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
     *         {@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
     * @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:	1.7
Committed:	Sun Jan 18 20:17:32 2015 UTC (9 years, 4 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.6:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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	jsr166	1.7
9	dl	1.1	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	jsr166	1.2	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
21	dl	1.1	* 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
27			* using a barrier in a 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			* barrier = new CyclicBarrier(N,
57			* new Runnable() {
58			* public void run() {
59			* mergeRows(...);
60			* }
61			* });
62			* for (int i = 0; i < N; ++i)
63			* new Thread(new Worker(i)).start();
64			*
65			* waitUntilDone();
66			* }
67			* }}</pre>
68			*
69			* Here, each worker thread processes a row of the matrix then waits at the
70			* barrier until all rows have been processed. When all rows are processed
71			* the supplied {@link Runnable} barrier action is executed and merges the
72			* rows. If the merger
73	jsr166	1.2	* determines that a solution has been found then {@code done()} will return
74			* {@code true} and each worker will terminate.
75	dl	1.1	*
76			* <p>If the barrier action does not rely on the parties being suspended when
77			* it is executed, then any of the threads in the party could execute that
78			* action when it is released. To facilitate this, each invocation of
79			* {@link #await} returns the arrival index of that thread at the barrier.
80			* You can then choose which thread should execute the barrier action, for
81			* example:
82			* <pre> {@code
83			* if (barrier.await() == 0) {
84			* // log the completion of this iteration
85			* }}</pre>
86			*
87	jsr166	1.2	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
88	dl	1.1	* for failed synchronization attempts: If a thread leaves a barrier
89			* point prematurely because of interruption, failure, or timeout, all
90			* other threads waiting at that barrier point will also leave
91			* abnormally via {@link BrokenBarrierException} (or
92			* {@link InterruptedException} if they too were interrupted at about
93			* the same time).
94			*
95			* <p>Memory consistency effects: Actions in a thread prior to calling
96			* {@code await()}
97			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
98			* actions that are part of the barrier action, which in turn
99			* <i>happen-before</i> actions following a successful return from the
100			* corresponding {@code await()} in other threads.
101			*
102			* @since 1.5
103			* @see CountDownLatch
104			*
105			* @author Doug Lea
106			*/
107			public class CyclicBarrier {
108			/**
109			* Each use of the barrier is represented as a generation instance.
110			* The generation changes whenever the barrier is tripped, or
111			* is reset. There can be many generations associated with threads
112			* using the barrier - due to the non-deterministic way the lock
113			* may be allocated to waiting threads - but only one of these
114	jsr166	1.2	* can be active at a time (the one to which {@code count} applies)
115	dl	1.1	* and all the rest are either broken or tripped.
116			* There need not be an active generation if there has been a break
117			* but no subsequent reset.
118			*/
119			private static class Generation {
120			boolean broken = false;
121			}
122
123			/** The lock for guarding barrier entry */
124			private final ReentrantLock lock = new ReentrantLock();
125			/** Condition to wait on until tripped */
126			private final Condition trip = lock.newCondition();
127			/** The number of parties */
128			private final int parties;
129	jsr166	1.6	/** The command to run when tripped */
130	dl	1.1	private final Runnable barrierCommand;
131			/** The current generation */
132			private Generation generation = new Generation();
133
134			/**
135			* Number of parties still waiting. Counts down from parties to 0
136			* on each generation. It is reset to parties on each new
137			* generation or when broken.
138			*/
139			private int count;
140
141			/**
142			* Updates state on barrier trip and wakes up everyone.
143			* Called only while holding lock.
144			*/
145			private void nextGeneration() {
146			// signal completion of last generation
147			trip.signalAll();
148			// set up next generation
149			count = parties;
150			generation = new Generation();
151			}
152
153			/**
154			* Sets current barrier generation as broken and wakes up everyone.
155			* Called only while holding lock.
156			*/
157			private void breakBarrier() {
158			generation.broken = true;
159			count = parties;
160			trip.signalAll();
161			}
162
163			/**
164			* Main barrier code, covering the various policies.
165			*/
166			private int dowait(boolean timed, long nanos)
167			throws InterruptedException, BrokenBarrierException,
168			TimeoutException {
169			final ReentrantLock lock = this.lock;
170			lock.lock();
171			try {
172			final Generation g = generation;
173
174			if (g.broken)
175			throw new BrokenBarrierException();
176
177			if (Thread.interrupted()) {
178			breakBarrier();
179			throw new InterruptedException();
180			}
181
182			int index = --count;
183			if (index == 0) { // tripped
184			boolean ranAction = false;
185			try {
186			final Runnable command = barrierCommand;
187			if (command != null)
188			command.run();
189			ranAction = true;
190			nextGeneration();
191			return 0;
192			} finally {
193			if (!ranAction)
194			breakBarrier();
195			}
196			}
197
198			// loop until tripped, broken, interrupted, or timed out
199			for (;;) {
200			try {
201			if (!timed)
202			trip.await();
203			else if (nanos > 0L)
204			nanos = trip.awaitNanos(nanos);
205			} catch (InterruptedException ie) {
206			if (g == generation && ! g.broken) {
207			breakBarrier();
208			throw ie;
209			} else {
210			// We're about to finish waiting even if we had not
211			// been interrupted, so this interrupt is deemed to
212			// "belong" to subsequent execution.
213			Thread.currentThread().interrupt();
214			}
215			}
216
217			if (g.broken)
218			throw new BrokenBarrierException();
219
220			if (g != generation)
221			return index;
222
223			if (timed && nanos <= 0L) {
224			breakBarrier();
225			throw new TimeoutException();
226			}
227			}
228			} finally {
229			lock.unlock();
230			}
231			}
232
233			/**
234	jsr166	1.2	* Creates a new {@code CyclicBarrier} that will trip when the
235	dl	1.1	* given number of parties (threads) are waiting upon it, and which
236			* will execute the given barrier action when the barrier is tripped,
237			* performed by the last thread entering the barrier.
238			*
239			* @param parties the number of threads that must invoke {@link #await}
240			* before the barrier is tripped
241			* @param barrierAction the command to execute when the barrier is
242			* tripped, or {@code null} if there is no action
243			* @throws IllegalArgumentException if {@code parties} is less than 1
244			*/
245			public CyclicBarrier(int parties, Runnable barrierAction) {
246			if (parties <= 0) throw new IllegalArgumentException();
247			this.parties = parties;
248			this.count = parties;
249			this.barrierCommand = barrierAction;
250			}
251
252			/**
253	jsr166	1.2	* Creates a new {@code CyclicBarrier} that will trip when the
254	dl	1.1	* given number of parties (threads) are waiting upon it, and
255			* does not perform a predefined action when the barrier is tripped.
256			*
257			* @param parties the number of threads that must invoke {@link #await}
258			* before the barrier is tripped
259			* @throws IllegalArgumentException if {@code parties} is less than 1
260			*/
261			public CyclicBarrier(int parties) {
262			this(parties, null);
263			}
264
265			/**
266			* Returns the number of parties required to trip this barrier.
267			*
268			* @return the number of parties required to trip this barrier
269			*/
270			public int getParties() {
271			return parties;
272			}
273
274			/**
275			* Waits until all {@linkplain #getParties parties} have invoked
276	jsr166	1.2	* {@code await} on this barrier.
277	dl	1.1	*
278			* <p>If the current thread is not the last to arrive then it is
279			* disabled for thread scheduling purposes and lies dormant until
280			* one of the following things happens:
281			* <ul>
282			* <li>The last thread arrives; or
283			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
284			* the current thread; or
285			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
286			* one of the other waiting threads; or
287			* <li>Some other thread times out while waiting for barrier; or
288			* <li>Some other thread invokes {@link #reset} on this barrier.
289			* </ul>
290			*
291			* <p>If the current thread:
292			* <ul>
293			* <li>has its interrupted status set on entry to this method; or
294			* <li>is {@linkplain Thread#interrupt interrupted} while waiting
295			* </ul>
296			* then {@link InterruptedException} is thrown and the current thread's
297			* interrupted status is cleared.
298			*
299			* <p>If the barrier is {@link #reset} while any thread is waiting,
300			* or if the barrier {@linkplain #isBroken is broken} when
301	jsr166	1.2	* {@code await} is invoked, or while any thread is waiting, then
302	dl	1.1	* {@link BrokenBarrierException} is thrown.
303			*
304			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
305			* then all other waiting threads will throw
306			* {@link BrokenBarrierException} and the barrier is placed in the broken
307			* state.
308			*
309			* <p>If the current thread is the last thread to arrive, and a
310			* non-null barrier action was supplied in the constructor, then the
311			* current thread runs the action before allowing the other threads to
312			* continue.
313			* If an exception occurs during the barrier action then that exception
314			* will be propagated in the current thread and the barrier is placed in
315			* the broken state.
316			*
317			* @return the arrival index of the current thread, where index
318	jsr166	1.5	* {@code getParties() - 1} indicates the first
319	dl	1.1	* to arrive and zero indicates the last to arrive
320			* @throws InterruptedException if the current thread was interrupted
321			* while waiting
322			* @throws BrokenBarrierException if <em>another</em> thread was
323			* interrupted or timed out while the current thread was
324			* waiting, or the barrier was reset, or the barrier was
325			* broken when {@code await} was called, or the barrier
326	jsr166	1.4	* action (if present) failed due to an exception
327	dl	1.1	*/
328			public int await() throws InterruptedException, BrokenBarrierException {
329			try {
330			return dowait(false, 0L);
331			} catch (TimeoutException toe) {
332			throw new Error(toe); // cannot happen
333			}
334			}
335
336			/**
337			* Waits until all {@linkplain #getParties parties} have invoked
338	jsr166	1.2	* {@code await} on this barrier, or the specified waiting time elapses.
339	dl	1.1	*
340			* <p>If the current thread is not the last to arrive then it is
341			* disabled for thread scheduling purposes and lies dormant until
342			* one of the following things happens:
343			* <ul>
344			* <li>The last thread arrives; or
345			* <li>The specified timeout elapses; or
346			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
347			* the current thread; or
348			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
349			* one of the other waiting threads; or
350			* <li>Some other thread times out while waiting for barrier; or
351			* <li>Some other thread invokes {@link #reset} on this barrier.
352			* </ul>
353			*
354			* <p>If the current thread:
355			* <ul>
356			* <li>has its interrupted status set on entry to this method; or
357			* <li>is {@linkplain Thread#interrupt interrupted} while waiting
358			* </ul>
359			* then {@link InterruptedException} is thrown and the current thread's
360			* interrupted status is cleared.
361			*
362			* <p>If the specified waiting time elapses then {@link TimeoutException}
363			* is thrown. If the time is less than or equal to zero, the
364			* method will not wait at all.
365			*
366			* <p>If the barrier is {@link #reset} while any thread is waiting,
367			* or if the barrier {@linkplain #isBroken is broken} when
368	jsr166	1.2	* {@code await} is invoked, or while any thread is waiting, then
369	dl	1.1	* {@link BrokenBarrierException} is thrown.
370			*
371			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
372			* waiting, then all other waiting threads will throw {@link
373			* BrokenBarrierException} and the barrier is placed in the broken
374			* state.
375			*
376			* <p>If the current thread is the last thread to arrive, and a
377			* non-null barrier action was supplied in the constructor, then the
378			* current thread runs the action before allowing the other threads to
379			* continue.
380			* If an exception occurs during the barrier action then that exception
381			* will be propagated in the current thread and the barrier is placed in
382			* the broken state.
383			*
384			* @param timeout the time to wait for the barrier
385			* @param unit the time unit of the timeout parameter
386			* @return the arrival index of the current thread, where index
387	jsr166	1.5	* {@code getParties() - 1} indicates the first
388	dl	1.1	* to arrive and zero indicates the last to arrive
389			* @throws InterruptedException if the current thread was interrupted
390			* while waiting
391			* @throws TimeoutException if the specified timeout elapses
392			* @throws BrokenBarrierException if <em>another</em> thread was
393			* interrupted or timed out while the current thread was
394			* waiting, or the barrier was reset, or the barrier was broken
395			* when {@code await} was called, or the barrier action (if
396	jsr166	1.3	* present) failed due to an exception
397	dl	1.1	*/
398			public int await(long timeout, TimeUnit unit)
399			throws InterruptedException,
400			BrokenBarrierException,
401			TimeoutException {
402			return dowait(true, unit.toNanos(timeout));
403			}
404
405			/**
406			* Queries if this barrier is in a broken state.
407			*
408			* @return {@code true} if one or more parties broke out of this
409			* barrier due to interruption or timeout since
410			* construction or the last reset, or a barrier action
411			* failed due to an exception; {@code false} otherwise.
412			*/
413			public boolean isBroken() {
414			final ReentrantLock lock = this.lock;
415			lock.lock();
416			try {
417			return generation.broken;
418			} finally {
419			lock.unlock();
420			}
421			}
422
423			/**
424			* Resets the barrier to its initial state. If any parties are
425			* currently waiting at the barrier, they will return with a
426			* {@link BrokenBarrierException}. Note that resets <em>after</em>
427			* a breakage has occurred for other reasons can be complicated to
428			* carry out; threads need to re-synchronize in some other way,
429			* and choose one to perform the reset. It may be preferable to
430			* instead create a new barrier for subsequent use.
431			*/
432			public void reset() {
433			final ReentrantLock lock = this.lock;
434			lock.lock();
435			try {
436			breakBarrier(); // break the current generation
437			nextGeneration(); // start a new generation
438			} finally {
439			lock.unlock();
440			}
441			}
442
443			/**
444			* Returns the number of parties currently waiting at the barrier.
445			* This method is primarily useful for debugging and assertions.
446			*
447			* @return the number of parties currently blocked in {@link #await}
448			*/
449			public int getNumberWaiting() {
450			final ReentrantLock lock = this.lock;
451			lock.lock();
452			try {
453			return parties - count;
454			} finally {
455			lock.unlock();
456			}
457			}
458			}