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:	1.58
Committed:	Thu Nov 24 02:35:13 2011 UTC (12 years, 6 months ago) by jsr166
Branch:	MAIN
Changes since 1.57:	+0 -1 lines
Log Message:	whitespace
#	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	package java.util.concurrent;
8	import static java.util.concurrent.TimeUnit.NANOSECONDS;
9	import java.util.concurrent.locks.Condition;
10	import java.util.concurrent.locks.ReentrantLock;
11	import java.util.*;
12
13	/**
14	* An unbounded {@linkplain BlockingQueue blocking queue} of
15	* <tt>Delayed</tt> elements, in which an element can only be taken
16	* when its delay has expired. The <em>head</em> of the queue is that
17	* <tt>Delayed</tt> element whose delay expired furthest in the
18	* past. If no delay has expired there is no head and <tt>poll</tt>
19	* will return <tt>null</tt>. Expiration occurs when an element's
20	* <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
21	* than or equal to zero. Even though unexpired elements cannot be
22	* removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
23	* treated as normal elements. For example, the <tt>size</tt> method
24	* returns the count of both expired and unexpired elements.
25	* This queue does not permit null elements.
26	*
27	* <p>This class and its iterator implement all of the
28	* <em>optional</em> methods of the {@link Collection} and {@link
29	* Iterator} interfaces. The Iterator provided in method {@link
30	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
31	* the DelayQueue in any particular order.
32	*
33	* <p>This class is a member of the
34	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
35	* Java Collections Framework</a>.
36	*
37	* @since 1.5
38	* @author Doug Lea
39	* @param <E> the type of elements held in this collection
40	*/
41
42	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
43	implements BlockingQueue<E> {
44
45	private transient final ReentrantLock lock = new ReentrantLock();
46	private final PriorityQueue<E> q = new PriorityQueue<E>();
47
48	/**
49	* Thread designated to wait for the element at the head of
50	* the queue. This variant of the Leader-Follower pattern
51	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
52	* minimize unnecessary timed waiting. When a thread becomes
53	* the leader, it waits only for the next delay to elapse, but
54	* other threads await indefinitely. The leader thread must
55	* signal some other thread before returning from take() or
56	* poll(...), unless some other thread becomes leader in the
57	* interim. Whenever the head of the queue is replaced with
58	* an element with an earlier expiration time, the leader
59	* field is invalidated by being reset to null, and some
60	* waiting thread, but not necessarily the current leader, is
61	* signalled. So waiting threads must be prepared to acquire
62	* and lose leadership while waiting.
63	*/
64	private Thread leader = null;
65
66	/**
67	* Condition signalled when a newer element becomes available
68	* at the head of the queue or a new thread may need to
69	* become leader.
70	*/
71	private final Condition available = lock.newCondition();
72
73	/**
74	* Creates a new <tt>DelayQueue</tt> that is initially empty.
75	*/
76	public DelayQueue() {}
77
78	/**
79	* Creates a <tt>DelayQueue</tt> initially containing the elements of the
80	* given collection of {@link Delayed} instances.
81	*
82	* @param c the collection of elements to initially contain
83	* @throws NullPointerException if the specified collection or any
84	* of its elements are null
85	*/
86	public DelayQueue(Collection<? extends E> c) {
87	this.addAll(c);
88	}
89
90	/**
91	* Inserts the specified element into this delay queue.
92	*
93	* @param e the element to add
94	* @return <tt>true</tt> (as specified by {@link Collection#add})
95	* @throws NullPointerException if the specified element is null
96	*/
97	public boolean add(E e) {
98	return offer(e);
99	}
100
101	/**
102	* Inserts the specified element into this delay queue.
103	*
104	* @param e the element to add
105	* @return <tt>true</tt>
106	* @throws NullPointerException if the specified element is null
107	*/
108	public boolean offer(E e) {
109	final ReentrantLock lock = this.lock;
110	lock.lock();
111	try {
112	q.offer(e);
113	if (q.peek() == e) {
114	leader = null;
115	available.signal();
116	}
117	return true;
118	} finally {
119	lock.unlock();
120	}
121	}
122
123	/**
124	* Inserts the specified element into this delay queue. As the queue is
125	* unbounded this method will never block.
126	*
127	* @param e the element to add
128	* @throws NullPointerException {@inheritDoc}
129	*/
130	public void put(E e) {
131	offer(e);
132	}
133
134	/**
135	* Inserts the specified element into this delay queue. As the queue is
136	* unbounded this method will never block.
137	*
138	* @param e the element to add
139	* @param timeout This parameter is ignored as the method never blocks
140	* @param unit This parameter is ignored as the method never blocks
141	* @return <tt>true</tt>
142	* @throws NullPointerException {@inheritDoc}
143	*/
144	public boolean offer(E e, long timeout, TimeUnit unit) {
145	return offer(e);
146	}
147
148	/**
149	* Retrieves and removes the head of this queue, or returns <tt>null</tt>
150	* if this queue has no elements with an expired delay.
151	*
152	* @return the head of this queue, or <tt>null</tt> if this
153	* queue has no elements with an expired delay
154	*/
155	public E poll() {
156	final ReentrantLock lock = this.lock;
157	lock.lock();
158	try {
159	E first = q.peek();
160	if (first == null \|\| first.getDelay(NANOSECONDS) > 0)
161	return null;
162	else
163	return q.poll();
164	} finally {
165	lock.unlock();
166	}
167	}
168
169	/**
170	* Retrieves and removes the head of this queue, waiting if necessary
171	* until an element with an expired delay is available on this queue.
172	*
173	* @return the head of this queue
174	* @throws InterruptedException {@inheritDoc}
175	*/
176	public E take() throws InterruptedException {
177	final ReentrantLock lock = this.lock;
178	lock.lockInterruptibly();
179	try {
180	for (;;) {
181	E first = q.peek();
182	if (first == null)
183	available.await();
184	else {
185	long delay = first.getDelay(NANOSECONDS);
186	if (delay <= 0)
187	return q.poll();
188	else if (leader != null)
189	available.await();
190	else {
191	Thread thisThread = Thread.currentThread();
192	leader = thisThread;
193	try {
194	available.awaitNanos(delay);
195	} finally {
196	if (leader == thisThread)
197	leader = null;
198	}
199	}
200	}
201	}
202	} finally {
203	if (leader == null && q.peek() != null)
204	available.signal();
205	lock.unlock();
206	}
207	}
208
209	/**
210	* Retrieves and removes the head of this queue, waiting if necessary
211	* until an element with an expired delay is available on this queue,
212	* or the specified wait time expires.
213	*
214	* @return the head of this queue, or <tt>null</tt> if the
215	* specified waiting time elapses before an element with
216	* an expired delay becomes available
217	* @throws InterruptedException {@inheritDoc}
218	*/
219	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
220	long nanos = unit.toNanos(timeout);
221	final ReentrantLock lock = this.lock;
222	lock.lockInterruptibly();
223	try {
224	for (;;) {
225	E first = q.peek();
226	if (first == null) {
227	if (nanos <= 0)
228	return null;
229	else
230	nanos = available.awaitNanos(nanos);
231	} else {
232	long delay = first.getDelay(NANOSECONDS);
233	if (delay <= 0)
234	return q.poll();
235	if (nanos <= 0)
236	return null;
237	if (nanos < delay \|\| leader != null)
238	nanos = available.awaitNanos(nanos);
239	else {
240	Thread thisThread = Thread.currentThread();
241	leader = thisThread;
242	try {
243	long timeLeft = available.awaitNanos(delay);
244	nanos -= delay - timeLeft;
245	} finally {
246	if (leader == thisThread)
247	leader = null;
248	}
249	}
250	}
251	}
252	} finally {
253	if (leader == null && q.peek() != null)
254	available.signal();
255	lock.unlock();
256	}
257	}
258
259	/**
260	* Retrieves, but does not remove, the head of this queue, or
261	* returns <tt>null</tt> if this queue is empty. Unlike
262	* <tt>poll</tt>, if no expired elements are available in the queue,
263	* this method returns the element that will expire next,
264	* if one exists.
265	*
266	* @return the head of this queue, or <tt>null</tt> if this
267	* queue is empty.
268	*/
269	public E peek() {
270	final ReentrantLock lock = this.lock;
271	lock.lock();
272	try {
273	return q.peek();
274	} finally {
275	lock.unlock();
276	}
277	}
278
279	public int size() {
280	final ReentrantLock lock = this.lock;
281	lock.lock();
282	try {
283	return q.size();
284	} finally {
285	lock.unlock();
286	}
287	}
288
289	/**
290	* Return first element only if it is expired.
291	* Used only by drainTo. Call only when holding lock.
292	*/
293	private E peekExpired() {
294	// assert lock.isHeldByCurrentThread();
295	E first = q.peek();
296	return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
297	null : first;
298	}
299
300	/**
301	* @throws UnsupportedOperationException {@inheritDoc}
302	* @throws ClassCastException {@inheritDoc}
303	* @throws NullPointerException {@inheritDoc}
304	* @throws IllegalArgumentException {@inheritDoc}
305	*/
306	public int drainTo(Collection<? super E> c) {
307	if (c == null)
308	throw new NullPointerException();
309	if (c == this)
310	throw new IllegalArgumentException();
311	final ReentrantLock lock = this.lock;
312	lock.lock();
313	try {
314	int n = 0;
315	for (E e; (e = peekExpired()) != null;) {
316	c.add(e); // In this order, in case add() throws.
317	q.poll();
318	++n;
319	}
320	return n;
321	} finally {
322	lock.unlock();
323	}
324	}
325
326	/**
327	* @throws UnsupportedOperationException {@inheritDoc}
328	* @throws ClassCastException {@inheritDoc}
329	* @throws NullPointerException {@inheritDoc}
330	* @throws IllegalArgumentException {@inheritDoc}
331	*/
332	public int drainTo(Collection<? super E> c, int maxElements) {
333	if (c == null)
334	throw new NullPointerException();
335	if (c == this)
336	throw new IllegalArgumentException();
337	if (maxElements <= 0)
338	return 0;
339	final ReentrantLock lock = this.lock;
340	lock.lock();
341	try {
342	int n = 0;
343	for (E e; n < maxElements && (e = peekExpired()) != null;) {
344	c.add(e); // In this order, in case add() throws.
345	q.poll();
346	++n;
347	}
348	return n;
349	} finally {
350	lock.unlock();
351	}
352	}
353
354	/**
355	* Atomically removes all of the elements from this delay queue.
356	* The queue will be empty after this call returns.
357	* Elements with an unexpired delay are not waited for; they are
358	* simply discarded from the queue.
359	*/
360	public void clear() {
361	final ReentrantLock lock = this.lock;
362	lock.lock();
363	try {
364	q.clear();
365	} finally {
366	lock.unlock();
367	}
368	}
369
370	/**
371	* Always returns <tt>Integer.MAX_VALUE</tt> because
372	* a <tt>DelayQueue</tt> is not capacity constrained.
373	*
374	* @return <tt>Integer.MAX_VALUE</tt>
375	*/
376	public int remainingCapacity() {
377	return Integer.MAX_VALUE;
378	}
379
380	/**
381	* Returns an array containing all of the elements in this queue.
382	* The returned array elements are in no particular order.
383	*
384	* <p>The returned array will be "safe" in that no references to it are
385	* maintained by this queue. (In other words, this method must allocate
386	* a new array). The caller is thus free to modify the returned array.
387	*
388	* <p>This method acts as bridge between array-based and collection-based
389	* APIs.
390	*
391	* @return an array containing all of the elements in this queue
392	*/
393	public Object[] toArray() {
394	final ReentrantLock lock = this.lock;
395	lock.lock();
396	try {
397	return q.toArray();
398	} finally {
399	lock.unlock();
400	}
401	}
402
403	/**
404	* Returns an array containing all of the elements in this queue; the
405	* runtime type of the returned array is that of the specified array.
406	* The returned array elements are in no particular order.
407	* If the queue fits in the specified array, it is returned therein.
408	* Otherwise, a new array is allocated with the runtime type of the
409	* specified array and the size of this queue.
410	*
411	* <p>If this queue fits in the specified array with room to spare
412	* (i.e., the array has more elements than this queue), the element in
413	* the array immediately following the end of the queue is set to
414	* <tt>null</tt>.
415	*
416	* <p>Like the {@link #toArray()} method, this method acts as bridge between
417	* array-based and collection-based APIs. Further, this method allows
418	* precise control over the runtime type of the output array, and may,
419	* under certain circumstances, be used to save allocation costs.
420	*
421	* <p>The following code can be used to dump a delay queue into a newly
422	* allocated array of <tt>Delayed</tt>:
423	*
424	* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
425	*
426	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
427	* <tt>toArray()</tt>.
428	*
429	* @param a the array into which the elements of the queue are to
430	* be stored, if it is big enough; otherwise, a new array of the
431	* same runtime type is allocated for this purpose
432	* @return an array containing all of the elements in this queue
433	* @throws ArrayStoreException if the runtime type of the specified array
434	* is not a supertype of the runtime type of every element in
435	* this queue
436	* @throws NullPointerException if the specified array is null
437	*/
438	public <T> T[] toArray(T[] a) {
439	final ReentrantLock lock = this.lock;
440	lock.lock();
441	try {
442	return q.toArray(a);
443	} finally {
444	lock.unlock();
445	}
446	}
447
448	/**
449	* Removes a single instance of the specified element from this
450	* queue, if it is present, whether or not it has expired.
451	*/
452	public boolean remove(Object o) {
453	final ReentrantLock lock = this.lock;
454	lock.lock();
455	try {
456	return q.remove(o);
457	} finally {
458	lock.unlock();
459	}
460	}
461
462	/**
463	* Identity-based version for use in Itr.remove
464	*/
465	void removeEQ(Object o) {
466	final ReentrantLock lock = this.lock;
467	lock.lock();
468	try {
469	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
470	if (o == it.next()) {
471	it.remove();
472	break;
473	}
474	}
475	} finally {
476	lock.unlock();
477	}
478	}
479
480	/**
481	* Returns an iterator over all the elements (both expired and
482	* unexpired) in this queue. The iterator does not return the
483	* elements in any particular order.
484	*
485	* <p>The returned iterator is a "weakly consistent" iterator that
486	* will never throw {@link java.util.ConcurrentModificationException
487	* ConcurrentModificationException}, and guarantees to traverse
488	* elements as they existed upon construction of the iterator, and
489	* may (but is not guaranteed to) reflect any modifications
490	* subsequent to construction.
491	*
492	* @return an iterator over the elements in this queue
493	*/
494	public Iterator<E> iterator() {
495	return new Itr(toArray());
496	}
497
498	/**
499	* Snapshot iterator that works off copy of underlying q array.
500	*/
501	private class Itr implements Iterator<E> {
502	final Object[] array; // Array of all elements
503	int cursor; // index of next element to return
504	int lastRet; // index of last element, or -1 if no such
505
506	Itr(Object[] array) {
507	lastRet = -1;
508	this.array = array;
509	}
510
511	public boolean hasNext() {
512	return cursor < array.length;
513	}
514
515	@SuppressWarnings("unchecked")
516	public E next() {
517	if (cursor >= array.length)
518	throw new NoSuchElementException();
519	lastRet = cursor;
520	return (E)array[cursor++];
521	}
522
523	public void remove() {
524	if (lastRet < 0)
525	throw new IllegalStateException();
526	removeEQ(array[lastRet]);
527	lastRet = -1;
528	}
529	}
530
531	}