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 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 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.
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/08/23 19:47:29 $
 * @editor $Author: tim $
 * @see CountDownLatch
 *
 * @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 policies.
     */
    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;
                }
                catch (Error 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 == g) 
                    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}. Note that resets <em>after</em>
     * a breakage 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() {
        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:	1.11
Committed:	Mon Aug 25 22:32:40 2003 UTC (20 years, 9 months ago) by dholmes
Branch:	MAIN
Changes since 1.10:	+11 -5 lines
Log Message:	Added missing "catch (Error ex)" clause in dowAwait after running barrier action. Minor formatting changes.
#	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 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 or timeout, all others
86	* will also leave abnormally (via {@link BrokenBarrierException}),
87	* until the barrier is {@link #reset}. This is usually the simplest
88	* and best strategy for sharing knowledge about failures among
89	* cooperating threads in the most common usage contexts of barriers.
90	*
91	* @since 1.5
92	* @spec JSR-166
93	* @revised $Date: 2003/08/23 19:47:29 $
94	* @editor $Author: tim $
95	* @see CountDownLatch
96	*
97	* @author Doug Lea
98	*/
99	public class CyclicBarrier {
100	/** The lock for guarding barrier entry */
101	private final ReentrantLock lock = new ReentrantLock();
102	/** Condition to wait on until tripped */
103	private final Condition trip = lock.newCondition();
104	/** The number of parties */
105	private final int parties;
106	/* The command to run when tripped */
107	private Runnable barrierCommand;
108
109	/**
110	* The generation number. Incremented mod Integer.MAX_VALUE every
111	* time barrier tripped. Starts at 1 to simplify handling of
112	* breakage indicator
113	*/
114	private int generation = 1;
115
116	/**
117	* Breakage indicator: last generation of breakage, propagated
118	* across barrier generations until reset.
119	*/
120	private int broken = 0;
121
122	/**
123	* Number of parties still waiting. Counts down from parties to 0
124	* on each cycle.
125	*/
126	private int count;
127
128	/**
129	* Update state on barrier trip.
130	*/
131	private void nextGeneration() {
132	count = parties;
133	int g = generation;
134	// avoid generation == 0
135	if (++generation < 0) generation = 1;
136	// propagate breakage
137	if (broken == g) broken = generation;
138	}
139
140	/**
141	* Main barrier code, covering the various policies.
142	*/
143	private int dowait(boolean timed, long nanos)
144	throws InterruptedException, BrokenBarrierException, TimeoutException {
145	lock.lock();
146	try {
147	int index = --count;
148	int g = generation;
149
150	if (broken == g)
151	throw new BrokenBarrierException();
152
153	if (Thread.interrupted()) {
154	broken = g;
155	trip.signalAll();
156	throw new InterruptedException();
157	}
158
159	if (index == 0) { // tripped
160	nextGeneration();
161	trip.signalAll();
162	try {
163	if (barrierCommand != null)
164	barrierCommand.run();
165	return 0;
166	} catch (RuntimeException ex) {
167	broken = generation; // next generation is broken
168	throw ex;
169	}
170	catch (Error ex) {
171	broken = generation; // next generation is broken
172	throw ex;
173	}
174	}
175
176	while (generation == g) {
177	try {
178	if (!timed)
179	trip.await();
180	else if (nanos > 0)
181	nanos = trip.awaitNanos(nanos);
182	} catch (InterruptedException ex) {
183	// Only claim that broken if interrupted before reset
184	if (generation == g) {
185	broken = g;
186	trip.signalAll();
187	throw ex;
188	} else {
189	Thread.currentThread().interrupt(); // propagate
190	break;
191	}
192	}
193
194	if (timed && nanos <= 0) {
195	broken = g;
196	trip.signalAll();
197	throw new TimeoutException();
198	}
199
200	if (broken == g)
201	throw new BrokenBarrierException();
202
203	}
204	return index;
205
206	} finally {
207	lock.unlock();
208	}
209	}
210
211	/**
212	* Create a new <tt>CyclicBarrier</tt> that will trip when the
213	* given number of parties (threads) are waiting upon it, and which
214	* will execute the given barrier action when the barrier is tripped.
215	*
216	* @param parties the number of threads that must invoke {@link #await}
217	* before the barrier is tripped.
218	* @param barrierAction the command to execute when the barrier is
219	* tripped.
220	*
221	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
222	*/
223	public CyclicBarrier(int parties, Runnable barrierAction) {
224	if (parties <= 0) throw new IllegalArgumentException();
225	this.parties = parties;
226	this.count = parties;
227	this.barrierCommand = barrierAction;
228	}
229
230	/**
231	* Create a new <tt>CyclicBarrier</tt> that will trip when the
232	* given number of parties (threads) are waiting upon it.
233	*
234	* <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
235	*
236	* @param parties the number of threads that must invoke {@link #await}
237	* before the barrier is tripped.
238	*
239	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
240	*/
241	public CyclicBarrier(int parties) {
242	this(parties, null);
243	}
244
245	/**
246	* Return the number of parties required to trip this barrier.
247	* @return the number of parties required to trip this barrier.
248	**/
249	public int getParties() {
250	return parties;
251	}
252
253	/**
254	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
255	* on this barrier.
256	*
257	* <p>If the current thread is not the last to arrive then it is
258	* disabled for thread scheduling purposes and lies dormant until
259	* one of following things happens:
260	* <ul>
261	* <li>The last thread arrives; or
262	* <li>Some other thread {@link Thread#interrupt interrupts} the current
263	* thread; or
264	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
265	* other waiting threads; or
266	* <li>Some other thread times out while waiting for barrier; or
267	* <li>Some other thread invokes {@link #reset} on this barrier.
268	* </ul>
269	* <p>If the current thread:
270	* <ul>
271	* <li>has its interrupted status set on entry to this method; or
272	* <li>is {@link Thread#interrupt interrupted} while waiting
273	* </ul>
274	* then {@link InterruptedException} is thrown and the current thread's
275	* interrupted status is cleared.
276	*
277	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
278	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked
279	* then {@link BrokenBarrierException} is thrown.
280	*
281	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
282	* then all other waiting threads will throw
283	* {@link BrokenBarrierException} and the barrier is placed in the broken
284	* state.
285	*
286	* <p>If the current thread is the last thread to arrive, and a
287	* non-null barrier action was supplied in the constructor, then the
288	* current thread runs the action before allowing the other threads to
289	* continue.
290	* If an exception occurs during the barrier action then that exception
291	* will be propagated in the current thread.
292	*
293	* @return the arrival index of the current thread, where index
294	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
295	* zero indicates the last to arrive.
296	*
297	* @throws InterruptedException if the current thread was interrupted
298	* while waiting
299	* @throws BrokenBarrierException if <em>another</em> thread was
300	* interrupted while the current thread was waiting, or the barrier was
301	* reset, or the barrier was broken when <tt>await</tt> was called.
302	*/
303	public int await() throws InterruptedException, BrokenBarrierException {
304	try {
305	return dowait(false, 0);
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	* @param timeout the time to wait for the barrier
353	* @param unit the time unit of the timeout parameter
354	* @return the arrival index of the current thread, where index
355	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
356	* zero indicates the last to arrive.
357	*
358	* @throws InterruptedException if the current thread was interrupted
359	* while waiting
360	* @throws TimeoutException if the specified timeout elapses.
361	* @throws BrokenBarrierException if <em>another</em> thread was
362	* interrupted while the current thread was waiting, or the barrier was
363	* reset, or the barrier was broken when <tt>await</tt> was called.
364	*/
365	public int await(long timeout, TimeUnit unit)
366	throws InterruptedException,
367	BrokenBarrierException,
368	TimeoutException {
369	return dowait(true, unit.toNanos(timeout));
370	}
371
372	/**
373	* Query if this barrier is in a broken state.
374	* @return <tt>true</tt> if one or more parties broke out of this
375	* barrier due to interruption or timeout since construction or
376	* the last reset; and <tt>false</tt> otherwise.
377	*/
378	public boolean isBroken() {
379	lock.lock();
380	try {
381	return broken >= generation;
382	} finally {
383	lock.unlock();
384	}
385	}
386
387	/**
388	* Reset the barrier to its initial state. If any parties are
389	* currently waiting at the barrier, they will return with a
390	* {@link BrokenBarrierException}. Note that resets <em>after</em>
391	* a breakage can be complicated to carry out; threads need to
392	* re-synchronize in some other way, and choose one to perform the
393	* reset. It may be preferable to instead create a new barrier
394	* for subsequent use.
395	*/
396	public void reset() {
397	lock.lock();
398	try {
399	int g = generation;
400	nextGeneration();
401	broken = g; // cause brokenness setting to stop at previous gen.
402	trip.signalAll();
403	} finally {
404	lock.unlock();
405	}
406	}
407
408	/**
409	* Return the number of parties currently waiting at the barrier.
410	* This method is primarily useful for debugging and assertions.
411	*
412	* @return the number of parties currently blocked in {@link #await}
413	**/
414	public int getNumberWaiting() {
415	lock.lock();
416	try {
417	return parties - count;
418	} finally {
419	lock.unlock();
420	}
421	}
422
423	}