util/concurrent/CyclicBarrier.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

package java.util.concurrent;

/**
 * A <tt>CyclicBarrier</tt> allows a set threads to all wait for each 
 * other to reach a common barrier point.  They are useful in programs
 * involving a fixed sized party of threads that must occasionally
 * wait for each other. The barrier is <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 a code sketch 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 or timeout, all others
 * will also leave abnormally (via {@link BrokenBarrierException}),
 * until the barrier is {@link #reset}. This is usually the simplest
 * and best strategy for sharing knowledge about failures among
 * cooperating threads in the most common usage contexts of barriers.
 *
 * <h3>Implementation Considerations</h3>
 * <p>This implementation has the property that interruptions among newly
 * arriving threads can cause as-yet-unresumed threads from a previous
 * barrier cycle to return out as broken. This transmits breakage as
 * early as possible, but with the possible byproduct that only some
 * threads returning out of a barrier will realize that it is newly
 * broken. (Others will not realize this until a future cycle.)
 *
 *
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/01/28 06:56:53 $
 * @editor $Author: dholmes $
 *
 * @fixme Is the above property actually true in this implementation?
 * @fixme Should we have a timeout version of await()?
 */
public class CyclicBarrier {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition trip = lock.newCondition();
    private final int parties;
    private Runnable barrierCommand;

    /**
     * The generation number. Incremented mod Integer.MAX_VALUE every
     * time barrier tripped. Starts at 1 to simplify handling of
     * breakage indicator
     */
    private int generation = 1; 

    /** 
     * Breakage indicator: last generation of breakage, propagated
     * across barrier generations until reset. 
     */
    private int broken = 0; 

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each cycle.
     */
    private int count; 

    /**
     * Update state on barrier trip.
     */  
    private void nextGeneration() {
        count = parties;
        int g = generation;
        // avoid generation == 0
        if (++generation < 0) generation = 1;
        // propagate breakage
        if (broken == g) broken = generation;
    }

    private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException {
        lock.lock();
        try {
            int index = --count;
            int g = generation;

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

            if (Thread.interrupted()) {
                broken = g;
                trip.signalAll();
                throw new InterruptedException();
            }

            if (index == 0) {  // tripped
                nextGeneration();
                trip.signalAll();
                try {
                    if (barrierCommand != null) 
                        barrierCommand.run();
                    return 0;
                }
                catch (RuntimeException ex) {
                    broken = generation; // next generation is broken
                    throw ex;
                }
            }

            while (generation == g) {
                try {
                    if (!timed) 
                        trip.await();
                    else if (nanos > 0)
                        nanos = trip.awaitNanos(nanos);
                }
                catch (InterruptedException ex) {
                    // Only claim that broken if interrupted before reset
                    if (generation == g) { 
                        broken = g;
                        trip.signalAll();
                        throw ex;
                    }
                    else {
                        Thread.currentThread().interrupt(); // propagate
                        break;
                    }
                }
                
                if (timed && nanos <= 0) {
                    broken = g;
                    trip.signalAll();
                    throw new TimeoutException();
                }

                if (broken == generation) 
                    throw new BrokenBarrierException();
                
            }
            return index;

        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Create 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.
     *
     * @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.
     *
     * @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;
    }

    /**
     * Create a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it.
     *
     * <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
     *
     * @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);
    }

    /**
     * Return the number of parties required to trip this barrier.
     * @return the number of parties required to trip this barrier.
     **/
    public int getParties() {
        return parties;
    }

    /**
     * Wait 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
     * 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.
     *
     * @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 while the current thread was waiting, or the barrier was
     * reset, or the barrier was broken when <tt>await</tt> was called.
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0);
        }
        catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen;
        }
    }

    /**
     * Wait 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 speceified 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 barrier is {@link #reset} while any thread is waiting, or if 
     * the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
     * 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.
     *
     * @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 while the current thread was waiting, or the barrier was
     * reset, or the barrier was broken when <tt>await</tt> was called.
     */
    public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * Query 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; and <tt>false</tt> otherwise.
     */
    public boolean isBroken() {
        lock.lock();
        try {
            return broken >= generation;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Reset the barrier to its initial state.  If any parties are
     * currently waiting at the barrier, they will return with a
     * {@link BrokenBarrierException}.
     */
    public void reset() {
        lock.lock();
        try {
            int g = generation;
            nextGeneration();
            broken = g; // cause brokenness setting to stop at previous gen.
            trip.signalAll();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Return 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() {
        lock.lock();
        try {
            return parties - count;
        }
        finally {
            lock.unlock();
        }
    }

}


Revision:	1.2
Committed:	Tue May 27 18:14:39 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRERELEASE_0_1
Changes since 1.1:	+213 -17 lines
Log Message:	re-check-in initial implementations
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain. Use, modify, and
4	* redistribute this code in any way without acknowledgement.
5	*/
6
7	package java.util.concurrent;
8
9	/**
10	* A <tt>CyclicBarrier</tt> allows a set threads to all wait for each
11	* other to reach a common barrier point. They are useful in programs
12	* involving a fixed sized party of threads that must occasionally
13	* wait for each other. The barrier is <em>cyclic</em> because it can
14	* be re-used after the waiting threads are released.
15	*
16	* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
17	* that is run once per barrier point, after the last thread in the party
18	* arrives, but before any threads are released.
19	* This <em>barrier action</em> is useful
20	* for updating shared-state before any of the parties continue.
21	*
22	* <p><b>Sample usage:</b> Here is a code sketch of
23	* using a barrier in a parallel decomposition design:
24	* <pre>
25	* class Solver {
26	* final int N;
27	* final float[][] data;
28	* final CyclicBarrier barrier;
29	*
30	* class Worker implements Runnable {
31	* int myRow;
32	* Worker(int row) { myRow = row; }
33	* public void run() {
34	* while (!done()) {
35	* processRow(myRow);
36	*
37	* try {
38	* barrier.await();
39	* }
40	* catch (InterruptedException ex) { return; }
41	* catch (BrokenBarrierException ex) { return; }
42	* }
43	* }
44	* }
45	*
46	* public Solver(float[][] matrix) {
47	* data = matrix;
48	* N = matrix.length;
49	* barrier = new CyclicBarrier(N,
50	* new Runnable() {
51	* public void run() {
52	* mergeRows(...);
53	* }
54	* });
55	* for (int i = 0; i < N; ++i)
56	* new Thread(new Worker(i)).start();
57	*
58	* waitUntilDone();
59	* }
60	* }
61	* </pre>
62	* Here, each worker thread processes a row of the matrix then waits at the
63	* barrier until all rows have been processed. When all rows are processed
64	* the supplied {@link Runnable} barrier action is executed and merges the
65	* rows. If the merger
66	* determines that a solution has been found then <tt>done()</tt> will return
67	* <tt>true</tt> and each worker will terminate.
68	*
69	* <p>If the barrier action does not rely on the parties being suspended when
70	* it is executed, then any of the threads in the party could execute that
71	* action when it is released. To facilitate this, each invocation of
72	* {@link #await} returns the arrival index of that thread at the barrier.
73	* You can then choose which thread should execute the barrier action, for
74	* example:
75	* <pre> if (barrier.await() == 0) {
76	* // log the completion of this iteration
77	* }</pre>
78	*
79	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
80	* for failed synchronization attempts: If a thread leaves a barrier
81	* point prematurely because of interruption or timeout, all others
82	* will also leave abnormally (via {@link BrokenBarrierException}),
83	* until the barrier is {@link #reset}. This is usually the simplest
84	* and best strategy for sharing knowledge about failures among
85	* cooperating threads in the most common usage contexts of barriers.
86	*
87	* <h3>Implementation Considerations</h3>
88	* <p>This implementation has the property that interruptions among newly
89	* arriving threads can cause as-yet-unresumed threads from a previous
90	* barrier cycle to return out as broken. This transmits breakage as
91	* early as possible, but with the possible byproduct that only some
92	* threads returning out of a barrier will realize that it is newly
93	* broken. (Others will not realize this until a future cycle.)
94	*
95	*
96	*
97	* @since 1.5
98	* @spec JSR-166
99	* @revised $Date: 2003/01/28 06:56:53 $
100	* @editor $Author: dholmes $
101	*
102	* @fixme Is the above property actually true in this implementation?
103	* @fixme Should we have a timeout version of await()?
104	*/
105	public class CyclicBarrier {
106	private final ReentrantLock lock = new ReentrantLock();
107	private final Condition trip = lock.newCondition();
108	private final int parties;
109	private Runnable barrierCommand;
110
111	/**
112	* The generation number. Incremented mod Integer.MAX_VALUE every
113	* time barrier tripped. Starts at 1 to simplify handling of
114	* breakage indicator
115	*/
116	private int generation = 1;
117
118	/**
119	* Breakage indicator: last generation of breakage, propagated
120	* across barrier generations until reset.
121	*/
122	private int broken = 0;
123
124	/**
125	* Number of parties still waiting. Counts down from parties to 0
126	* on each cycle.
127	*/
128	private int count;
129
130	/**
131	* Update state on barrier trip.
132	*/
133	private void nextGeneration() {
134	count = parties;
135	int g = generation;
136	// avoid generation == 0
137	if (++generation < 0) generation = 1;
138	// propagate breakage
139	if (broken == g) broken = generation;
140	}
141
142	private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException {
143	lock.lock();
144	try {
145	int index = --count;
146	int g = generation;
147
148	if (broken == g)
149	throw new BrokenBarrierException();
150
151	if (Thread.interrupted()) {
152	broken = g;
153	trip.signalAll();
154	throw new InterruptedException();
155	}
156
157	if (index == 0) { // tripped
158	nextGeneration();
159	trip.signalAll();
160	try {
161	if (barrierCommand != null)
162	barrierCommand.run();
163	return 0;
164	}
165	catch (RuntimeException ex) {
166	broken = generation; // next generation is broken
167	throw ex;
168	}
169	}
170
171	while (generation == g) {
172	try {
173	if (!timed)
174	trip.await();
175	else if (nanos > 0)
176	nanos = trip.awaitNanos(nanos);
177	}
178	catch (InterruptedException ex) {
179	// Only claim that broken if interrupted before reset
180	if (generation == g) {
181	broken = g;
182	trip.signalAll();
183	throw ex;
184	}
185	else {
186	Thread.currentThread().interrupt(); // propagate
187	break;
188	}
189	}
190
191	if (timed && nanos <= 0) {
192	broken = g;
193	trip.signalAll();
194	throw new TimeoutException();
195	}
196
197	if (broken == generation)
198	throw new BrokenBarrierException();
199
200	}
201	return index;
202
203	}
204	finally {
205	lock.unlock();
206	}
207	}
208
209	/**
210	* Create a new <tt>CyclicBarrier</tt> that will trip when the
211	* given number of parties (threads) are waiting upon it, and which
212	* will execute the given barrier action when the barrier is tripped.
213	*
214	* @param parties the number of threads that must invoke {@link #await}
215	* before the barrier is tripped.
216	* @param barrierAction the command to execute when the barrier is
217	* tripped.
218	*
219	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
220	*/
221	public CyclicBarrier(int parties, Runnable barrierAction) {
222	if (parties <= 0) throw new IllegalArgumentException();
223	this.parties = parties;
224	this.count = parties;
225	this.barrierCommand = barrierAction;
226	}
227
228	/**
229	* Create a new <tt>CyclicBarrier</tt> that will trip when the
230	* given number of parties (threads) are waiting upon it.
231	*
232	* <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
233	*
234	* @param parties the number of threads that must invoke {@link #await}
235	* before the barrier is tripped.
236	*
237	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
238	*/
239	public CyclicBarrier(int parties) {
240	this(parties, null);
241	}
242
243	/**
244	* Return the number of parties required to trip this barrier.
245	* @return the number of parties required to trip this barrier.
246	**/
247	public int getParties() {
248	return parties;
249	}
250
251	/**
252	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
253	* on this barrier.
254	*
255	* <p>If the current thread is not the last to arrive then it is
256	* disabled for thread scheduling purposes and lies dormant until
257	* one of following things happens:
258	* <ul>
259	* <li>The last thread arrives; or
260	* <li>Some other thread {@link Thread#interrupt interrupts} the current
261	* thread; or
262	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
263	* other waiting threads; or
264	* <li>Some other thread times out while waiting for barrier; or
265	* <li>Some other thread invokes {@link #reset} on this barrier.
266	* </ul>
267	* <p>If the current thread:
268	* <ul>
269	* <li>has its interrupted status set on entry to this method; or
270	* <li>is {@link Thread#interrupt interrupted} while waiting
271	* </ul>
272	* then {@link InterruptedException} is thrown and the current thread's
273	* interrupted status is cleared.
274	*
275	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
276	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
277	* then {@link BrokenBarrierException} is thrown.
278	*
279	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
280	* then all other waiting threads will throw
281	* {@link BrokenBarrierException} and the barrier is placed in the broken
282	* state.
283	*
284	* <p>If the current thread is the last thread to arrive, and a
285	* non-null barrier action was supplied in the constructor, then the
286	* current thread runs the action before allowing the other threads to
287	* continue.
288	* If an exception occurs during the barrier action then that exception
289	* will be propagated in the current thread.
290	*
291	* @return the arrival index of the current thread, where index
292	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
293	* zero indicates the last to arrive.
294	*
295	* @throws InterruptedException if the current thread was interrupted
296	* while waiting
297	* @throws BrokenBarrierException if <em>another</em> thread was
298	* interrupted while the current thread was waiting, or the barrier was
299	* reset, or the barrier was broken when <tt>await</tt> was called.
300	*/
301	public int await() throws InterruptedException, BrokenBarrierException {
302	try {
303	return dowait(false, 0);
304	}
305	catch (TimeoutException toe) {
306	throw new Error(toe); // cannot happen;
307	}
308	}
309
310	/**
311	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
312	* on this barrier.
313	*
314	* <p>If the current thread is not the last to arrive then it is
315	* disabled for thread scheduling purposes and lies dormant until
316	* one of the following things happens:
317	* <ul>
318	* <li>The last thread arrives; or
319	* <li>The speceified timeout elapses; or
320	* <li>Some other thread {@link Thread#interrupt interrupts} the current
321	* thread; or
322	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
323	* other waiting threads; or
324	* <li>Some other thread times out while waiting for barrier; or
325	* <li>Some other thread invokes {@link #reset} on this barrier.
326	* </ul>
327	* <p>If the current thread:
328	* <ul>
329	* <li>has its interrupted status set on entry to this method; or
330	* <li>is {@link Thread#interrupt interrupted} while waiting
331	* </ul>
332	* then {@link InterruptedException} is thrown and the current thread's
333	* interrupted status is cleared.
334	*
335	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
336	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
337	* then {@link BrokenBarrierException} is thrown.
338	*
339	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
340	* then all other waiting threads will throw
341	* {@link BrokenBarrierException} and the barrier is placed in the broken
342	* state.
343	*
344	* <p>If the current thread is the last thread to arrive, and a
345	* non-null barrier action was supplied in the constructor, then the
346	* current thread runs the action before allowing the other threads to
347	* continue.
348	* If an exception occurs during the barrier action then that exception
349	* will be propagated in the current thread.
350	*
351	* @return the arrival index of the current thread, where index
352	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
353	* zero indicates the last to arrive.
354	*
355	* @throws InterruptedException if the current thread was interrupted
356	* while waiting
357	* @throws TimeoutException if the specified timeout elapses.
358	* @throws BrokenBarrierException if <em>another</em> thread was
359	* interrupted while the current thread was waiting, or the barrier was
360	* reset, or the barrier was broken when <tt>await</tt> was called.
361	*/
362	public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException {
363	return dowait(true, unit.toNanos(timeout));
364	}
365
366	/**
367	* Query if this barrier is in a broken state.
368	* @return <tt>true</tt> if one or more parties broke out of this
369	* barrier due to interruption or timeout since construction or
370	* the last reset; and <tt>false</tt> otherwise.
371	*/
372	public boolean isBroken() {
373	lock.lock();
374	try {
375	return broken >= generation;
376	}
377	finally {
378	lock.unlock();
379	}
380	}
381
382	/**
383	* Reset the barrier to its initial state. If any parties are
384	* currently waiting at the barrier, they will return with a
385	* {@link BrokenBarrierException}.
386	*/
387	public void reset() {
388	lock.lock();
389	try {
390	int g = generation;
391	nextGeneration();
392	broken = g; // cause brokenness setting to stop at previous gen.
393	trip.signalAll();
394	}
395	finally {
396	lock.unlock();
397	}
398	}
399
400	/**
401	* Return the number of parties currently waiting at the barrier.
402	* This method is primarily useful for debugging and assertions.
403	*
404	* @return the number of parties currently blocked in {@link #await}
405	**/
406	public int getNumberWaiting() {
407	lock.lock();
408	try {
409	return parties - count;
410	}
411	finally {
412	lock.unlock();
413	}
414	}
415
416	}
417
418