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).
 *
 * @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;
        boolean tripped = 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
        generation.tripped = true;
        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) {
                    breakBarrier();
                    throw ie;
                }

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

                if (g.tripped)
                    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 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:	1.32
Committed:	Tue Apr 26 01:55:35 2005 UTC (19 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.31:	+1 -1 lines
Log Message:	doc fixes
#	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	* @since 1.5
92	* @see CountDownLatch
93	*
94	* @author Doug Lea
95	*/
96	public class CyclicBarrier {
97	/**
98	* Each use of the barrier is represented as a generation instance.
99	* The generation changes whenever the barrier is tripped, or
100	* is reset. There can be many generations associated with threads
101	* using the barrier - due to the non-deterministic way the lock
102	* may be allocated to waiting threads - but only one of these
103	* can be active at a time (the one to which <tt>count</tt> applies)
104	* and all the rest are either broken or tripped.
105	* There need not be an active generation if there has been a break
106	* but no subsequent reset.
107	*/
108	private static class Generation {
109	boolean broken = false;
110	boolean tripped = false;
111	}
112
113	/** The lock for guarding barrier entry */
114	private final ReentrantLock lock = new ReentrantLock();
115	/** Condition to wait on until tripped */
116	private final Condition trip = lock.newCondition();
117	/** The number of parties */
118	private final int parties;
119	/* The command to run when tripped */
120	private final Runnable barrierCommand;
121	/** The current generation */
122	private Generation generation = new Generation();
123
124	/**
125	* Number of parties still waiting. Counts down from parties to 0
126	* on each generation. It is reset to parties on each new
127	* generation or when broken.
128	*/
129	private int count;
130
131	/**
132	* Updates state on barrier trip and wakes up everyone.
133	* Called only while holding lock.
134	*/
135	private void nextGeneration() {
136	// signal completion of last generation
137	generation.tripped = true;
138	trip.signalAll();
139	// set up next generation
140	count = parties;
141	generation = new Generation();
142	}
143
144	/**
145	* Sets current barrier generation as broken and wakes up everyone.
146	* Called only while holding lock.
147	*/
148	private void breakBarrier() {
149	generation.broken = true;
150	count = parties;
151	trip.signalAll();
152	}
153
154	/**
155	* Main barrier code, covering the various policies.
156	*/
157	private int dowait(boolean timed, long nanos)
158	throws InterruptedException, BrokenBarrierException,
159	TimeoutException {
160	final ReentrantLock lock = this.lock;
161	lock.lock();
162	try {
163	final Generation g = generation;
164
165	if (g.broken)
166	throw new BrokenBarrierException();
167
168	if (Thread.interrupted()) {
169	breakBarrier();
170	throw new InterruptedException();
171	}
172
173	int index = --count;
174	if (index == 0) { // tripped
175	boolean ranAction = false;
176	try {
177	final Runnable command = barrierCommand;
178	if (command != null)
179	command.run();
180	ranAction = true;
181	nextGeneration();
182	return 0;
183	} finally {
184	if (!ranAction)
185	breakBarrier();
186	}
187	}
188
189	// loop until tripped, broken, interrupted, or timed out
190	for (;;) {
191	try {
192	if (!timed)
193	trip.await();
194	else if (nanos > 0L)
195	nanos = trip.awaitNanos(nanos);
196	} catch (InterruptedException ie) {
197	breakBarrier();
198	throw ie;
199	}
200
201	if (g.broken)
202	throw new BrokenBarrierException();
203
204	if (g.tripped)
205	return index;
206
207	if (timed && nanos <= 0L) {
208	breakBarrier();
209	throw new TimeoutException();
210	}
211	}
212	} finally {
213	lock.unlock();
214	}
215	}
216
217	/**
218	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
219	* given number of parties (threads) are waiting upon it, and which
220	* will execute the given barrier action when the barrier is tripped,
221	* performed by the last thread entering the barrier.
222	*
223	* @param parties the number of threads that must invoke {@link #await}
224	* before the barrier is tripped.
225	* @param barrierAction the command to execute when the barrier is
226	* tripped, or <tt>null</tt> if there is no action.
227	*
228	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
229	*/
230	public CyclicBarrier(int parties, Runnable barrierAction) {
231	if (parties <= 0) throw new IllegalArgumentException();
232	this.parties = parties;
233	this.count = parties;
234	this.barrierCommand = barrierAction;
235	}
236
237	/**
238	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
239	* given number of parties (threads) are waiting upon it, and
240	* does not perform a predefined action when the barrier is tripped.
241	*
242	* @param parties the number of threads that must invoke {@link #await}
243	* before the barrier is tripped.
244	*
245	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
246	*/
247	public CyclicBarrier(int parties) {
248	this(parties, null);
249	}
250
251	/**
252	* Returns the number of parties required to trip this barrier.
253	* @return the number of parties required to trip this barrier.
254	*/
255	public int getParties() {
256	return parties;
257	}
258
259	/**
260	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
261	* on this barrier.
262	*
263	* <p>If the current thread is not the last to arrive then it is
264	* disabled for thread scheduling purposes and lies dormant until
265	* one of following things happens:
266	* <ul>
267	* <li>The last thread arrives; or
268	* <li>Some other thread {@link Thread#interrupt interrupts} the current
269	* thread; or
270	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
271	* other waiting threads; or
272	* <li>Some other thread times out while waiting for barrier; or
273	* <li>Some other thread invokes {@link #reset} on this barrier.
274	* </ul>
275	* <p>If the current thread:
276	* <ul>
277	* <li>has its interrupted status set on entry to this method; or
278	* <li>is {@link Thread#interrupt interrupted} while waiting
279	* </ul>
280	* then {@link InterruptedException} is thrown and the current thread's
281	* interrupted status is cleared.
282	*
283	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
284	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
285	* or while any thread is waiting,
286	* then {@link BrokenBarrierException} is thrown.
287	*
288	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
289	* then all other waiting threads will throw
290	* {@link BrokenBarrierException} and the barrier is placed in the broken
291	* state.
292	*
293	* <p>If the current thread is the last thread to arrive, and a
294	* non-null barrier action was supplied in the constructor, then the
295	* current thread runs the action before allowing the other threads to
296	* continue.
297	* If an exception occurs during the barrier action then that exception
298	* will be propagated in the current thread and the barrier is placed in
299	* the broken state.
300	*
301	* @return the arrival index of the current thread, where index
302	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
303	* zero indicates the last to arrive.
304	*
305	* @throws InterruptedException if the current thread was interrupted
306	* while waiting.
307	* @throws BrokenBarrierException if <em>another</em> thread was
308	* interrupted or timed out while the current thread was waiting,
309	* or the barrier was reset, or the barrier was broken when
310	* <tt>await</tt> was called, or the barrier action (if present)
311	* failed due an exception.
312	*/
313	public int await() throws InterruptedException, BrokenBarrierException {
314	try {
315	return dowait(false, 0L);
316	} catch (TimeoutException toe) {
317	throw new Error(toe); // cannot happen;
318	}
319	}
320
321	/**
322	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
323	* on this barrier.
324	*
325	* <p>If the current thread is not the last to arrive then it is
326	* disabled for thread scheduling purposes and lies dormant until
327	* one of the following things happens:
328	* <ul>
329	* <li>The last thread arrives; or
330	* <li>The specified timeout elapses; or
331	* <li>Some other thread {@link Thread#interrupt interrupts} the current
332	* thread; or
333	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
334	* other waiting threads; or
335	* <li>Some other thread times out while waiting for barrier; or
336	* <li>Some other thread invokes {@link #reset} on this barrier.
337	* </ul>
338	* <p>If the current thread:
339	* <ul>
340	* <li>has its interrupted status set on entry to this method; or
341	* <li>is {@link Thread#interrupt interrupted} while waiting
342	* </ul>
343	* then {@link InterruptedException} is thrown and the current thread's
344	* interrupted status is cleared.
345	*
346	* <p>If the specified waiting time elapses then {@link TimeoutException}
347	* is thrown. If the time is less than or equal to zero, the
348	* method will not wait at all.
349	*
350	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
351	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
352	* or while any thread is waiting,
353	* then {@link BrokenBarrierException} is thrown.
354	*
355	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
356	* then all other waiting threads will throw
357	* {@link BrokenBarrierException} and the barrier is placed in the broken
358	* state.
359	*
360	* <p>If the current thread is the last thread to arrive, and a
361	* non-null barrier action was supplied in the constructor, then the
362	* current thread runs the action before allowing the other threads to
363	* continue.
364	* If an exception occurs during the barrier action then that exception
365	* will be propagated in the current thread and the barrier is placed in
366	* the broken state.
367	*
368	* @param timeout the time to wait for the barrier
369	* @param unit the time unit of the timeout parameter
370	* @return the arrival index of the current thread, where index
371	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
372	* zero indicates the last to arrive.
373	*
374	* @throws InterruptedException if the current thread was interrupted
375	* while waiting.
376	* @throws TimeoutException if the specified timeout elapses.
377	* @throws BrokenBarrierException if <em>another</em> thread was
378	* interrupted or timed out while the current thread was waiting,
379	* or the barrier was reset, or the barrier was broken when
380	* <tt>await</tt> was called, or the barrier action (if present)
381	* failed due an exception.
382	*/
383	public int await(long timeout, TimeUnit unit)
384	throws InterruptedException,
385	BrokenBarrierException,
386	TimeoutException {
387	return dowait(true, unit.toNanos(timeout));
388	}
389
390	/**
391	* Queries if this barrier is in a broken state.
392	* @return <tt>true</tt> if one or more parties broke out of this
393	* barrier due to interruption or timeout since construction or
394	* the last reset, or a barrier action failed due to an exception;
395	* <tt>false</tt> otherwise.
396	*/
397	public boolean isBroken() {
398	final ReentrantLock lock = this.lock;
399	lock.lock();
400	try {
401	return generation.broken;
402	} finally {
403	lock.unlock();
404	}
405	}
406
407	/**
408	* Resets the barrier to its initial state. If any parties are
409	* currently waiting at the barrier, they will return with a
410	* {@link BrokenBarrierException}. Note that resets <em>after</em>
411	* a breakage has occurred for other reasons can be complicated to
412	* carry out; threads need to re-synchronize in some other way,
413	* and choose one to perform the reset. It may be preferable to
414	* instead create a new barrier for subsequent use.
415	*/
416	public void reset() {
417	final ReentrantLock lock = this.lock;
418	lock.lock();
419	try {
420	breakBarrier(); // break the current generation
421	nextGeneration(); // start a new generation
422	} finally {
423	lock.unlock();
424	}
425	}
426
427	/**
428	* Returns the number of parties currently waiting at the barrier.
429	* This method is primarily useful for debugging and assertions.
430	*
431	* @return the number of parties currently blocked in {@link #await}.
432	*/
433	public int getNumberWaiting() {
434	final ReentrantLock lock = this.lock;
435	lock.lock();
436	try {
437	return parties - count;
438	} finally {
439	lock.unlock();
440	}
441	}
442	}