util/concurrent/DelayQueue.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 static java.util.concurrent.TimeUnit.NANOSECONDS;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} of
 * {@code Delayed} elements, in which an element can only be taken
 * when its delay has expired.  The <em>head</em> of the queue is that
 * {@code Delayed} element whose delay expired furthest in the
 * past.  If no delay has expired there is no head and {@code poll}
 * will return {@code null}. Expiration occurs when an element's
 * {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
 * than or equal to zero.  Even though unexpired elements cannot be
 * removed using {@code take} or {@code poll}, they are otherwise
 * treated as normal elements. For example, the {@code size} method
 * returns the count of both expired and unexpired elements.
 * This queue does not permit null elements.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.  The Iterator provided in method {@link
 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
 * the DelayQueue in any particular order.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this queue
 */
public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
    implements BlockingQueue<E> {

    private final transient ReentrantLock lock = new ReentrantLock();
    private final PriorityQueue<E> q = new PriorityQueue<E>();

    /**
     * Thread designated to wait for the element at the head of
     * the queue.  This variant of the Leader-Follower pattern
     * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
     * minimize unnecessary timed waiting.  When a thread becomes
     * the leader, it waits only for the next delay to elapse, but
     * other threads await indefinitely.  The leader thread must
     * signal some other thread before returning from take() or
     * poll(...), unless some other thread becomes leader in the
     * interim.  Whenever the head of the queue is replaced with
     * an element with an earlier expiration time, the leader
     * field is invalidated by being reset to null, and some
     * waiting thread, but not necessarily the current leader, is
     * signalled.  So waiting threads must be prepared to acquire
     * and lose leadership while waiting.
     */
    private Thread leader;

    /**
     * Condition signalled when a newer element becomes available
     * at the head of the queue or a new thread may need to
     * become leader.
     */
    private final Condition available = lock.newCondition();

    /**
     * Creates a new {@code DelayQueue} that is initially empty.
     */
    public DelayQueue() {}

    /**
     * Creates a {@code DelayQueue} initially containing the elements of the
     * given collection of {@link Delayed} instances.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public DelayQueue(Collection<? extends E> c) {
        this.addAll(c);
    }

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return {@code true}
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.offer(e);
            if (q.peek() == e) {
                leader = null;
                available.signal();
            }
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Inserts the specified element into this delay queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) {
        offer(e);
    }

    /**
     * Inserts the specified element into this delay queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @param timeout This parameter is ignored as the method never blocks
     * @param unit This parameter is ignored as the method never blocks
     * @return {@code true}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit) {
        return offer(e);
    }

    /**
     * Retrieves and removes the head of this queue, or returns {@code null}
     * if this queue has no elements with an expired delay.
     *
     * @return the head of this queue, or {@code null} if this
     *         queue has no elements with an expired delay
     */
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E first = q.peek();
            return (first == null || first.getDelay(NANOSECONDS) > 0)
                ? null
                : q.poll();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * until an element with an expired delay is available on this queue.
     *
     * @return the head of this queue
     * @throws InterruptedException {@inheritDoc}
     */
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E first = q.peek();
                if (first == null)
                    available.await();
                else {
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0L)
                        return q.poll();
                    first = null; // don't retain ref while waiting
                    if (leader != null)
                        available.await();
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            available.awaitNanos(delay);
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && q.peek() != null)
                available.signal();
            lock.unlock();
        }
    }

    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * until an element with an expired delay is available on this queue,
     * or the specified wait time expires.
     *
     * @return the head of this queue, or {@code null} if the
     *         specified waiting time elapses before an element with
     *         an expired delay becomes available
     * @throws InterruptedException {@inheritDoc}
     */
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E first = q.peek();
                if (first == null) {
                    if (nanos <= 0L)
                        return null;
                    else
                        nanos = available.awaitNanos(nanos);
                } else {
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0L)
                        return q.poll();
                    if (nanos <= 0L)
                        return null;
                    first = null; // don't retain ref while waiting
                    if (nanos < delay || leader != null)
                        nanos = available.awaitNanos(nanos);
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            long timeLeft = available.awaitNanos(delay);
                            nanos -= delay - timeLeft;
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && q.peek() != null)
                available.signal();
            lock.unlock();
        }
    }

    /**
     * Retrieves, but does not remove, the head of this queue, or
     * returns {@code null} if this queue is empty.  Unlike
     * {@code poll}, if no expired elements are available in the queue,
     * this method returns the element that will expire next,
     * if one exists.
     *
     * @return the head of this queue, or {@code null} if this
     *         queue is empty
     */
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.peek();
        } finally {
            lock.unlock();
        }
    }

    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.size();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns first element only if it is expired.
     * Used only by drainTo.  Call only when holding lock.
     */
    private E peekExpired() {
        // assert lock.isHeldByCurrentThread();
        E first = q.peek();
        return (first == null || first.getDelay(NANOSECONDS) > 0) ?
            null : first;
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = 0;
            for (E e; (e = peekExpired()) != null;) {
                c.add(e);       // In this order, in case add() throws.
                q.poll();
                ++n;
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = 0;
            for (E e; n < maxElements && (e = peekExpired()) != null;) {
                c.add(e);       // In this order, in case add() throws.
                q.poll();
                ++n;
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Atomically removes all of the elements from this delay queue.
     * The queue will be empty after this call returns.
     * Elements with an unexpired delay are not waited for; they are
     * simply discarded from the queue.
     */
    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.clear();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Always returns {@code Integer.MAX_VALUE} because
     * a {@code DelayQueue} is not capacity constrained.
     *
     * @return {@code Integer.MAX_VALUE}
     */
    public int remainingCapacity() {
        return Integer.MAX_VALUE;
    }

    /**
     * Returns an array containing all of the elements in this queue.
     * The returned array elements are in no particular order.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this queue.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this queue
     */
    public Object[] toArray() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.toArray();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns an array containing all of the elements in this queue; the
     * runtime type of the returned array is that of the specified array.
     * The returned array elements are in no particular order.
     * If the queue fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this queue.
     *
     * <p>If this queue fits in the specified array with room to spare
     * (i.e., the array has more elements than this queue), the element in
     * the array immediately following the end of the queue is set to
     * {@code null}.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>The following code can be used to dump a delay queue into a newly
     * allocated array of {@code Delayed}:
     *
     * <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of the queue are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose
     * @return an array containing all of the elements in this queue
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this queue
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.toArray(a);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Removes a single instance of the specified element from this
     * queue, if it is present, whether or not it has expired.
     */
    public boolean remove(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.remove(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Identity-based version for use in Itr.remove.
     */
    void removeEQ(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
                if (o == it.next()) {
                    it.remove();
                    break;
                }
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns an iterator over all the elements (both expired and
     * unexpired) in this queue. The iterator does not return the
     * elements in any particular order.
     *
     * <p>The returned iterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * @return an iterator over the elements in this queue
     */
    public Iterator<E> iterator() {
        return new Itr(toArray());
    }

    /**
     * Snapshot iterator that works off copy of underlying q array.
     */
    private class Itr implements Iterator<E> {
        final Object[] array; // Array of all elements
        int cursor;           // index of next element to return
        int lastRet;          // index of last element, or -1 if no such

        Itr(Object[] array) {
            lastRet = -1;
            this.array = array;
        }

        public boolean hasNext() {
            return cursor < array.length;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (cursor >= array.length)
                throw new NoSuchElementException();
            lastRet = cursor;
            return (E)array[cursor++];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            removeEQ(array[lastRet]);
            lastRet = -1;
        }
    }

}
Revision:	1.1
Committed:	Sat Mar 26 06:22:50 2016 UTC (8 years, 1 month ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	User	Rev	Content
1	jsr166	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent;
8
9			import static java.util.concurrent.TimeUnit.NANOSECONDS;
10
11			import java.util.AbstractQueue;
12			import java.util.Collection;
13			import java.util.Iterator;
14			import java.util.NoSuchElementException;
15			import java.util.PriorityQueue;
16			import java.util.concurrent.locks.Condition;
17			import java.util.concurrent.locks.ReentrantLock;
18
19			/**
20			* An unbounded {@linkplain BlockingQueue blocking queue} of
21			* {@code Delayed} elements, in which an element can only be taken
22			* when its delay has expired. The <em>head</em> of the queue is that
23			* {@code Delayed} element whose delay expired furthest in the
24			* past. If no delay has expired there is no head and {@code poll}
25			* will return {@code null}. Expiration occurs when an element's
26			* {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
27			* than or equal to zero. Even though unexpired elements cannot be
28			* removed using {@code take} or {@code poll}, they are otherwise
29			* treated as normal elements. For example, the {@code size} method
30			* returns the count of both expired and unexpired elements.
31			* This queue does not permit null elements.
32			*
33			* <p>This class and its iterator implement all of the
34			* <em>optional</em> methods of the {@link Collection} and {@link
35			* Iterator} interfaces. The Iterator provided in method {@link
36			* #iterator()} is <em>not</em> guaranteed to traverse the elements of
37			* the DelayQueue in any particular order.
38			*
39			* <p>This class is a member of the
40			* <a href="{@docRoot}/../technotes/guides/collections/index.html">
41			* Java Collections Framework</a>.
42			*
43			* @since 1.5
44			* @author Doug Lea
45			* @param <E> the type of elements held in this queue
46			*/
47			public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
48			implements BlockingQueue<E> {
49
50			private final transient ReentrantLock lock = new ReentrantLock();
51			private final PriorityQueue<E> q = new PriorityQueue<E>();
52
53			/**
54			* Thread designated to wait for the element at the head of
55			* the queue. This variant of the Leader-Follower pattern
56			* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
57			* minimize unnecessary timed waiting. When a thread becomes
58			* the leader, it waits only for the next delay to elapse, but
59			* other threads await indefinitely. The leader thread must
60			* signal some other thread before returning from take() or
61			* poll(...), unless some other thread becomes leader in the
62			* interim. Whenever the head of the queue is replaced with
63			* an element with an earlier expiration time, the leader
64			* field is invalidated by being reset to null, and some
65			* waiting thread, but not necessarily the current leader, is
66			* signalled. So waiting threads must be prepared to acquire
67			* and lose leadership while waiting.
68			*/
69			private Thread leader;
70
71			/**
72			* Condition signalled when a newer element becomes available
73			* at the head of the queue or a new thread may need to
74			* become leader.
75			*/
76			private final Condition available = lock.newCondition();
77
78			/**
79			* Creates a new {@code DelayQueue} that is initially empty.
80			*/
81			public DelayQueue() {}
82
83			/**
84			* Creates a {@code DelayQueue} initially containing the elements of the
85			* given collection of {@link Delayed} instances.
86			*
87			* @param c the collection of elements to initially contain
88			* @throws NullPointerException if the specified collection or any
89			* of its elements are null
90			*/
91			public DelayQueue(Collection<? extends E> c) {
92			this.addAll(c);
93			}
94
95			/**
96			* Inserts the specified element into this delay queue.
97			*
98			* @param e the element to add
99			* @return {@code true} (as specified by {@link Collection#add})
100			* @throws NullPointerException if the specified element is null
101			*/
102			public boolean add(E e) {
103			return offer(e);
104			}
105
106			/**
107			* Inserts the specified element into this delay queue.
108			*
109			* @param e the element to add
110			* @return {@code true}
111			* @throws NullPointerException if the specified element is null
112			*/
113			public boolean offer(E e) {
114			final ReentrantLock lock = this.lock;
115			lock.lock();
116			try {
117			q.offer(e);
118			if (q.peek() == e) {
119			leader = null;
120			available.signal();
121			}
122			return true;
123			} finally {
124			lock.unlock();
125			}
126			}
127
128			/**
129			* Inserts the specified element into this delay queue. As the queue is
130			* unbounded this method will never block.
131			*
132			* @param e the element to add
133			* @throws NullPointerException {@inheritDoc}
134			*/
135			public void put(E e) {
136			offer(e);
137			}
138
139			/**
140			* Inserts the specified element into this delay queue. As the queue is
141			* unbounded this method will never block.
142			*
143			* @param e the element to add
144			* @param timeout This parameter is ignored as the method never blocks
145			* @param unit This parameter is ignored as the method never blocks
146			* @return {@code true}
147			* @throws NullPointerException {@inheritDoc}
148			*/
149			public boolean offer(E e, long timeout, TimeUnit unit) {
150			return offer(e);
151			}
152
153			/**
154			* Retrieves and removes the head of this queue, or returns {@code null}
155			* if this queue has no elements with an expired delay.
156			*
157			* @return the head of this queue, or {@code null} if this
158			* queue has no elements with an expired delay
159			*/
160			public E poll() {
161			final ReentrantLock lock = this.lock;
162			lock.lock();
163			try {
164			E first = q.peek();
165			return (first == null \|\| first.getDelay(NANOSECONDS) > 0)
166			? null
167			: q.poll();
168			} finally {
169			lock.unlock();
170			}
171			}
172
173			/**
174			* Retrieves and removes the head of this queue, waiting if necessary
175			* until an element with an expired delay is available on this queue.
176			*
177			* @return the head of this queue
178			* @throws InterruptedException {@inheritDoc}
179			*/
180			public E take() throws InterruptedException {
181			final ReentrantLock lock = this.lock;
182			lock.lockInterruptibly();
183			try {
184			for (;;) {
185			E first = q.peek();
186			if (first == null)
187			available.await();
188			else {
189			long delay = first.getDelay(NANOSECONDS);
190			if (delay <= 0L)
191			return q.poll();
192			first = null; // don't retain ref while waiting
193			if (leader != null)
194			available.await();
195			else {
196			Thread thisThread = Thread.currentThread();
197			leader = thisThread;
198			try {
199			available.awaitNanos(delay);
200			} finally {
201			if (leader == thisThread)
202			leader = null;
203			}
204			}
205			}
206			}
207			} finally {
208			if (leader == null && q.peek() != null)
209			available.signal();
210			lock.unlock();
211			}
212			}
213
214			/**
215			* Retrieves and removes the head of this queue, waiting if necessary
216			* until an element with an expired delay is available on this queue,
217			* or the specified wait time expires.
218			*
219			* @return the head of this queue, or {@code null} if the
220			* specified waiting time elapses before an element with
221			* an expired delay becomes available
222			* @throws InterruptedException {@inheritDoc}
223			*/
224			public E poll(long timeout, TimeUnit unit) throws InterruptedException {
225			long nanos = unit.toNanos(timeout);
226			final ReentrantLock lock = this.lock;
227			lock.lockInterruptibly();
228			try {
229			for (;;) {
230			E first = q.peek();
231			if (first == null) {
232			if (nanos <= 0L)
233			return null;
234			else
235			nanos = available.awaitNanos(nanos);
236			} else {
237			long delay = first.getDelay(NANOSECONDS);
238			if (delay <= 0L)
239			return q.poll();
240			if (nanos <= 0L)
241			return null;
242			first = null; // don't retain ref while waiting
243			if (nanos < delay \|\| leader != null)
244			nanos = available.awaitNanos(nanos);
245			else {
246			Thread thisThread = Thread.currentThread();
247			leader = thisThread;
248			try {
249			long timeLeft = available.awaitNanos(delay);
250			nanos -= delay - timeLeft;
251			} finally {
252			if (leader == thisThread)
253			leader = null;
254			}
255			}
256			}
257			}
258			} finally {
259			if (leader == null && q.peek() != null)
260			available.signal();
261			lock.unlock();
262			}
263			}
264
265			/**
266			* Retrieves, but does not remove, the head of this queue, or
267			* returns {@code null} if this queue is empty. Unlike
268			* {@code poll}, if no expired elements are available in the queue,
269			* this method returns the element that will expire next,
270			* if one exists.
271			*
272			* @return the head of this queue, or {@code null} if this
273			* queue is empty
274			*/
275			public E peek() {
276			final ReentrantLock lock = this.lock;
277			lock.lock();
278			try {
279			return q.peek();
280			} finally {
281			lock.unlock();
282			}
283			}
284
285			public int size() {
286			final ReentrantLock lock = this.lock;
287			lock.lock();
288			try {
289			return q.size();
290			} finally {
291			lock.unlock();
292			}
293			}
294
295			/**
296			* Returns first element only if it is expired.
297			* Used only by drainTo. Call only when holding lock.
298			*/
299			private E peekExpired() {
300			// assert lock.isHeldByCurrentThread();
301			E first = q.peek();
302			return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
303			null : first;
304			}
305
306			/**
307			* @throws UnsupportedOperationException {@inheritDoc}
308			* @throws ClassCastException {@inheritDoc}
309			* @throws NullPointerException {@inheritDoc}
310			* @throws IllegalArgumentException {@inheritDoc}
311			*/
312			public int drainTo(Collection<? super E> c) {
313			if (c == null)
314			throw new NullPointerException();
315			if (c == this)
316			throw new IllegalArgumentException();
317			final ReentrantLock lock = this.lock;
318			lock.lock();
319			try {
320			int n = 0;
321			for (E e; (e = peekExpired()) != null;) {
322			c.add(e); // In this order, in case add() throws.
323			q.poll();
324			++n;
325			}
326			return n;
327			} finally {
328			lock.unlock();
329			}
330			}
331
332			/**
333			* @throws UnsupportedOperationException {@inheritDoc}
334			* @throws ClassCastException {@inheritDoc}
335			* @throws NullPointerException {@inheritDoc}
336			* @throws IllegalArgumentException {@inheritDoc}
337			*/
338			public int drainTo(Collection<? super E> c, int maxElements) {
339			if (c == null)
340			throw new NullPointerException();
341			if (c == this)
342			throw new IllegalArgumentException();
343			if (maxElements <= 0)
344			return 0;
345			final ReentrantLock lock = this.lock;
346			lock.lock();
347			try {
348			int n = 0;
349			for (E e; n < maxElements && (e = peekExpired()) != null;) {
350			c.add(e); // In this order, in case add() throws.
351			q.poll();
352			++n;
353			}
354			return n;
355			} finally {
356			lock.unlock();
357			}
358			}
359
360			/**
361			* Atomically removes all of the elements from this delay queue.
362			* The queue will be empty after this call returns.
363			* Elements with an unexpired delay are not waited for; they are
364			* simply discarded from the queue.
365			*/
366			public void clear() {
367			final ReentrantLock lock = this.lock;
368			lock.lock();
369			try {
370			q.clear();
371			} finally {
372			lock.unlock();
373			}
374			}
375
376			/**
377			* Always returns {@code Integer.MAX_VALUE} because
378			* a {@code DelayQueue} is not capacity constrained.
379			*
380			* @return {@code Integer.MAX_VALUE}
381			*/
382			public int remainingCapacity() {
383			return Integer.MAX_VALUE;
384			}
385
386			/**
387			* Returns an array containing all of the elements in this queue.
388			* The returned array elements are in no particular order.
389			*
390			* <p>The returned array will be "safe" in that no references to it are
391			* maintained by this queue. (In other words, this method must allocate
392			* a new array). The caller is thus free to modify the returned array.
393			*
394			* <p>This method acts as bridge between array-based and collection-based
395			* APIs.
396			*
397			* @return an array containing all of the elements in this queue
398			*/
399			public Object[] toArray() {
400			final ReentrantLock lock = this.lock;
401			lock.lock();
402			try {
403			return q.toArray();
404			} finally {
405			lock.unlock();
406			}
407			}
408
409			/**
410			* Returns an array containing all of the elements in this queue; the
411			* runtime type of the returned array is that of the specified array.
412			* The returned array elements are in no particular order.
413			* If the queue fits in the specified array, it is returned therein.
414			* Otherwise, a new array is allocated with the runtime type of the
415			* specified array and the size of this queue.
416			*
417			* <p>If this queue fits in the specified array with room to spare
418			* (i.e., the array has more elements than this queue), the element in
419			* the array immediately following the end of the queue is set to
420			* {@code null}.
421			*
422			* <p>Like the {@link #toArray()} method, this method acts as bridge between
423			* array-based and collection-based APIs. Further, this method allows
424			* precise control over the runtime type of the output array, and may,
425			* under certain circumstances, be used to save allocation costs.
426			*
427			* <p>The following code can be used to dump a delay queue into a newly
428			* allocated array of {@code Delayed}:
429			*
430			* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
431			*
432			* Note that {@code toArray(new Object[0])} is identical in function to
433			* {@code toArray()}.
434			*
435			* @param a the array into which the elements of the queue are to
436			* be stored, if it is big enough; otherwise, a new array of the
437			* same runtime type is allocated for this purpose
438			* @return an array containing all of the elements in this queue
439			* @throws ArrayStoreException if the runtime type of the specified array
440			* is not a supertype of the runtime type of every element in
441			* this queue
442			* @throws NullPointerException if the specified array is null
443			*/
444			public <T> T[] toArray(T[] a) {
445			final ReentrantLock lock = this.lock;
446			lock.lock();
447			try {
448			return q.toArray(a);
449			} finally {
450			lock.unlock();
451			}
452			}
453
454			/**
455			* Removes a single instance of the specified element from this
456			* queue, if it is present, whether or not it has expired.
457			*/
458			public boolean remove(Object o) {
459			final ReentrantLock lock = this.lock;
460			lock.lock();
461			try {
462			return q.remove(o);
463			} finally {
464			lock.unlock();
465			}
466			}
467
468			/**
469			* Identity-based version for use in Itr.remove.
470			*/
471			void removeEQ(Object o) {
472			final ReentrantLock lock = this.lock;
473			lock.lock();
474			try {
475			for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
476			if (o == it.next()) {
477			it.remove();
478			break;
479			}
480			}
481			} finally {
482			lock.unlock();
483			}
484			}
485
486			/**
487			* Returns an iterator over all the elements (both expired and
488			* unexpired) in this queue. The iterator does not return the
489			* elements in any particular order.
490			*
491			* <p>The returned iterator is
492			* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
493			*
494			* @return an iterator over the elements in this queue
495			*/
496			public Iterator<E> iterator() {
497			return new Itr(toArray());
498			}
499
500			/**
501			* Snapshot iterator that works off copy of underlying q array.
502			*/
503			private class Itr implements Iterator<E> {
504			final Object[] array; // Array of all elements
505			int cursor; // index of next element to return
506			int lastRet; // index of last element, or -1 if no such
507
508			Itr(Object[] array) {
509			lastRet = -1;
510			this.array = array;
511			}
512
513			public boolean hasNext() {
514			return cursor < array.length;
515			}
516
517			@SuppressWarnings("unchecked")
518			public E next() {
519			if (cursor >= array.length)
520			throw new NoSuchElementException();
521			lastRet = cursor;
522			return (E)array[cursor++];
523			}
524
525			public void remove() {
526			if (lastRet < 0)
527			throw new IllegalStateException();
528			removeEQ(array[lastRet]);
529			lastRet = -1;
530			}
531			}
532
533			}