util/concurrent/PriorityBlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;

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

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} that uses
 * the same ordering rules as class {@link PriorityQueue} and supplies
 * blocking retrieval operations.  While this queue is logically
 * unbounded, attempted additions may fail due to resource exhaustion
 * (causing <tt>OutOfMemoryError</tt>). This class does not permit
 * <tt>null</tt> elements.  A priority queue relying on {@linkplain
 * Comparable natural ordering} also does not permit insertion of
 * non-comparable objects (doing so results in
 * <tt>ClassCastException</tt>).
 *
 * <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 PriorityBlockingQueue in any particular order. If you need
 * ordered traversal, consider using
 * <tt>Arrays.sort(pq.toArray())</tt>.  Also, method <tt>drainTo</tt>
 * can be used to <em>remove</em> some or all elements in priority
 * order and place them in another collection.
 *
 * <p>Operations on this class make no guarantees about the ordering
 * of elements with equal priority. If you need to enforce an
 * ordering, you can define custom classes or comparators that use a
 * secondary key to break ties in primary priority values.  For
 * example, here is a class that applies first-in-first-out
 * tie-breaking to comparable elements. To use it, you would insert a
 * <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.
 *
 * <pre>
 * class FIFOEntry&lt;E extends Comparable&lt;? super E&gt;&gt;
 *     implements Comparable&lt;FIFOEntry&lt;E&gt;&gt; {
 *   final static AtomicLong seq = new AtomicLong();
 *   final long seqNum;
 *   final E entry;
 *   public FIFOEntry(E entry) {
 *     seqNum = seq.getAndIncrement();
 *     this.entry = entry;
 *   }
 *   public E getEntry() { return entry; }
 *   public int compareTo(FIFOEntry&lt;E&gt; other) {
 *     int res = entry.compareTo(other.entry);
 *     if (res == 0 &amp;&amp; other.entry != this.entry)
 *       res = (seqNum &lt; other.seqNum ? -1 : 1);
 *     return res;
 *   }
 * }</pre>
 *
 * <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 PriorityBlockingQueue<E> extends AbstractQueue<E>
    implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = 5595510919245408276L;

    private final PriorityQueue<E> q;
    private final ReentrantLock lock = new ReentrantLock(true);
    private final Condition notEmpty = lock.newCondition();

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the default
     * initial capacity (11) that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public PriorityBlockingQueue() {
        q = new PriorityQueue<E>();
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the specified
     * initial capacity that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity) {
        q = new PriorityQueue<E>(initialCapacity, null);
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the specified initial
     * capacity that orders its elements according to the specified
     * comparator.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @param  comparator the comparator that will be used to order this
     *         priority queue.  If {@code null}, the {@linkplain Comparable
     *         natural ordering} of the elements will be used.
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity,
                                 Comparator<? super E> comparator) {
        q = new PriorityQueue<E>(initialCapacity, comparator);
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> containing the elements
     * in the specified collection.  If the specified collection is a
     * {@link SortedSet} or a {@link PriorityQueue},  this
     * priority queue will be ordered according to the same ordering.
     * Otherwise, this priority queue will be ordered according to the
     * {@linkplain Comparable natural ordering} of its elements.
     *
     * @param  c the collection whose elements are to be placed
     *         into this priority queue
     * @throws ClassCastException if elements of the specified collection
     *         cannot be compared to one another according to the priority
     *         queue's ordering
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public PriorityBlockingQueue(Collection<? extends E> c) {
        q = new PriorityQueue<E>(c);
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Queue#offer})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            boolean ok = q.offer(e);
            assert ok;
            notEmpty.signal();
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Inserts the specified element into this priority queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public void put(E e) {
        offer(e); // never need to block
    }

    /**
     * Inserts the specified element into this priority 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 ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e, long timeout, TimeUnit unit) {
        return offer(e); // never need to block
    }

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

    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            try {
                while (q.size() == 0)
                    notEmpty.await();
            } catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            E x = q.poll();
            assert x != null;
            return x;
        } finally {
            lock.unlock();
        }
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E x = q.poll();
                if (x != null)
                    return x;
                if (nanos <= 0)
                    return null;
                try {
                    nanos = notEmpty.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to non-interrupted thread
                    throw ie;
                }
            }
        } finally {
            lock.unlock();
        }
    }

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

    /**
     * Returns the comparator used to order the elements in this queue,
     * or <tt>null</tt> if this queue uses the {@linkplain Comparable
     * natural ordering} of its elements.
     *
     * @return the comparator used to order the elements in this queue,
     *         or <tt>null</tt> if this queue uses the natural
     *         ordering of its elements
     */
    public Comparator<? super E> comparator() {
        return q.comparator();
    }

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

    /**
     * Always returns <tt>Integer.MAX_VALUE</tt> because
     * a <tt>PriorityBlockingQueue</tt> is not capacity constrained.
     * @return <tt>Integer.MAX_VALUE</tt>
     */
    public int remainingCapacity() {
        return Integer.MAX_VALUE;
    }

    /**
     * Removes a single instance of the specified element from this queue,
     * if it is present.  More formally, removes an element {@code e} such
     * that {@code o.equals(e)}, if this queue contains one or more such
     * elements.  Returns {@code true} if and only if this queue contained
     * the specified element (or equivalently, if this queue changed as a
     * result of the call).
     *
     * @param o element to be removed from this queue, if present
     * @return <tt>true</tt> if this queue changed as a result of the call
     */
    public boolean remove(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.remove(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns {@code true} if this queue contains the specified element.
     * More formally, returns {@code true} if and only if this queue contains
     * at least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this queue
     * @return <tt>true</tt> if this queue contains the specified element
     */
    public boolean contains(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.contains(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * 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();
        }
    }


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

    /**
     * @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;
            E e;
            while ( (e = q.poll()) != null) {
                c.add(e);
                ++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;
            E e;
            while (n < maxElements && (e = q.poll()) != null) {
                c.add(e);
                ++n;
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Atomically removes all of the elements from this queue.
     * The queue will be empty after this call returns.
     */
    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.clear();
        } 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>Suppose <tt>x</tt> is a queue known to contain only strings.
     * The following code can be used to dump the queue into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[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();
        }
    }

    /**
     * Returns an iterator over the elements in this queue. The
     * iterator does not return the elements in any particular order.
     * The returned <tt>Iterator</tt> is a "weakly consistent"
     * iterator that will never throw {@link
     * 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;
        }

        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();
            Object x = array[lastRet];
            lastRet = -1;
            // Traverse underlying queue to find == element,
            // not just a .equals element.
            lock.lock();
            try {
                for (Iterator it = q.iterator(); it.hasNext(); ) {
                    if (it.next() == x) {
                        it.remove();
                        return;
                    }
                }
            } finally {
                lock.unlock();
            }
        }
    }

    /**
     * Saves the state to a stream (that is, serializes it).  This
     * merely wraps default serialization within lock.  The
     * serialization strategy for items is left to underlying
     * Queue. Note that locking is not needed on deserialization, so
     * readObject is not defined, just relying on default.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        lock.lock();
        try {
            s.defaultWriteObject();
        } finally {
            lock.unlock();
        }
    }

}
Revision:	1.54
Committed:	Sun May 18 23:47:56 2008 UTC (16 years ago) by jsr166
Branch:	MAIN
Changes since 1.53:	+3 -3 lines
Log Message:	Sync with OpenJDK; untabify
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.concurrent.locks.*;
10	import java.util.*;
11
12	/**
13	* An unbounded {@linkplain BlockingQueue blocking queue} that uses
14	* the same ordering rules as class {@link PriorityQueue} and supplies
15	* blocking retrieval operations. While this queue is logically
16	* unbounded, attempted additions may fail due to resource exhaustion
17	* (causing <tt>OutOfMemoryError</tt>). This class does not permit
18	* <tt>null</tt> elements. A priority queue relying on {@linkplain
19	* Comparable natural ordering} also does not permit insertion of
20	* non-comparable objects (doing so results in
21	* <tt>ClassCastException</tt>).
22	*
23	* <p>This class and its iterator implement all of the
24	* <em>optional</em> methods of the {@link Collection} and {@link
25	* Iterator} interfaces. The Iterator provided in method {@link
26	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
27	* the PriorityBlockingQueue in any particular order. If you need
28	* ordered traversal, consider using
29	* <tt>Arrays.sort(pq.toArray())</tt>. Also, method <tt>drainTo</tt>
30	* can be used to <em>remove</em> some or all elements in priority
31	* order and place them in another collection.
32	*
33	* <p>Operations on this class make no guarantees about the ordering
34	* of elements with equal priority. If you need to enforce an
35	* ordering, you can define custom classes or comparators that use a
36	* secondary key to break ties in primary priority values. For
37	* example, here is a class that applies first-in-first-out
38	* tie-breaking to comparable elements. To use it, you would insert a
39	* <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.
40	*
41	* <pre>
42	* class FIFOEntry<E extends Comparable<? super E>>
43	* implements Comparable<FIFOEntry<E>> {
44	* final static AtomicLong seq = new AtomicLong();
45	* final long seqNum;
46	* final E entry;
47	* public FIFOEntry(E entry) {
48	* seqNum = seq.getAndIncrement();
49	* this.entry = entry;
50	* }
51	* public E getEntry() { return entry; }
52	* public int compareTo(FIFOEntry<E> other) {
53	* int res = entry.compareTo(other.entry);
54	* if (res == 0 && other.entry != this.entry)
55	* res = (seqNum < other.seqNum ? -1 : 1);
56	* return res;
57	* }
58	* }</pre>
59	*
60	* <p>This class is a member of the
61	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
62	* Java Collections Framework</a>.
63	*
64	* @since 1.5
65	* @author Doug Lea
66	* @param <E> the type of elements held in this collection
67	*/
68	public class PriorityBlockingQueue<E> extends AbstractQueue<E>
69	implements BlockingQueue<E>, java.io.Serializable {
70	private static final long serialVersionUID = 5595510919245408276L;
71
72	private final PriorityQueue<E> q;
73	private final ReentrantLock lock = new ReentrantLock(true);
74	private final Condition notEmpty = lock.newCondition();
75
76	/**
77	* Creates a <tt>PriorityBlockingQueue</tt> with the default
78	* initial capacity (11) that orders its elements according to
79	* their {@linkplain Comparable natural ordering}.
80	*/
81	public PriorityBlockingQueue() {
82	q = new PriorityQueue<E>();
83	}
84
85	/**
86	* Creates a <tt>PriorityBlockingQueue</tt> with the specified
87	* initial capacity that orders its elements according to their
88	* {@linkplain Comparable natural ordering}.
89	*
90	* @param initialCapacity the initial capacity for this priority queue
91	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
92	* than 1
93	*/
94	public PriorityBlockingQueue(int initialCapacity) {
95	q = new PriorityQueue<E>(initialCapacity, null);
96	}
97
98	/**
99	* Creates a <tt>PriorityBlockingQueue</tt> with the specified initial
100	* capacity that orders its elements according to the specified
101	* comparator.
102	*
103	* @param initialCapacity the initial capacity for this priority queue
104	* @param comparator the comparator that will be used to order this
105	* priority queue. If {@code null}, the {@linkplain Comparable
106	* natural ordering} of the elements will be used.
107	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
108	* than 1
109	*/
110	public PriorityBlockingQueue(int initialCapacity,
111	Comparator<? super E> comparator) {
112	q = new PriorityQueue<E>(initialCapacity, comparator);
113	}
114
115	/**
116	* Creates a <tt>PriorityBlockingQueue</tt> containing the elements
117	* in the specified collection. If the specified collection is a
118	* {@link SortedSet} or a {@link PriorityQueue}, this
119	* priority queue will be ordered according to the same ordering.
120	* Otherwise, this priority queue will be ordered according to the
121	* {@linkplain Comparable natural ordering} of its elements.
122	*
123	* @param c the collection whose elements are to be placed
124	* into this priority queue
125	* @throws ClassCastException if elements of the specified collection
126	* cannot be compared to one another according to the priority
127	* queue's ordering
128	* @throws NullPointerException if the specified collection or any
129	* of its elements are null
130	*/
131	public PriorityBlockingQueue(Collection<? extends E> c) {
132	q = new PriorityQueue<E>(c);
133	}
134
135	/**
136	* Inserts the specified element into this priority queue.
137	*
138	* @param e the element to add
139	* @return <tt>true</tt> (as specified by {@link Collection#add})
140	* @throws ClassCastException if the specified element cannot be compared
141	* with elements currently in the priority queue according to the
142	* priority queue's ordering
143	* @throws NullPointerException if the specified element is null
144	*/
145	public boolean add(E e) {
146	return offer(e);
147	}
148
149	/**
150	* Inserts the specified element into this priority queue.
151	*
152	* @param e the element to add
153	* @return <tt>true</tt> (as specified by {@link Queue#offer})
154	* @throws ClassCastException if the specified element cannot be compared
155	* with elements currently in the priority queue according to the
156	* priority queue's ordering
157	* @throws NullPointerException if the specified element is null
158	*/
159	public boolean offer(E e) {
160	final ReentrantLock lock = this.lock;
161	lock.lock();
162	try {
163	boolean ok = q.offer(e);
164	assert ok;
165	notEmpty.signal();
166	return true;
167	} finally {
168	lock.unlock();
169	}
170	}
171
172	/**
173	* Inserts the specified element into this priority queue. As the queue is
174	* unbounded this method will never block.
175	*
176	* @param e the element to add
177	* @throws ClassCastException if the specified element cannot be compared
178	* with elements currently in the priority queue according to the
179	* priority queue's ordering
180	* @throws NullPointerException if the specified element is null
181	*/
182	public void put(E e) {
183	offer(e); // never need to block
184	}
185
186	/**
187	* Inserts the specified element into this priority queue. As the queue is
188	* unbounded this method will never block.
189	*
190	* @param e the element to add
191	* @param timeout This parameter is ignored as the method never blocks
192	* @param unit This parameter is ignored as the method never blocks
193	* @return <tt>true</tt>
194	* @throws ClassCastException if the specified element cannot be compared
195	* with elements currently in the priority queue according to the
196	* priority queue's ordering
197	* @throws NullPointerException if the specified element is null
198	*/
199	public boolean offer(E e, long timeout, TimeUnit unit) {
200	return offer(e); // never need to block
201	}
202
203	public E poll() {
204	final ReentrantLock lock = this.lock;
205	lock.lock();
206	try {
207	return q.poll();
208	} finally {
209	lock.unlock();
210	}
211	}
212
213	public E take() throws InterruptedException {
214	final ReentrantLock lock = this.lock;
215	lock.lockInterruptibly();
216	try {
217	try {
218	while (q.size() == 0)
219	notEmpty.await();
220	} catch (InterruptedException ie) {
221	notEmpty.signal(); // propagate to non-interrupted thread
222	throw ie;
223	}
224	E x = q.poll();
225	assert x != null;
226	return x;
227	} finally {
228	lock.unlock();
229	}
230	}
231
232	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
233	long nanos = unit.toNanos(timeout);
234	final ReentrantLock lock = this.lock;
235	lock.lockInterruptibly();
236	try {
237	for (;;) {
238	E x = q.poll();
239	if (x != null)
240	return x;
241	if (nanos <= 0)
242	return null;
243	try {
244	nanos = notEmpty.awaitNanos(nanos);
245	} catch (InterruptedException ie) {
246	notEmpty.signal(); // propagate to non-interrupted thread
247	throw ie;
248	}
249	}
250	} finally {
251	lock.unlock();
252	}
253	}
254
255	public E peek() {
256	final ReentrantLock lock = this.lock;
257	lock.lock();
258	try {
259	return q.peek();
260	} finally {
261	lock.unlock();
262	}
263	}
264
265	/**
266	* Returns the comparator used to order the elements in this queue,
267	* or <tt>null</tt> if this queue uses the {@linkplain Comparable
268	* natural ordering} of its elements.
269	*
270	* @return the comparator used to order the elements in this queue,
271	* or <tt>null</tt> if this queue uses the natural
272	* ordering of its elements
273	*/
274	public Comparator<? super E> comparator() {
275	return q.comparator();
276	}
277
278	public int size() {
279	final ReentrantLock lock = this.lock;
280	lock.lock();
281	try {
282	return q.size();
283	} finally {
284	lock.unlock();
285	}
286	}
287
288	/**
289	* Always returns <tt>Integer.MAX_VALUE</tt> because
290	* a <tt>PriorityBlockingQueue</tt> is not capacity constrained.
291	* @return <tt>Integer.MAX_VALUE</tt>
292	*/
293	public int remainingCapacity() {
294	return Integer.MAX_VALUE;
295	}
296
297	/**
298	* Removes a single instance of the specified element from this queue,
299	* if it is present. More formally, removes an element {@code e} such
300	* that {@code o.equals(e)}, if this queue contains one or more such
301	* elements. Returns {@code true} if and only if this queue contained
302	* the specified element (or equivalently, if this queue changed as a
303	* result of the call).
304	*
305	* @param o element to be removed from this queue, if present
306	* @return <tt>true</tt> if this queue changed as a result of the call
307	*/
308	public boolean remove(Object o) {
309	final ReentrantLock lock = this.lock;
310	lock.lock();
311	try {
312	return q.remove(o);
313	} finally {
314	lock.unlock();
315	}
316	}
317
318	/**
319	* Returns {@code true} if this queue contains the specified element.
320	* More formally, returns {@code true} if and only if this queue contains
321	* at least one element {@code e} such that {@code o.equals(e)}.
322	*
323	* @param o object to be checked for containment in this queue
324	* @return <tt>true</tt> if this queue contains the specified element
325	*/
326	public boolean contains(Object o) {
327	final ReentrantLock lock = this.lock;
328	lock.lock();
329	try {
330	return q.contains(o);
331	} finally {
332	lock.unlock();
333	}
334	}
335
336	/**
337	* Returns an array containing all of the elements in this queue.
338	* The returned array elements are in no particular order.
339	*
340	* <p>The returned array will be "safe" in that no references to it are
341	* maintained by this queue. (In other words, this method must allocate
342	* a new array). The caller is thus free to modify the returned array.
343	*
344	* <p>This method acts as bridge between array-based and collection-based
345	* APIs.
346	*
347	* @return an array containing all of the elements in this queue
348	*/
349	public Object[] toArray() {
350	final ReentrantLock lock = this.lock;
351	lock.lock();
352	try {
353	return q.toArray();
354	} finally {
355	lock.unlock();
356	}
357	}
358
359
360	public String toString() {
361	final ReentrantLock lock = this.lock;
362	lock.lock();
363	try {
364	return q.toString();
365	} finally {
366	lock.unlock();
367	}
368	}
369
370	/**
371	* @throws UnsupportedOperationException {@inheritDoc}
372	* @throws ClassCastException {@inheritDoc}
373	* @throws NullPointerException {@inheritDoc}
374	* @throws IllegalArgumentException {@inheritDoc}
375	*/
376	public int drainTo(Collection<? super E> c) {
377	if (c == null)
378	throw new NullPointerException();
379	if (c == this)
380	throw new IllegalArgumentException();
381	final ReentrantLock lock = this.lock;
382	lock.lock();
383	try {
384	int n = 0;
385	E e;
386	while ( (e = q.poll()) != null) {
387	c.add(e);
388	++n;
389	}
390	return n;
391	} finally {
392	lock.unlock();
393	}
394	}
395
396	/**
397	* @throws UnsupportedOperationException {@inheritDoc}
398	* @throws ClassCastException {@inheritDoc}
399	* @throws NullPointerException {@inheritDoc}
400	* @throws IllegalArgumentException {@inheritDoc}
401	*/
402	public int drainTo(Collection<? super E> c, int maxElements) {
403	if (c == null)
404	throw new NullPointerException();
405	if (c == this)
406	throw new IllegalArgumentException();
407	if (maxElements <= 0)
408	return 0;
409	final ReentrantLock lock = this.lock;
410	lock.lock();
411	try {
412	int n = 0;
413	E e;
414	while (n < maxElements && (e = q.poll()) != null) {
415	c.add(e);
416	++n;
417	}
418	return n;
419	} finally {
420	lock.unlock();
421	}
422	}
423
424	/**
425	* Atomically removes all of the elements from this queue.
426	* The queue will be empty after this call returns.
427	*/
428	public void clear() {
429	final ReentrantLock lock = this.lock;
430	lock.lock();
431	try {
432	q.clear();
433	} finally {
434	lock.unlock();
435	}
436	}
437
438	/**
439	* Returns an array containing all of the elements in this queue; the
440	* runtime type of the returned array is that of the specified array.
441	* The returned array elements are in no particular order.
442	* If the queue fits in the specified array, it is returned therein.
443	* Otherwise, a new array is allocated with the runtime type of the
444	* specified array and the size of this queue.
445	*
446	* <p>If this queue fits in the specified array with room to spare
447	* (i.e., the array has more elements than this queue), the element in
448	* the array immediately following the end of the queue is set to
449	* <tt>null</tt>.
450	*
451	* <p>Like the {@link #toArray()} method, this method acts as bridge between
452	* array-based and collection-based APIs. Further, this method allows
453	* precise control over the runtime type of the output array, and may,
454	* under certain circumstances, be used to save allocation costs.
455	*
456	* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
457	* The following code can be used to dump the queue into a newly
458	* allocated array of <tt>String</tt>:
459	*
460	* <pre>
461	* String[] y = x.toArray(new String[0]);</pre>
462	*
463	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
464	* <tt>toArray()</tt>.
465	*
466	* @param a the array into which the elements of the queue are to
467	* be stored, if it is big enough; otherwise, a new array of the
468	* same runtime type is allocated for this purpose
469	* @return an array containing all of the elements in this queue
470	* @throws ArrayStoreException if the runtime type of the specified array
471	* is not a supertype of the runtime type of every element in
472	* this queue
473	* @throws NullPointerException if the specified array is null
474	*/
475	public <T> T[] toArray(T[] a) {
476	final ReentrantLock lock = this.lock;
477	lock.lock();
478	try {
479	return q.toArray(a);
480	} finally {
481	lock.unlock();
482	}
483	}
484
485	/**
486	* Returns an iterator over the elements in this queue. The
487	* iterator does not return the elements in any particular order.
488	* The returned <tt>Iterator</tt> is a "weakly consistent"
489	* iterator that will never throw {@link
490	* ConcurrentModificationException}, and guarantees to traverse
491	* elements as they existed upon construction of the iterator, and
492	* may (but is not guaranteed to) reflect any modifications
493	* subsequent to construction.
494	*
495	* @return an iterator over the elements in this queue
496	*/
497	public Iterator<E> iterator() {
498	return new Itr(toArray());
499	}
500
501	/**
502	* Snapshot iterator that works off copy of underlying q array.
503	*/
504	private class Itr implements Iterator<E> {
505	final Object[] array; // Array of all elements
506	int cursor; // index of next element to return;
507	int lastRet; // index of last element, or -1 if no such
508
509	Itr(Object[] array) {
510	lastRet = -1;
511	this.array = array;
512	}
513
514	public boolean hasNext() {
515	return cursor < array.length;
516	}
517
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	Object x = array[lastRet];
529	lastRet = -1;
530	// Traverse underlying queue to find == element,
531	// not just a .equals element.
532	lock.lock();
533	try {
534	for (Iterator it = q.iterator(); it.hasNext(); ) {
535	if (it.next() == x) {
536	it.remove();
537	return;
538	}
539	}
540	} finally {
541	lock.unlock();
542	}
543	}
544	}
545
546	/**
547	* Saves the state to a stream (that is, serializes it). This
548	* merely wraps default serialization within lock. The
549	* serialization strategy for items is left to underlying
550	* Queue. Note that locking is not needed on deserialization, so
551	* readObject is not defined, just relying on default.
552	*/
553	private void writeObject(java.io.ObjectOutputStream s)
554	throws java.io.IOException {
555	lock.lock();
556	try {
557	s.defaultWriteObject();
558	} finally {
559	lock.unlock();
560	}
561	}
562
563	}