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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * 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 <tt>CyclicBarrier</tt> 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>
 * 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 <tt>done()</tt> will return
 * <tt>true</tt> 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>  if (barrier.await() == 0) {
 *     // log the completion of this iteration
 *   }</pre>
 *
 * <p>The <tt>CyclicBarrier</tt> 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 <tt>count</tt> 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 <tt>CyclicBarrier</tt> 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 <tt>null</tt> if there is no action.
     *
     * @throws IllegalArgumentException if <tt>parties</tt> 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 <tt>CyclicBarrier</tt> 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 <tt>parties</tt> 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 {@link #getParties parties} have invoked <tt>await</tt>
     * 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 {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>Some other thread  {@link 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 {@link 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 {@link #isBroken is broken} when <tt>await</tt> is invoked,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link 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
     * <tt>await</tt> was called, or the barrier action (if present)
     * failed due 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 {@link #getParties parties} have invoked <tt>await</tt>
     * 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>The specified timeout elapses; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>Some other thread  {@link 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 {@link 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 {@link #isBroken is broken} when <tt>await</tt> is invoked,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link 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
     * <tt>await</tt> was called, or the barrier action (if present)
     * failed due 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 <tt>true</tt> 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;
     * <tt>false</tt> 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:	1.36
Committed:	Tue Jan 10 21:30:33 2006 UTC (18 years, 4 months ago) by jsr166
Branch:	MAIN
Changes since 1.35:	+11 -6 lines
Log Message:	final revision after review: 6366811: CyclicBarrier behaviour incorrect if interruption occurs during barrier "trip"
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9
10	/**
11	* A synchronization aid that allows a set of threads to all wait for
12	* each other to reach a common barrier point. CyclicBarriers are
13	* useful in programs involving a fixed sized party of threads that
14	* must occasionally wait for each other. The barrier is called
15	* <em>cyclic</em> because it can be re-used after the waiting threads
16	* are released.
17	*
18	* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
19	* that is run once per barrier point, after the last thread in the party
20	* arrives, but before any threads are released.
21	* This <em>barrier action</em> is useful
22	* for updating shared-state before any of the parties continue.
23	*
24	* <p><b>Sample usage:</b> Here is an example of
25	* using a barrier in a parallel decomposition design:
26	* <pre>
27	* class Solver {
28	* final int N;
29	* final float[][] data;
30	* final CyclicBarrier barrier;
31	*
32	* class Worker implements Runnable {
33	* int myRow;
34	* Worker(int row) { myRow = row; }
35	* public void run() {
36	* while (!done()) {
37	* processRow(myRow);
38	*
39	* try {
40	* barrier.await();
41	* } catch (InterruptedException ex) {
42	* return;
43	* } catch (BrokenBarrierException ex) {
44	* return;
45	* }
46	* }
47	* }
48	* }
49	*
50	* public Solver(float[][] matrix) {
51	* data = matrix;
52	* N = matrix.length;
53	* barrier = new CyclicBarrier(N,
54	* new Runnable() {
55	* public void run() {
56	* mergeRows(...);
57	* }
58	* });
59	* for (int i = 0; i < N; ++i)
60	* new Thread(new Worker(i)).start();
61	*
62	* waitUntilDone();
63	* }
64	* }
65	* </pre>
66	* Here, each worker thread processes a row of the matrix then waits at the
67	* barrier until all rows have been processed. When all rows are processed
68	* the supplied {@link Runnable} barrier action is executed and merges the
69	* rows. If the merger
70	* determines that a solution has been found then <tt>done()</tt> will return
71	* <tt>true</tt> and each worker will terminate.
72	*
73	* <p>If the barrier action does not rely on the parties being suspended when
74	* it is executed, then any of the threads in the party could execute that
75	* action when it is released. To facilitate this, each invocation of
76	* {@link #await} returns the arrival index of that thread at the barrier.
77	* You can then choose which thread should execute the barrier action, for
78	* example:
79	* <pre> if (barrier.await() == 0) {
80	* // log the completion of this iteration
81	* }</pre>
82	*
83	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
84	* for failed synchronization attempts: If a thread leaves a barrier
85	* point prematurely because of interruption, failure, or timeout, all
86	* other threads waiting at that barrier point will also leave
87	* abnormally via {@link BrokenBarrierException} (or
88	* {@link InterruptedException} if they too were interrupted at about
89	* the same time).
90	*
91	* <p>Memory consistency effects: Actions in a thread prior to calling
92	* {@code await()}
93	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
94	* actions that are part of the barrier action, which in turn
95	* <i>happen-before</i> actions following a successful return from the
96	* corresponding {@code await()} in other threads.
97	*
98	* @since 1.5
99	* @see CountDownLatch
100	*
101	* @author Doug Lea
102	*/
103	public class CyclicBarrier {
104	/**
105	* Each use of the barrier is represented as a generation instance.
106	* The generation changes whenever the barrier is tripped, or
107	* is reset. There can be many generations associated with threads
108	* using the barrier - due to the non-deterministic way the lock
109	* may be allocated to waiting threads - but only one of these
110	* can be active at a time (the one to which <tt>count</tt> applies)
111	* and all the rest are either broken or tripped.
112	* There need not be an active generation if there has been a break
113	* but no subsequent reset.
114	*/
115	private static class Generation {
116	boolean broken = false;
117	}
118
119	/** The lock for guarding barrier entry */
120	private final ReentrantLock lock = new ReentrantLock();
121	/** Condition to wait on until tripped */
122	private final Condition trip = lock.newCondition();
123	/** The number of parties */
124	private final int parties;
125	/* The command to run when tripped */
126	private final Runnable barrierCommand;
127	/** The current generation */
128	private Generation generation = new Generation();
129
130	/**
131	* Number of parties still waiting. Counts down from parties to 0
132	* on each generation. It is reset to parties on each new
133	* generation or when broken.
134	*/
135	private int count;
136
137	/**
138	* Updates state on barrier trip and wakes up everyone.
139	* Called only while holding lock.
140	*/
141	private void nextGeneration() {
142	// signal completion of last generation
143	trip.signalAll();
144	// set up next generation
145	count = parties;
146	generation = new Generation();
147	}
148
149	/**
150	* Sets current barrier generation as broken and wakes up everyone.
151	* Called only while holding lock.
152	*/
153	private void breakBarrier() {
154	generation.broken = true;
155	count = parties;
156	trip.signalAll();
157	}
158
159	/**
160	* Main barrier code, covering the various policies.
161	*/
162	private int dowait(boolean timed, long nanos)
163	throws InterruptedException, BrokenBarrierException,
164	TimeoutException {
165	final ReentrantLock lock = this.lock;
166	lock.lock();
167	try {
168	final Generation g = generation;
169
170	if (g.broken)
171	throw new BrokenBarrierException();
172
173	if (Thread.interrupted()) {
174	breakBarrier();
175	throw new InterruptedException();
176	}
177
178	int index = --count;
179	if (index == 0) { // tripped
180	boolean ranAction = false;
181	try {
182	final Runnable command = barrierCommand;
183	if (command != null)
184	command.run();
185	ranAction = true;
186	nextGeneration();
187	return 0;
188	} finally {
189	if (!ranAction)
190	breakBarrier();
191	}
192	}
193
194	// loop until tripped, broken, interrupted, or timed out
195	for (;;) {
196	try {
197	if (!timed)
198	trip.await();
199	else if (nanos > 0L)
200	nanos = trip.awaitNanos(nanos);
201	} catch (InterruptedException ie) {
202	if (g == generation && ! g.broken) {
203	breakBarrier();
204	throw ie;
205	} else {
206	// We're about to finish waiting even if we had not
207	// been interrupted, so this interrupt is deemed to
208	// "belong" to subsequent execution.
209	Thread.currentThread().interrupt();
210	}
211	}
212
213	if (g.broken)
214	throw new BrokenBarrierException();
215
216	if (g != generation)
217	return index;
218
219	if (timed && nanos <= 0L) {
220	breakBarrier();
221	throw new TimeoutException();
222	}
223	}
224	} finally {
225	lock.unlock();
226	}
227	}
228
229	/**
230	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
231	* given number of parties (threads) are waiting upon it, and which
232	* will execute the given barrier action when the barrier is tripped,
233	* performed by the last thread entering the barrier.
234	*
235	* @param parties the number of threads that must invoke {@link #await}
236	* before the barrier is tripped.
237	* @param barrierAction the command to execute when the barrier is
238	* tripped, or <tt>null</tt> if there is no action.
239	*
240	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
241	*/
242	public CyclicBarrier(int parties, Runnable barrierAction) {
243	if (parties <= 0) throw new IllegalArgumentException();
244	this.parties = parties;
245	this.count = parties;
246	this.barrierCommand = barrierAction;
247	}
248
249	/**
250	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
251	* given number of parties (threads) are waiting upon it, and
252	* does not perform a predefined action when the barrier is tripped.
253	*
254	* @param parties the number of threads that must invoke {@link #await}
255	* before the barrier is tripped.
256	*
257	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
258	*/
259	public CyclicBarrier(int parties) {
260	this(parties, null);
261	}
262
263	/**
264	* Returns the number of parties required to trip this barrier.
265	* @return the number of parties required to trip this barrier.
266	*/
267	public int getParties() {
268	return parties;
269	}
270
271	/**
272	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
273	* on this barrier.
274	*
275	* <p>If the current thread is not the last to arrive then it is
276	* disabled for thread scheduling purposes and lies dormant until
277	* one of the following things happens:
278	* <ul>
279	* <li>The last thread arrives; or
280	* <li>Some other thread {@link Thread#interrupt interrupts} the current
281	* thread; or
282	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
283	* other waiting threads; or
284	* <li>Some other thread times out while waiting for barrier; or
285	* <li>Some other thread invokes {@link #reset} on this barrier.
286	* </ul>
287	* <p>If the current thread:
288	* <ul>
289	* <li>has its interrupted status set on entry to this method; or
290	* <li>is {@link Thread#interrupt interrupted} while waiting
291	* </ul>
292	* then {@link InterruptedException} is thrown and the current thread's
293	* interrupted status is cleared.
294	*
295	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
296	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
297	* or while any thread is waiting,
298	* then {@link BrokenBarrierException} is thrown.
299	*
300	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
301	* then all other waiting threads will throw
302	* {@link BrokenBarrierException} and the barrier is placed in the broken
303	* state.
304	*
305	* <p>If the current thread is the last thread to arrive, and a
306	* non-null barrier action was supplied in the constructor, then the
307	* current thread runs the action before allowing the other threads to
308	* continue.
309	* If an exception occurs during the barrier action then that exception
310	* will be propagated in the current thread and the barrier is placed in
311	* the broken state.
312	*
313	* @return the arrival index of the current thread, where index
314	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
315	* zero indicates the last to arrive.
316	*
317	* @throws InterruptedException if the current thread was interrupted
318	* while waiting.
319	* @throws BrokenBarrierException if <em>another</em> thread was
320	* interrupted or timed out while the current thread was waiting,
321	* or the barrier was reset, or the barrier was broken when
322	* <tt>await</tt> was called, or the barrier action (if present)
323	* failed due an exception.
324	*/
325	public int await() throws InterruptedException, BrokenBarrierException {
326	try {
327	return dowait(false, 0L);
328	} catch (TimeoutException toe) {
329	throw new Error(toe); // cannot happen;
330	}
331	}
332
333	/**
334	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
335	* on this barrier.
336	*
337	* <p>If the current thread is not the last to arrive then it is
338	* disabled for thread scheduling purposes and lies dormant until
339	* one of the following things happens:
340	* <ul>
341	* <li>The last thread arrives; or
342	* <li>The specified timeout elapses; or
343	* <li>Some other thread {@link Thread#interrupt interrupts} the current
344	* thread; or
345	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
346	* other waiting threads; or
347	* <li>Some other thread times out while waiting for barrier; or
348	* <li>Some other thread invokes {@link #reset} on this barrier.
349	* </ul>
350	* <p>If the current thread:
351	* <ul>
352	* <li>has its interrupted status set on entry to this method; or
353	* <li>is {@link Thread#interrupt interrupted} while waiting
354	* </ul>
355	* then {@link InterruptedException} is thrown and the current thread's
356	* interrupted status is cleared.
357	*
358	* <p>If the specified waiting time elapses then {@link TimeoutException}
359	* is thrown. If the time is less than or equal to zero, the
360	* method will not wait at all.
361	*
362	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
363	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
364	* or while any thread is waiting,
365	* then {@link BrokenBarrierException} is thrown.
366	*
367	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
368	* then all other waiting threads will throw
369	* {@link BrokenBarrierException} and the barrier is placed in the broken
370	* state.
371	*
372	* <p>If the current thread is the last thread to arrive, and a
373	* non-null barrier action was supplied in the constructor, then the
374	* current thread runs the action before allowing the other threads to
375	* continue.
376	* If an exception occurs during the barrier action then that exception
377	* will be propagated in the current thread and the barrier is placed in
378	* the broken state.
379	*
380	* @param timeout the time to wait for the barrier
381	* @param unit the time unit of the timeout parameter
382	* @return the arrival index of the current thread, where index
383	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
384	* zero indicates the last to arrive.
385	*
386	* @throws InterruptedException if the current thread was interrupted
387	* while waiting.
388	* @throws TimeoutException if the specified timeout elapses.
389	* @throws BrokenBarrierException if <em>another</em> thread was
390	* interrupted or timed out while the current thread was waiting,
391	* or the barrier was reset, or the barrier was broken when
392	* <tt>await</tt> was called, or the barrier action (if present)
393	* failed due an exception.
394	*/
395	public int await(long timeout, TimeUnit unit)
396	throws InterruptedException,
397	BrokenBarrierException,
398	TimeoutException {
399	return dowait(true, unit.toNanos(timeout));
400	}
401
402	/**
403	* Queries if this barrier is in a broken state.
404	* @return <tt>true</tt> if one or more parties broke out of this
405	* barrier due to interruption or timeout since construction or
406	* the last reset, or a barrier action failed due to an exception;
407	* <tt>false</tt> otherwise.
408	*/
409	public boolean isBroken() {
410	final ReentrantLock lock = this.lock;
411	lock.lock();
412	try {
413	return generation.broken;
414	} finally {
415	lock.unlock();
416	}
417	}
418
419	/**
420	* Resets the barrier to its initial state. If any parties are
421	* currently waiting at the barrier, they will return with a
422	* {@link BrokenBarrierException}. Note that resets <em>after</em>
423	* a breakage has occurred for other reasons can be complicated to
424	* carry out; threads need to re-synchronize in some other way,
425	* and choose one to perform the reset. It may be preferable to
426	* instead create a new barrier for subsequent use.
427	*/
428	public void reset() {
429	final ReentrantLock lock = this.lock;
430	lock.lock();
431	try {
432	breakBarrier(); // break the current generation
433	nextGeneration(); // start a new generation
434	} finally {
435	lock.unlock();
436	}
437	}
438
439	/**
440	* Returns the number of parties currently waiting at the barrier.
441	* This method is primarily useful for debugging and assertions.
442	*
443	* @return the number of parties currently blocked in {@link #await}.
444	*/
445	public int getNumberWaiting() {
446	final ReentrantLock lock = this.lock;
447	lock.lock();
448	try {
449	return parties - count;
450	} finally {
451	lock.unlock();
452	}
453	}
454	}