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<>(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:	1.54
Committed:	Tue Feb 17 18:55:39 2015 UTC (9 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.53:	+2 -2 lines
Log Message:	standardize code sample idiom: * <pre> {@code
#	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	1.44	* Runnable barrierAction =
57			* new Runnable() { public void run() { mergeRows(...); }};
58			* barrier = new CyclicBarrier(N, barrierAction);
59	tim	1.1	*
60	jsr166	1.52	* List<Thread> threads = new ArrayList<>(N);
61	jsr166	1.44	* for (int i = 0; i < N; i++) {
62			* Thread thread = new Thread(new Worker(i));
63			* threads.add(thread);
64			* thread.start();
65			* }
66			*
67			* // wait until done
68			* for (Thread thread : threads)
69			* thread.join();
70	tim	1.1	* }
71	jsr166	1.41	* }}</pre>
72			*
73	dl	1.28	* Here, each worker thread processes a row of the matrix then waits at the
74	tim	1.1	* barrier until all rows have been processed. When all rows are processed
75	dl	1.28	* the supplied {@link Runnable} barrier action is executed and merges the
76	tim	1.1	* rows. If the merger
77	jsr166	1.45	* determines that a solution has been found then {@code done()} will return
78			* {@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	tim	1.1	* @since 1.5
107	brian	1.4	* @see CountDownLatch
108	tim	1.1	*
109	dl	1.5	* @author Doug Lea
110	tim	1.1	*/
111			public class CyclicBarrier {
112	dl	1.28	/**
113			* Each use of the barrier is represented as a generation instance.
114			* The generation changes whenever the barrier is tripped, or
115			* is reset. There can be many generations associated with threads
116			* using the barrier - due to the non-deterministic way the lock
117			* may be allocated to waiting threads - but only one of these
118	jsr166	1.45	* can be active at a time (the one to which {@code count} applies)
119	dl	1.28	* and all the rest are either broken or tripped.
120			* There need not be an active generation if there has been a break
121			* but no subsequent reset.
122			*/
123			private static class Generation {
124			boolean broken = false;
125			}
126
127	dl	1.5	/** The lock for guarding barrier entry */
128	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
129	dl	1.5	/** Condition to wait on until tripped */
130	dl	1.21	private final Condition trip = lock.newCondition();
131	dl	1.5	/** The number of parties */
132	dl	1.2	private final int parties;
133	jsr166	1.51	/** The command to run when tripped */
134	dl	1.12	private final Runnable barrierCommand;
135	dl	1.28	/** The current generation */
136			private Generation generation = new Generation();
137	dl	1.2
138			/**
139			* Number of parties still waiting. Counts down from parties to 0
140	dl	1.31	* on each generation. It is reset to parties on each new
141			* generation or when broken.
142	dl	1.2	*/
143	dl	1.28	private int count;
144	dl	1.2
145			/**
146	jsr166	1.29	* Updates state on barrier trip and wakes up everyone.
147	dl	1.28	* Called only while holding lock.
148			*/
149	dl	1.2	private void nextGeneration() {
150	dl	1.28	// signal completion of last generation
151			trip.signalAll();
152			// set up next generation
153	dl	1.2	count = parties;
154	dl	1.28	generation = new Generation();
155	dl	1.12	}
156
157			/**
158	jsr166	1.29	* Sets current barrier generation as broken and wakes up everyone.
159	dl	1.28	* Called only while holding lock.
160	dl	1.12	*/
161			private void breakBarrier() {
162	dl	1.28	generation.broken = true;
163	jsr166	1.38	count = parties;
164	dl	1.12	trip.signalAll();
165	dl	1.2	}
166
167	dl	1.5	/**
168	dl	1.7	* Main barrier code, covering the various policies.
169	dl	1.5	*/
170	dl	1.28	private int dowait(boolean timed, long nanos)
171			throws InterruptedException, BrokenBarrierException,
172			TimeoutException {
173	dl	1.20	final ReentrantLock lock = this.lock;
174	dl	1.2	lock.lock();
175			try {
176	dl	1.30	final Generation g = generation;
177	dl	1.2
178	dl	1.28	if (g.broken)
179	dl	1.2	throw new BrokenBarrierException();
180
181			if (Thread.interrupted()) {
182	dl	1.12	breakBarrier();
183	dl	1.2	throw new InterruptedException();
184			}
185
186	jsr166	1.40	int index = --count;
187			if (index == 0) { // tripped
188			boolean ranAction = false;
189			try {
190			final Runnable command = barrierCommand;
191			if (command != null)
192			command.run();
193			ranAction = true;
194			nextGeneration();
195			return 0;
196			} finally {
197			if (!ranAction)
198			breakBarrier();
199			}
200			}
201	dl	1.2
202	dl	1.28	// loop until tripped, broken, interrupted, or timed out
203	dl	1.12	for (;;) {
204	dl	1.2	try {
205	dl	1.28	if (!timed)
206	dl	1.2	trip.await();
207	dl	1.23	else if (nanos > 0L)
208	dl	1.2	nanos = trip.awaitNanos(nanos);
209	dl	1.12	} catch (InterruptedException ie) {
210	jsr166	1.36	if (g == generation && ! g.broken) {
211			breakBarrier();
212	jsr166	1.38	throw ie;
213			} else {
214			// We're about to finish waiting even if we had not
215			// been interrupted, so this interrupt is deemed to
216			// "belong" to subsequent execution.
217			Thread.currentThread().interrupt();
218			}
219	dl	1.2	}
220	dl	1.28
221	dl	1.30	if (g.broken)
222	dl	1.12	throw new BrokenBarrierException();
223
224	jsr166	1.36	if (g != generation)
225	dl	1.12	return index;
226
227	dl	1.23	if (timed && nanos <= 0L) {
228	dl	1.12	breakBarrier();
229	dl	1.2	throw new TimeoutException();
230			}
231			}
232	tim	1.9	} finally {
233	dl	1.2	lock.unlock();
234			}
235			}
236	tim	1.1
237			/**
238	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
239	tim	1.1	* given number of parties (threads) are waiting upon it, and which
240	dl	1.17	* will execute the given barrier action when the barrier is tripped,
241	dl	1.19	* performed by the last thread entering the barrier.
242	tim	1.1	*
243			* @param parties the number of threads that must invoke {@link #await}
244	jsr166	1.37	* before the barrier is tripped
245	tim	1.1	* @param barrierAction the command to execute when the barrier is
246	jsr166	1.37	* tripped, or {@code null} if there is no action
247			* @throws IllegalArgumentException if {@code parties} is less than 1
248	tim	1.1	*/
249			public CyclicBarrier(int parties, Runnable barrierAction) {
250	dl	1.2	if (parties <= 0) throw new IllegalArgumentException();
251	dl	1.28	this.parties = parties;
252	dl	1.2	this.count = parties;
253			this.barrierCommand = barrierAction;
254	tim	1.1	}
255
256			/**
257	jsr166	1.45	* Creates a new {@code CyclicBarrier} that will trip when the
258	dl	1.14	* given number of parties (threads) are waiting upon it, and
259	dl	1.31	* does not perform a predefined action when the barrier is tripped.
260	tim	1.1	*
261			* @param parties the number of threads that must invoke {@link #await}
262	jsr166	1.37	* before the barrier is tripped
263			* @throws IllegalArgumentException if {@code parties} is less than 1
264	tim	1.1	*/
265			public CyclicBarrier(int parties) {
266	dl	1.2	this(parties, null);
267	tim	1.1	}
268
269			/**
270	dl	1.25	* Returns the number of parties required to trip this barrier.
271	jsr166	1.37	*
272			* @return the number of parties required to trip this barrier
273	dl	1.31	*/
274	tim	1.1	public int getParties() {
275	dl	1.2	return parties;
276	tim	1.1	}
277
278			/**
279	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
280	jsr166	1.45	* {@code await} on this barrier.
281	tim	1.1	*
282			* <p>If the current thread is not the last to arrive then it is
283			* disabled for thread scheduling purposes and lies dormant until
284	jsr166	1.36	* one of the following things happens:
285	tim	1.1	* <ul>
286			* <li>The last thread arrives; or
287	jsr166	1.37	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
288			* the current thread; or
289			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
290			* one of the other waiting threads; or
291	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
292	tim	1.1	* <li>Some other thread invokes {@link #reset} on this barrier.
293			* </ul>
294	jsr166	1.37	*
295	tim	1.1	* <p>If the current thread:
296			* <ul>
297			* <li>has its interrupted status set on entry to this method; or
298	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
299	tim	1.1	* </ul>
300			* then {@link InterruptedException} is thrown and the current thread's
301			* interrupted status is cleared.
302			*
303	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
304			* or if the barrier {@linkplain #isBroken is broken} when
305	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
306	jsr166	1.37	* {@link BrokenBarrierException} is thrown.
307	tim	1.1	*
308	jsr166	1.37	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
309	dl	1.28	* then all other waiting threads will throw
310	tim	1.1	* {@link BrokenBarrierException} and the barrier is placed in the broken
311			* state.
312			*
313			* <p>If the current thread is the last thread to arrive, and a
314			* non-null barrier action was supplied in the constructor, then the
315	dl	1.28	* current thread runs the action before allowing the other threads to
316	tim	1.1	* continue.
317			* If an exception occurs during the barrier action then that exception
318	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
319			* the broken state.
320	tim	1.1	*
321			* @return the arrival index of the current thread, where index
322	jsr166	1.49	* {@code getParties() - 1} indicates the first
323	jsr166	1.37	* to arrive and zero indicates the last to arrive
324	dl	1.28	* @throws InterruptedException if the current thread was interrupted
325	jsr166	1.37	* while waiting
326	tim	1.1	* @throws BrokenBarrierException if <em>another</em> thread was
327	jsr166	1.37	* interrupted or timed out while the current thread was
328			* waiting, or the barrier was reset, or the barrier was
329			* broken when {@code await} was called, or the barrier
330	jsr166	1.47	* action (if present) failed due to an exception
331	tim	1.1	*/
332			public int await() throws InterruptedException, BrokenBarrierException {
333	dl	1.2	try {
334	dl	1.23	return dowait(false, 0L);
335	tim	1.9	} catch (TimeoutException toe) {
336	jsr166	1.42	throw new Error(toe); // cannot happen
337	dl	1.2	}
338			}
339
340			/**
341	jsr166	1.37	* Waits until all {@linkplain #getParties parties} have invoked
342	jsr166	1.45	* {@code await} on this barrier, or the specified waiting time elapses.
343	dl	1.2	*
344			* <p>If the current thread is not the last to arrive then it is
345			* disabled for thread scheduling purposes and lies dormant until
346			* one of the following things happens:
347			* <ul>
348			* <li>The last thread arrives; or
349	dholmes	1.13	* <li>The specified timeout elapses; or
350	jsr166	1.37	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
351			* the current thread; or
352			* <li>Some other thread {@linkplain Thread#interrupt interrupts}
353			* one of the other waiting threads; or
354	dl	1.24	* <li>Some other thread times out while waiting for barrier; or
355	dl	1.2	* <li>Some other thread invokes {@link #reset} on this barrier.
356			* </ul>
357	jsr166	1.37	*
358	dl	1.2	* <p>If the current thread:
359			* <ul>
360			* <li>has its interrupted status set on entry to this method; or
361	jsr166	1.37	* <li>is {@linkplain Thread#interrupt interrupted} while waiting
362	dl	1.2	* </ul>
363			* then {@link InterruptedException} is thrown and the current thread's
364			* interrupted status is cleared.
365			*
366	dl	1.26	* <p>If the specified waiting time elapses then {@link TimeoutException}
367			* is thrown. If the time is less than or equal to zero, the
368			* method will not wait at all.
369			*
370	jsr166	1.37	* <p>If the barrier is {@link #reset} while any thread is waiting,
371			* or if the barrier {@linkplain #isBroken is broken} when
372	jsr166	1.45	* {@code await} is invoked, or while any thread is waiting, then
373	jsr166	1.37	* {@link BrokenBarrierException} is thrown.
374			*
375			* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
376			* waiting, then all other waiting threads will throw {@link
377			* BrokenBarrierException} and the barrier is placed in the broken
378	dl	1.2	* state.
379			*
380			* <p>If the current thread is the last thread to arrive, and a
381			* non-null barrier action was supplied in the constructor, then the
382	dl	1.28	* current thread runs the action before allowing the other threads to
383	dl	1.2	* continue.
384			* If an exception occurs during the barrier action then that exception
385	dholmes	1.13	* will be propagated in the current thread and the barrier is placed in
386			* the broken state.
387	dl	1.2	*
388	dl	1.5	* @param timeout the time to wait for the barrier
389			* @param unit the time unit of the timeout parameter
390	dl	1.2	* @return the arrival index of the current thread, where index
391	jsr166	1.49	* {@code getParties() - 1} indicates the first
392	jsr166	1.37	* to arrive and zero indicates the last to arrive
393	dl	1.28	* @throws InterruptedException if the current thread was interrupted
394	jsr166	1.37	* while waiting
395	jsr166	1.50	* @throws TimeoutException if the specified timeout elapses.
396			* In this case the barrier will be broken.
397	dl	1.2	* @throws BrokenBarrierException if <em>another</em> thread was
398	jsr166	1.37	* interrupted or timed out while the current thread was
399			* waiting, or the barrier was reset, or the barrier was broken
400			* when {@code await} was called, or the barrier action (if
401	jsr166	1.46	* present) failed due to an exception
402	dl	1.28	*/
403			public int await(long timeout, TimeUnit unit)
404			throws InterruptedException,
405			BrokenBarrierException,
406			TimeoutException {
407	dl	1.2	return dowait(true, unit.toNanos(timeout));
408	tim	1.1	}
409
410			/**
411	dl	1.25	* Queries if this barrier is in a broken state.
412	jsr166	1.37	*
413			* @return {@code true} if one or more parties broke out of this
414			* barrier due to interruption or timeout since
415			* construction or the last reset, or a barrier action
416			* failed due to an exception; {@code false} otherwise.
417	tim	1.1	*/
418			public boolean isBroken() {
419	dl	1.20	final ReentrantLock lock = this.lock;
420	dl	1.2	lock.lock();
421			try {
422	dl	1.28	return generation.broken;
423	tim	1.9	} finally {
424	dl	1.2	lock.unlock();
425			}
426	tim	1.1	}
427
428			/**
429	dl	1.25	* Resets the barrier to its initial state. If any parties are
430	tim	1.1	* currently waiting at the barrier, they will return with a
431	dl	1.8	* {@link BrokenBarrierException}. Note that resets <em>after</em>
432	dl	1.12	* a breakage has occurred for other reasons can be complicated to
433			* carry out; threads need to re-synchronize in some other way,
434			* and choose one to perform the reset. It may be preferable to
435			* instead create a new barrier for subsequent use.
436	tim	1.1	*/
437			public void reset() {
438	dl	1.20	final ReentrantLock lock = this.lock;
439	dl	1.2	lock.lock();
440			try {
441	dl	1.28	breakBarrier(); // break the current generation
442			nextGeneration(); // start a new generation
443	tim	1.9	} finally {
444	dl	1.2	lock.unlock();
445			}
446	tim	1.1	}
447
448			/**
449	dl	1.25	* Returns the number of parties currently waiting at the barrier.
450	tim	1.1	* This method is primarily useful for debugging and assertions.
451			*
452	jsr166	1.37	* @return the number of parties currently blocked in {@link #await}
453	jsr166	1.32	*/
454	tim	1.1	public int getNumberWaiting() {
455	dl	1.20	final ReentrantLock lock = this.lock;
456	dl	1.2	lock.lock();
457			try {
458	dl	1.31	return parties - count;
459	tim	1.9	} finally {
460	dl	1.2	lock.unlock();
461			}
462	tim	1.1	}
463			}