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.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.*;

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} of
 * <tt>Delayed</tt> elements, in which an element can only be taken
 * when its delay has expired.  The <em>head</em> of the queue is that
 * <tt>Delayed</tt> element whose delay expired furthest in the
 * past.  If no delay has expired there is no head and <tt>poll</tt>
 * will return <tt>null</tt>. Expiration occurs when an element's
 * <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
 * than or equal to zero.  Even though unexpired elements cannot be
 * removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
 * treated as normal elements. For example, the <tt>size</tt> 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 collection
 */

public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
    implements BlockingQueue<E> {

    private transient final 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 = null;

    /**
     * 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 <tt>DelayQueue</tt> that is initially empty.
     */
    public DelayQueue() {}

    /**
     * Creates a <tt>DelayQueue</tt> 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 <tt>true</tt> (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 <tt>true</tt>
     * @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 <tt>true</tt>
     * @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 <tt>null</tt>
     * if this queue has no elements with an expired delay.
     *
     * @return the head of this queue, or <tt>null</tt> 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();
            if (first == null || first.getDelay(NANOSECONDS) > 0)
                return null;
            else
                return 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 <= 0)
                        return q.poll();
                    else 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 <tt>null</tt> 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 <= 0)
                        return null;
                    else
                        nanos = available.awaitNanos(nanos);
                } else {
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0)
                        return q.poll();
                    if (nanos <= 0)
                        return null;
                    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 <tt>null</tt> if this queue is empty.  Unlike
     * <tt>poll</tt>, 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 <tt>null</tt> 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();
        }
    }

    /**
     * Return 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 <tt>Integer.MAX_VALUE</tt> because
     * a <tt>DelayQueue</tt> is not capacity constrained.
     *
     * @return <tt>Integer.MAX_VALUE</tt>
     */
    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
     * <tt>null</tt>.
     *
     * <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 <tt>Delayed</tt>:
     *
     * <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @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 "weakly consistent" iterator that
     * will never throw {@link java.util.ConcurrentModificationException
     * ConcurrentModificationException}, and guarantees to traverse
     * elements as they existed upon construction of the iterator, and
     * may (but is not guaranteed to) reflect any modifications
     * subsequent to construction.
     *
     * @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:	1.57
Committed:	Sun Jul 3 06:04:06 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.56:	+20 -15 lines
Log Message:	Introduce removeEQ as in PriorityBlockingQueue
#	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/publicdomain/zero/1.0/
5	*/
6
7
8	package java.util.concurrent;
9	import static java.util.concurrent.TimeUnit.NANOSECONDS;
10	import java.util.concurrent.locks.Condition;
11	import java.util.concurrent.locks.ReentrantLock;
12	import java.util.*;
13
14	/**
15	* An unbounded {@linkplain BlockingQueue blocking queue} of
16	* <tt>Delayed</tt> elements, in which an element can only be taken
17	* when its delay has expired. The <em>head</em> of the queue is that
18	* <tt>Delayed</tt> element whose delay expired furthest in the
19	* past. If no delay has expired there is no head and <tt>poll</tt>
20	* will return <tt>null</tt>. Expiration occurs when an element's
21	* <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
22	* than or equal to zero. Even though unexpired elements cannot be
23	* removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
24	* treated as normal elements. For example, the <tt>size</tt> method
25	* returns the count of both expired and unexpired elements.
26	* This queue does not permit null elements.
27	*
28	* <p>This class and its iterator implement all of the
29	* <em>optional</em> methods of the {@link Collection} and {@link
30	* Iterator} interfaces. The Iterator provided in method {@link
31	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
32	* the DelayQueue in any particular order.
33	*
34	* <p>This class is a member of the
35	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
36	* Java Collections Framework</a>.
37	*
38	* @since 1.5
39	* @author Doug Lea
40	* @param <E> the type of elements held in this collection
41	*/
42
43	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
44	implements BlockingQueue<E> {
45
46	private transient final ReentrantLock lock = new ReentrantLock();
47	private final PriorityQueue<E> q = new PriorityQueue<E>();
48
49	/**
50	* Thread designated to wait for the element at the head of
51	* the queue. This variant of the Leader-Follower pattern
52	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
53	* minimize unnecessary timed waiting. When a thread becomes
54	* the leader, it waits only for the next delay to elapse, but
55	* other threads await indefinitely. The leader thread must
56	* signal some other thread before returning from take() or
57	* poll(...), unless some other thread becomes leader in the
58	* interim. Whenever the head of the queue is replaced with
59	* an element with an earlier expiration time, the leader
60	* field is invalidated by being reset to null, and some
61	* waiting thread, but not necessarily the current leader, is
62	* signalled. So waiting threads must be prepared to acquire
63	* and lose leadership while waiting.
64	*/
65	private Thread leader = null;
66
67	/**
68	* Condition signalled when a newer element becomes available
69	* at the head of the queue or a new thread may need to
70	* become leader.
71	*/
72	private final Condition available = lock.newCondition();
73
74	/**
75	* Creates a new <tt>DelayQueue</tt> that is initially empty.
76	*/
77	public DelayQueue() {}
78
79	/**
80	* Creates a <tt>DelayQueue</tt> initially containing the elements of the
81	* given collection of {@link Delayed} instances.
82	*
83	* @param c the collection of elements to initially contain
84	* @throws NullPointerException if the specified collection or any
85	* of its elements are null
86	*/
87	public DelayQueue(Collection<? extends E> c) {
88	this.addAll(c);
89	}
90
91	/**
92	* Inserts the specified element into this delay queue.
93	*
94	* @param e the element to add
95	* @return <tt>true</tt> (as specified by {@link Collection#add})
96	* @throws NullPointerException if the specified element is null
97	*/
98	public boolean add(E e) {
99	return offer(e);
100	}
101
102	/**
103	* Inserts the specified element into this delay queue.
104	*
105	* @param e the element to add
106	* @return <tt>true</tt>
107	* @throws NullPointerException if the specified element is null
108	*/
109	public boolean offer(E e) {
110	final ReentrantLock lock = this.lock;
111	lock.lock();
112	try {
113	q.offer(e);
114	if (q.peek() == e) {
115	leader = null;
116	available.signal();
117	}
118	return true;
119	} finally {
120	lock.unlock();
121	}
122	}
123
124	/**
125	* Inserts the specified element into this delay queue. As the queue is
126	* unbounded this method will never block.
127	*
128	* @param e the element to add
129	* @throws NullPointerException {@inheritDoc}
130	*/
131	public void put(E e) {
132	offer(e);
133	}
134
135	/**
136	* Inserts the specified element into this delay queue. As the queue is
137	* unbounded this method will never block.
138	*
139	* @param e the element to add
140	* @param timeout This parameter is ignored as the method never blocks
141	* @param unit This parameter is ignored as the method never blocks
142	* @return <tt>true</tt>
143	* @throws NullPointerException {@inheritDoc}
144	*/
145	public boolean offer(E e, long timeout, TimeUnit unit) {
146	return offer(e);
147	}
148
149	/**
150	* Retrieves and removes the head of this queue, or returns <tt>null</tt>
151	* if this queue has no elements with an expired delay.
152	*
153	* @return the head of this queue, or <tt>null</tt> if this
154	* queue has no elements with an expired delay
155	*/
156	public E poll() {
157	final ReentrantLock lock = this.lock;
158	lock.lock();
159	try {
160	E first = q.peek();
161	if (first == null \|\| first.getDelay(NANOSECONDS) > 0)
162	return null;
163	else
164	return q.poll();
165	} finally {
166	lock.unlock();
167	}
168	}
169
170	/**
171	* Retrieves and removes the head of this queue, waiting if necessary
172	* until an element with an expired delay is available on this queue.
173	*
174	* @return the head of this queue
175	* @throws InterruptedException {@inheritDoc}
176	*/
177	public E take() throws InterruptedException {
178	final ReentrantLock lock = this.lock;
179	lock.lockInterruptibly();
180	try {
181	for (;;) {
182	E first = q.peek();
183	if (first == null)
184	available.await();
185	else {
186	long delay = first.getDelay(NANOSECONDS);
187	if (delay <= 0)
188	return q.poll();
189	else if (leader != null)
190	available.await();
191	else {
192	Thread thisThread = Thread.currentThread();
193	leader = thisThread;
194	try {
195	available.awaitNanos(delay);
196	} finally {
197	if (leader == thisThread)
198	leader = null;
199	}
200	}
201	}
202	}
203	} finally {
204	if (leader == null && q.peek() != null)
205	available.signal();
206	lock.unlock();
207	}
208	}
209
210	/**
211	* Retrieves and removes the head of this queue, waiting if necessary
212	* until an element with an expired delay is available on this queue,
213	* or the specified wait time expires.
214	*
215	* @return the head of this queue, or <tt>null</tt> if the
216	* specified waiting time elapses before an element with
217	* an expired delay becomes available
218	* @throws InterruptedException {@inheritDoc}
219	*/
220	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
221	long nanos = unit.toNanos(timeout);
222	final ReentrantLock lock = this.lock;
223	lock.lockInterruptibly();
224	try {
225	for (;;) {
226	E first = q.peek();
227	if (first == null) {
228	if (nanos <= 0)
229	return null;
230	else
231	nanos = available.awaitNanos(nanos);
232	} else {
233	long delay = first.getDelay(NANOSECONDS);
234	if (delay <= 0)
235	return q.poll();
236	if (nanos <= 0)
237	return null;
238	if (nanos < delay \|\| leader != null)
239	nanos = available.awaitNanos(nanos);
240	else {
241	Thread thisThread = Thread.currentThread();
242	leader = thisThread;
243	try {
244	long timeLeft = available.awaitNanos(delay);
245	nanos -= delay - timeLeft;
246	} finally {
247	if (leader == thisThread)
248	leader = null;
249	}
250	}
251	}
252	}
253	} finally {
254	if (leader == null && q.peek() != null)
255	available.signal();
256	lock.unlock();
257	}
258	}
259
260	/**
261	* Retrieves, but does not remove, the head of this queue, or
262	* returns <tt>null</tt> if this queue is empty. Unlike
263	* <tt>poll</tt>, if no expired elements are available in the queue,
264	* this method returns the element that will expire next,
265	* if one exists.
266	*
267	* @return the head of this queue, or <tt>null</tt> if this
268	* queue is empty.
269	*/
270	public E peek() {
271	final ReentrantLock lock = this.lock;
272	lock.lock();
273	try {
274	return q.peek();
275	} finally {
276	lock.unlock();
277	}
278	}
279
280	public int size() {
281	final ReentrantLock lock = this.lock;
282	lock.lock();
283	try {
284	return q.size();
285	} finally {
286	lock.unlock();
287	}
288	}
289
290	/**
291	* Return first element only if it is expired.
292	* Used only by drainTo. Call only when holding lock.
293	*/
294	private E peekExpired() {
295	// assert lock.isHeldByCurrentThread();
296	E first = q.peek();
297	return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
298	null : first;
299	}
300
301	/**
302	* @throws UnsupportedOperationException {@inheritDoc}
303	* @throws ClassCastException {@inheritDoc}
304	* @throws NullPointerException {@inheritDoc}
305	* @throws IllegalArgumentException {@inheritDoc}
306	*/
307	public int drainTo(Collection<? super E> c) {
308	if (c == null)
309	throw new NullPointerException();
310	if (c == this)
311	throw new IllegalArgumentException();
312	final ReentrantLock lock = this.lock;
313	lock.lock();
314	try {
315	int n = 0;
316	for (E e; (e = peekExpired()) != null;) {
317	c.add(e); // In this order, in case add() throws.
318	q.poll();
319	++n;
320	}
321	return n;
322	} finally {
323	lock.unlock();
324	}
325	}
326
327	/**
328	* @throws UnsupportedOperationException {@inheritDoc}
329	* @throws ClassCastException {@inheritDoc}
330	* @throws NullPointerException {@inheritDoc}
331	* @throws IllegalArgumentException {@inheritDoc}
332	*/
333	public int drainTo(Collection<? super E> c, int maxElements) {
334	if (c == null)
335	throw new NullPointerException();
336	if (c == this)
337	throw new IllegalArgumentException();
338	if (maxElements <= 0)
339	return 0;
340	final ReentrantLock lock = this.lock;
341	lock.lock();
342	try {
343	int n = 0;
344	for (E e; n < maxElements && (e = peekExpired()) != null;) {
345	c.add(e); // In this order, in case add() throws.
346	q.poll();
347	++n;
348	}
349	return n;
350	} finally {
351	lock.unlock();
352	}
353	}
354
355	/**
356	* Atomically removes all of the elements from this delay queue.
357	* The queue will be empty after this call returns.
358	* Elements with an unexpired delay are not waited for; they are
359	* simply discarded from the queue.
360	*/
361	public void clear() {
362	final ReentrantLock lock = this.lock;
363	lock.lock();
364	try {
365	q.clear();
366	} finally {
367	lock.unlock();
368	}
369	}
370
371	/**
372	* Always returns <tt>Integer.MAX_VALUE</tt> because
373	* a <tt>DelayQueue</tt> is not capacity constrained.
374	*
375	* @return <tt>Integer.MAX_VALUE</tt>
376	*/
377	public int remainingCapacity() {
378	return Integer.MAX_VALUE;
379	}
380
381	/**
382	* Returns an array containing all of the elements in this queue.
383	* The returned array elements are in no particular order.
384	*
385	* <p>The returned array will be "safe" in that no references to it are
386	* maintained by this queue. (In other words, this method must allocate
387	* a new array). The caller is thus free to modify the returned array.
388	*
389	* <p>This method acts as bridge between array-based and collection-based
390	* APIs.
391	*
392	* @return an array containing all of the elements in this queue
393	*/
394	public Object[] toArray() {
395	final ReentrantLock lock = this.lock;
396	lock.lock();
397	try {
398	return q.toArray();
399	} finally {
400	lock.unlock();
401	}
402	}
403
404	/**
405	* Returns an array containing all of the elements in this queue; the
406	* runtime type of the returned array is that of the specified array.
407	* The returned array elements are in no particular order.
408	* If the queue fits in the specified array, it is returned therein.
409	* Otherwise, a new array is allocated with the runtime type of the
410	* specified array and the size of this queue.
411	*
412	* <p>If this queue fits in the specified array with room to spare
413	* (i.e., the array has more elements than this queue), the element in
414	* the array immediately following the end of the queue is set to
415	* <tt>null</tt>.
416	*
417	* <p>Like the {@link #toArray()} method, this method acts as bridge between
418	* array-based and collection-based APIs. Further, this method allows
419	* precise control over the runtime type of the output array, and may,
420	* under certain circumstances, be used to save allocation costs.
421	*
422	* <p>The following code can be used to dump a delay queue into a newly
423	* allocated array of <tt>Delayed</tt>:
424	*
425	* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
426	*
427	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
428	* <tt>toArray()</tt>.
429	*
430	* @param a the array into which the elements of the queue are to
431	* be stored, if it is big enough; otherwise, a new array of the
432	* same runtime type is allocated for this purpose
433	* @return an array containing all of the elements in this queue
434	* @throws ArrayStoreException if the runtime type of the specified array
435	* is not a supertype of the runtime type of every element in
436	* this queue
437	* @throws NullPointerException if the specified array is null
438	*/
439	public <T> T[] toArray(T[] a) {
440	final ReentrantLock lock = this.lock;
441	lock.lock();
442	try {
443	return q.toArray(a);
444	} finally {
445	lock.unlock();
446	}
447	}
448
449	/**
450	* Removes a single instance of the specified element from this
451	* queue, if it is present, whether or not it has expired.
452	*/
453	public boolean remove(Object o) {
454	final ReentrantLock lock = this.lock;
455	lock.lock();
456	try {
457	return q.remove(o);
458	} finally {
459	lock.unlock();
460	}
461	}
462
463	/**
464	* Identity-based version for use in Itr.remove
465	*/
466	void removeEQ(Object o) {
467	final ReentrantLock lock = this.lock;
468	lock.lock();
469	try {
470	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
471	if (o == it.next()) {
472	it.remove();
473	break;
474	}
475	}
476	} finally {
477	lock.unlock();
478	}
479	}
480
481	/**
482	* Returns an iterator over all the elements (both expired and
483	* unexpired) in this queue. The iterator does not return the
484	* elements in any particular order.
485	*
486	* <p>The returned iterator is a "weakly consistent" iterator that
487	* will never throw {@link java.util.ConcurrentModificationException
488	* ConcurrentModificationException}, and guarantees to traverse
489	* elements as they existed upon construction of the iterator, and
490	* may (but is not guaranteed to) reflect any modifications
491	* subsequent to construction.
492	*
493	* @return an iterator over the elements in this queue
494	*/
495	public Iterator<E> iterator() {
496	return new Itr(toArray());
497	}
498
499	/**
500	* Snapshot iterator that works off copy of underlying q array.
501	*/
502	private class Itr implements Iterator<E> {
503	final Object[] array; // Array of all elements
504	int cursor; // index of next element to return
505	int lastRet; // index of last element, or -1 if no such
506
507	Itr(Object[] array) {
508	lastRet = -1;
509	this.array = array;
510	}
511
512	public boolean hasNext() {
513	return cursor < array.length;
514	}
515
516	@SuppressWarnings("unchecked")
517	public E next() {
518	if (cursor >= array.length)
519	throw new NoSuchElementException();
520	lastRet = cursor;
521	return (E)array[cursor++];
522	}
523
524	public void remove() {
525	if (lastRet < 0)
526	throw new IllegalStateException();
527	removeEQ(array[lastRet]);
528	lastRet = -1;
529	}
530	}
531
532	}