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, 3 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
#	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
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	* determines that a solution has been found then {@code done()} will return
74	* {@code true} and each worker will terminate.
75	*
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	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
88	* 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	* can be active at a time (the one to which {@code count} applies)
115	* 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	/** The command to run when tripped */
130	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	* Creates a new {@code CyclicBarrier} that will trip when the
235	* 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	* Creates a new {@code CyclicBarrier} that will trip when the
254	* 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	* {@code await} on this barrier.
277	*
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	* {@code await} is invoked, or while any thread is waiting, then
302	* {@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	* {@code getParties() - 1} indicates the first
319	* 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	* action (if present) failed due to an exception
327	*/
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	* {@code await} on this barrier, or the specified waiting time elapses.
339	*
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	* {@code await} is invoked, or while any thread is waiting, then
369	* {@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	* {@code getParties() - 1} indicates the first
388	* 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	* present) failed due to an exception
397	*/
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	}