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

/**
 * A synchronization aid that allows a set 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 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: 2003/06/24 14:34:47 $
 * @editor $Author: dl $
 * @see CountDownLatch
 *
 * @fixme Is the above property actually true in this implementation?
 * @fixme Should we have a timeout version of await()?
 * @author Doug Lea
 */
public class CyclicBarrier {
    /** 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 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;
    }

    /**
     * Main barrier code, covering the various pilicies.
     */
    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.
     *
     * @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 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:	1.6
Committed:	Tue Jul 8 00:46:33 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.5:	+3 -2 lines
Log Message:	Locks in subpackage; fairness params added
#	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	import java.util.concurrent.locks.*;
9
10	/**
11	* A synchronization aid that allows a set 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	* }
42	* catch (InterruptedException ex) { return; }
43	* catch (BrokenBarrierException ex) { return; }
44	* }
45	* }
46	* }
47	*
48	* public Solver(float[][] matrix) {
49	* data = matrix;
50	* N = matrix.length;
51	* barrier = new CyclicBarrier(N,
52	* new Runnable() {
53	* public void run() {
54	* mergeRows(...);
55	* }
56	* });
57	* for (int i = 0; i < N; ++i)
58	* new Thread(new Worker(i)).start();
59	*
60	* waitUntilDone();
61	* }
62	* }
63	* </pre>
64	* Here, each worker thread processes a row of the matrix then waits at the
65	* barrier until all rows have been processed. When all rows are processed
66	* the supplied {@link Runnable} barrier action is executed and merges the
67	* rows. If the merger
68	* determines that a solution has been found then <tt>done()</tt> will return
69	* <tt>true</tt> and each worker will terminate.
70	*
71	* <p>If the barrier action does not rely on the parties being suspended when
72	* it is executed, then any of the threads in the party could execute that
73	* action when it is released. To facilitate this, each invocation of
74	* {@link #await} returns the arrival index of that thread at the barrier.
75	* You can then choose which thread should execute the barrier action, for
76	* example:
77	* <pre> if (barrier.await() == 0) {
78	* // log the completion of this iteration
79	* }</pre>
80	*
81	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
82	* for failed synchronization attempts: If a thread leaves a barrier
83	* point prematurely because of interruption or timeout, all others
84	* will also leave abnormally (via {@link BrokenBarrierException}),
85	* until the barrier is {@link #reset}. This is usually the simplest
86	* and best strategy for sharing knowledge about failures among
87	* cooperating threads in the most common usage contexts of barriers.
88	*
89	* <h3>Implementation Considerations</h3>
90	* <p>This implementation has the property that interruptions among newly
91	* arriving threads can cause as-yet-unresumed threads from a previous
92	* barrier cycle to return out as broken. This transmits breakage as
93	* early as possible, but with the possible byproduct that only some
94	* threads returning out of a barrier will realize that it is newly
95	* broken. (Others will not realize this until a future cycle.)
96	*
97	*
98	*
99	* @since 1.5
100	* @spec JSR-166
101	* @revised $Date: 2003/06/24 14:34:47 $
102	* @editor $Author: dl $
103	* @see CountDownLatch
104	*
105	* @fixme Is the above property actually true in this implementation?
106	* @fixme Should we have a timeout version of await()?
107	* @author Doug Lea
108	*/
109	public class CyclicBarrier {
110	/** The lock for guarding barrier entry */
111	private final ReentrantLock lock = new ReentrantLock();
112	/** Condition to wait on until tripped */
113	private final Condition trip = lock.newCondition();
114	/** The number of parties */
115	private final int parties;
116	/* The command to run when tripped */
117	private Runnable barrierCommand;
118
119	/**
120	* The generation number. Incremented mod Integer.MAX_VALUE every
121	* time barrier tripped. Starts at 1 to simplify handling of
122	* breakage indicator
123	*/
124	private int generation = 1;
125
126	/**
127	* Breakage indicator: last generation of breakage, propagated
128	* across barrier generations until reset.
129	*/
130	private int broken = 0;
131
132	/**
133	* Number of parties still waiting. Counts down from parties to 0
134	* on each cycle.
135	*/
136	private int count;
137
138	/**
139	* Update state on barrier trip.
140	*/
141	private void nextGeneration() {
142	count = parties;
143	int g = generation;
144	// avoid generation == 0
145	if (++generation < 0) generation = 1;
146	// propagate breakage
147	if (broken == g) broken = generation;
148	}
149
150	/**
151	* Main barrier code, covering the various pilicies.
152	*/
153	private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException {
154	lock.lock();
155	try {
156	int index = --count;
157	int g = generation;
158
159	if (broken == g)
160	throw new BrokenBarrierException();
161
162	if (Thread.interrupted()) {
163	broken = g;
164	trip.signalAll();
165	throw new InterruptedException();
166	}
167
168	if (index == 0) { // tripped
169	nextGeneration();
170	trip.signalAll();
171	try {
172	if (barrierCommand != null)
173	barrierCommand.run();
174	return 0;
175	}
176	catch (RuntimeException ex) {
177	broken = generation; // next generation is broken
178	throw ex;
179	}
180	}
181
182	while (generation == g) {
183	try {
184	if (!timed)
185	trip.await();
186	else if (nanos > 0)
187	nanos = trip.awaitNanos(nanos);
188	}
189	catch (InterruptedException ex) {
190	// Only claim that broken if interrupted before reset
191	if (generation == g) {
192	broken = g;
193	trip.signalAll();
194	throw ex;
195	}
196	else {
197	Thread.currentThread().interrupt(); // propagate
198	break;
199	}
200	}
201
202	if (timed && nanos <= 0) {
203	broken = g;
204	trip.signalAll();
205	throw new TimeoutException();
206	}
207
208	if (broken == generation)
209	throw new BrokenBarrierException();
210
211	}
212	return index;
213
214	}
215	finally {
216	lock.unlock();
217	}
218	}
219
220	/**
221	* Create a new <tt>CyclicBarrier</tt> that will trip when the
222	* given number of parties (threads) are waiting upon it, and which
223	* will execute the given barrier action when the barrier is tripped.
224	*
225	* @param parties the number of threads that must invoke {@link #await}
226	* before the barrier is tripped.
227	* @param barrierAction the command to execute when the barrier is
228	* tripped.
229	*
230	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
231	*/
232	public CyclicBarrier(int parties, Runnable barrierAction) {
233	if (parties <= 0) throw new IllegalArgumentException();
234	this.parties = parties;
235	this.count = parties;
236	this.barrierCommand = barrierAction;
237	}
238
239	/**
240	* Create a new <tt>CyclicBarrier</tt> that will trip when the
241	* given number of parties (threads) are waiting upon it.
242	*
243	* <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
244	*
245	* @param parties the number of threads that must invoke {@link #await}
246	* before the barrier is tripped.
247	*
248	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
249	*/
250	public CyclicBarrier(int parties) {
251	this(parties, null);
252	}
253
254	/**
255	* Return the number of parties required to trip this barrier.
256	* @return the number of parties required to trip this barrier.
257	**/
258	public int getParties() {
259	return parties;
260	}
261
262	/**
263	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
264	* on this barrier.
265	*
266	* <p>If the current thread is not the last to arrive then it is
267	* disabled for thread scheduling purposes and lies dormant until
268	* one of following things happens:
269	* <ul>
270	* <li>The last thread arrives; or
271	* <li>Some other thread {@link Thread#interrupt interrupts} the current
272	* thread; or
273	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
274	* other waiting threads; or
275	* <li>Some other thread times out while waiting for barrier; or
276	* <li>Some other thread invokes {@link #reset} on this barrier.
277	* </ul>
278	* <p>If the current thread:
279	* <ul>
280	* <li>has its interrupted status set on entry to this method; or
281	* <li>is {@link Thread#interrupt interrupted} while waiting
282	* </ul>
283	* then {@link InterruptedException} is thrown and the current thread's
284	* interrupted status is cleared.
285	*
286	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
287	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
288	* then {@link BrokenBarrierException} is thrown.
289	*
290	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
291	* then all other waiting threads will throw
292	* {@link BrokenBarrierException} and the barrier is placed in the broken
293	* state.
294	*
295	* <p>If the current thread is the last thread to arrive, and a
296	* non-null barrier action was supplied in the constructor, then the
297	* current thread runs the action before allowing the other threads to
298	* continue.
299	* If an exception occurs during the barrier action then that exception
300	* will be propagated in the current thread.
301	*
302	* @return the arrival index of the current thread, where index
303	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
304	* zero indicates the last to arrive.
305	*
306	* @throws InterruptedException if the current thread was interrupted
307	* while waiting
308	* @throws BrokenBarrierException if <em>another</em> thread was
309	* interrupted while the current thread was waiting, or the barrier was
310	* reset, or the barrier was broken when <tt>await</tt> was called.
311	*/
312	public int await() throws InterruptedException, BrokenBarrierException {
313	try {
314	return dowait(false, 0);
315	}
316	catch (TimeoutException toe) {
317	throw new Error(toe); // cannot happen;
318	}
319	}
320
321	/**
322	* Wait 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 speceified 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 barrier is {@link #reset} while any thread is waiting, or if
347	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
348	* then {@link BrokenBarrierException} is thrown.
349	*
350	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
351	* then all other waiting threads will throw
352	* {@link BrokenBarrierException} and the barrier is placed in the broken
353	* state.
354	*
355	* <p>If the current thread is the last thread to arrive, and a
356	* non-null barrier action was supplied in the constructor, then the
357	* current thread runs the action before allowing the other threads to
358	* continue.
359	* If an exception occurs during the barrier action then that exception
360	* will be propagated in the current thread.
361	*
362	* @param timeout the time to wait for the barrier
363	* @param unit the time unit of the timeout parameter
364	* @return the arrival index of the current thread, where index
365	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
366	* zero indicates the last to arrive.
367	*
368	* @throws InterruptedException if the current thread was interrupted
369	* while waiting
370	* @throws TimeoutException if the specified timeout elapses.
371	* @throws BrokenBarrierException if <em>another</em> thread was
372	* interrupted while the current thread was waiting, or the barrier was
373	* reset, or the barrier was broken when <tt>await</tt> was called.
374	*/
375	public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException {
376	return dowait(true, unit.toNanos(timeout));
377	}
378
379	/**
380	* Query if this barrier is in a broken state.
381	* @return <tt>true</tt> if one or more parties broke out of this
382	* barrier due to interruption or timeout since construction or
383	* the last reset; and <tt>false</tt> otherwise.
384	*/
385	public boolean isBroken() {
386	lock.lock();
387	try {
388	return broken >= generation;
389	}
390	finally {
391	lock.unlock();
392	}
393	}
394
395	/**
396	* Reset the barrier to its initial state. If any parties are
397	* currently waiting at the barrier, they will return with a
398	* {@link BrokenBarrierException}.
399	*/
400	public void reset() {
401	lock.lock();
402	try {
403	int g = generation;
404	nextGeneration();
405	broken = g; // cause brokenness setting to stop at previous gen.
406	trip.signalAll();
407	}
408	finally {
409	lock.unlock();
410	}
411	}
412
413	/**
414	* Return the number of parties currently waiting at the barrier.
415	* This method is primarily useful for debugging and assertions.
416	*
417	* @return the number of parties currently blocked in {@link #await}
418	**/
419	public int getNumberWaiting() {
420	lock.lock();
421	try {
422	return parties - count;
423	}
424	finally {
425	lock.unlock();
426	}
427	}
428
429	}
430
431