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
 * @spec JSR-166
 * @revised $Date: 2003/08/26 13:07:36 $
 * @editor $Author: dl $
 * @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 {
        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.
     *
     * @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,
     * or whilst 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 whilst 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:	1.13
Committed:	Wed Aug 27 01:45:28 2003 UTC (20 years, 9 months ago) by dholmes
Branch:	MAIN
Changes since 1.12:	+14 -9 lines
Log Message:	As per Eamonn's comments and further to Doug's code & doc changes, clarifications were made as to when BBE is thrown, when a barrier isBroken, and that a barrier action exception puts the barrier into the broken state.
#	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	* @spec JSR-166
93	* @revised $Date: 2003/08/26 13:07:36 $
94	* @editor $Author: dl $
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 final Runnable barrierCommand;
108
109	/**
110	* The generation number. Incremented upon barrier trip.
111	* Retracted upon reset.
112	*/
113	private long generation;
114
115	/**
116	* Breakage indicator.
117	*/
118	private boolean broken;
119
120	/**
121	* Number of parties still waiting. Counts down from parties to 0
122	* on each cycle.
123	*/
124	private int count;
125
126	/**
127	* Update state on barrier trip and wake up everyone.
128	*/
129	private void nextGeneration() {
130	count = parties;
131	++generation;
132	trip.signalAll();
133	}
134
135	/**
136	* Set barrier as broken and wake up everyone
137	*/
138	private void breakBarrier() {
139	broken = true;
140	trip.signalAll();
141	}
142
143	/**
144	* Main barrier code, covering the various policies.
145	*/
146	private int dowait(boolean timed, long nanos)
147	throws InterruptedException, BrokenBarrierException, TimeoutException {
148	lock.lock();
149	try {
150	int index = --count;
151	long g = generation;
152
153	if (broken)
154	throw new BrokenBarrierException();
155
156	if (Thread.interrupted()) {
157	breakBarrier();
158	throw new InterruptedException();
159	}
160
161	if (index == 0) { // tripped
162	nextGeneration();
163	boolean ranAction = false;
164	try {
165	if (barrierCommand != null)
166	barrierCommand.run();
167	ranAction = true;
168	return 0;
169	} finally {
170	if (!ranAction)
171	breakBarrier();
172	}
173	}
174
175	for (;;) {
176	try {
177	if (!timed)
178	trip.await();
179	else if (nanos > 0)
180	nanos = trip.awaitNanos(nanos);
181	} catch (InterruptedException ie) {
182	breakBarrier();
183	throw ie;
184	}
185
186	if (broken \|\|
187	g > generation) // true if a reset occurred while waiting
188	throw new BrokenBarrierException();
189
190	if (g < generation)
191	return index;
192
193	if (timed && nanos <= 0) {
194	breakBarrier();
195	throw new TimeoutException();
196	}
197	}
198
199	} finally {
200	lock.unlock();
201	}
202	}
203
204	/**
205	* Create a new <tt>CyclicBarrier</tt> that will trip when the
206	* given number of parties (threads) are waiting upon it, and which
207	* will execute the given barrier action when the barrier is tripped.
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.
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.
226	*
227	* <p>This is equivalent to <tt>CyclicBarrier(parties, null)</tt>.
228	*
229	* @param parties the number of threads that must invoke {@link #await}
230	* before the barrier is tripped.
231	*
232	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
233	*/
234	public CyclicBarrier(int parties) {
235	this(parties, null);
236	}
237
238	/**
239	* Return the number of parties required to trip this barrier.
240	* @return the number of parties required to trip this barrier.
241	**/
242	public int getParties() {
243	return parties;
244	}
245
246	/**
247	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
248	* on this barrier.
249	*
250	* <p>If the current thread is not the last to arrive then it is
251	* disabled for thread scheduling purposes and lies dormant until
252	* one of following things happens:
253	* <ul>
254	* <li>The last thread arrives; or
255	* <li>Some other thread {@link Thread#interrupt interrupts} the current
256	* thread; or
257	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
258	* other waiting threads; or
259	* <li>Some other thread times out while waiting for barrier; or
260	* <li>Some other thread invokes {@link #reset} on this barrier.
261	* </ul>
262	* <p>If the current thread:
263	* <ul>
264	* <li>has its interrupted status set on entry to this method; or
265	* <li>is {@link Thread#interrupt interrupted} while waiting
266	* </ul>
267	* then {@link InterruptedException} is thrown and the current thread's
268	* interrupted status is cleared.
269	*
270	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
271	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
272	* or whilst any thread is waiting,
273	* then {@link BrokenBarrierException} is thrown.
274	*
275	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
276	* then all other waiting threads will throw
277	* {@link BrokenBarrierException} and the barrier is placed in the broken
278	* state.
279	*
280	* <p>If the current thread is the last thread to arrive, and a
281	* non-null barrier action was supplied in the constructor, then the
282	* current thread runs the action before allowing the other threads to
283	* continue.
284	* If an exception occurs during the barrier action then that exception
285	* will be propagated in the current thread and the barrier is placed in
286	* the broken state.
287	*
288	* @return the arrival index of the current thread, where index
289	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
290	* zero indicates the last to arrive.
291	*
292	* @throws InterruptedException if the current thread was interrupted
293	* while waiting
294	* @throws BrokenBarrierException if <em>another</em> thread was
295	* interrupted while the current thread was waiting, or the barrier was
296	* reset, or the barrier was broken when <tt>await</tt> was called,
297	* or the barrier action (if present) failed due an exception.
298	*/
299	public int await() throws InterruptedException, BrokenBarrierException {
300	try {
301	return dowait(false, 0);
302	} catch (TimeoutException toe) {
303	throw new Error(toe); // cannot happen;
304	}
305	}
306
307	/**
308	* Wait until all {@link #getParties parties} have invoked <tt>await</tt>
309	* on this barrier.
310	*
311	* <p>If the current thread is not the last to arrive then it is
312	* disabled for thread scheduling purposes and lies dormant until
313	* one of the following things happens:
314	* <ul>
315	* <li>The last thread arrives; or
316	* <li>The specified timeout elapses; or
317	* <li>Some other thread {@link Thread#interrupt interrupts} the current
318	* thread; or
319	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
320	* other waiting threads; or
321	* <li>Some other thread times out while waiting for barrier; or
322	* <li>Some other thread invokes {@link #reset} on this barrier.
323	* </ul>
324	* <p>If the current thread:
325	* <ul>
326	* <li>has its interrupted status set on entry to this method; or
327	* <li>is {@link Thread#interrupt interrupted} while waiting
328	* </ul>
329	* then {@link InterruptedException} is thrown and the current thread's
330	* interrupted status is cleared.
331	*
332	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
333	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
334	* or whilst any thread is waiting,
335	* then {@link BrokenBarrierException} is thrown.
336	*
337	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
338	* then all other waiting threads will throw
339	* {@link BrokenBarrierException} and the barrier is placed in the broken
340	* state.
341	*
342	* <p>If the current thread is the last thread to arrive, and a
343	* non-null barrier action was supplied in the constructor, then the
344	* current thread runs the action before allowing the other threads to
345	* continue.
346	* If an exception occurs during the barrier action then that exception
347	* will be propagated in the current thread and the barrier is placed in
348	* the broken state.
349	*
350	* @param timeout the time to wait for the barrier
351	* @param unit the time unit of the timeout parameter
352	* @return the arrival index of the current thread, where index
353	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
354	* zero indicates the last to arrive.
355	*
356	* @throws InterruptedException if the current thread was interrupted
357	* while waiting
358	* @throws TimeoutException if the specified timeout elapses.
359	* @throws BrokenBarrierException if <em>another</em> thread was
360	* interrupted while the current thread was waiting, or the barrier was
361	* reset, or the barrier was broken when <tt>await</tt> was called,
362	* or the barrier action (if present) failed due an exception.
363	*/
364	public int await(long timeout, TimeUnit unit)
365	throws InterruptedException,
366	BrokenBarrierException,
367	TimeoutException {
368	return dowait(true, unit.toNanos(timeout));
369	}
370
371	/**
372	* Query if this barrier is in a broken state.
373	* @return <tt>true</tt> if one or more parties broke out of this
374	* barrier due to interruption or timeout since construction or
375	* the last reset, or a barrier action failed due to an exception;
376	* and <tt>false</tt> otherwise.
377	*/
378	public boolean isBroken() {
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	lock.lock();
398	try {
399	/*
400	* Retract generation number enough to cover threads
401	* currently waiting on current and still resuming from
402	* previous generation, plus similarly accommodating spans
403	* after the reset.
404	*/
405	generation -= 4;
406	broken = false;
407	trip.signalAll();
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	} finally {
424	lock.unlock();
425	}
426	}
427
428	}