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 synchronization aid that allows a set threads to all wait for
 * each other to reach a common barrier point.  CyckicBarriers 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: 2003/05/27 18:14:39 $
 * @editor $Author: dl $
 *
 * @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:	1.3
Committed:	Sat Jun 7 18:20:20 2003 UTC (21 years ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_1
Changes since 1.2:	+8 -7 lines
Log Message:	Misc documentation updates
#	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 synchronization aid that allows a set threads to all wait for
11	* each other to reach a common barrier point. CyckicBarriers are
12	* useful in programs involving a fixed sized party of threads that
13	* must occasionally wait for each other. The barrier is
14	* <em>cyclic</em> because it can be re-used after the waiting threads
15	* are released.
16	*
17	* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
18	* that is run once per barrier point, after the last thread in the party
19	* arrives, but before any threads are released.
20	* This <em>barrier action</em> is useful
21	* for updating shared-state before any of the parties continue.
22	*
23	* <p><b>Sample usage:</b> Here is a code sketch of
24	* using a barrier in a parallel decomposition design:
25	* <pre>
26	* class Solver {
27	* final int N;
28	* final float[][] data;
29	* final CyclicBarrier barrier;
30	*
31	* class Worker implements Runnable {
32	* int myRow;
33	* Worker(int row) { myRow = row; }
34	* public void run() {
35	* while (!done()) {
36	* processRow(myRow);
37	*
38	* try {
39	* barrier.await();
40	* }
41	* catch (InterruptedException ex) { return; }
42	* catch (BrokenBarrierException ex) { return; }
43	* }
44	* }
45	* }
46	*
47	* public Solver(float[][] matrix) {
48	* data = matrix;
49	* N = matrix.length;
50	* barrier = new CyclicBarrier(N,
51	* new Runnable() {
52	* public void run() {
53	* mergeRows(...);
54	* }
55	* });
56	* for (int i = 0; i < N; ++i)
57	* new Thread(new Worker(i)).start();
58	*
59	* waitUntilDone();
60	* }
61	* }
62	* </pre>
63	* Here, each worker thread processes a row of the matrix then waits at the
64	* barrier until all rows have been processed. When all rows are processed
65	* the supplied {@link Runnable} barrier action is executed and merges the
66	* rows. If the merger
67	* determines that a solution has been found then <tt>done()</tt> will return
68	* <tt>true</tt> and each worker will terminate.
69	*
70	* <p>If the barrier action does not rely on the parties being suspended when
71	* it is executed, then any of the threads in the party could execute that
72	* action when it is released. To facilitate this, each invocation of
73	* {@link #await} returns the arrival index of that thread at the barrier.
74	* You can then choose which thread should execute the barrier action, for
75	* example:
76	* <pre> if (barrier.await() == 0) {
77	* // log the completion of this iteration
78	* }</pre>
79	*
80	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
81	* for failed synchronization attempts: If a thread leaves a barrier
82	* point prematurely because of interruption or timeout, all others
83	* will also leave abnormally (via {@link BrokenBarrierException}),
84	* until the barrier is {@link #reset}. This is usually the simplest
85	* and best strategy for sharing knowledge about failures among
86	* cooperating threads in the most common usage contexts of barriers.
87	*
88	* <h3>Implementation Considerations</h3>
89	* <p>This implementation has the property that interruptions among newly
90	* arriving threads can cause as-yet-unresumed threads from a previous
91	* barrier cycle to return out as broken. This transmits breakage as
92	* early as possible, but with the possible byproduct that only some
93	* threads returning out of a barrier will realize that it is newly
94	* broken. (Others will not realize this until a future cycle.)
95	*
96	*
97	*
98	* @since 1.5
99	* @spec JSR-166
100	* @revised $Date: 2003/05/27 18:14:39 $
101	* @editor $Author: dl $
102	*
103	* @fixme Is the above property actually true in this implementation?
104	* @fixme Should we have a timeout version of await()?
105	*/
106	public class CyclicBarrier {
107	private final ReentrantLock lock = new ReentrantLock();
108	private final Condition trip = lock.newCondition();
109	private final int parties;
110	private Runnable barrierCommand;
111
112	/**
113	* The generation number. Incremented mod Integer.MAX_VALUE every
114	* time barrier tripped. Starts at 1 to simplify handling of
115	* breakage indicator
116	*/
117	private int generation = 1;
118
119	/**
120	* Breakage indicator: last generation of breakage, propagated
121	* across barrier generations until reset.
122	*/
123	private int broken = 0;
124
125	/**
126	* Number of parties still waiting. Counts down from parties to 0
127	* on each cycle.
128	*/
129	private int count;
130
131	/**
132	* Update state on barrier trip.
133	*/
134	private void nextGeneration() {
135	count = parties;
136	int g = generation;
137	// avoid generation == 0
138	if (++generation < 0) generation = 1;
139	// propagate breakage
140	if (broken == g) broken = generation;
141	}
142
143	private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException {
144	lock.lock();
145	try {
146	int index = --count;
147	int g = generation;
148
149	if (broken == g)
150	throw new BrokenBarrierException();
151
152	if (Thread.interrupted()) {
153	broken = g;
154	trip.signalAll();
155	throw new InterruptedException();
156	}
157
158	if (index == 0) { // tripped
159	nextGeneration();
160	trip.signalAll();
161	try {
162	if (barrierCommand != null)
163	barrierCommand.run();
164	return 0;
165	}
166	catch (RuntimeException ex) {
167	broken = generation; // next generation is broken
168	throw ex;
169	}
170	}
171
172	while (generation == g) {
173	try {
174	if (!timed)
175	trip.await();
176	else if (nanos > 0)
177	nanos = trip.awaitNanos(nanos);
178	}
179	catch (InterruptedException ex) {
180	// Only claim that broken if interrupted before reset
181	if (generation == g) {
182	broken = g;
183	trip.signalAll();
184	throw ex;
185	}
186	else {
187	Thread.currentThread().interrupt(); // propagate
188	break;
189	}
190	}
191
192	if (timed && nanos <= 0) {
193	broken = g;
194	trip.signalAll();
195	throw new TimeoutException();
196	}
197
198	if (broken == generation)
199	throw new BrokenBarrierException();
200
201	}
202	return index;
203
204	}
205	finally {
206	lock.unlock();
207	}
208	}
209
210	/**
211	* Create a new <tt>CyclicBarrier</tt> that will trip when the
212	* given number of parties (threads) are waiting upon it, and which
213	* will execute the given barrier action when the barrier is tripped.
214	*
215	* @param parties the number of threads that must invoke {@link #await}
216	* before the barrier is tripped.
217	* @param barrierAction the command to execute when the barrier is
218	* tripped.
219	*
220	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
221	*/
222	public CyclicBarrier(int parties, Runnable barrierAction) {
223	if (parties <= 0) throw new IllegalArgumentException();
224	this.parties = parties;
225	this.count = parties;
226	this.barrierCommand = barrierAction;
227	}
228
229	/**
230	* Create a new <tt>CyclicBarrier</tt> that will trip when the
231	* given number of parties (threads) are waiting upon it.
232	*
233	* <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
234	*
235	* @param parties the number of threads that must invoke {@link #await}
236	* before the barrier is tripped.
237	*
238	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
239	*/
240	public CyclicBarrier(int parties) {
241	this(parties, null);
242	}
243
244	/**
245	* Return the number of parties required to trip this barrier.
246	* @return the number of parties required to trip this barrier.
247	**/
248	public int getParties() {
249	return parties;
250	}
251
252	/**
253	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
254	* on this barrier.
255	*
256	* <p>If the current thread is not the last to arrive then it is
257	* disabled for thread scheduling purposes and lies dormant until
258	* one of following things happens:
259	* <ul>
260	* <li>The last thread arrives; or
261	* <li>Some other thread {@link Thread#interrupt interrupts} the current
262	* thread; or
263	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
264	* other waiting threads; or
265	* <li>Some other thread times out while waiting for barrier; or
266	* <li>Some other thread invokes {@link #reset} on this barrier.
267	* </ul>
268	* <p>If the current thread:
269	* <ul>
270	* <li>has its interrupted status set on entry to this method; or
271	* <li>is {@link Thread#interrupt interrupted} while waiting
272	* </ul>
273	* then {@link InterruptedException} is thrown and the current thread's
274	* interrupted status is cleared.
275	*
276	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
277	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
278	* then {@link BrokenBarrierException} is thrown.
279	*
280	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
281	* then all other waiting threads will throw
282	* {@link BrokenBarrierException} and the barrier is placed in the broken
283	* state.
284	*
285	* <p>If the current thread is the last thread to arrive, and a
286	* non-null barrier action was supplied in the constructor, then the
287	* current thread runs the action before allowing the other threads to
288	* continue.
289	* If an exception occurs during the barrier action then that exception
290	* will be propagated in the current thread.
291	*
292	* @return the arrival index of the current thread, where index
293	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
294	* zero indicates the last to arrive.
295	*
296	* @throws InterruptedException if the current thread was interrupted
297	* while waiting
298	* @throws BrokenBarrierException if <em>another</em> thread was
299	* interrupted while the current thread was waiting, or the barrier was
300	* reset, or the barrier was broken when <tt>await</tt> was called.
301	*/
302	public int await() throws InterruptedException, BrokenBarrierException {
303	try {
304	return dowait(false, 0);
305	}
306	catch (TimeoutException toe) {
307	throw new Error(toe); // cannot happen;
308	}
309	}
310
311	/**
312	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
313	* on this barrier.
314	*
315	* <p>If the current thread is not the last to arrive then it is
316	* disabled for thread scheduling purposes and lies dormant until
317	* one of the following things happens:
318	* <ul>
319	* <li>The last thread arrives; or
320	* <li>The speceified timeout elapses; or
321	* <li>Some other thread {@link Thread#interrupt interrupts} the current
322	* thread; or
323	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
324	* other waiting threads; or
325	* <li>Some other thread times out while waiting for barrier; or
326	* <li>Some other thread invokes {@link #reset} on this barrier.
327	* </ul>
328	* <p>If the current thread:
329	* <ul>
330	* <li>has its interrupted status set on entry to this method; or
331	* <li>is {@link Thread#interrupt interrupted} while waiting
332	* </ul>
333	* then {@link InterruptedException} is thrown and the current thread's
334	* interrupted status is cleared.
335	*
336	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
337	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
338	* then {@link BrokenBarrierException} is thrown.
339	*
340	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
341	* then all other waiting threads will throw
342	* {@link BrokenBarrierException} and the barrier is placed in the broken
343	* state.
344	*
345	* <p>If the current thread is the last thread to arrive, and a
346	* non-null barrier action was supplied in the constructor, then the
347	* current thread runs the action before allowing the other threads to
348	* continue.
349	* If an exception occurs during the barrier action then that exception
350	* will be propagated in the current thread.
351	*
352	* @return the arrival index of the current thread, where index
353	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
354	* zero indicates the last to arrive.
355	*
356	* @throws InterruptedException if the current thread was interrupted
357	* while waiting
358	* @throws TimeoutException if the specified timeout elapses.
359	* @throws BrokenBarrierException if <em>another</em> thread was
360	* interrupted while the current thread was waiting, or the barrier was
361	* reset, or the barrier was broken when <tt>await</tt> was called.
362	*/
363	public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException {
364	return dowait(true, unit.toNanos(timeout));
365	}
366
367	/**
368	* Query if this barrier is in a broken state.
369	* @return <tt>true</tt> if one or more parties broke out of this
370	* barrier due to interruption or timeout since construction or
371	* the last reset; and <tt>false</tt> otherwise.
372	*/
373	public boolean isBroken() {
374	lock.lock();
375	try {
376	return broken >= generation;
377	}
378	finally {
379	lock.unlock();
380	}
381	}
382
383	/**
384	* Reset the barrier to its initial state. If any parties are
385	* currently waiting at the barrier, they will return with a
386	* {@link BrokenBarrierException}.
387	*/
388	public void reset() {
389	lock.lock();
390	try {
391	int g = generation;
392	nextGeneration();
393	broken = g; // cause brokenness setting to stop at previous gen.
394	trip.signalAll();
395	}
396	finally {
397	lock.unlock();
398	}
399	}
400
401	/**
402	* Return the number of parties currently waiting at the barrier.
403	* This method is primarily useful for debugging and assertions.
404	*
405	* @return the number of parties currently blocked in {@link #await}
406	**/
407	public int getNumberWaiting() {
408	lock.lock();
409	try {
410	return parties - count;
411	}
412	finally {
413	lock.unlock();
414	}
415	}
416
417	}
418
419