrier(int parties, Runnable barrierAction) {
245			if (parties <= 0) throw new IllegalArgumentException();
246			this.parties = parties;
247			this.count = parties;
248			this.barrierCommand = barrierAction;
249			}
250
251			/**
252	jsr166	1.2	* Creates a new {@code CyclicBarrier} that will trip when the
253	dl	1.1	* given number of parties (threads) are waiting upon it, and
254			* does not perform a predefined action when the barrier is tripped.
255			*
256			* @param parties the number of threads that must invoke {@link #await}
257			* before the barrier is tripped
258			* @throws IllegalArgumentException if {@code parties} is less than 1
259			*/
260			public CyclicBarrier(int parties) {
261			this(parties, null);
262			}
263
264			/**
265			* Returns the number of parties required to trip this barrier.
266			*
267			* @return the number of parties required to trip this barrier
268			*/
269			public int getParties() {
270			return parties;
271			}
272
273			/**
274			* Waits until all {@linkplain #getParties parties} have invoked
275	jsr166	1.2	* {@code await} on this barrier.
276	dl	1.1	*
277			* <p>If the current thread is not the last to arrive then it is
278			* disabled for thread scheduling purposes and lies dormant until
279			* one of the following things happens:
280			* <ul>
281			* <li>The last thread arrives; or
282			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
283			* the current thread; or
284			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
285			* one of the other waiting threads; or
286			* <li>Some other thread times out while waiting for barrier; or
287			* <li>Some other thread invokes {@link #reset} on this barrier.
288			* </ul>
289			*
290			* <p>If the current thread:
291			* <ul>
292			* <li>has its interrupted status set on entry to this method; or
293			* <li>is {@linkplain Thread#interrupt interrupted} while waiting
294			* </ul>
295			* then {@link InterruptedException} is thrown and the current thread's
296			* interrupted status is cleared.
297			*
298			* <p>If the barrier is {@link #reset} while any thread is waiting,
299			* or if the barrier {@linkplain #isBroken is broken} when
300	jsr166	1.2	* {@code await} is invoked, or while any thread is waiting, then
301	dl	1.1	* {@link BrokenBarrierException} is thrown.
302			*
303			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
304			* then all other waiting threads will throw
305			* {@link BrokenBarrierException} and the barrier is placed in the broken
306			* state.
307			*
308			* <p>If the current thread is the last thread to arrive, and a
309			* non-null barrier action was supplied in the constructor, then the
310			* current thread runs the action before allowing the other threads to
311			* continue.
312			* If an exception occurs during the barrier action then that exception
313			* will be propagated in the current thread and the barrier is placed in
314			* the broken state.
315			*
316			* @return the arrival index of the current thread, where index
317			* <tt>{@link #getParties()} - 1</tt> indicates the first
318			* to arrive and zero indicates the last to arrive
319			* @throws InterruptedException if the current thread was interrupted
320			* while waiting
321			* @throws BrokenBarrierException if <em>another</em> thread was
322			* interrupted or timed out while the current thread was
323			* waiting, or the barrier was reset, or the barrier was
324			* broken when {@code await} was called, or the barrier
325	jsr166	1.3	* action (if present) failed due to an exception.
326	dl	1.1	*/
327			public int await() throws InterruptedException, BrokenBarrierException {
328			try {
329			return dowait(false, 0L);
330			} catch (TimeoutException toe) {
331			throw new Error(toe); // cannot happen
332			}
333			}
334
335			/**
336			* Waits until all {@linkplain #getParties parties} have invoked
337	jsr166	1.2	* {@code await} on this barrier, or the specified waiting time elapses.
338	dl	1.1	*
339			* <p>If the current thread is not the last to arrive then it is
340			* disabled for thread scheduling purposes and lies dormant until
341			* one of the following things happens:
342			* <ul>
343			* <li>The last thread arrives; or
344			* <li>The specified timeout elapses; or
345			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
346			* the current thread; or
347			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
348			* one of the other waiting threads; or
349			* <li>Some other thread times out while waiting for barrier; or
350			* <li>Some other thread invokes {@link #reset} on this barrier.
351			* </ul>
352			*
353			* <p>If the current thread:
354			* <ul>
355			* <li>has its interrupted status set on entry to this method; or
356			* <li>is {@linkplain Thread#interrupt interrupted} while waiting
357			* </ul>
358			* then {@link InterruptedException} is thrown and the current thread's
359			* interrupted status is cleared.
360			*
361			* <p>If the specified waiting time elapses then {@link TimeoutException}
362			* is thrown. If the time is less than or equal to zero, the
363			* method will not wait at all.
364			*
365			* <p>If the barrier is {@link #reset} while any thread is waiting,
366			* or if the barrier {@linkplain #isBroken is broken} when
367	jsr166	1.2	* {@code await} is invoked, or while any thread is waiting, then
368	dl	1.1	* {@link BrokenBarrierException} is thrown.
369			*
370			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
371			* waiting, then all other waiting threads will throw {@link
372			* BrokenBarrierException} and the barrier is placed in the broken
373			* state.
374			*
375			* <p>If the current thread is the last thread to arrive, and a
376			* non-null barrier action was supplied in the constructor, then the
377			* current thread runs the action before allowing the other threads to
378			* continue.
379			* If an exception occurs during the barrier action then that exception
380			* will be propagated in the current thread and the barrier is placed in
381			* the broken state.
382			*
383			* @param timeout the time to wait for the barrier
384			* @param unit the time unit of the timeout parameter
385			* @return the arrival index of the current thread, where index
386			* <tt>{@link #getParties()} - 1</tt> indicates the first
387			* to arrive and zero indicates the last to arrive
388			* @throws InterruptedException if the current thread was interrupted
389			* while waiting
390			* @throws TimeoutException if the specified timeout elapses
391			* @throws BrokenBarrierException if <em>another</em> thread was
392			* interrupted or timed out while the current thread was
393			* waiting, or the barrier was reset, or the barrier was broken
394			* when {@code await} was called, or the barrier action (if
395	jsr166	1.3	* present) failed due to an exception
396	dl	1.1	*/
397			public int await(long timeout, TimeUnit unit)
398			throws InterruptedException,
399			BrokenBarrierException,
400			TimeoutException {
401			return dowait(true, unit.toNanos(timeout));
402			}
403
404			/**
405			* Queries if this barrier is in a broken state.
406			*
407			* @return {@code true} if one or more parties broke out of this
408			* barrier due to interruption or timeout since
409			* construction or the last reset, or a barrier action
410			* failed due to an exception; {@code false} otherwise.
411			*/
412			public boolean isBroken() {
413			final ReentrantLock lock = this.lock;
414			lock.lock();
415			try {
416			return generation.broken;
417			} finally {
418			lock.unlock();
419			}
420			}
421
422			/**
423			* Resets the barrier to its initial state. If any parties are
424			* currently waiting at the barrier, they will return with a
425			* {@link BrokenBarrierException}. Note that resets <em>after</em>
426			* a breakage has occurred for other reasons can be complicated to
427			* carry out; threads need to re-synchronize in some other way,
428			* and choose one to perform the reset. It may be preferable to
429			* instead create a new barrier for subsequent use.
430			*/
431			public void reset() {
432			final ReentrantLock lock = this.lock;
433			lock.lock();
434			try {
435			breakBarrier(); // break the current generation
436			nextGeneration(); // start a new generation
437			} finally {
438			lock.unlock();
439			}
440			}
441
442			/**
443			* Returns the number of parties currently waiting at the barrier.
444			* This method is primarily useful for debugging and assertions.
445			*
446			* @return the number of parties currently blocked in {@link #await}
447			*/
448			public int getNumberWaiting() {
449			final ReentrantLock lock = this.lock;
450			lock.lock();
451			try {
452			return parties - count;
453			} finally {
454			lock.unlock();
455			}
456			}
457			}