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 =
 *       new Runnable() { public void run() { mergeRows(...); }};
 *     barrier = new CyclicBarrier(N, barrierAction);
 *
 *     List<Thread> threads = new ArrayList<Thread>(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.
 *
 * @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 {@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 {
        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 {@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:	1.51
Committed:	Sat May 3 21:05:50 2014 UTC (10 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.50:	+1 -1 lines
Log Message:	/* => /** for javadoc
#	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	1.43	import java.util.concurrent.locks.Condition;
9			import java.util.concurrent.locks.ReentrantLock;
10	tim	1.1
11			/**
12	tim	1.10	* A synchronization aid that allows a set of threads to all wait for
13	brian	1.4	* each other to reach a common barrier point. CyclicBarriers are
14	dl	1.3	* useful in programs involving a fixed sized party of threads that
15	brian	1.4	* must occasionally wait for each other. The barrier is called
16	dl	1.3	* <em>cyclic</em> because it can be re-used after the waiting threads
17			* are released.
18	tim	1.1	*
19	jsr166	1.45	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
20	tim	1.1	* that is run once per barrier point, after the last thread in the party
21	dl	1.28	* arrives, but before any threads are released.
22	tim	1.1	* This <em>barrier action</em> is useful
23			* for updating shared-state before any of the parties continue.
24	dl	1.28	*
25	jsr166	1.48	* <p><b>Sample usage:</b> Here is an example of using a barrier in a
26			* parallel decomposition design:
27	jsr166	1.41	*
28			* <pre> {@code
29	tim	1.1	* class Solver {
30			* final int N;
31			* final float[][] data;
32			* final CyclicBarrier barrier;
33	dl	1.28	*
34	tim	1.1	* class Worker implements Runnable {
35			* int myRow;
36			* Worker(int row) { myRow = row; }
37			* public void run() {
38			* while (!done()) {
39			* processRow(myRow);
40			*
41			* try {
42	dl	1.28	* barrier.await();
43			* } catch (InterruptedException ex) {
44			* return;
45			* } catch (BrokenBarrierException ex) {
46			* return;
47	tim	1.1	* }
48			* }
49			* }
50			* }
51			*
52			* public Solver(float[][] matrix) {
53			* data = matrix;
54			* N = matrix.length;
55	jsr166	1.44	* Runnable barrierAction =
56			* new Runnable() { public void run() { mergeRows(...); }};
57			* barrier = new CyclicBarrier(N, barrierAction);
58	tim	1.1	*
59	jsr166	1.44	* List<Thread> threads = new ArrayList<Thread>(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	tim	1.1	* }
70	jsr166	1.41	* }}</pre>
71			*
72	dl	1.28	* Here, each worker thread processes a row of the matrix then waits at the
73	tim	1.1	* barrier until all rows have been processed. When all rows are processed
74	dl	1.28	* the supplied {@link Runnable} barrier action is executed and merges the
75	tim	1.1	* rows. If the merger
76	jsr166	1.45	* determines that a solution has been found then {@code done()} will return
77			* {@code true} and each worker will terminate.
78	tim	1.1	*
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	dl	1.28	* You can then choose which thread should execute the barrier action, for
84	tim	1.1	* example:
85	jsr166	1.41	* <pre> {@code
86			* if (barrier.await() == 0) {
87			* // log the completion of this iteration
88			* }}</pre>
89	tim	1.1	*
90	jsr166	1.45	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
91	dl	1.28	* 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	dl	1.31	* {@link InterruptedException} if they too were interrupted at about
96	dl	1.28	* the same time).
97	tim	1.1	*
98	jsr166	1.35	* <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	brian	1.33	* actions that are part of the barrier action, which in turn
102	jsr166	1.35	* <i>happen-before</i> actions following a successful return from the
103			* corresponding {@code await()} in other threads.
104	brian	1.33	*
105	tim	1.1	* @since 1.5
106	brian	1.4	* @see CountDownLatch
107	tim	1.1	*
108	dl	1.5	* @author Doug Lea
109	tim	1.1	*/
110			public class CyclicBarrier {
111	dl	1.28	/**
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	jsr166	1.45	* can be active at a time (the one to which {@code count} applies)
118	dl	1.28	* 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			boolean broken = false;
124			}
125
126	dl	1.5	/** The lock for guarding barrier entry */
127	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
128	dl	1.5	/** Condition to wait on until tripped */
129	dl	1.21	private final Condition trip = lock.newCondition();
130	dl	1.5	/** The number of parties */
131	dl	1.2	private final int parties;
132	jsr166	1.51	/** The command to run when tripped */
133	dl	1.12	private final Runnable barrierCommand;
134	dl	1.28	/** The current generation */
135			private Generation generation = new Generation();
136	dl	1.2
137			/**
138			* Number of parties still waiting. Counts down from parties to 0
139	dl	1.31	* on each generation. It is reset to parties on each new
140			* generation or when broken.
141	dl	1.2	*/
142	dl	1.28	private int count;
143	dl	1.2
144			/**
145	jsr166	1.29	* Updates state on barrier trip and wakes up everyone.
146	dl	1.28	* Called only while holding lock.
147			*/
148	dl	1.2	private void nextGeneration() {
149	dl	1.28	// signal completion of last generation
150			trip.signalAll();
151			// set up next generation
152	dl	1.2	count = parties;
153	dl	1.28	generation = new Generation();
154	dl	1.12	}
155
156			/**
157	jsr166	1.29	* Sets current barrier generation as broken and wakes up everyone.
158	dl	1.28	* Called only while holding lock.
159	dl	1.12	*/
160			private void breakBarrier() {
161	dl	1.28	generation.broken = true;
162	jsr166	1.38	count = parties;
163	dl	1.12	trip.signalAll();
164	dl	1.2	}
165
166	dl	1.5	/**
167	dl	1.7	* Main barrier code, covering the various policies.
168	dl	1.5	*/
169	dl	1.28	private int dowait(boolean timed, long nanos)
170			throws InterruptedException, BrokenBarrierException,
171			TimeoutException {
172	dl	1.20	final ReentrantLock lock = this.lock;
173	dl	1.2	lock.lock();
174			try {
175	dl	1.30	final Generation g = generation;
176	dl	1.2
177	dl	1.28	if (g.broken)
178	dl	1.2	throw new BrokenBarrierException();
179
180			if (Thread.interrupted()) {
181	dl	1.12	breakBarrier();
182	dl	1.2	throw new InterruptedException();
183			}
184
185	jsr166	1.40	int index = --count;
186			if (index == 0) { // tripped
187			boolean ranAction = false;
188			try {
189			final Runnable command = barrierCommand;
190			if (command != null)
191			command.run();
192			ranAction = true;
193			nextGeneration();
194			return 0;
195			} finally {
196			if (!ranAction)
197			breakBarrier();
198			}
199			}
200	dl	1.2
201	dl	1.28	// loop until tripped, broken, interrupted, or timed out
202	dl	1.12	for (;;) {
203	dl	1.2	try {
204	dl	1.28	if (!timed)
205	dl	1.2	trip.await();
206	dl	1.23	else if (nanos > 0L)
207	dl	1.2	nanos = trip.awaitNanos(nanos);
208	dl	1.12	} catch (InterruptedException ie) {
209	jsr166	1.36	if (g == generation && ! g.broken) {
210			breakBarrier();
211	jsr166	1.38	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	dl	1.2	}
219	dl	1.28
220	dl	1.30	if (g.broken)
221	dl	1.12	throw new BrokenBarrierException();
222
223	jsr166	1.36	if (g != generation)
224	dl	1.12	return index;
225
226	dl	1.23	if (timed && nanos <= 0L) {
227	dl	1.12	breakBarrier();
228	dl	1.2	throw new TimeoutException();
229			}
230			}
231	tim	1.9	} finally {
232	dl	1.2	lock.unlock();
233			}
234			}
235	tim	1.1
236			/**
237	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
238	tim	1.1	* given number of parties (threads) are waiting upon it, and which
239	dl	1.17	* will execute the given barrier action when the barrier is tripped,
240	dl	1.19	* performed by the last thread entering the barrier.
241	tim	1.1	*
242			* @param parties the number of threads that must invoke {@link #await}
243	jsr166	1.37	* before the barrier is tripped
244	tim	1.1	* @param barrierAction the command to execute when the barrier is
245	jsr166	1.37	* tripped, or {@code null} if there is no action
246			* @throws IllegalArgumentException if {@code parties} is less than 1
247	tim	1.1	*/
248			public CyclicBarrier(int parties, Runnable barrierAction) {
249	dl	1.2	if (parties <= 0) throw new IllegalArgumentException();
250	dl	1.28	this.parties = parties;
251	dl	1.2	this.count = parties;
252			this.barrierCommand = barrierAction;
253	tim	1.1	}
254
255			/**
256	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
257	dl	1.14	* given number of parties (threads) are waiting upon it, and
258	dl	1.31	* does not perform a predefined action when the barrier is tripped.
259	tim	1.1	*
260			* @param parties the number of threads that must invoke {@link #await}
261	jsr166	1.37	* before the barrier is tripped
262			* @throws IllegalArgumentException if {@code parties} is less than 1
263	tim	1.1	*/
264			public CyclicBarrier(int parties) {
265	dl	1.2	this(parties, null);
266	tim	1.1	}
267
268			/**
269	dl	1.25	* Returns the number of parties required to trip this barrier.
270	jsr166	1.37	*
271			* @return the number of parties required to trip this barrier
272	dl	1.31	*/
273	tim	1.1	public int getParties() {
274	dl	1.2	return parties;
275	tim	1.1	}
276
277			/**
278	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
279	jsr166	1.45	* {@code await} on this barrier.
280	tim	1.1	*
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	jsr166	1.36	* one of the following things happens:
284	tim	1.1	* <ul>
285			* <li>The last thread arrives; or
286	jsr166	1.37	* <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	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
291	tim	1.1	* <li>Some other thread invokes {@link #reset} on this barrier.
292			* </ul>
293	jsr166	1.37	*
294	tim	1.1	* <p>If the current thread:
295			* <ul>
296			* <li>has its interrupted status set on entry to this method; or
297	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
298	tim	1.1	* </ul>
299			* then {@link InterruptedException} is thrown and the current thread's
300			* interrupted status is cleared.
301			*
302	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
303			* or if the barrier {@linkplain #isBroken is broken} when
304	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
305	jsr166	1.37	* {@link BrokenBarrierException} is thrown.
306	tim	1.1	*
307	jsr166	1.37	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
308	dl	1.28	* then all other waiting threads will throw
309	tim	1.1	* {@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	dl	1.28	* current thread runs the action before allowing the other threads to
315	tim	1.1	* continue.
316			* If an exception occurs during the barrier action then that exception
317	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
318			* the broken state.
319	tim	1.1	*
320			* @return the arrival index of the current thread, where index
321	jsr166	1.49	* {@code getParties() - 1} indicates the first
322	jsr166	1.37	* to arrive and zero indicates the last to arrive
323	dl	1.28	* @throws InterruptedException if the current thread was interrupted
324	jsr166	1.37	* while waiting
325	tim	1.1	* @throws BrokenBarrierException if <em>another</em> thread was
326	jsr166	1.37	* 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	jsr166	1.47	* action (if present) failed due to an exception
330	tim	1.1	*/
331			public int await() throws InterruptedException, BrokenBarrierException {
332	dl	1.2	try {
333	dl	1.23	return dowait(false, 0L);
334	tim	1.9	} catch (TimeoutException toe) {
335	jsr166	1.42	throw new Error(toe); // cannot happen
336	dl	1.2	}
337			}
338
339			/**
340	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
341	jsr166	1.45	* {@code await} on this barrier, or the specified waiting time elapses.
342	dl	1.2	*
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	dholmes	1.13	* <li>The specified timeout elapses; or
349	jsr166	1.37	* <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	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
354	dl	1.2	* <li>Some other thread invokes {@link #reset} on this barrier.
355			* </ul>
356	jsr166	1.37	*
357	dl	1.2	* <p>If the current thread:
358			* <ul>
359			* <li>has its interrupted status set on entry to this method; or
360	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
361	dl	1.2	* </ul>
362			* then {@link InterruptedException} is thrown and the current thread's
363			* interrupted status is cleared.
364			*
365	dl	1.26	* <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	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
370			* or if the barrier {@linkplain #isBroken is broken} when
371	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
372	jsr166	1.37	* {@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	dl	1.2	* 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	dl	1.28	* current thread runs the action before allowing the other threads to
382	dl	1.2	* continue.
383			* If an exception occurs during the barrier action then that exception
384	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
385			* the broken state.
386	dl	1.2	*
387	dl	1.5	* @param timeout the time to wait for the barrier
388			* @param unit the time unit of the timeout parameter
389	dl	1.2	* @return the arrival index of the current thread, where index
390	jsr166	1.49	* {@code getParties() - 1} indicates the first
391	jsr166	1.37	* to arrive and zero indicates the last to arrive
392	dl	1.28	* @throws InterruptedException if the current thread was interrupted
393	jsr166	1.37	* while waiting
394	jsr166	1.50	* @throws TimeoutException if the specified timeout elapses.
395			* In this case the barrier will be broken.
396	dl	1.2	* @throws BrokenBarrierException if <em>another</em> thread was
397	jsr166	1.37	* 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	jsr166	1.46	* present) failed due to an exception
401	dl	1.28	*/
402			public int await(long timeout, TimeUnit unit)
403			throws InterruptedException,
404			BrokenBarrierException,
405			TimeoutException {
406	dl	1.2	return dowait(true, unit.toNanos(timeout));
407	tim	1.1	}
408
409			/**
410	dl	1.25	* Queries if this barrier is in a broken state.
411	jsr166	1.37	*
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	tim	1.1	*/
417			public boolean isBroken() {
418	dl	1.20	final ReentrantLock lock = this.lock;
419	dl	1.2	lock.lock();
420			try {
421	dl	1.28	return generation.broken;
422	tim	1.9	} finally {
423	dl	1.2	lock.unlock();
424			}
425	tim	1.1	}
426
427			/**
428	dl	1.25	* Resets the barrier to its initial state. If any parties are
429	tim	1.1	* currently waiting at the barrier, they will return with a
430	dl	1.8	* {@link BrokenBarrierException}. Note that resets <em>after</em>
431	dl	1.12	* 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	tim	1.1	*/
436			public void reset() {
437	dl	1.20	final ReentrantLock lock = this.lock;
438	dl	1.2	lock.lock();
439			try {
440	dl	1.28	breakBarrier(); // break the current generation
441			nextGeneration(); // start a new generation
442	tim	1.9	} finally {
443	dl	1.2	lock.unlock();
444			}
445	tim	1.1	}
446
447			/**
448	dl	1.25	* Returns the number of parties currently waiting at the barrier.
449	tim	1.1	* This method is primarily useful for debugging and assertions.
450			*
451	jsr166	1.37	* @return the number of parties currently blocked in {@link #await}
452	jsr166	1.32	*/
453	tim	1.1	public int getNumberWaiting() {
454	dl	1.20	final ReentrantLock lock = this.lock;
455	dl	1.2	lock.lock();
456			try {
457	dl	1.31	return parties - count;
458	tim	1.9	} finally {
459	dl	1.2	lock.unlock();
460			}
461	tim	1.1	}
462			}