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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * A synchronization aid that allows a set of threads to all wait for
 * each other to reach a common barrier point.  CyclicBarriers are
 * useful in programs involving a fixed sized party of threads that
 * must occasionally wait for each other. The barrier is called
 * <em>cyclic</em> because it can be re-used after the waiting threads
 * are released.
 *
 * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
 * that is run once per barrier point, after the last thread in the party
 * arrives, but before any threads are released.
 * This <em>barrier action</em> is useful
 * for updating shared-state before any of the parties continue.
 *
 * <p><b>Sample usage:</b> Here is an example of
 *  using a barrier in a parallel decomposition design:
 * <pre>
 * class Solver {
 *   final int N;
 *   final float[][] data;
 *   final CyclicBarrier barrier;
 *
 *   class Worker implements Runnable {
 *     int myRow;
 *     Worker(int row) { myRow = row; }
 *     public void run() {
 *       while (!done()) {
 *         processRow(myRow);
 *
 *         try {
 *           barrier.await();
 *         } catch (InterruptedException ex) {
 *           return;
 *         } catch (BrokenBarrierException ex) {
 *           return;
 *         }
 *       }
 *     }
 *   }
 *
 *   public Solver(float[][] matrix) {
 *     data = matrix;
 *     N = matrix.length;
 *     barrier = new CyclicBarrier(N,
 *                                 new Runnable() {
 *                                   public void run() {
 *                                     mergeRows(...);
 *                                   }
 *                                 });
 *     for (int i = 0; i < N; ++i)
 *       new Thread(new Worker(i)).start();
 *
 *     waitUntilDone();
 *   }
 * }
 * </pre>
 * Here, each worker thread processes a row of the matrix then waits at the
 * barrier until all rows have been processed. When all rows are processed
 * the supplied {@link Runnable} barrier action is executed and merges the
 * rows. If the merger
 * determines that a solution has been found then <tt>done()</tt> will return
 * <tt>true</tt> and each worker will terminate.
 *
 * <p>If the barrier action does not rely on the parties being suspended when
 * it is executed, then any of the threads in the party could execute that
 * action when it is released. To facilitate this, each invocation of
 * {@link #await} returns the arrival index of that thread at the barrier.
 * You can then choose which thread should execute the barrier action, for
 * example:
 * <pre>  if (barrier.await() == 0) {
 *     // log the completion of this iteration
 *   }</pre>
 *
 * <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
 * for failed synchronization attempts: If a thread leaves a barrier
 * point prematurely because of interruption, failure, or timeout, all
 * other threads waiting at that barrier point will also leave
 * abnormally via {@link BrokenBarrierException} (or
 * {@link InterruptedException} if they too were interrupted at about
 * the same time).
 *
 * <p> Memory visibility effects: Actions in a thread prior to calling
 * <tt>await()</tt> <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 <tt>await()</tt> in other threads.
 *
 * @since 1.5
 * @see CountDownLatch
 *
 * @author Doug Lea
 */
public class CyclicBarrier {
    /**
     * Each use of the barrier is represented as a generation instance.
     * The generation changes whenever the barrier is tripped, or
     * is reset. There can be many generations associated with threads
     * using the barrier - due to the non-deterministic way the lock
     * may be allocated to waiting threads - but only one of these
     * can be active at a time (the one to which <tt>count</tt> applies)
     * and all the rest are either broken or tripped.
     * There need not be an active generation if there has been a break
     * but no subsequent reset.
     */
    private static class Generation {
        boolean broken = false;
        boolean tripped = 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
        generation.tripped = true;
        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) {
                    breakBarrier();
                    throw ie;
                }

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

                if (g.tripped)
                    return index;

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

    /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     * before the barrier is tripped.
     * @param barrierAction the command to execute when the barrier is
     * tripped, or <tt>null</tt> if there is no action.
     *
     * @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     * before the barrier is tripped.
     *
     * @throws IllegalArgumentException if <tt>parties</tt> 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 {@link #getParties parties} have invoked <tt>await</tt>
     * on this barrier.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>Some other thread  {@link 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 {@link 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 {@link #isBroken is broken} when <tt>await</tt> is invoked,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @return the arrival index of the current thread, where index
     *  <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
     * zero indicates the last to arrive.
     *
     * @throws InterruptedException if the current thread was interrupted
     * while waiting.
     * @throws BrokenBarrierException if <em>another</em> thread was
     * interrupted or timed out while the current thread was waiting,
     * or the barrier was reset, or the barrier was broken when
     * <tt>await</tt> was called, or the barrier action (if present)
     * failed due an exception.
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen;
        }
    }

    /**
     * Waits until all {@link #getParties parties} have invoked <tt>await</tt>
     * on this barrier.
     *
     * <p>If the current thread is not the last to arrive then it is
     * disabled for thread scheduling purposes and lies dormant until
     * one of the following things happens:
     * <ul>
     * <li>The last thread arrives; or
     * <li>The specified timeout elapses; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>Some other thread  {@link 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 {@link 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 {@link #isBroken is broken} when <tt>await</tt> is invoked,
     * or while any thread is waiting,
     * then {@link BrokenBarrierException} is thrown.
     *
     * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
     * then all other waiting threads will throw
     * {@link BrokenBarrierException} and the barrier is placed in the broken
     * state.
     *
     * <p>If the current thread is the last thread to arrive, and a
     * non-null barrier action was supplied in the constructor, then the
     * current thread runs the action before allowing the other threads to
     * continue.
     * If an exception occurs during the barrier action then that exception
     * will be propagated in the current thread and the barrier is placed in
     * the broken state.
     *
     * @param timeout the time to wait for the barrier
     * @param unit the time unit of the timeout parameter
     * @return the arrival index of the current thread, where index
     * <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
     * zero indicates the last to arrive.
     *
     * @throws InterruptedException if the current thread was interrupted
     * while waiting.
     * @throws TimeoutException if the specified timeout elapses.
     * @throws BrokenBarrierException if <em>another</em> thread was
     * interrupted or timed out while the current thread was waiting,
     * or the barrier was reset, or the barrier was broken when
     * <tt>await</tt> was called, or the barrier action (if present)
     * failed due an exception.
     */
    public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * Queries if this barrier is in a broken state.
     * @return <tt>true</tt> 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;
     * <tt>false</tt> 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:	1.33
Committed:	Fri Sep 2 01:03:08 2005 UTC (18 years, 9 months ago) by brian
Branch:	MAIN
Changes since 1.32:	+7 -0 lines
Log Message:	Happens-before markup
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9
10	/**
11	* A synchronization aid that allows a set of threads to all wait for
12	* each other to reach a common barrier point. CyclicBarriers are
13	* useful in programs involving a fixed sized party of threads that
14	* must occasionally wait for each other. The barrier is called
15	* <em>cyclic</em> because it can be re-used after the waiting threads
16	* are released.
17	*
18	* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
19	* that is run once per barrier point, after the last thread in the party
20	* arrives, but before any threads are released.
21	* This <em>barrier action</em> is useful
22	* for updating shared-state before any of the parties continue.
23	*
24	* <p><b>Sample usage:</b> Here is an example of
25	* using a barrier in a parallel decomposition design:
26	* <pre>
27	* class Solver {
28	* final int N;
29	* final float[][] data;
30	* final CyclicBarrier barrier;
31	*
32	* class Worker implements Runnable {
33	* int myRow;
34	* Worker(int row) { myRow = row; }
35	* public void run() {
36	* while (!done()) {
37	* processRow(myRow);
38	*
39	* try {
40	* barrier.await();
41	* } catch (InterruptedException ex) {
42	* return;
43	* } catch (BrokenBarrierException ex) {
44	* return;
45	* }
46	* }
47	* }
48	* }
49	*
50	* public Solver(float[][] matrix) {
51	* data = matrix;
52	* N = matrix.length;
53	* barrier = new CyclicBarrier(N,
54	* new Runnable() {
55	* public void run() {
56	* mergeRows(...);
57	* }
58	* });
59	* for (int i = 0; i < N; ++i)
60	* new Thread(new Worker(i)).start();
61	*
62	* waitUntilDone();
63	* }
64	* }
65	* </pre>
66	* Here, each worker thread processes a row of the matrix then waits at the
67	* barrier until all rows have been processed. When all rows are processed
68	* the supplied {@link Runnable} barrier action is executed and merges the
69	* rows. If the merger
70	* determines that a solution has been found then <tt>done()</tt> will return
71	* <tt>true</tt> and each worker will terminate.
72	*
73	* <p>If the barrier action does not rely on the parties being suspended when
74	* it is executed, then any of the threads in the party could execute that
75	* action when it is released. To facilitate this, each invocation of
76	* {@link #await} returns the arrival index of that thread at the barrier.
77	* You can then choose which thread should execute the barrier action, for
78	* example:
79	* <pre> if (barrier.await() == 0) {
80	* // log the completion of this iteration
81	* }</pre>
82	*
83	* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
84	* for failed synchronization attempts: If a thread leaves a barrier
85	* point prematurely because of interruption, failure, or timeout, all
86	* other threads waiting at that barrier point will also leave
87	* abnormally via {@link BrokenBarrierException} (or
88	* {@link InterruptedException} if they too were interrupted at about
89	* the same time).
90	*
91	* <p> Memory visibility effects: Actions in a thread prior to calling
92	* <tt>await()</tt> <a
93	* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
94	* actions that are part of the barrier action, which in turn
95	* <i>happen-before</I> actions following a successful return from the
96	* corresponding <tt>await()</tt> in other threads.
97	*
98	* @since 1.5
99	* @see CountDownLatch
100	*
101	* @author Doug Lea
102	*/
103	public class CyclicBarrier {
104	/**
105	* Each use of the barrier is represented as a generation instance.
106	* The generation changes whenever the barrier is tripped, or
107	* is reset. There can be many generations associated with threads
108	* using the barrier - due to the non-deterministic way the lock
109	* may be allocated to waiting threads - but only one of these
110	* can be active at a time (the one to which <tt>count</tt> applies)
111	* and all the rest are either broken or tripped.
112	* There need not be an active generation if there has been a break
113	* but no subsequent reset.
114	*/
115	private static class Generation {
116	boolean broken = false;
117	boolean tripped = false;
118	}
119
120	/** The lock for guarding barrier entry */
121	private final ReentrantLock lock = new ReentrantLock();
122	/** Condition to wait on until tripped */
123	private final Condition trip = lock.newCondition();
124	/** The number of parties */
125	private final int parties;
126	/* The command to run when tripped */
127	private final Runnable barrierCommand;
128	/** The current generation */
129	private Generation generation = new Generation();
130
131	/**
132	* Number of parties still waiting. Counts down from parties to 0
133	* on each generation. It is reset to parties on each new
134	* generation or when broken.
135	*/
136	private int count;
137
138	/**
139	* Updates state on barrier trip and wakes up everyone.
140	* Called only while holding lock.
141	*/
142	private void nextGeneration() {
143	// signal completion of last generation
144	generation.tripped = true;
145	trip.signalAll();
146	// set up next generation
147	count = parties;
148	generation = new Generation();
149	}
150
151	/**
152	* Sets current barrier generation as broken and wakes up everyone.
153	* Called only while holding lock.
154	*/
155	private void breakBarrier() {
156	generation.broken = true;
157	count = parties;
158	trip.signalAll();
159	}
160
161	/**
162	* Main barrier code, covering the various policies.
163	*/
164	private int dowait(boolean timed, long nanos)
165	throws InterruptedException, BrokenBarrierException,
166	TimeoutException {
167	final ReentrantLock lock = this.lock;
168	lock.lock();
169	try {
170	final Generation g = generation;
171
172	if (g.broken)
173	throw new BrokenBarrierException();
174
175	if (Thread.interrupted()) {
176	breakBarrier();
177	throw new InterruptedException();
178	}
179
180	int index = --count;
181	if (index == 0) { // tripped
182	boolean ranAction = false;
183	try {
184	final Runnable command = barrierCommand;
185	if (command != null)
186	command.run();
187	ranAction = true;
188	nextGeneration();
189	return 0;
190	} finally {
191	if (!ranAction)
192	breakBarrier();
193	}
194	}
195
196	// loop until tripped, broken, interrupted, or timed out
197	for (;;) {
198	try {
199	if (!timed)
200	trip.await();
201	else if (nanos > 0L)
202	nanos = trip.awaitNanos(nanos);
203	} catch (InterruptedException ie) {
204	breakBarrier();
205	throw ie;
206	}
207
208	if (g.broken)
209	throw new BrokenBarrierException();
210
211	if (g.tripped)
212	return index;
213
214	if (timed && nanos <= 0L) {
215	breakBarrier();
216	throw new TimeoutException();
217	}
218	}
219	} finally {
220	lock.unlock();
221	}
222	}
223
224	/**
225	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
226	* given number of parties (threads) are waiting upon it, and which
227	* will execute the given barrier action when the barrier is tripped,
228	* performed by the last thread entering the barrier.
229	*
230	* @param parties the number of threads that must invoke {@link #await}
231	* before the barrier is tripped.
232	* @param barrierAction the command to execute when the barrier is
233	* tripped, or <tt>null</tt> if there is no action.
234	*
235	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
236	*/
237	public CyclicBarrier(int parties, Runnable barrierAction) {
238	if (parties <= 0) throw new IllegalArgumentException();
239	this.parties = parties;
240	this.count = parties;
241	this.barrierCommand = barrierAction;
242	}
243
244	/**
245	* Creates a new <tt>CyclicBarrier</tt> that will trip when the
246	* given number of parties (threads) are waiting upon it, and
247	* does not perform a predefined action when the barrier is tripped.
248	*
249	* @param parties the number of threads that must invoke {@link #await}
250	* before the barrier is tripped.
251	*
252	* @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
253	*/
254	public CyclicBarrier(int parties) {
255	this(parties, null);
256	}
257
258	/**
259	* Returns the number of parties required to trip this barrier.
260	* @return the number of parties required to trip this barrier.
261	*/
262	public int getParties() {
263	return parties;
264	}
265
266	/**
267	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
268	* on this barrier.
269	*
270	* <p>If the current thread is not the last to arrive then it is
271	* disabled for thread scheduling purposes and lies dormant until
272	* one of following things happens:
273	* <ul>
274	* <li>The last thread arrives; or
275	* <li>Some other thread {@link Thread#interrupt interrupts} the current
276	* thread; or
277	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
278	* other waiting threads; or
279	* <li>Some other thread times out while waiting for barrier; or
280	* <li>Some other thread invokes {@link #reset} on this barrier.
281	* </ul>
282	* <p>If the current thread:
283	* <ul>
284	* <li>has its interrupted status set on entry to this method; or
285	* <li>is {@link Thread#interrupt interrupted} while waiting
286	* </ul>
287	* then {@link InterruptedException} is thrown and the current thread's
288	* interrupted status is cleared.
289	*
290	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
291	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
292	* or while any thread is waiting,
293	* then {@link BrokenBarrierException} is thrown.
294	*
295	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
296	* then all other waiting threads will throw
297	* {@link BrokenBarrierException} and the barrier is placed in the broken
298	* state.
299	*
300	* <p>If the current thread is the last thread to arrive, and a
301	* non-null barrier action was supplied in the constructor, then the
302	* current thread runs the action before allowing the other threads to
303	* continue.
304	* If an exception occurs during the barrier action then that exception
305	* will be propagated in the current thread and the barrier is placed in
306	* the broken state.
307	*
308	* @return the arrival index of the current thread, where index
309	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
310	* zero indicates the last to arrive.
311	*
312	* @throws InterruptedException if the current thread was interrupted
313	* while waiting.
314	* @throws BrokenBarrierException if <em>another</em> thread was
315	* interrupted or timed out while the current thread was waiting,
316	* or the barrier was reset, or the barrier was broken when
317	* <tt>await</tt> was called, or the barrier action (if present)
318	* failed due an exception.
319	*/
320	public int await() throws InterruptedException, BrokenBarrierException {
321	try {
322	return dowait(false, 0L);
323	} catch (TimeoutException toe) {
324	throw new Error(toe); // cannot happen;
325	}
326	}
327
328	/**
329	* Waits until all {@link #getParties parties} have invoked <tt>await</tt>
330	* on this barrier.
331	*
332	* <p>If the current thread is not the last to arrive then it is
333	* disabled for thread scheduling purposes and lies dormant until
334	* one of the following things happens:
335	* <ul>
336	* <li>The last thread arrives; or
337	* <li>The specified timeout elapses; or
338	* <li>Some other thread {@link Thread#interrupt interrupts} the current
339	* thread; or
340	* <li>Some other thread {@link Thread#interrupt interrupts} one of the
341	* other waiting threads; or
342	* <li>Some other thread times out while waiting for barrier; or
343	* <li>Some other thread invokes {@link #reset} on this barrier.
344	* </ul>
345	* <p>If the current thread:
346	* <ul>
347	* <li>has its interrupted status set on entry to this method; or
348	* <li>is {@link Thread#interrupt interrupted} while waiting
349	* </ul>
350	* then {@link InterruptedException} is thrown and the current thread's
351	* interrupted status is cleared.
352	*
353	* <p>If the specified waiting time elapses then {@link TimeoutException}
354	* is thrown. If the time is less than or equal to zero, the
355	* method will not wait at all.
356	*
357	* <p>If the barrier is {@link #reset} while any thread is waiting, or if
358	* the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
359	* or while any thread is waiting,
360	* then {@link BrokenBarrierException} is thrown.
361	*
362	* <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
363	* then all other waiting threads will throw
364	* {@link BrokenBarrierException} and the barrier is placed in the broken
365	* state.
366	*
367	* <p>If the current thread is the last thread to arrive, and a
368	* non-null barrier action was supplied in the constructor, then the
369	* current thread runs the action before allowing the other threads to
370	* continue.
371	* If an exception occurs during the barrier action then that exception
372	* will be propagated in the current thread and the barrier is placed in
373	* the broken state.
374	*
375	* @param timeout the time to wait for the barrier
376	* @param unit the time unit of the timeout parameter
377	* @return the arrival index of the current thread, where index
378	* <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
379	* zero indicates the last to arrive.
380	*
381	* @throws InterruptedException if the current thread was interrupted
382	* while waiting.
383	* @throws TimeoutException if the specified timeout elapses.
384	* @throws BrokenBarrierException if <em>another</em> thread was
385	* interrupted or timed out while the current thread was waiting,
386	* or the barrier was reset, or the barrier was broken when
387	* <tt>await</tt> was called, or the barrier action (if present)
388	* failed due an exception.
389	*/
390	public int await(long timeout, TimeUnit unit)
391	throws InterruptedException,
392	BrokenBarrierException,
393	TimeoutException {
394	return dowait(true, unit.toNanos(timeout));
395	}
396
397	/**
398	* Queries if this barrier is in a broken state.
399	* @return <tt>true</tt> if one or more parties broke out of this
400	* barrier due to interruption or timeout since construction or
401	* the last reset, or a barrier action failed due to an exception;
402	* <tt>false</tt> otherwise.
403	*/
404	public boolean isBroken() {
405	final ReentrantLock lock = this.lock;
406	lock.lock();
407	try {
408	return generation.broken;
409	} finally {
410	lock.unlock();
411	}
412	}
413
414	/**
415	* Resets the barrier to its initial state. If any parties are
416	* currently waiting at the barrier, they will return with a
417	* {@link BrokenBarrierException}. Note that resets <em>after</em>
418	* a breakage has occurred for other reasons can be complicated to
419	* carry out; threads need to re-synchronize in some other way,
420	* and choose one to perform the reset. It may be preferable to
421	* instead create a new barrier for subsequent use.
422	*/
423	public void reset() {
424	final ReentrantLock lock = this.lock;
425	lock.lock();
426	try {
427	breakBarrier(); // break the current generation
428	nextGeneration(); // start a new generation
429	} finally {
430	lock.unlock();
431	}
432	}
433
434	/**
435	* Returns the number of parties currently waiting at the barrier.
436	* This method is primarily useful for debugging and assertions.
437	*
438	* @return the number of parties currently blocked in {@link #await}.
439	*/
440	public int getNumberWaiting() {
441	final ReentrantLock lock = this.lock;
442	lock.lock();
443	try {
444	return parties - count;
445	} finally {
446	lock.unlock();
447	}
448	}
449	}