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/publicdomain/zero/1.0/
 */

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> {@code
 * 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> {@code
 * 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, failure, or timeout, all
 * other threads waiting at that barrier point will also leave
 * abnormally via {@link BrokenBarrierException} (or
 * {@link InterruptedException} if they too were interrupted at about
 * the same time).
 *
 * <p>Memory consistency effects: Actions in a thread prior to calling
 * {@code await()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions that are part of the barrier action, which in turn
 * <i>happen-before</i> actions following a successful return from the
 * corresponding {@code await()} in other threads.
 *
 * @since 1.5
 * @see CountDownLatch
 *
 * @author Doug Lea
 */
public class CyclicBarrier {
    /**
     * Each use of the barrier is represented as a generation instance.
     * The generation changes whenever the barrier is tripped, or
     * is reset. There can be many generations associated with threads
     * using the barrier - due to the non-deterministic way the lock
     * may be allocated to waiting threads - but only one of these
     * can be active at a time (the one to which <tt>count</tt> applies)
     * and all the rest are either broken or tripped.
     * There need not be an active generation if there has been a break
     * but no subsequent reset.
     */
    private static class Generation {
        boolean broken = false;
    }

    /** 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 current generation */
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

    /**
     * Updates state on barrier trip and wakes up everyone.
     * Called only while holding lock.
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }

    /**
     * Sets current barrier generation as broken and wakes up everyone.
     * Called only while holding lock.
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        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 {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

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

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

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    if (g == generation && ! g.broken) {
                        breakBarrier();
                        throw ie;
                    } else {
                        // We're about to finish waiting even if we had not
                        // been interrupted, so this interrupt is deemed to
                        // "belong" to subsequent execution.
                        Thread.currentThread().interrupt();
                    }
                }

                if (g.broken)
                    throw new BrokenBarrierException();

                if (g != generation)
                    return index;

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

    /**
     * Creates 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 {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} 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;
    }

    /**
     * Creates 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 when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

    /**
     * Returns the number of parties required to trip this barrier.
     *
     * @return the number of parties required to trip this barrier
     */
    public int getParties() {
        return parties;
    }

    /**
     * Waits until all {@linkplain #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>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain 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 {@linkplain 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 {@linkplain #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 {@linkplain 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 or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was
     *         broken when {@code await} 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;
        }
    }

    /**
     * Waits until all {@linkplain #getParties parties} have invoked
     * <tt>await</tt> on this barrier, or the specified waiting time elapses.
     *
     * <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 {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * <li>Some other thread {@linkplain 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 {@linkplain Thread#interrupt interrupted} while waiting
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown. If the time is less than or equal to zero, the
     * method will not wait at all.
     *
     * <p>If the barrier is {@link #reset} while any thread is waiting,
     * or if the barrier {@linkplain #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 {@linkplain 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 or timed out while the current thread was
     *         waiting, or the barrier was reset, or the barrier was broken
     *         when {@code await} 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));
    }

    /**
     * Queries if this barrier is in a broken state.
     *
     * @return {@code true} 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; {@code false} otherwise.
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Resets 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 {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns 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:	1.41
Committed:	Thu Jun 9 07:48:43 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.40:	+8 -6 lines
Log Message:	consistent style for code snippets
#	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/publicdomain/zero/1.0/
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	*
27	* <pre> {@code
28	* class Solver {
29	* final int N;
30	* final float[][] data;
31	* final CyclicBarrier barrier;
32	*
33	* class Worker implements Runnable {
34	* int myRow;
35	* Worker(int row) { myRow = row; }
36	* public void run() {
37	* while (!done()) {
38	* processRow(myRow);
39	*
40	* try {
41	* barrier.await();
42	* } catch (InterruptedException ex) {
43	* return;
44	* } catch (BrokenBarrierException ex) {
45	* return;
46	* }
47	* }
48	* }
49	* }
50	*
51	* public Solver(float[][] matrix) {
52	* data = matrix;
53	* N = matrix.length;
54	* barrier = new CyclicBarrier(N,
55	* new Runnable() {
56	* public void run() {
57	* mergeRows(...);
58	* }
59	* });
60	* for (int i = 0; i < N; ++i)
61	* new Thread(new Worker(i)).start();
62	*
63	* waitUntilDone();
64	* }
65	* }}</pre>
66	*
67	* Here, each worker thread processes a row of the matrix then waits at the
68	* barrier until all rows have been processed. When all rows are processed
69	* the supplied {@link Runnable} barrier action is executed and merges the
70	* rows. If the merger
71	* determines that a solution has been found then <tt>done()</tt> will return
72	* <tt>true</tt> and each worker will terminate.
73	*
74	* <p>If the barrier action does not rely on the parties being suspended when
75	* it is executed, then any of the threads in the party could execute that
76	* action when it is released. To facilitate this, each invocation of
77	* {@link #await} returns the arrival index of that thread at the barrier.
78	* You can then choose which thread should execute the barrier action, for
79	* example:
80	* <pre> {@code
81	* if (barrier.await() == 0) {
82	* // log the completion of this iteration
83	* }}</pre>
84	*
85	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
86	* for failed synchronization attempts: If a thread leaves a barrier
87	* point prematurely because of interruption, failure, or timeout, all
88	* other threads waiting at that barrier point will also leave
89	* abnormally via {@link BrokenBarrierException} (or
90	* {@link InterruptedException} if they too were interrupted at about
91	* the same time).
92	*
93	* <p>Memory consistency effects: Actions in a thread prior to calling
94	* {@code await()}
95	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
96	* actions that are part of the barrier action, which in turn
97	* <i>happen-before</i> actions following a successful return from the
98	* corresponding {@code await()} in other threads.
99	*
100	* @since 1.5
101	* @see CountDownLatch
102	*
103	* @author Doug Lea
104	*/
105	public class CyclicBarrier {
106	/**
107	* Each use of the barrier is represented as a generation instance.
108	* The generation changes whenever the barrier is tripped, or
109	* is reset. There can be many generations associated with threads
110	* using the barrier - due to the non-deterministic way the lock
111	* may be allocated to waiting threads - but only one of these
112	* can be active at a time (the one to which <tt>count</tt> applies)
113	* and all the rest are either broken or tripped.
114	* There need not be an active generation if there has been a break
115	* but no subsequent reset.
116	*/
117	private static class Generation {
118	boolean broken = false;
119	}
120
121	/** The lock for guarding barrier entry */
122	private final ReentrantLock lock = new ReentrantLock();
123	/** Condition to wait on until tripped */
124	private final Condition trip = lock.newCondition();
125	/** The number of parties */
126	private final int parties;
127	/* The command to run when tripped */
128	private final Runnable barrierCommand;
129	/** The current generation */
130	private Generation generation = new Generation();
131
132	/**
133	* Number of parties still waiting. Counts down from parties to 0
134	* on each generation. It is reset to parties on each new
135	* generation or when broken.
136	*/
137	private int count;
138
139	/**
140	* Updates state on barrier trip and wakes up everyone.
141	* Called only while holding lock.
142	*/
143	private void nextGeneration() {
144	// signal completion of last generation
145	trip.signalAll();
146	// set up next generation
147	count = parties;
148	generation = new Generation();
149	}
150
151	/**
152	* Sets current barrier generation as broken and wakes up everyone.
153	* Called only while holding lock.
154	*/
155	private void breakBarrier() {
156	generation.broken = true;
157	count = parties;
158	trip.signalAll();
159	}
160
161	/**
162	* Main barrier code, covering the various policies.
163	*/
164	private int dowait(boolean timed, long nanos)
165	throws InterruptedException, BrokenBarrierException,
166	TimeoutException {
167	final ReentrantLock lock = this.lock;
168	lock.lock();
169	try {
170	final Generation g = generation;
171
172	if (g.broken)
173	throw new BrokenBarrierException();
174
175	if (Thread.interrupted()) {
176	breakBarrier();
177	throw new InterruptedException();
178	}
179
180	int index = --count;
181	if (index == 0) { // tripped
182	boolean ranAction = false;
183	try {
184	final Runnable command = barrierCommand;
185	if (command != null)
186	command.run();
187	ranAction = true;
188	nextGeneration();
189	return 0;
190	} finally {
191	if (!ranAction)
192	breakBarrier();
193	}
194	}
195
196	// loop until tripped, broken, interrupted, or timed out
197	for (;;) {
198	try {
199	if (!timed)
200	trip.await();
201	else if (nanos > 0L)
202	nanos = trip.awaitNanos(nanos);
203	} catch (InterruptedException ie) {
204	if (g == generation && ! g.broken) {
205	breakBarrier();
206	throw ie;
207	} else {
208	// We're about to finish waiting even if we had not
209	// been interrupted, so this interrupt is deemed to
210	// "belong" to subsequent execution.
211	Thread.currentThread().interrupt();
212	}
213	}
214
215	if (g.broken)
216	throw new BrokenBarrierException();
217
218	if (g != generation)
219	return index;
220
221	if (timed && nanos <= 0L) {
222	breakBarrier();
223	throw new TimeoutException();
224	}
225	}
226	} finally {
227	lock.unlock();
228	}
229	}
230
231	/**
232	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
233	* given number of parties (threads) are waiting upon it, and which
234	* will execute the given barrier action when the barrier is tripped,
235	* performed by the last thread entering the barrier.
236	*
237	* @param parties the number of threads that must invoke {@link #await}
238	* before the barrier is tripped
239	* @param barrierAction the command to execute when the barrier is
240	* tripped, or {@code null} if there is no action
241	* @throws IllegalArgumentException if {@code parties} is less than 1
242	*/
243	public CyclicBarrier(int parties, Runnable barrierAction) {
244	if (parties <= 0) throw new IllegalArgumentException();
245	this.parties = parties;
246	this.count = parties;
247	this.barrierCommand = barrierAction;
248	}
249
250	/**
251	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
252	* given number of parties (threads) are waiting upon it, and
253	* does not perform a predefined action when the barrier is tripped.
254	*
255	* @param parties the number of threads that must invoke {@link #await}
256	* before the barrier is tripped
257	* @throws IllegalArgumentException if {@code parties} is less than 1
258	*/
259	public CyclicBarrier(int parties) {
260	this(parties, null);
261	}
262
263	/**
264	* Returns the number of parties required to trip this barrier.
265	*
266	* @return the number of parties required to trip this barrier
267	*/
268	public int getParties() {
269	return parties;
270	}
271
272	/**
273	* Waits until all {@linkplain #getParties parties} have invoked
274	* <tt>await</tt> on this barrier.
275	*
276	* <p>If the current thread is not the last to arrive then it is
277	* disabled for thread scheduling purposes and lies dormant until
278	* one of the following things happens:
279	* <ul>
280	* <li>The last thread arrives; or
281	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
282	* the current thread; or
283	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
284	* one of the other waiting threads; or
285	* <li>Some other thread times out while waiting for barrier; or
286	* <li>Some other thread invokes {@link #reset} on this barrier.
287	* </ul>
288	*
289	* <p>If the current thread:
290	* <ul>
291	* <li>has its interrupted status set on entry to this method; or
292	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
293	* </ul>
294	* then {@link InterruptedException} is thrown and the current thread's
295	* interrupted status is cleared.
296	*
297	* <p>If the barrier is {@link #reset} while any thread is waiting,
298	* or if the barrier {@linkplain #isBroken is broken} when
299	* <tt>await</tt> is invoked, or while any thread is waiting, then
300	* {@link BrokenBarrierException} is thrown.
301	*
302	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
303	* then all other waiting threads will throw
304	* {@link BrokenBarrierException} and the barrier is placed in the broken
305	* state.
306	*
307	* <p>If the current thread is the last thread to arrive, and a
308	* non-null barrier action was supplied in the constructor, then the
309	* current thread runs the action before allowing the other threads to
310	* continue.
311	* If an exception occurs during the barrier action then that exception
312	* will be propagated in the current thread and the barrier is placed in
313	* the broken state.
314	*
315	* @return the arrival index of the current thread, where index
316	* <tt>{@link #getParties()} - 1</tt> indicates the first
317	* to arrive and zero indicates the last to arrive
318	* @throws InterruptedException if the current thread was interrupted
319	* while waiting
320	* @throws BrokenBarrierException if <em>another</em> thread was
321	* interrupted or timed out while the current thread was
322	* waiting, or the barrier was reset, or the barrier was
323	* broken when {@code await} was called, or the barrier
324	* action (if present) failed due an exception.
325	*/
326	public int await() throws InterruptedException, BrokenBarrierException {
327	try {
328	return dowait(false, 0L);
329	} catch (TimeoutException toe) {
330	throw new Error(toe); // cannot happen;
331	}
332	}
333
334	/**
335	* Waits until all {@linkplain #getParties parties} have invoked
336	* <tt>await</tt> on this barrier, or the specified waiting time elapses.
337	*
338	* <p>If the current thread is not the last to arrive then it is
339	* disabled for thread scheduling purposes and lies dormant until
340	* one of the following things happens:
341	* <ul>
342	* <li>The last thread arrives; or
343	* <li>The specified timeout elapses; or
344	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
345	* the current thread; or
346	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
347	* one of the other waiting threads; or
348	* <li>Some other thread times out while waiting for barrier; or
349	* <li>Some other thread invokes {@link #reset} on this barrier.
350	* </ul>
351	*
352	* <p>If the current thread:
353	* <ul>
354	* <li>has its interrupted status set on entry to this method; or
355	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
356	* </ul>
357	* then {@link InterruptedException} is thrown and the current thread's
358	* interrupted status is cleared.
359	*
360	* <p>If the specified waiting time elapses then {@link TimeoutException}
361	* is thrown. If the time is less than or equal to zero, the
362	* method will not wait at all.
363	*
364	* <p>If the barrier is {@link #reset} while any thread is waiting,
365	* or if the barrier {@linkplain #isBroken is broken} when
366	* <tt>await</tt> is invoked, or while any thread is waiting, then
367	* {@link BrokenBarrierException} is thrown.
368	*
369	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
370	* waiting, then all other waiting threads will throw {@link
371	* BrokenBarrierException} and the barrier is placed in the broken
372	* state.
373	*
374	* <p>If the current thread is the last thread to arrive, and a
375	* non-null barrier action was supplied in the constructor, then the
376	* current thread runs the action before allowing the other threads to
377	* continue.
378	* If an exception occurs during the barrier action then that exception
379	* will be propagated in the current thread and the barrier is placed in
380	* the broken state.
381	*
382	* @param timeout the time to wait for the barrier
383	* @param unit the time unit of the timeout parameter
384	* @return the arrival index of the current thread, where index
385	* <tt>{@link #getParties()} - 1</tt> indicates the first
386	* to arrive and zero indicates the last to arrive
387	* @throws InterruptedException if the current thread was interrupted
388	* while waiting
389	* @throws TimeoutException if the specified timeout elapses
390	* @throws BrokenBarrierException if <em>another</em> thread was
391	* interrupted or timed out while the current thread was
392	* waiting, or the barrier was reset, or the barrier was broken
393	* when {@code await} was called, or the barrier action (if
394	* present) failed due an exception
395	*/
396	public int await(long timeout, TimeUnit unit)
397	throws InterruptedException,
398	BrokenBarrierException,
399	TimeoutException {
400	return dowait(true, unit.toNanos(timeout));
401	}
402
403	/**
404	* Queries if this barrier is in a broken state.
405	*
406	* @return {@code true} if one or more parties broke out of this
407	* barrier due to interruption or timeout since
408	* construction or the last reset, or a barrier action
409	* failed due to an exception; {@code false} otherwise.
410	*/
411	public boolean isBroken() {
412	final ReentrantLock lock = this.lock;
413	lock.lock();
414	try {
415	return generation.broken;
416	} finally {
417	lock.unlock();
418	}
419	}
420
421	/**
422	* Resets the barrier to its initial state. If any parties are
423	* currently waiting at the barrier, they will return with a
424	* {@link BrokenBarrierException}. Note that resets <em>after</em>
425	* a breakage has occurred for other reasons can be complicated to
426	* carry out; threads need to re-synchronize in some other way,
427	* and choose one to perform the reset. It may be preferable to
428	* instead create a new barrier for subsequent use.
429	*/
430	public void reset() {
431	final ReentrantLock lock = this.lock;
432	lock.lock();
433	try {
434	breakBarrier(); // break the current generation
435	nextGeneration(); // start a new generation
436	} finally {
437	lock.unlock();
438	}
439	}
440
441	/**
442	* Returns the number of parties currently waiting at the barrier.
443	* This method is primarily useful for debugging and assertions.
444	*
445	* @return the number of parties currently blocked in {@link #await}
446	*/
447	public int getNumberWaiting() {
448	final ReentrantLock lock = this.lock;
449	lock.lock();
450	try {
451	return parties - count;
452	} finally {
453	lock.unlock();
454	}
455	}
456	}