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 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 ReentrantLock.ConditionObject 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 {
        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 {
                    if (barrierCommand != null) 
                        barrierCommand.run();
                    ranAction = true;
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }

            for (;;) {
                try {
                    if (!timed) 
                        trip.await();
                    else if (nanos > 0)
                        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 <= 0) {
                    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, 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 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() {
        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() {
        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() {
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }

}
Revision:	1.19
Committed:	Sat Oct 18 12:29:33 2003 UTC (20 years, 7 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_NOV3_FREEZE, JSR166_DEC9_PRE_ES_SUBMIT, JSR166_DEC9_POST_ES_SUBMIT
Changes since 1.18:	+1 -1 lines
Log Message:	Added docs for type params
#	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 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 ReentrantLock.ConditionObject 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	lock.lock();
146	try {
147	int index = --count;
148	long g = generation;
149
150	if (broken)
151	throw new BrokenBarrierException();
152
153	if (Thread.interrupted()) {
154	breakBarrier();
155	throw new InterruptedException();
156	}
157
158	if (index == 0) { // tripped
159	nextGeneration();
160	boolean ranAction = false;
161	try {
162	if (barrierCommand != null)
163	barrierCommand.run();
164	ranAction = true;
165	return 0;
166	} finally {
167	if (!ranAction)
168	breakBarrier();
169	}
170	}
171
172	for (;;) {
173	try {
174	if (!timed)
175	trip.await();
176	else if (nanos > 0)
177	nanos = trip.awaitNanos(nanos);
178	} catch (InterruptedException ie) {
179	breakBarrier();
180	throw ie;
181	}
182
183	if (broken \|\|
184	g > generation) // true if a reset occurred while waiting
185	throw new BrokenBarrierException();
186
187	if (g < generation)
188	return index;
189
190	if (timed && nanos <= 0) {
191	breakBarrier();
192	throw new TimeoutException();
193	}
194	}
195
196	} finally {
197	lock.unlock();
198	}
199	}
200
201	/**
202	* Create a new <tt>CyclicBarrier</tt> that will trip when the
203	* given number of parties (threads) are waiting upon it, and which
204	* will execute the given barrier action when the barrier is tripped,
205	* performed by the last thread entering the barrier.
206	*
207	* @param parties the number of threads that must invoke {@link #await}
208	* before the barrier is tripped.
209	* @param barrierAction the command to execute when the barrier is
210	* tripped, or <tt>null</tt> if there is no action.
211	*
212	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
213	*/
214	public CyclicBarrier(int parties, Runnable barrierAction) {
215	if (parties <= 0) throw new IllegalArgumentException();
216	this.parties = parties;
217	this.count = parties;
218	this.barrierCommand = barrierAction;
219	}
220
221	/**
222	* Create a new <tt>CyclicBarrier</tt> that will trip when the
223	* given number of parties (threads) are waiting upon it, and
224	* does not perform a predefined action upon each barrier.
225	*
226	* @param parties the number of threads that must invoke {@link #await}
227	* before the barrier is tripped.
228	*
229	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
230	*/
231	public CyclicBarrier(int parties) {
232	this(parties, null);
233	}
234
235	/**
236	* Return the number of parties required to trip this barrier.
237	* @return the number of parties required to trip this barrier.
238	**/
239	public int getParties() {
240	return parties;
241	}
242
243	/**
244	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
245	* on this barrier.
246	*
247	* <p>If the current thread is not the last to arrive then it is
248	* disabled for thread scheduling purposes and lies dormant until
249	* one of following things happens:
250	* <ul>
251	* <li>The last thread arrives; or
252	* <li>Some other thread {@link Thread#interrupt interrupts} the current
253	* thread; or
254	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
255	* other waiting threads; or
256	* <li>Some other thread times out while waiting for barrier; or
257	* <li>Some other thread invokes {@link #reset} on this barrier.
258	* </ul>
259	* <p>If the current thread:
260	* <ul>
261	* <li>has its interrupted status set on entry to this method; or
262	* <li>is {@link Thread#interrupt interrupted} while waiting
263	* </ul>
264	* then {@link InterruptedException} is thrown and the current thread's
265	* interrupted status is cleared.
266	*
267	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
268	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
269	* or while any thread is waiting,
270	* then {@link BrokenBarrierException} is thrown.
271	*
272	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
273	* then all other waiting threads will throw
274	* {@link BrokenBarrierException} and the barrier is placed in the broken
275	* state.
276	*
277	* <p>If the current thread is the last thread to arrive, and a
278	* non-null barrier action was supplied in the constructor, then the
279	* current thread runs the action before allowing the other threads to
280	* continue.
281	* If an exception occurs during the barrier action then that exception
282	* will be propagated in the current thread and the barrier is placed in
283	* the broken state.
284	*
285	* @return the arrival index of the current thread, where index
286	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
287	* zero indicates the last to arrive.
288	*
289	* @throws InterruptedException if the current thread was interrupted
290	* while waiting
291	* @throws BrokenBarrierException if <em>another</em> thread was
292	* interrupted while the current thread was waiting, or the barrier was
293	* reset, or the barrier was broken when <tt>await</tt> was called,
294	* or the barrier action (if present) failed due an exception.
295	*/
296	public int await() throws InterruptedException, BrokenBarrierException {
297	try {
298	return dowait(false, 0);
299	} catch (TimeoutException toe) {
300	throw new Error(toe); // cannot happen;
301	}
302	}
303
304	/**
305	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
306	* on this barrier.
307	*
308	* <p>If the current thread is not the last to arrive then it is
309	* disabled for thread scheduling purposes and lies dormant until
310	* one of the following things happens:
311	* <ul>
312	* <li>The last thread arrives; or
313	* <li>The specified timeout elapses; or
314	* <li>Some other thread {@link Thread#interrupt interrupts} the current
315	* thread; or
316	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
317	* other waiting threads; or
318	* <li>Some other thread times out while waiting for barrier; or
319	* <li>Some other thread invokes {@link #reset} on this barrier.
320	* </ul>
321	* <p>If the current thread:
322	* <ul>
323	* <li>has its interrupted status set on entry to this method; or
324	* <li>is {@link Thread#interrupt interrupted} while waiting
325	* </ul>
326	* then {@link InterruptedException} is thrown and the current thread's
327	* interrupted status is cleared.
328	*
329	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
330	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
331	* or while any thread is waiting,
332	* then {@link BrokenBarrierException} is thrown.
333	*
334	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
335	* then all other waiting threads will throw
336	* {@link BrokenBarrierException} and the barrier is placed in the broken
337	* state.
338	*
339	* <p>If the current thread is the last thread to arrive, and a
340	* non-null barrier action was supplied in the constructor, then the
341	* current thread runs the action before allowing the other threads to
342	* continue.
343	* If an exception occurs during the barrier action then that exception
344	* will be propagated in the current thread and the barrier is placed in
345	* the broken state.
346	*
347	* @param timeout the time to wait for the barrier
348	* @param unit the time unit of the timeout parameter
349	* @return the arrival index of the current thread, where index
350	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
351	* zero indicates the last to arrive.
352	*
353	* @throws InterruptedException if the current thread was interrupted
354	* while waiting
355	* @throws TimeoutException if the specified timeout elapses.
356	* @throws BrokenBarrierException if <em>another</em> thread was
357	* interrupted while the current thread was waiting, or the barrier was
358	* reset, or the barrier was broken when <tt>await</tt> was called,
359	* or the barrier action (if present) failed due an exception.
360	*/
361	public int await(long timeout, TimeUnit unit)
362	throws InterruptedException,
363	BrokenBarrierException,
364	TimeoutException {
365	return dowait(true, unit.toNanos(timeout));
366	}
367
368	/**
369	* Query if this barrier is in a broken state.
370	* @return <tt>true</tt> if one or more parties broke out of this
371	* barrier due to interruption or timeout since construction or
372	* the last reset, or a barrier action failed due to an exception;
373	* and <tt>false</tt> otherwise.
374	*/
375	public boolean isBroken() {
376	lock.lock();
377	try {
378	return broken;
379	} finally {
380	lock.unlock();
381	}
382	}
383
384	/**
385	* Reset the barrier to its initial state. If any parties are
386	* currently waiting at the barrier, they will return with a
387	* {@link BrokenBarrierException}. Note that resets <em>after</em>
388	* a breakage has occurred for other reasons can be complicated to
389	* carry out; threads need to re-synchronize in some other way,
390	* and choose one to perform the reset. It may be preferable to
391	* instead create a new barrier for subsequent use.
392	*/
393	public void reset() {
394	lock.lock();
395	try {
396	/*
397	* Retract generation number enough to cover threads
398	* currently waiting on current and still resuming from
399	* previous generation, plus similarly accommodating spans
400	* after the reset.
401	*/
402	generation -= 4;
403	broken = false;
404	trip.signalAll();
405	} finally {
406	lock.unlock();
407	}
408	}
409
410	/**
411	* Return the number of parties currently waiting at the barrier.
412	* This method is primarily useful for debugging and assertions.
413	*
414	* @return the number of parties currently blocked in {@link #await}
415	**/
416	public int getNumberWaiting() {
417	lock.lock();
418	try {
419	return parties - count;
420	} finally {
421	lock.unlock();
422	}
423	}
424
425	}