util/concurrent/CyclicBarrier.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

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

/**
 * A synchronization aid that allows a set of threads to all wait for
 * each other to reach a common barrier point.  CyclicBarriers are
 * useful in programs involving a fixed sized party of threads that
 * must occasionally wait for each other. The barrier is called
 * <em>cyclic</em> because it can be re-used after the waiting threads
 * are released.
 *
 * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
 * that is run once per barrier point, after the last thread in the party
 * arrives, but before any threads are released. 
 * This <em>barrier action</em> is useful
 * for updating shared-state before any of the parties continue.
 * 
 * <p><b>Sample usage:</b> Here is an example of
 *  using a barrier in a parallel decomposition design:
 * <pre>
 * class Solver {
 *   final int N;
 *   final float[][] data;
 *   final CyclicBarrier barrier;
 *   
 *   class Worker implements Runnable {
 *     int myRow;
 *     Worker(int row) { myRow = row; }
 *     public void run() {
 *       while (!done()) {
 *         processRow(myRow);
 *
 *         try {
 *           barrier.await(); 
 *         } catch (InterruptedException ex) { 
 *           return; 
 *         } catch (BrokenBarrierException ex) { 
 *           return; 
 *         }
 *       }
 *     }
 *   }
 *
 *   public Solver(float[][] matrix) {
 *     data = matrix;
 *     N = matrix.length;
 *     barrier = new CyclicBarrier(N, 
 *                                 new Runnable() {
 *                                   public void run() { 
 *                                     mergeRows(...); 
 *                                   }
 *                                 });
 *     for (int i = 0; i < N; ++i) 
 *       new Thread(new Worker(i)).start();
 *
 *     waitUntilDone();
 *   }
 * }
 * </pre>
 * Here, each worker thread processes a row of the matrix then waits at the 
 * barrier until all rows have been processed. When all rows are processed
 * the supplied {@link Runnable} barrier action is executed and merges the 
 * rows. If the merger
 * determines that a solution has been found then <tt>done()</tt> will return
 * <tt>true</tt> and each worker will terminate.
 *
 * <p>If the barrier action does not rely on the parties being suspended when
 * it is executed, then any of the threads in the party could execute that
 * action when it is released. To facilitate this, each invocation of
 * {@link #await} returns the arrival index of that thread at the barrier.
 * You can then choose which thread should execute the barrier action, for 
 * example:
 * <pre>  if (barrier.await() == 0) {
 *     // log the completion of this iteration
 *   }</pre>
 *
 * <p>The <tt>CyclicBarrier</tt> uses a fast-fail all-or-none breakage
 * model for failed synchronization attempts: If a thread leaves a
 * barrier point prematurely because of interruption, failure, or
 * timeout, all other threads, even those that have not yet resumed
 * from a previous {@link #await}. will also leave abnormally via
 * {@link BrokenBarrierException} (or <tt>InterruptedException</tt> if
 * they too were interrupted at about the same time).
 *
 * @since 1.5
 * @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 final Runnable barrierCommand;

    /**
     * The generation number. Incremented upon barrier trip.
     * Retracted upon reset.
     */
    private long generation; 

    /** 
     * Breakage indicator.
     */
    private boolean broken; 

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

    /**
     * Update state on barrier trip and wake up everyone.
     */  
    private void nextGeneration() {
        count = parties;
        ++generation;
        trip.signalAll();
    }

    /**
     * Set barrier as broken and wake up everyone
     */
    private void breakBarrier() {
        broken = true;
        trip.signalAll();
    }

    /**
     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos) 
        throws InterruptedException, BrokenBarrierException, TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int index = --count;
            long g = generation;

            if (broken) 
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }

            if (index == 0) {  // tripped
                nextGeneration();
                boolean ranAction = false;
                try {
                    Runnable command = barrierCommand;
                    if (command != null) 
                        command.run();
                    ranAction = true;
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            for (;;) {
                try {
                    if (!timed) 
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    breakBarrier();
                    throw ie;
                }
                
                if (broken || 
                    g > generation) // true if a reset occurred while waiting
                    throw new BrokenBarrierException();

                if (g < generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }

        } 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,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     * before the barrier is tripped.
     * @param barrierAction the command to execute when the barrier is
     * tripped, or <tt>null</tt> if there is no action.
     *
     * @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties; 
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Create a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action upon each barrier.
     *
     * @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,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw 
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to 
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @return the arrival index of the current thread, where index
     *  <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and 
     * zero indicates the last to arrive.
     *
     * @throws InterruptedException if the current thread was interrupted 
     * while waiting
     * @throws BrokenBarrierException if <em>another</em> thread was
     * interrupted while the current thread was waiting, or the barrier was
     * reset, or the barrier was broken when <tt>await</tt> was called,
     * or the barrier action (if present) failed due an exception.
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen;
        }
    }

    /**
     * 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 specified timeout elapses; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>Some other thread  {@link Thread#interrupt interrupts} one of the
     * other waiting threads; or
     * <li>Some other thread times out while waiting for barrier; or
     * <li>Some other thread invokes {@link #reset} on this barrier.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting, or if 
     * the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw 
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to 
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @param timeout the time to wait for the barrier
     * @param unit the time unit of the timeout parameter
     * @return the arrival index of the current thread, where index
     *  <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and 
     * zero indicates the last to arrive.
     *
     * @throws InterruptedException if the current thread was interrupted 
     * while waiting
     * @throws TimeoutException if the specified timeout elapses.
     * @throws BrokenBarrierException if <em>another</em> thread was
     * interrupted while the current thread was waiting, or the barrier was
     * reset, or the barrier was broken when <tt>await</tt> was called,
     * or the barrier action (if present) failed due an exception.
     */
    public int await(long timeout, TimeUnit unit) 
        throws InterruptedException, 
        BrokenBarrierException, 
        TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * 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, or a barrier action failed due to an exception; 
     * and <tt>false</tt> otherwise.
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return broken;
        } 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 has occurred for other reasons can be complicated to
     * carry out; threads need to re-synchronize in some other way,
     * and choose one to perform the reset.  It may be preferable to
     * instead create a new barrier for subsequent use.
     */
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            /*
             * Retract generation number enough to cover threads
             * currently waiting on current and still resuming from
             * previous generation, plus similarly accommodating spans
             * after the reset.
             */
            generation -= 4;
            broken = false;
            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() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }

}
Revision:	1.24
Committed:	Mon Feb 9 00:23:55 2004 UTC (20 years, 3 months ago) by dl
Branch:	MAIN
Changes since 1.23:	+2 -2 lines
Log Message:	Wording improvements and fixes
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9
10	/**
11	* A synchronization aid that allows a set of threads to all wait for
12	* each other to reach a common barrier point. CyclicBarriers are
13	* useful in programs involving a fixed sized party of threads that
14	* must occasionally wait for each other. The barrier is called
15	* <em>cyclic</em> because it can be re-used after the waiting threads
16	* are released.
17	*
18	* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
19	* that is run once per barrier point, after the last thread in the party
20	* arrives, but before any threads are released.
21	* This <em>barrier action</em> is useful
22	* for updating shared-state before any of the parties continue.
23	*
24	* <p><b>Sample usage:</b> Here is an example of
25	* using a barrier in a parallel decomposition design:
26	* <pre>
27	* class Solver {
28	* final int N;
29	* final float[][] data;
30	* final CyclicBarrier barrier;
31	*
32	* class Worker implements Runnable {
33	* int myRow;
34	* Worker(int row) { myRow = row; }
35	* public void run() {
36	* while (!done()) {
37	* processRow(myRow);
38	*
39	* try {
40	* barrier.await();
41	* } catch (InterruptedException ex) {
42	* return;
43	* } catch (BrokenBarrierException ex) {
44	* return;
45	* }
46	* }
47	* }
48	* }
49	*
50	* public Solver(float[][] matrix) {
51	* data = matrix;
52	* N = matrix.length;
53	* barrier = new CyclicBarrier(N,
54	* new Runnable() {
55	* public void run() {
56	* mergeRows(...);
57	* }
58	* });
59	* for (int i = 0; i < N; ++i)
60	* new Thread(new Worker(i)).start();
61	*
62	* waitUntilDone();
63	* }
64	* }
65	* </pre>
66	* Here, each worker thread processes a row of the matrix then waits at the
67	* barrier until all rows have been processed. When all rows are processed
68	* the supplied {@link Runnable} barrier action is executed and merges the
69	* rows. If the merger
70	* determines that a solution has been found then <tt>done()</tt> will return
71	* <tt>true</tt> and each worker will terminate.
72	*
73	* <p>If the barrier action does not rely on the parties being suspended when
74	* it is executed, then any of the threads in the party could execute that
75	* action when it is released. To facilitate this, each invocation of
76	* {@link #await} returns the arrival index of that thread at the barrier.
77	* You can then choose which thread should execute the barrier action, for
78	* example:
79	* <pre> if (barrier.await() == 0) {
80	* // log the completion of this iteration
81	* }</pre>
82	*
83	* <p>The <tt>CyclicBarrier</tt> uses a fast-fail all-or-none breakage
84	* model for failed synchronization attempts: If a thread leaves a
85	* barrier point prematurely because of interruption, failure, or
86	* timeout, all other threads, even those that have not yet resumed
87	* from a previous {@link #await}. will also leave abnormally via
88	* {@link BrokenBarrierException} (or <tt>InterruptedException</tt> if
89	* they too were interrupted at about the same time).
90	*
91	* @since 1.5
92	* @see CountDownLatch
93	*
94	* @author Doug Lea
95	*/
96	public class CyclicBarrier {
97	/** The lock for guarding barrier entry */
98	private final ReentrantLock lock = new ReentrantLock();
99	/** Condition to wait on until tripped */
100	private final Condition trip = lock.newCondition();
101	/** The number of parties */
102	private final int parties;
103	/* The command to run when tripped */
104	private final Runnable barrierCommand;
105
106	/**
107	* The generation number. Incremented upon barrier trip.
108	* Retracted upon reset.
109	*/
110	private long generation;
111
112	/**
113	* Breakage indicator.
114	*/
115	private boolean broken;
116
117	/**
118	* Number of parties still waiting. Counts down from parties to 0
119	* on each cycle.
120	*/
121	private int count;
122
123	/**
124	* Update state on barrier trip and wake up everyone.
125	*/
126	private void nextGeneration() {
127	count = parties;
128	++generation;
129	trip.signalAll();
130	}
131
132	/**
133	* Set barrier as broken and wake up everyone
134	*/
135	private void breakBarrier() {
136	broken = true;
137	trip.signalAll();
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	final ReentrantLock lock = this.lock;
146	lock.lock();
147	try {
148	int index = --count;
149	long g = generation;
150
151	if (broken)
152	throw new BrokenBarrierException();
153
154	if (Thread.interrupted()) {
155	breakBarrier();
156	throw new InterruptedException();
157	}
158
159	if (index == 0) { // tripped
160	nextGeneration();
161	boolean ranAction = false;
162	try {
163	Runnable command = barrierCommand;
164	if (command != null)
165	command.run();
166	ranAction = true;
167	return 0;
168	} finally {
169	if (!ranAction)
170	breakBarrier();
171	}
172	}
173
174	for (;;) {
175	try {
176	if (!timed)
177	trip.await();
178	else if (nanos > 0L)
179	nanos = trip.awaitNanos(nanos);
180	} catch (InterruptedException ie) {
181	breakBarrier();
182	throw ie;
183	}
184
185	if (broken \|\|
186	g > generation) // true if a reset occurred while waiting
187	throw new BrokenBarrierException();
188
189	if (g < generation)
190	return index;
191
192	if (timed && nanos <= 0L) {
193	breakBarrier();
194	throw new TimeoutException();
195	}
196	}
197
198	} finally {
199	lock.unlock();
200	}
201	}
202
203	/**
204	* Create a new <tt>CyclicBarrier</tt> that will trip when the
205	* given number of parties (threads) are waiting upon it, and which
206	* will execute the given barrier action when the barrier is tripped,
207	* performed by the last thread entering the barrier.
208	*
209	* @param parties the number of threads that must invoke {@link #await}
210	* before the barrier is tripped.
211	* @param barrierAction the command to execute when the barrier is
212	* tripped, or <tt>null</tt> if there is no action.
213	*
214	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
215	*/
216	public CyclicBarrier(int parties, Runnable barrierAction) {
217	if (parties <= 0) throw new IllegalArgumentException();
218	this.parties = parties;
219	this.count = parties;
220	this.barrierCommand = barrierAction;
221	}
222
223	/**
224	* Create a new <tt>CyclicBarrier</tt> that will trip when the
225	* given number of parties (threads) are waiting upon it, and
226	* does not perform a predefined action upon each barrier.
227	*
228	* @param parties the number of threads that must invoke {@link #await}
229	* before the barrier is tripped.
230	*
231	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
232	*/
233	public CyclicBarrier(int parties) {
234	this(parties, null);
235	}
236
237	/**
238	* Return the number of parties required to trip this barrier.
239	* @return the number of parties required to trip this barrier.
240	**/
241	public int getParties() {
242	return parties;
243	}
244
245	/**
246	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
247	* on this barrier.
248	*
249	* <p>If the current thread is not the last to arrive then it is
250	* disabled for thread scheduling purposes and lies dormant until
251	* one of following things happens:
252	* <ul>
253	* <li>The last thread arrives; or
254	* <li>Some other thread {@link Thread#interrupt interrupts} the current
255	* thread; or
256	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
257	* other waiting threads; or
258	* <li>Some other thread times out while waiting for barrier; or
259	* <li>Some other thread invokes {@link #reset} on this barrier.
260	* </ul>
261	* <p>If the current thread:
262	* <ul>
263	* <li>has its interrupted status set on entry to this method; or
264	* <li>is {@link Thread#interrupt interrupted} while waiting
265	* </ul>
266	* then {@link InterruptedException} is thrown and the current thread's
267	* interrupted status is cleared.
268	*
269	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
270	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
271	* or while any thread is waiting,
272	* then {@link BrokenBarrierException} is thrown.
273	*
274	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
275	* then all other waiting threads will throw
276	* {@link BrokenBarrierException} and the barrier is placed in the broken
277	* state.
278	*
279	* <p>If the current thread is the last thread to arrive, and a
280	* non-null barrier action was supplied in the constructor, then the
281	* current thread runs the action before allowing the other threads to
282	* continue.
283	* If an exception occurs during the barrier action then that exception
284	* will be propagated in the current thread and the barrier is placed in
285	* the broken state.
286	*
287	* @return the arrival index of the current thread, where index
288	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
289	* zero indicates the last to arrive.
290	*
291	* @throws InterruptedException if the current thread was interrupted
292	* while waiting
293	* @throws BrokenBarrierException if <em>another</em> thread was
294	* interrupted while the current thread was waiting, or the barrier was
295	* reset, or the barrier was broken when <tt>await</tt> was called,
296	* or the barrier action (if present) failed due an exception.
297	*/
298	public int await() throws InterruptedException, BrokenBarrierException {
299	try {
300	return dowait(false, 0L);
301	} catch (TimeoutException toe) {
302	throw new Error(toe); // cannot happen;
303	}
304	}
305
306	/**
307	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
308	* on this barrier.
309	*
310	* <p>If the current thread is not the last to arrive then it is
311	* disabled for thread scheduling purposes and lies dormant until
312	* one of the following things happens:
313	* <ul>
314	* <li>The last thread arrives; or
315	* <li>The specified timeout elapses; or
316	* <li>Some other thread {@link Thread#interrupt interrupts} the current
317	* thread; or
318	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
319	* other waiting threads; or
320	* <li>Some other thread times out while waiting for barrier; or
321	* <li>Some other thread invokes {@link #reset} on this barrier.
322	* </ul>
323	* <p>If the current thread:
324	* <ul>
325	* <li>has its interrupted status set on entry to this method; or
326	* <li>is {@link Thread#interrupt interrupted} while waiting
327	* </ul>
328	* then {@link InterruptedException} is thrown and the current thread's
329	* interrupted status is cleared.
330	*
331	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
332	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
333	* or while any thread is waiting,
334	* then {@link BrokenBarrierException} is thrown.
335	*
336	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
337	* then all other waiting threads will throw
338	* {@link BrokenBarrierException} and the barrier is placed in the broken
339	* state.
340	*
341	* <p>If the current thread is the last thread to arrive, and a
342	* non-null barrier action was supplied in the constructor, then the
343	* current thread runs the action before allowing the other threads to
344	* continue.
345	* If an exception occurs during the barrier action then that exception
346	* will be propagated in the current thread and the barrier is placed in
347	* the broken state.
348	*
349	* @param timeout the time to wait for the barrier
350	* @param unit the time unit of the timeout parameter
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	* or the barrier action (if present) failed due an exception.
362	*/
363	public int await(long timeout, TimeUnit unit)
364	throws InterruptedException,
365	BrokenBarrierException,
366	TimeoutException {
367	return dowait(true, unit.toNanos(timeout));
368	}
369
370	/**
371	* Query if this barrier is in a broken state.
372	* @return <tt>true</tt> if one or more parties broke out of this
373	* barrier due to interruption or timeout since construction or
374	* the last reset, or a barrier action failed due to an exception;
375	* and <tt>false</tt> otherwise.
376	*/
377	public boolean isBroken() {
378	final ReentrantLock lock = this.lock;
379	lock.lock();
380	try {
381	return broken;
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 has occurred for other reasons can be complicated to
392	* carry out; threads need to re-synchronize in some other way,
393	* and choose one to perform the reset. It may be preferable to
394	* instead create a new barrier for subsequent use.
395	*/
396	public void reset() {
397	final ReentrantLock lock = this.lock;
398	lock.lock();
399	try {
400	/*
401	* Retract generation number enough to cover threads
402	* currently waiting on current and still resuming from
403	* previous generation, plus similarly accommodating spans
404	* after the reset.
405	*/
406	generation -= 4;
407	broken = false;
408	trip.signalAll();
409	} finally {
410	lock.unlock();
411	}
412	}
413
414	/**
415	* Return the number of parties currently waiting at the barrier.
416	* This method is primarily useful for debugging and assertions.
417	*
418	* @return the number of parties currently blocked in {@link #await}
419	**/
420	public int getNumberWaiting() {
421	final ReentrantLock lock = this.lock;
422	lock.lock();
423	try {
424	return parties - count;
425	} finally {
426	lock.unlock();
427	}
428	}
429
430	}