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.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * 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 {@code CyclicBarrier} 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;
 *     Runnable barrierAction = () -> mergeRows(...);
 *     barrier = new CyclicBarrier(N, barrierAction);
 *
 *     List<Thread> threads = new ArrayList<>(N);
 *     for (int i = 0; i < N; i++) {
 *       Thread thread = new Thread(new Worker(i));
 *       threads.add(thread);
 *       thread.start();
 *     }
 *
 *     // wait until done
 *     for (Thread thread : threads)
 *       thread.join();
 *   }
 * }}</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 {@code done()} will return
 * {@code true} 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 {@code CyclicBarrier} 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.
 *
 * @see CountDownLatch
 *
 * @author Doug Lea
 * @since 1.5
 */
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 {@code count} 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 {
        Generation() {}                 // prevent access constructor creation
        boolean broken;                 // initially 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
                Runnable command = barrierCommand;
                if (command != null) {
                    try {
                        command.run();
                    } catch (Throwable ex) {
                        breakBarrier();
                        throw ex;
                    }
                }
                nextGeneration();
                return 0;
            }

            // 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 {@code CyclicBarrier} 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 {@code CyclicBarrier} 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
     * {@code await} 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
     * {@code await} 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
     *         {@code getParties() - 1} 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 to 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
     * {@code await} 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
     * {@code await} 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
     *         {@code getParties() - 1} 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.
     *         In this case the barrier will be broken.
     * @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 to 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:	1.59
Committed:	Fri Feb 1 18:26:06 2019 UTC (5 years, 4 months ago) by jsr166
Branch:	MAIN
Changes since 1.58:	+8 -9 lines
Log Message:	dowait: use catch + rethrow instead of a boolean + finally
#	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
9	import java.util.concurrent.locks.Condition;
10	import java.util.concurrent.locks.ReentrantLock;
11
12	/**
13	* A synchronization aid that allows a set of threads to all wait for
14	* each other to reach a common barrier point. CyclicBarriers are
15	* useful in programs involving a fixed sized party of threads that
16	* must occasionally wait for each other. The barrier is called
17	* <em>cyclic</em> because it can be re-used after the waiting threads
18	* are released.
19	*
20	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
21	* that is run once per barrier point, after the last thread in the party
22	* arrives, but before any threads are released.
23	* This <em>barrier action</em> is useful
24	* for updating shared-state before any of the parties continue.
25	*
26	* <p><b>Sample usage:</b> Here is an example of using a barrier in a
27	* parallel decomposition design:
28	*
29	* <pre> {@code
30	* class Solver {
31	* final int N;
32	* final float[][] data;
33	* final CyclicBarrier barrier;
34	*
35	* class Worker implements Runnable {
36	* int myRow;
37	* Worker(int row) { myRow = row; }
38	* public void run() {
39	* while (!done()) {
40	* processRow(myRow);
41	*
42	* try {
43	* barrier.await();
44	* } catch (InterruptedException ex) {
45	* return;
46	* } catch (BrokenBarrierException ex) {
47	* return;
48	* }
49	* }
50	* }
51	* }
52	*
53	* public Solver(float[][] matrix) {
54	* data = matrix;
55	* N = matrix.length;
56	* Runnable barrierAction = () -> mergeRows(...);
57	* barrier = new CyclicBarrier(N, barrierAction);
58	*
59	* List<Thread> threads = new ArrayList<>(N);
60	* for (int i = 0; i < N; i++) {
61	* Thread thread = new Thread(new Worker(i));
62	* threads.add(thread);
63	* thread.start();
64	* }
65	*
66	* // wait until done
67	* for (Thread thread : threads)
68	* thread.join();
69	* }
70	* }}</pre>
71	*
72	* Here, each worker thread processes a row of the matrix then waits at the
73	* barrier until all rows have been processed. When all rows are processed
74	* the supplied {@link Runnable} barrier action is executed and merges the
75	* rows. If the merger
76	* determines that a solution has been found then {@code done()} will return
77	* {@code true} and each worker will terminate.
78	*
79	* <p>If the barrier action does not rely on the parties being suspended when
80	* it is executed, then any of the threads in the party could execute that
81	* action when it is released. To facilitate this, each invocation of
82	* {@link #await} returns the arrival index of that thread at the barrier.
83	* You can then choose which thread should execute the barrier action, for
84	* example:
85	* <pre> {@code
86	* if (barrier.await() == 0) {
87	* // log the completion of this iteration
88	* }}</pre>
89	*
90	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
91	* for failed synchronization attempts: If a thread leaves a barrier
92	* point prematurely because of interruption, failure, or timeout, all
93	* other threads waiting at that barrier point will also leave
94	* abnormally via {@link BrokenBarrierException} (or
95	* {@link InterruptedException} if they too were interrupted at about
96	* the same time).
97	*
98	* <p>Memory consistency effects: Actions in a thread prior to calling
99	* {@code await()}
100	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
101	* actions that are part of the barrier action, which in turn
102	* <i>happen-before</i> actions following a successful return from the
103	* corresponding {@code await()} in other threads.
104	*
105	* @see CountDownLatch
106	*
107	* @author Doug Lea
108	* @since 1.5
109	*/
110	public class CyclicBarrier {
111	/**
112	* Each use of the barrier is represented as a generation instance.
113	* The generation changes whenever the barrier is tripped, or
114	* is reset. There can be many generations associated with threads
115	* using the barrier - due to the non-deterministic way the lock
116	* may be allocated to waiting threads - but only one of these
117	* can be active at a time (the one to which {@code count} applies)
118	* and all the rest are either broken or tripped.
119	* There need not be an active generation if there has been a break
120	* but no subsequent reset.
121	*/
122	private static class Generation {
123	Generation() {} // prevent access constructor creation
124	boolean broken; // initially false
125	}
126
127	/** The lock for guarding barrier entry */
128	private final ReentrantLock lock = new ReentrantLock();
129	/** Condition to wait on until tripped */
130	private final Condition trip = lock.newCondition();
131	/** The number of parties */
132	private final int parties;
133	/** The command to run when tripped */
134	private final Runnable barrierCommand;
135	/** The current generation */
136	private Generation generation = new Generation();
137
138	/**
139	* Number of parties still waiting. Counts down from parties to 0
140	* on each generation. It is reset to parties on each new
141	* generation or when broken.
142	*/
143	private int count;
144
145	/**
146	* Updates state on barrier trip and wakes up everyone.
147	* Called only while holding lock.
148	*/
149	private void nextGeneration() {
150	// signal completion of last generation
151	trip.signalAll();
152	// set up next generation
153	count = parties;
154	generation = new Generation();
155	}
156
157	/**
158	* Sets current barrier generation as broken and wakes up everyone.
159	* Called only while holding lock.
160	*/
161	private void breakBarrier() {
162	generation.broken = true;
163	count = parties;
164	trip.signalAll();
165	}
166
167	/**
168	* Main barrier code, covering the various policies.
169	*/
170	private int dowait(boolean timed, long nanos)
171	throws InterruptedException, BrokenBarrierException,
172	TimeoutException {
173	final ReentrantLock lock = this.lock;
174	lock.lock();
175	try {
176	final Generation g = generation;
177
178	if (g.broken)
179	throw new BrokenBarrierException();
180
181	if (Thread.interrupted()) {
182	breakBarrier();
183	throw new InterruptedException();
184	}
185
186	int index = --count;
187	if (index == 0) { // tripped
188	Runnable command = barrierCommand;
189	if (command != null) {
190	try {
191	command.run();
192	} catch (Throwable ex) {
193	breakBarrier();
194	throw ex;
195	}
196	}
197	nextGeneration();
198	return 0;
199	}
200
201	// loop until tripped, broken, interrupted, or timed out
202	for (;;) {
203	try {
204	if (!timed)
205	trip.await();
206	else if (nanos > 0L)
207	nanos = trip.awaitNanos(nanos);
208	} catch (InterruptedException ie) {
209	if (g == generation && ! g.broken) {
210	breakBarrier();
211	throw ie;
212	} else {
213	// We're about to finish waiting even if we had not
214	// been interrupted, so this interrupt is deemed to
215	// "belong" to subsequent execution.
216	Thread.currentThread().interrupt();
217	}
218	}
219
220	if (g.broken)
221	throw new BrokenBarrierException();
222
223	if (g != generation)
224	return index;
225
226	if (timed && nanos <= 0L) {
227	breakBarrier();
228	throw new TimeoutException();
229	}
230	}
231	} finally {
232	lock.unlock();
233	}
234	}
235
236	/**
237	* Creates a new {@code CyclicBarrier} that will trip when the
238	* given number of parties (threads) are waiting upon it, and which
239	* will execute the given barrier action when the barrier is tripped,
240	* performed by the last thread entering the barrier.
241	*
242	* @param parties the number of threads that must invoke {@link #await}
243	* before the barrier is tripped
244	* @param barrierAction the command to execute when the barrier is
245	* tripped, or {@code null} if there is no action
246	* @throws IllegalArgumentException if {@code parties} is less than 1
247	*/
248	public CyclicBarrier(int parties, Runnable barrierAction) {
249	if (parties <= 0) throw new IllegalArgumentException();
250	this.parties = parties;
251	this.count = parties;
252	this.barrierCommand = barrierAction;
253	}
254
255	/**
256	* Creates a new {@code CyclicBarrier} that will trip when the
257	* given number of parties (threads) are waiting upon it, and
258	* does not perform a predefined action when the barrier is tripped.
259	*
260	* @param parties the number of threads that must invoke {@link #await}
261	* before the barrier is tripped
262	* @throws IllegalArgumentException if {@code parties} is less than 1
263	*/
264	public CyclicBarrier(int parties) {
265	this(parties, null);
266	}
267
268	/**
269	* Returns the number of parties required to trip this barrier.
270	*
271	* @return the number of parties required to trip this barrier
272	*/
273	public int getParties() {
274	return parties;
275	}
276
277	/**
278	* Waits until all {@linkplain #getParties parties} have invoked
279	* {@code await} on this barrier.
280	*
281	* <p>If the current thread is not the last to arrive then it is
282	* disabled for thread scheduling purposes and lies dormant until
283	* one of the following things happens:
284	* <ul>
285	* <li>The last thread arrives; or
286	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
287	* the current thread; or
288	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
289	* one of the other waiting threads; or
290	* <li>Some other thread times out while waiting for barrier; or
291	* <li>Some other thread invokes {@link #reset} on this barrier.
292	* </ul>
293	*
294	* <p>If the current thread:
295	* <ul>
296	* <li>has its interrupted status set on entry to this method; or
297	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
298	* </ul>
299	* then {@link InterruptedException} is thrown and the current thread's
300	* interrupted status is cleared.
301	*
302	* <p>If the barrier is {@link #reset} while any thread is waiting,
303	* or if the barrier {@linkplain #isBroken is broken} when
304	* {@code await} is invoked, or while any thread is waiting, then
305	* {@link BrokenBarrierException} is thrown.
306	*
307	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
308	* then all other waiting threads will throw
309	* {@link BrokenBarrierException} and the barrier is placed in the broken
310	* state.
311	*
312	* <p>If the current thread is the last thread to arrive, and a
313	* non-null barrier action was supplied in the constructor, then the
314	* current thread runs the action before allowing the other threads to
315	* continue.
316	* If an exception occurs during the barrier action then that exception
317	* will be propagated in the current thread and the barrier is placed in
318	* the broken state.
319	*
320	* @return the arrival index of the current thread, where index
321	* {@code getParties() - 1} indicates the first
322	* to arrive and zero indicates the last to arrive
323	* @throws InterruptedException if the current thread was interrupted
324	* while waiting
325	* @throws BrokenBarrierException if <em>another</em> thread was
326	* interrupted or timed out while the current thread was
327	* waiting, or the barrier was reset, or the barrier was
328	* broken when {@code await} was called, or the barrier
329	* action (if present) failed due to an exception
330	*/
331	public int await() throws InterruptedException, BrokenBarrierException {
332	try {
333	return dowait(false, 0L);
334	} catch (TimeoutException toe) {
335	throw new Error(toe); // cannot happen
336	}
337	}
338
339	/**
340	* Waits until all {@linkplain #getParties parties} have invoked
341	* {@code await} on this barrier, or the specified waiting time elapses.
342	*
343	* <p>If the current thread is not the last to arrive then it is
344	* disabled for thread scheduling purposes and lies dormant until
345	* one of the following things happens:
346	* <ul>
347	* <li>The last thread arrives; or
348	* <li>The specified timeout elapses; or
349	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
350	* the current thread; or
351	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
352	* one of the other waiting threads; or
353	* <li>Some other thread times out while waiting for barrier; or
354	* <li>Some other thread invokes {@link #reset} on this barrier.
355	* </ul>
356	*
357	* <p>If the current thread:
358	* <ul>
359	* <li>has its interrupted status set on entry to this method; or
360	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
361	* </ul>
362	* then {@link InterruptedException} is thrown and the current thread's
363	* interrupted status is cleared.
364	*
365	* <p>If the specified waiting time elapses then {@link TimeoutException}
366	* is thrown. If the time is less than or equal to zero, the
367	* method will not wait at all.
368	*
369	* <p>If the barrier is {@link #reset} while any thread is waiting,
370	* or if the barrier {@linkplain #isBroken is broken} when
371	* {@code await} is invoked, or while any thread is waiting, then
372	* {@link BrokenBarrierException} is thrown.
373	*
374	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
375	* waiting, then all other waiting threads will throw {@link
376	* BrokenBarrierException} and the barrier is placed in the broken
377	* state.
378	*
379	* <p>If the current thread is the last thread to arrive, and a
380	* non-null barrier action was supplied in the constructor, then the
381	* current thread runs the action before allowing the other threads to
382	* continue.
383	* If an exception occurs during the barrier action then that exception
384	* will be propagated in the current thread and the barrier is placed in
385	* the broken state.
386	*
387	* @param timeout the time to wait for the barrier
388	* @param unit the time unit of the timeout parameter
389	* @return the arrival index of the current thread, where index
390	* {@code getParties() - 1} indicates the first
391	* to arrive and zero indicates the last to arrive
392	* @throws InterruptedException if the current thread was interrupted
393	* while waiting
394	* @throws TimeoutException if the specified timeout elapses.
395	* In this case the barrier will be broken.
396	* @throws BrokenBarrierException if <em>another</em> thread was
397	* interrupted or timed out while the current thread was
398	* waiting, or the barrier was reset, or the barrier was broken
399	* when {@code await} was called, or the barrier action (if
400	* present) failed due to an exception
401	*/
402	public int await(long timeout, TimeUnit unit)
403	throws InterruptedException,
404	BrokenBarrierException,
405	TimeoutException {
406	return dowait(true, unit.toNanos(timeout));
407	}
408
409	/**
410	* Queries if this barrier is in a broken state.
411	*
412	* @return {@code true} if one or more parties broke out of this
413	* barrier due to interruption or timeout since
414	* construction or the last reset, or a barrier action
415	* failed due to an exception; {@code false} otherwise.
416	*/
417	public boolean isBroken() {
418	final ReentrantLock lock = this.lock;
419	lock.lock();
420	try {
421	return generation.broken;
422	} finally {
423	lock.unlock();
424	}
425	}
426
427	/**
428	* Resets the barrier to its initial state. If any parties are
429	* currently waiting at the barrier, they will return with a
430	* {@link BrokenBarrierException}. Note that resets <em>after</em>
431	* a breakage has occurred for other reasons can be complicated to
432	* carry out; threads need to re-synchronize in some other way,
433	* and choose one to perform the reset. It may be preferable to
434	* instead create a new barrier for subsequent use.
435	*/
436	public void reset() {
437	final ReentrantLock lock = this.lock;
438	lock.lock();
439	try {
440	breakBarrier(); // break the current generation
441	nextGeneration(); // start a new generation
442	} finally {
443	lock.unlock();
444	}
445	}
446
447	/**
448	* Returns the number of parties currently waiting at the barrier.
449	* This method is primarily useful for debugging and assertions.
450	*
451	* @return the number of parties currently blocked in {@link #await}
452	*/
453	public int getNumberWaiting() {
454	final ReentrantLock lock = this.lock;
455	lock.lock();
456	try {
457	return parties - count;
458	} finally {
459	lock.unlock();
460	}
461	}
462	}