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)
 *       try {
 *         thread.join();
 *       } catch (InterruptedException ex) { }
 *   }
 * }}</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
 * @see Phaser
 *
 * @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:	1.62
Committed:	Fri Nov 27 17:41:59 2020 UTC (3 years, 6 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.61:	+3 -1 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.22	* Expert Group and released to the public domain, as explained at
4	jsr166	1.39	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	jsr166	1.53
9	jsr166	1.43	import java.util.concurrent.locks.Condition;
10			import java.util.concurrent.locks.ReentrantLock;
11	tim	1.1
12			/**
13	tim	1.10	* A synchronization aid that allows a set of threads to all wait for
14	brian	1.4	* each other to reach a common barrier point. CyclicBarriers are
15	dl	1.3	* useful in programs involving a fixed sized party of threads that
16	brian	1.4	* must occasionally wait for each other. The barrier is called
17	dl	1.3	* <em>cyclic</em> because it can be re-used after the waiting threads
18			* are released.
19	tim	1.1	*
20	jsr166	1.45	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
21	tim	1.1	* that is run once per barrier point, after the last thread in the party
22	dl	1.28	* arrives, but before any threads are released.
23	tim	1.1	* This <em>barrier action</em> is useful
24			* for updating shared-state before any of the parties continue.
25	dl	1.28	*
26	jsr166	1.48	* <p><b>Sample usage:</b> Here is an example of using a barrier in a
27			* parallel decomposition design:
28	jsr166	1.41	*
29	jsr166	1.54	* <pre> {@code
30	tim	1.1	* class Solver {
31			* final int N;
32			* final float[][] data;
33			* final CyclicBarrier barrier;
34	dl	1.28	*
35	tim	1.1	* 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	dl	1.28	* barrier.await();
44			* } catch (InterruptedException ex) {
45			* return;
46			* } catch (BrokenBarrierException ex) {
47			* return;
48	tim	1.1	* }
49			* }
50			* }
51			* }
52			*
53			* public Solver(float[][] matrix) {
54			* data = matrix;
55			* N = matrix.length;
56	jsr166	1.57	* Runnable barrierAction = () -> mergeRows(...);
57	jsr166	1.44	* barrier = new CyclicBarrier(N, barrierAction);
58	tim	1.1	*
59	jsr166	1.52	* List<Thread> threads = new ArrayList<>(N);
60	jsr166	1.44	* 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	dl	1.62	* try {
69			* thread.join();
70			* } catch (InterruptedException ex) { }
71	tim	1.1	* }
72	jsr166	1.41	* }}</pre>
73			*
74	jsr166	1.60	* Here, each worker thread processes a row of the matrix, then waits at the
75			* barrier until all rows have been processed. When all rows are processed the
76			* supplied {@link Runnable} barrier action is executed and merges the rows.
77			* If the merger determines that a solution has been found then {@code done()}
78			* will return {@code true} and each worker will terminate.
79	tim	1.1	*
80			* <p>If the barrier action does not rely on the parties being suspended when
81			* it is executed, then any of the threads in the party could execute that
82			* action when it is released. To facilitate this, each invocation of
83			* {@link #await} returns the arrival index of that thread at the barrier.
84	dl	1.28	* You can then choose which thread should execute the barrier action, for
85	tim	1.1	* example:
86	jsr166	1.54	* <pre> {@code
87	jsr166	1.41	* if (barrier.await() == 0) {
88			* // log the completion of this iteration
89			* }}</pre>
90	tim	1.1	*
91	jsr166	1.45	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
92	dl	1.28	* for failed synchronization attempts: If a thread leaves a barrier
93			* point prematurely because of interruption, failure, or timeout, all
94			* other threads waiting at that barrier point will also leave
95			* abnormally via {@link BrokenBarrierException} (or
96	dl	1.31	* {@link InterruptedException} if they too were interrupted at about
97	dl	1.28	* the same time).
98	tim	1.1	*
99	jsr166	1.35	* <p>Memory consistency effects: Actions in a thread prior to calling
100			* {@code await()}
101			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
102	brian	1.33	* actions that are part of the barrier action, which in turn
103	jsr166	1.35	* <i>happen-before</i> actions following a successful return from the
104			* corresponding {@code await()} in other threads.
105	brian	1.33	*
106	brian	1.4	* @see CountDownLatch
107	jsr166	1.61	* @see Phaser
108	tim	1.1	*
109	dl	1.5	* @author Doug Lea
110	jsr166	1.56	* @since 1.5
111	tim	1.1	*/
112			public class CyclicBarrier {
113	dl	1.28	/**
114			* Each use of the barrier is represented as a generation instance.
115			* The generation changes whenever the barrier is tripped, or
116			* is reset. There can be many generations associated with threads
117			* using the barrier - due to the non-deterministic way the lock
118			* may be allocated to waiting threads - but only one of these
119	jsr166	1.45	* can be active at a time (the one to which {@code count} applies)
120	dl	1.28	* and all the rest are either broken or tripped.
121			* There need not be an active generation if there has been a break
122			* but no subsequent reset.
123			*/
124			private static class Generation {
125	jsr166	1.58	Generation() {} // prevent access constructor creation
126			boolean broken; // initially false
127	dl	1.28	}
128
129	dl	1.5	/** The lock for guarding barrier entry */
130	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
131	dl	1.5	/** Condition to wait on until tripped */
132	dl	1.21	private final Condition trip = lock.newCondition();
133	dl	1.5	/** The number of parties */
134	dl	1.2	private final int parties;
135	jsr166	1.51	/** The command to run when tripped */
136	dl	1.12	private final Runnable barrierCommand;
137	dl	1.28	/** The current generation */
138			private Generation generation = new Generation();
139	dl	1.2
140			/**
141			* Number of parties still waiting. Counts down from parties to 0
142	dl	1.31	* on each generation. It is reset to parties on each new
143			* generation or when broken.
144	dl	1.2	*/
145	dl	1.28	private int count;
146	dl	1.2
147			/**
148	jsr166	1.29	* Updates state on barrier trip and wakes up everyone.
149	dl	1.28	* Called only while holding lock.
150			*/
151	dl	1.2	private void nextGeneration() {
152	dl	1.28	// signal completion of last generation
153			trip.signalAll();
154			// set up next generation
155	dl	1.2	count = parties;
156	dl	1.28	generation = new Generation();
157	dl	1.12	}
158
159			/**
160	jsr166	1.29	* Sets current barrier generation as broken and wakes up everyone.
161	dl	1.28	* Called only while holding lock.
162	dl	1.12	*/
163			private void breakBarrier() {
164	dl	1.28	generation.broken = true;
165	jsr166	1.38	count = parties;
166	dl	1.12	trip.signalAll();
167	dl	1.2	}
168
169	dl	1.5	/**
170	dl	1.7	* Main barrier code, covering the various policies.
171	dl	1.5	*/
172	dl	1.28	private int dowait(boolean timed, long nanos)
173			throws InterruptedException, BrokenBarrierException,
174			TimeoutException {
175	dl	1.20	final ReentrantLock lock = this.lock;
176	dl	1.2	lock.lock();
177			try {
178	dl	1.30	final Generation g = generation;
179	dl	1.2
180	dl	1.28	if (g.broken)
181	dl	1.2	throw new BrokenBarrierException();
182
183			if (Thread.interrupted()) {
184	dl	1.12	breakBarrier();
185	dl	1.2	throw new InterruptedException();
186			}
187
188	jsr166	1.40	int index = --count;
189			if (index == 0) { // tripped
190	jsr166	1.59	Runnable command = barrierCommand;
191			if (command != null) {
192			try {
193	jsr166	1.40	command.run();
194	jsr166	1.59	} catch (Throwable ex) {
195	jsr166	1.40	breakBarrier();
196	jsr166	1.59	throw ex;
197			}
198	jsr166	1.40	}
199	jsr166	1.59	nextGeneration();
200			return 0;
201	jsr166	1.40	}
202	dl	1.2
203	dl	1.28	// loop until tripped, broken, interrupted, or timed out
204	dl	1.12	for (;;) {
205	dl	1.2	try {
206	dl	1.28	if (!timed)
207	dl	1.2	trip.await();
208	dl	1.23	else if (nanos > 0L)
209	dl	1.2	nanos = trip.awaitNanos(nanos);
210	dl	1.12	} catch (InterruptedException ie) {
211	jsr166	1.36	if (g == generation && ! g.broken) {
212			breakBarrier();
213	jsr166	1.38	throw ie;
214			} else {
215			// We're about to finish waiting even if we had not
216			// been interrupted, so this interrupt is deemed to
217			// "belong" to subsequent execution.
218			Thread.currentThread().interrupt();
219			}
220	dl	1.2	}
221	dl	1.28
222	dl	1.30	if (g.broken)
223	dl	1.12	throw new BrokenBarrierException();
224
225	jsr166	1.36	if (g != generation)
226	dl	1.12	return index;
227
228	dl	1.23	if (timed && nanos <= 0L) {
229	dl	1.12	breakBarrier();
230	dl	1.2	throw new TimeoutException();
231			}
232			}
233	tim	1.9	} finally {
234	dl	1.2	lock.unlock();
235			}
236			}
237	tim	1.1
238			/**
239	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
240	tim	1.1	* given number of parties (threads) are waiting upon it, and which
241	dl	1.17	* will execute the given barrier action when the barrier is tripped,
242	dl	1.19	* performed by the last thread entering the barrier.
243	tim	1.1	*
244			* @param parties the number of threads that must invoke {@link #await}
245	jsr166	1.37	* before the barrier is tripped
246	tim	1.1	* @param barrierAction the command to execute when the barrier is
247	jsr166	1.37	* tripped, or {@code null} if there is no action
248			* @throws IllegalArgumentException if {@code parties} is less than 1
249	tim	1.1	*/
250			public CyclicBarrier(int parties, Runnable barrierAction) {
251	dl	1.2	if (parties <= 0) throw new IllegalArgumentException();
252	dl	1.28	this.parties = parties;
253	dl	1.2	this.count = parties;
254			this.barrierCommand = barrierAction;
255	tim	1.1	}
256
257			/**
258	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
259	dl	1.14	* given number of parties (threads) are waiting upon it, and
260	dl	1.31	* does not perform a predefined action when the barrier is tripped.
261	tim	1.1	*
262			* @param parties the number of threads that must invoke {@link #await}
263	jsr166	1.37	* before the barrier is tripped
264			* @throws IllegalArgumentException if {@code parties} is less than 1
265	tim	1.1	*/
266			public CyclicBarrier(int parties) {
267	dl	1.2	this(parties, null);
268	tim	1.1	}
269
270			/**
271	dl	1.25	* Returns the number of parties required to trip this barrier.
272	jsr166	1.37	*
273			* @return the number of parties required to trip this barrier
274	dl	1.31	*/
275	tim	1.1	public int getParties() {
276	dl	1.2	return parties;
277	tim	1.1	}
278
279			/**
280	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
281	jsr166	1.45	* {@code await} on this barrier.
282	tim	1.1	*
283			* <p>If the current thread is not the last to arrive then it is
284			* disabled for thread scheduling purposes and lies dormant until
285	jsr166	1.36	* one of the following things happens:
286	tim	1.1	* <ul>
287			* <li>The last thread arrives; or
288	jsr166	1.37	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
289			* the current thread; or
290			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
291			* one of the other waiting threads; or
292	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
293	tim	1.1	* <li>Some other thread invokes {@link #reset} on this barrier.
294			* </ul>
295	jsr166	1.37	*
296	tim	1.1	* <p>If the current thread:
297			* <ul>
298			* <li>has its interrupted status set on entry to this method; or
299	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
300	tim	1.1	* </ul>
301			* then {@link InterruptedException} is thrown and the current thread's
302			* interrupted status is cleared.
303			*
304	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
305			* or if the barrier {@linkplain #isBroken is broken} when
306	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
307	jsr166	1.37	* {@link BrokenBarrierException} is thrown.
308	tim	1.1	*
309	jsr166	1.37	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
310	dl	1.28	* then all other waiting threads will throw
311	tim	1.1	* {@link BrokenBarrierException} and the barrier is placed in the broken
312			* state.
313			*
314			* <p>If the current thread is the last thread to arrive, and a
315			* non-null barrier action was supplied in the constructor, then the
316	dl	1.28	* current thread runs the action before allowing the other threads to
317	tim	1.1	* continue.
318			* If an exception occurs during the barrier action then that exception
319	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
320			* the broken state.
321	tim	1.1	*
322			* @return the arrival index of the current thread, where index
323	jsr166	1.49	* {@code getParties() - 1} indicates the first
324	jsr166	1.37	* to arrive and zero indicates the last to arrive
325	dl	1.28	* @throws InterruptedException if the current thread was interrupted
326	jsr166	1.37	* while waiting
327	tim	1.1	* @throws BrokenBarrierException if <em>another</em> thread was
328	jsr166	1.37	* interrupted or timed out while the current thread was
329			* waiting, or the barrier was reset, or the barrier was
330			* broken when {@code await} was called, or the barrier
331	jsr166	1.47	* action (if present) failed due to an exception
332	tim	1.1	*/
333			public int await() throws InterruptedException, BrokenBarrierException {
334	dl	1.2	try {
335	dl	1.23	return dowait(false, 0L);
336	tim	1.9	} catch (TimeoutException toe) {
337	jsr166	1.42	throw new Error(toe); // cannot happen
338	dl	1.2	}
339			}
340
341			/**
342	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
343	jsr166	1.45	* {@code await} on this barrier, or the specified waiting time elapses.
344	dl	1.2	*
345			* <p>If the current thread is not the last to arrive then it is
346			* disabled for thread scheduling purposes and lies dormant until
347			* one of the following things happens:
348			* <ul>
349			* <li>The last thread arrives; or
350	dholmes	1.13	* <li>The specified timeout elapses; or
351	jsr166	1.37	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
352			* the current thread; or
353			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
354			* one of the other waiting threads; or
355	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
356	dl	1.2	* <li>Some other thread invokes {@link #reset} on this barrier.
357			* </ul>
358	jsr166	1.37	*
359	dl	1.2	* <p>If the current thread:
360			* <ul>
361			* <li>has its interrupted status set on entry to this method; or
362	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
363	dl	1.2	* </ul>
364			* then {@link InterruptedException} is thrown and the current thread's
365			* interrupted status is cleared.
366			*
367	dl	1.26	* <p>If the specified waiting time elapses then {@link TimeoutException}
368			* is thrown. If the time is less than or equal to zero, the
369			* method will not wait at all.
370			*
371	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
372			* or if the barrier {@linkplain #isBroken is broken} when
373	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
374	jsr166	1.37	* {@link BrokenBarrierException} is thrown.
375			*
376			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
377			* waiting, then all other waiting threads will throw {@link
378			* BrokenBarrierException} and the barrier is placed in the broken
379	dl	1.2	* state.
380			*
381			* <p>If the current thread is the last thread to arrive, and a
382			* non-null barrier action was supplied in the constructor, then the
383	dl	1.28	* current thread runs the action before allowing the other threads to
384	dl	1.2	* continue.
385			* If an exception occurs during the barrier action then that exception
386	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
387			* the broken state.
388	dl	1.2	*
389	dl	1.5	* @param timeout the time to wait for the barrier
390			* @param unit the time unit of the timeout parameter
391	dl	1.2	* @return the arrival index of the current thread, where index
392	jsr166	1.49	* {@code getParties() - 1} indicates the first
393	jsr166	1.37	* to arrive and zero indicates the last to arrive
394	dl	1.28	* @throws InterruptedException if the current thread was interrupted
395	jsr166	1.37	* while waiting
396	jsr166	1.50	* @throws TimeoutException if the specified timeout elapses.
397			* In this case the barrier will be broken.
398	dl	1.2	* @throws BrokenBarrierException if <em>another</em> thread was
399	jsr166	1.37	* interrupted or timed out while the current thread was
400			* waiting, or the barrier was reset, or the barrier was broken
401			* when {@code await} was called, or the barrier action (if
402	jsr166	1.46	* present) failed due to an exception
403	dl	1.28	*/
404			public int await(long timeout, TimeUnit unit)
405			throws InterruptedException,
406			BrokenBarrierException,
407			TimeoutException {
408	dl	1.2	return dowait(true, unit.toNanos(timeout));
409	tim	1.1	}
410
411			/**
412	dl	1.25	* Queries if this barrier is in a broken state.
413	jsr166	1.37	*
414			* @return {@code true} if one or more parties broke out of this
415			* barrier due to interruption or timeout since
416			* construction or the last reset, or a barrier action
417			* failed due to an exception; {@code false} otherwise.
418	tim	1.1	*/
419			public boolean isBroken() {
420	dl	1.20	final ReentrantLock lock = this.lock;
421	dl	1.2	lock.lock();
422			try {
423	dl	1.28	return generation.broken;
424	tim	1.9	} finally {
425	dl	1.2	lock.unlock();
426			}
427	tim	1.1	}
428
429			/**
430	dl	1.25	* Resets the barrier to its initial state. If any parties are
431	tim	1.1	* currently waiting at the barrier, they will return with a
432	dl	1.8	* {@link BrokenBarrierException}. Note that resets <em>after</em>
433	dl	1.12	* a breakage has occurred for other reasons can be complicated to
434			* carry out; threads need to re-synchronize in some other way,
435			* and choose one to perform the reset. It may be preferable to
436			* instead create a new barrier for subsequent use.
437	tim	1.1	*/
438			public void reset() {
439	dl	1.20	final ReentrantLock lock = this.lock;
440	dl	1.2	lock.lock();
441			try {
442	dl	1.28	breakBarrier(); // break the current generation
443			nextGeneration(); // start a new generation
444	tim	1.9	} finally {
445	dl	1.2	lock.unlock();
446			}
447	tim	1.1	}
448
449			/**
450	dl	1.25	* Returns the number of parties currently waiting at the barrier.
451	tim	1.1	* This method is primarily useful for debugging and assertions.
452			*
453	jsr166	1.37	* @return the number of parties currently blocked in {@link #await}
454	jsr166	1.32	*/
455	tim	1.1	public int getNumberWaiting() {
456	dl	1.20	final ReentrantLock lock = this.lock;
457	dl	1.2	lock.lock();
458			try {
459	dl	1.31	return parties - count;
460	tim	1.9	} finally {
461	dl	1.2	lock.unlock();
462			}
463	tim	1.1	}
464			}