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}/../guide/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 used to order this priority queue.
     * If <tt>null</tt> then the order depends on the elements' natural
     * ordering.
     * @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.  The priority queue has an initial
     * capacity of 110% of the size of the specified collection.  If
     * the specified collection is a {@link SortedSet} or a {@link
     * PriorityQueue}, this priority queue will be sorted according to
     * the same comparator, or according to the natural ordering of its
     * elements if the collection is sorted according to the natural
     * ordering of its elements.  Otherwise, this priority queue is
     * ordered according to the 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 <tt>e</tt> such
     * that <tt>o.equals(e)</tt>, if this queue contains one or more such
     * elements.
     * Returns <tt>true</tt> 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 <tt>true</tt> if this queue contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this queue contains
     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
     *
     * @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.
     *
     * @return an iterator over the elements in this queue
     */
    public Iterator<E> iterator() {
        return new Itr<E>(toArray());
    }

    /**
     * Snapshot iterator that works off copy of underlying q array.
     */
    private class Itr<E> 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();
            }
        }
    }

    /**
     * Save the state to a stream (that is, serialize 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:	1.50
Committed:	Wed Nov 23 05:33:03 2005 UTC (18 years, 6 months ago) by jsr166
Branch:	MAIN
Changes since 1.49:	+2 -2 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/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}/../guide/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 used to order this priority queue.
105	* If <tt>null</tt> then the order depends on the elements' natural
106	* ordering.
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. The priority queue has an initial
118	* capacity of 110% of the size of the specified collection. If
119	* the specified collection is a {@link SortedSet} or a {@link
120	* PriorityQueue}, this priority queue will be sorted according to
121	* the same comparator, or according to the natural ordering of its
122	* elements if the collection is sorted according to the natural
123	* ordering of its elements. Otherwise, this priority queue is
124	* ordered according to the natural ordering of its elements.
125	*
126	* @param c the collection whose elements are to be placed
127	* into this priority queue.
128	* @throws ClassCastException if elements of the specified collection
129	* cannot be compared to one another according to the priority
130	* queue's ordering.
131	* @throws NullPointerException if the specified collection or any
132	* of its elements are null
133	*/
134	public PriorityBlockingQueue(Collection<? extends E> c) {
135	q = new PriorityQueue<E>(c);
136	}
137
138	/**
139	* Inserts the specified element into this priority queue.
140	*
141	* @param e the element to add
142	* @return <tt>true</tt> (as specified by {@link Collection#add})
143	* @throws ClassCastException if the specified element cannot be compared
144	* with elements currently in the priority queue according to the
145	* priority queue's ordering
146	* @throws NullPointerException if the specified element is null
147	*/
148	public boolean add(E e) {
149	return offer(e);
150	}
151
152	/**
153	* Inserts the specified element into this priority queue.
154	*
155	* @param e the element to add
156	* @return <tt>true</tt> (as specified by {@link Queue#offer})
157	* @throws ClassCastException if the specified element cannot be compared
158	* with elements currently in the priority queue according to the
159	* priority queue's ordering
160	* @throws NullPointerException if the specified element is null
161	*/
162	public boolean offer(E e) {
163	final ReentrantLock lock = this.lock;
164	lock.lock();
165	try {
166	boolean ok = q.offer(e);
167	assert ok;
168	notEmpty.signal();
169	return true;
170	} finally {
171	lock.unlock();
172	}
173	}
174
175	/**
176	* Inserts the specified element into this priority queue. As the queue is
177	* unbounded this method will never block.
178	*
179	* @param e the element to add
180	* @throws ClassCastException if the specified element cannot be compared
181	* with elements currently in the priority queue according to the
182	* priority queue's ordering
183	* @throws NullPointerException if the specified element is null
184	*/
185	public void put(E e) {
186	offer(e); // never need to block
187	}
188
189	/**
190	* Inserts the specified element into this priority queue. As the queue is
191	* unbounded this method will never block.
192	*
193	* @param e the element to add
194	* @param timeout This parameter is ignored as the method never blocks
195	* @param unit This parameter is ignored as the method never blocks
196	* @return <tt>true</tt>
197	* @throws ClassCastException if the specified element cannot be compared
198	* with elements currently in the priority queue according to the
199	* priority queue's ordering
200	* @throws NullPointerException if the specified element is null
201	*/
202	public boolean offer(E e, long timeout, TimeUnit unit) {
203	return offer(e); // never need to block
204	}
205
206	public E poll() {
207	final ReentrantLock lock = this.lock;
208	lock.lock();
209	try {
210	return q.poll();
211	} finally {
212	lock.unlock();
213	}
214	}
215
216	public E take() throws InterruptedException {
217	final ReentrantLock lock = this.lock;
218	lock.lockInterruptibly();
219	try {
220	try {
221	while (q.size() == 0)
222	notEmpty.await();
223	} catch (InterruptedException ie) {
224	notEmpty.signal(); // propagate to non-interrupted thread
225	throw ie;
226	}
227	E x = q.poll();
228	assert x != null;
229	return x;
230	} finally {
231	lock.unlock();
232	}
233	}
234
235	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
236	long nanos = unit.toNanos(timeout);
237	final ReentrantLock lock = this.lock;
238	lock.lockInterruptibly();
239	try {
240	for (;;) {
241	E x = q.poll();
242	if (x != null)
243	return x;
244	if (nanos <= 0)
245	return null;
246	try {
247	nanos = notEmpty.awaitNanos(nanos);
248	} catch (InterruptedException ie) {
249	notEmpty.signal(); // propagate to non-interrupted thread
250	throw ie;
251	}
252	}
253	} finally {
254	lock.unlock();
255	}
256	}
257
258	public E peek() {
259	final ReentrantLock lock = this.lock;
260	lock.lock();
261	try {
262	return q.peek();
263	} finally {
264	lock.unlock();
265	}
266	}
267
268	/**
269	* Returns the comparator used to order the elements in this queue,
270	* or <tt>null</tt> if this queue uses the {@linkplain Comparable
271	* natural ordering} of its elements.
272	*
273	* @return the comparator used to order the elements in this queue,
274	* or <tt>null</tt> if this queue uses the natural
275	* ordering of its elements.
276	*/
277	public Comparator<? super E> comparator() {
278	return q.comparator();
279	}
280
281	public int size() {
282	final ReentrantLock lock = this.lock;
283	lock.lock();
284	try {
285	return q.size();
286	} finally {
287	lock.unlock();
288	}
289	}
290
291	/**
292	* Always returns <tt>Integer.MAX_VALUE</tt> because
293	* a <tt>PriorityBlockingQueue</tt> is not capacity constrained.
294	* @return <tt>Integer.MAX_VALUE</tt>
295	*/
296	public int remainingCapacity() {
297	return Integer.MAX_VALUE;
298	}
299
300	/**
301	* Removes a single instance of the specified element from this queue,
302	* if it is present. More formally, removes an element <tt>e</tt> such
303	* that <tt>o.equals(e)</tt>, if this queue contains one or more such
304	* elements.
305	* Returns <tt>true</tt> if this queue contained the specified element
306	* (or equivalently, if this queue changed as a result of the call).
307	*
308	* @param o element to be removed from this queue, if present
309	* @return <tt>true</tt> if this queue changed as a result of the call
310	*/
311	public boolean remove(Object o) {
312	final ReentrantLock lock = this.lock;
313	lock.lock();
314	try {
315	return q.remove(o);
316	} finally {
317	lock.unlock();
318	}
319	}
320
321	/**
322	* Returns <tt>true</tt> if this queue contains the specified element.
323	* More formally, returns <tt>true</tt> if and only if this queue contains
324	* at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
325	*
326	* @param o object to be checked for containment in this queue
327	* @return <tt>true</tt> if this queue contains the specified element
328	*/
329	public boolean contains(Object o) {
330	final ReentrantLock lock = this.lock;
331	lock.lock();
332	try {
333	return q.contains(o);
334	} finally {
335	lock.unlock();
336	}
337	}
338
339	/**
340	* Returns an array containing all of the elements in this queue.
341	* The returned array elements are in no particular order.
342	*
343	* <p>The returned array will be "safe" in that no references to it are
344	* maintained by this queue. (In other words, this method must allocate
345	* a new array). The caller is thus free to modify the returned array.
346	*
347	* <p>This method acts as bridge between array-based and collection-based
348	* APIs.
349	*
350	* @return an array containing all of the elements in this queue
351	*/
352	public Object[] toArray() {
353	final ReentrantLock lock = this.lock;
354	lock.lock();
355	try {
356	return q.toArray();
357	} finally {
358	lock.unlock();
359	}
360	}
361
362	public String toString() {
363	final ReentrantLock lock = this.lock;
364	lock.lock();
365	try {
366	return q.toString();
367	} finally {
368	lock.unlock();
369	}
370	}
371
372	/**
373	* @throws UnsupportedOperationException {@inheritDoc}
374	* @throws ClassCastException {@inheritDoc}
375	* @throws NullPointerException {@inheritDoc}
376	* @throws IllegalArgumentException {@inheritDoc}
377	*/
378	public int drainTo(Collection<? super E> c) {
379	if (c == null)
380	throw new NullPointerException();
381	if (c == this)
382	throw new IllegalArgumentException();
383	final ReentrantLock lock = this.lock;
384	lock.lock();
385	try {
386	int n = 0;
387	E e;
388	while ( (e = q.poll()) != null) {
389	c.add(e);
390	++n;
391	}
392	return n;
393	} finally {
394	lock.unlock();
395	}
396	}
397
398	/**
399	* @throws UnsupportedOperationException {@inheritDoc}
400	* @throws ClassCastException {@inheritDoc}
401	* @throws NullPointerException {@inheritDoc}
402	* @throws IllegalArgumentException {@inheritDoc}
403	*/
404	public int drainTo(Collection<? super E> c, int maxElements) {
405	if (c == null)
406	throw new NullPointerException();
407	if (c == this)
408	throw new IllegalArgumentException();
409	if (maxElements <= 0)
410	return 0;
411	final ReentrantLock lock = this.lock;
412	lock.lock();
413	try {
414	int n = 0;
415	E e;
416	while (n < maxElements && (e = q.poll()) != null) {
417	c.add(e);
418	++n;
419	}
420	return n;
421	} finally {
422	lock.unlock();
423	}
424	}
425
426	/**
427	* Atomically removes all of the elements from this queue.
428	* The queue will be empty after this call returns.
429	*/
430	public void clear() {
431	final ReentrantLock lock = this.lock;
432	lock.lock();
433	try {
434	q.clear();
435	} finally {
436	lock.unlock();
437	}
438	}
439
440	/**
441	* Returns an array containing all of the elements in this queue; the
442	* runtime type of the returned array is that of the specified array.
443	* The returned array elements are in no particular order.
444	* If the queue fits in the specified array, it is returned therein.
445	* Otherwise, a new array is allocated with the runtime type of the
446	* specified array and the size of this queue.
447	*
448	* <p>If this queue fits in the specified array with room to spare
449	* (i.e., the array has more elements than this queue), the element in
450	* the array immediately following the end of the queue is set to
451	* <tt>null</tt>.
452	*
453	* <p>Like the {@link #toArray()} method, this method acts as bridge between
454	* array-based and collection-based APIs. Further, this method allows
455	* precise control over the runtime type of the output array, and may,
456	* under certain circumstances, be used to save allocation costs.
457	*
458	* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
459	* The following code can be used to dump the queue into a newly
460	* allocated array of <tt>String</tt>:
461	*
462	* <pre>
463	* String[] y = x.toArray(new String[0]);</pre>
464	*
465	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
466	* <tt>toArray()</tt>.
467	*
468	* @param a the array into which the elements of the queue are to
469	* be stored, if it is big enough; otherwise, a new array of the
470	* same runtime type is allocated for this purpose
471	* @return an array containing all of the elements in this queue
472	* @throws ArrayStoreException if the runtime type of the specified array
473	* is not a supertype of the runtime type of every element in
474	* this queue
475	* @throws NullPointerException if the specified array is null
476	*/
477	public <T> T[] toArray(T[] a) {
478	final ReentrantLock lock = this.lock;
479	lock.lock();
480	try {
481	return q.toArray(a);
482	} finally {
483	lock.unlock();
484	}
485	}
486
487	/**
488	* Returns an iterator over the elements in this queue. The
489	* iterator does not return the elements in any particular order.
490	*
491	* @return an iterator over the elements in this queue
492	*/
493	public Iterator<E> iterator() {
494	return new Itr<E>(toArray());
495	}
496
497	/**
498	* Snapshot iterator that works off copy of underlying q array.
499	*/
500	private class Itr<E> implements Iterator<E> {
501	final Object[] array; // Array of all elements
502	int cursor; // index of next element to return;
503	int lastRet; // index of last element, or -1 if no such
504
505	Itr(Object[] array) {
506	lastRet = -1;
507	this.array = array;
508	}
509
510	public boolean hasNext() {
511	return cursor < array.length;
512	}
513
514	public E next() {
515	if (cursor >= array.length)
516	throw new NoSuchElementException();
517	lastRet = cursor;
518	return (E)array[cursor++];
519	}
520
521	public void remove() {
522	if (lastRet < 0)
523	throw new IllegalStateException();
524	Object x = array[lastRet];
525	lastRet = -1;
526	// Traverse underlying queue to find == element,
527	// not just a .equals element.
528	lock.lock();
529	try {
530	for (Iterator it = q.iterator(); it.hasNext(); ) {
531	if (it.next() == x) {
532	it.remove();
533	return;
534	}
535	}
536	} finally {
537	lock.unlock();
538	}
539	}
540	}
541
542	/**
543	* Save the state to a stream (that is, serialize it). This
544	* merely wraps default serialization within lock. The
545	* serialization strategy for items is left to underlying
546	* Queue. Note that locking is not needed on deserialization, so
547	* readObject is not defined, just relying on default.
548	*/
549	private void writeObject(java.io.ObjectOutputStream s)
550	throws java.io.IOException {
551	lock.lock();
552	try {
553	s.defaultWriteObject();
554	} finally {
555	lock.unlock();
556	}
557	}
558
559	}