util/concurrent/LinkedBlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

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

/**
 * An optionally-bounded {@linkplain BlockingQueue blocking queue} based on
 * linked nodes.
 * This queue orders elements FIFO (first-in-first-out).
 * The <em>head</em> of the queue is that element that has been on the
 * queue the longest time.
 * The <em>tail</em> of the queue is that element that has been on the
 * queue the shortest time. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 * Linked queues typically have higher throughput than array-based queues but
 * less predictable performance in most concurrent applications.
 *
 * <p> The optional capacity bound constructor argument serves as a
 * way to prevent excessive queue expansion. The capacity, if unspecified,
 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * queue above capacity.
 *
 * <p>This class implements all of the <em>optional</em> methods
 * of the {@link Collection} and {@link Iterator} interfaces.
 *
 * @since 1.5
 * @author Doug Lea
 *
 **/
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;

    /*
     * A variant of the "two lock queue" algorithm.  The putLock gates
     * entry to put (and offer), and has an associated condition for
     * waiting puts.  Similarly for the takeLock.  The "count" field
     * that they both rely on is maintained as an atomic to avoid
     * needing to get both locks in most cases. Also, to minimize need
     * for puts to get takeLock and vice-versa, cascading notifies are
     * used. When a put notices that it has enabled at least one take,
     * it signals taker. That taker in turn signals others if more
     * items have been entered since the signal. And symmetrically for
     * takes signalling puts. Operations such as remove(Object) and
     * iterators acquire both locks.
    */

    /**
     * Linked list node class
     */
    static class Node<E> {
        /** The item, volatile to ensure barrier separating write and read */
        volatile E item;
        Node<E> next;
        Node(E x) { item = x; }
    }

    /** The capacity bound, or Integer.MAX_VALUE if none */
    private final int capacity;

    /** Current number of elements */
    private final AtomicInteger count = new AtomicInteger(0);

    /** Head of linked list */
    private transient Node<E> head;

    /** Tail of linked list */
    private transient Node<E> last;

    /** Lock held by take, poll, etc */
    private final ReentrantLock takeLock = new ReentrantLock();

    /** Wait queue for waiting takes */
    private final Condition notEmpty = takeLock.newCondition();

    /** Lock held by put, offer, etc */
    private final ReentrantLock putLock = new ReentrantLock();

    /** Wait queue for waiting puts */
    private final Condition notFull = putLock.newCondition();

    /**
     * Signal a waiting take. Called only from put/offer (which do not
     * otherwise ordinarily lock takeLock.)
     */
    private void signalNotEmpty() {
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }

    /**
     * Signal a waiting put. Called only from take/poll.
     */
    private void signalNotFull() {
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

    /**
     * Create a node and link it at end of queue
     * @param x the item
     */
    private void insert(E x) {
        last = last.next = new Node<E>(x);
    }

    /**
     * Remove a node from head of queue,
     * @return the node
     */
    private E extract() {
        Node<E> first = head.next;
        head = first;
        E x = (E)first.item;
        first.item = null;
        return x;
    }

    /**
     * Lock to prevent both puts and takes.
     */
    private void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

    /**
     * Unlock to allow both puts and takes.
     */
    private void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }


    /**
     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
     * {@link Integer#MAX_VALUE}.
     */
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }

    /**
     * Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
     *
     * @param capacity the capacity of this queue.
     * @throws IllegalArgumentException if <tt>capacity</tt> is not greater
     *         than zero.
     */
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }

    /**
     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
     * {@link Integer#MAX_VALUE}, initially containing the elements of the
     * given collection,
     * added in traversal order of the collection's iterator.
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public LinkedBlockingQueue(Collection<? extends E> c) {
        this(Integer.MAX_VALUE);
        for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
            add(it.next());
    }


    // this doc comment is overridden to remove the reference to collections
    // greater in size than Integer.MAX_VALUE
    /**
     * Returns the number of elements in this queue.
     *
     * @return  the number of elements in this queue.
     */
    public int size() {
        return count.get();
    }

    // this doc comment is a modified copy of the inherited doc comment,
    // without the reference to unlimited queues.
    /**
     * Returns the number of elements that this queue can ideally (in
     * the absence of memory or resource constraints) accept without
     * blocking. This is always equal to the initial capacity of this queue
     * less the current <tt>size</tt> of this queue.
     * <p>Note that you <em>cannot</em> always tell if
     * an attempt to <tt>add</tt> an element will succeed by
     * inspecting <tt>remainingCapacity</tt> because it may be the
     * case that a waiting consumer is ready to <tt>take</tt> an
     * element out of an otherwise full queue.
     */
    public int remainingCapacity() {
        return capacity - count.get();
    }

    public void put(E o) throws InterruptedException {
        if (o == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset
        // local var holding count  negative to indicate failure unless set.
        int c = -1;
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from
             * capacity. Similarly for all other uses of count in
             * other wait guards.
             */
            try {
                while (count.get() == capacity)
                    notFull.await();
            } catch (InterruptedException ie) {
                notFull.signal(); // propagate to a non-interrupted thread
                throw ie;
            }
            insert(o);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

    public boolean offer(E o, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (o == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        putLock.lockInterruptibly();
        try {
            for (;;) {
                if (count.get() < capacity) {
                    insert(o);
                    c = count.getAndIncrement();
                    if (c + 1 < capacity)
                        notFull.signal();
                    break;
                }
                if (nanos <= 0)
                    return false;
                try {
                    nanos = notFull.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    notFull.signal(); // propagate to a non-interrupted thread
                    throw ie;
                }
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }

   /**
    * Adds the specified element to the tail of this queue if possible,
    * returning immediately if this queue is full.
    *
    * @param o the element to add.
    * @return <tt>true</tt> if it was possible to add the element to
    *         this queue, else <tt>false</tt>
    * @throws NullPointerException if the specified element is <tt>null</tt>
    */
    public boolean offer(E o) {
        if (o == null) throw new NullPointerException();
        if (count.get() == capacity)
            return false;
        int c = -1;
        putLock.lock();
        try {
            if (count.get() < capacity) {
                insert(o);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    }


    public E take() throws InterruptedException {
        E x;
        int c = -1;
        takeLock.lockInterruptibly();
        try {
            try {
                while (count.get() == 0)
                    notEmpty.await();
            } catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to a non-interrupted thread
                throw ie;
            }

            x = extract();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E x = null;
        int c = -1;
        long nanos = unit.toNanos(timeout);
        takeLock.lockInterruptibly();
        try {
            for (;;) {
                if (count.get() > 0) {
                    x = extract();
                    c = count.getAndDecrement();
                    if (c > 1)
                        notEmpty.signal();
                    break;
                }
                if (nanos <= 0)
                    return null;
                try {
                    nanos = notEmpty.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to a non-interrupted thread
                    throw ie;
                }
            }
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }

    public E poll() {
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1;
        takeLock.tryLock();
        try {
            if (count.get() > 0) {
                x = extract();
                c = count.getAndDecrement();
                if (c > 1)
                    notEmpty.signal();
            }
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }


    public E peek() {
        if (count.get() == 0)
            return null;
        takeLock.lock();
        try {
            Node<E> first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
        } finally {
            takeLock.unlock();
        }
    }

    public boolean remove(Object o) {
        if (o == null) return false;
        boolean removed = false;
        fullyLock();
        try {
            Node<E> trail = head;
            Node<E> p = head.next;
            while (p != null) {
                if (o.equals(p.item)) {
                    removed = true;
                    break;
                }
                trail = p;
                p = p.next;
            }
            if (removed) {
                p.item = null;
                trail.next = p.next;
                if (count.getAndDecrement() == capacity)
                    notFull.signalAll();
            }
        } finally {
            fullyUnlock();
        }
        return removed;
    }

    public Object[] toArray() {
        fullyLock();
        try {
            int size = count.get();
            Object[] a = new Object[size];
            int k = 0;
            for (Node<E> p = head.next; p != null; p = p.next)
                a[k++] = p.item;
            return a;
        } finally {
            fullyUnlock();
        }
    }

    public <T> T[] toArray(T[] a) {
        fullyLock();
        try {
            int size = count.get();
            if (a.length < size)
                a = (T[])java.lang.reflect.Array.newInstance
                    (a.getClass().getComponentType(), size);

            int k = 0;
            for (Node p = head.next; p != null; p = p.next)
                a[k++] = (T)p.item;
            return a;
        } finally {
            fullyUnlock();
        }
    }

    public String toString() {
        fullyLock();
        try {
            return super.toString();
        } finally {
            fullyUnlock();
        }
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
     * will never throw {@link java.util.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 in proper sequence.
     */
    public Iterator<E> iterator() {
      return new Itr();
    }

    private class Itr implements Iterator<E> {
        /*
         * Basic weak-consistent iterator.  At all times hold the next
         * item to hand out so that if hasNext() reports true, we will
         * still have it to return even if lost race with a take etc.
         */
        Node<E> current;
        Node<E> lastRet;
        E currentElement;

        Itr() {
            fullyLock();
            try {
                current = head.next;
                if (current != null)
                    currentElement = current.item;
            } finally {
                fullyUnlock();
            }
        }

        public boolean hasNext() {
            return current != null;
        }

        public E next() {
            fullyLock();
            try {
                if (current == null)
                    throw new NoSuchElementException();
                E x = currentElement;
                lastRet = current;
                current = current.next;
                if (current != null)
                    currentElement = current.item;
                return x;
            } finally {
                fullyUnlock();
            }

        }

        public void remove() {
            if (lastRet == null)
                throw new IllegalStateException();
            fullyLock();
            try {
                Node<E> node = lastRet;
                lastRet = null;
                Node<E> trail = head;
                Node<E> p = head.next;
                while (p != null && p != node) {
                    trail = p;
                    p = p.next;
                }
                if (p == node) {
                    p.item = null;
                    trail.next = p.next;
                    int c = count.getAndDecrement();
                    if (c == capacity)
                        notFull.signalAll();
                }
            } finally {
                fullyUnlock();
            }
        }
    }

    /**
     * Save the state to a stream (that is, serialize it).
     *
     * @serialData The capacity is emitted (int), followed by all of
     * its elements (each an <tt>Object</tt>) in the proper order,
     * followed by a null
     * @param s the stream
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

        fullyLock();
        try {
            // Write out any hidden stuff, plus capacity
            s.defaultWriteObject();

            // Write out all elements in the proper order.
            for (Node<E> p = head.next; p != null; p = p.next)
                s.writeObject(p.item);

            // Use trailing null as sentinel
            s.writeObject(null);
        } finally {
            fullyUnlock();
        }
    }

    /**
     * Reconstitute this queue instance from a stream (that is,
     * deserialize it).
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in capacity, and any hidden stuff
        s.defaultReadObject();

        count.set(0);
        last = head = new Node<E>(null);

        // Read in all elements and place in queue
        for (;;) {
            E item = (E)s.readObject();
            if (item == null)
                break;
            add(item);
        }
    }
}
Revision:	1.21
Committed:	Sat Sep 13 18:51:11 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.20:	+3 -0 lines
Log Message:	Proofreading pass -- many minor adjustments
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain. Use, modify, and
4	* redistribute this code in any way without acknowledgement.
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.atomic.*;
9	import java.util.concurrent.locks.*;
10	import java.util.*;
11
12	/**
13	* An optionally-bounded {@linkplain BlockingQueue blocking queue} based on
14	* linked nodes.
15	* This queue orders elements FIFO (first-in-first-out).
16	* The <em>head</em> of the queue is that element that has been on the
17	* queue the longest time.
18	* The <em>tail</em> of the queue is that element that has been on the
19	* queue the shortest time. New elements
20	* are inserted at the tail of the queue, and the queue retrieval
21	* operations obtain elements at the head of the queue.
22	* Linked queues typically have higher throughput than array-based queues but
23	* less predictable performance in most concurrent applications.
24	*
25	* <p> The optional capacity bound constructor argument serves as a
26	* way to prevent excessive queue expansion. The capacity, if unspecified,
27	* is equal to {@link Integer#MAX_VALUE}. Linked nodes are
28	* dynamically created upon each insertion unless this would bring the
29	* queue above capacity.
30	*
31	* <p>This class implements all of the <em>optional</em> methods
32	* of the {@link Collection} and {@link Iterator} interfaces.
33	*
34	* @since 1.5
35	* @author Doug Lea
36	*
37	**/
38	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
39	implements BlockingQueue<E>, java.io.Serializable {
40	private static final long serialVersionUID = -6903933977591709194L;
41
42	/*
43	* A variant of the "two lock queue" algorithm. The putLock gates
44	* entry to put (and offer), and has an associated condition for
45	* waiting puts. Similarly for the takeLock. The "count" field
46	* that they both rely on is maintained as an atomic to avoid
47	* needing to get both locks in most cases. Also, to minimize need
48	* for puts to get takeLock and vice-versa, cascading notifies are
49	* used. When a put notices that it has enabled at least one take,
50	* it signals taker. That taker in turn signals others if more
51	* items have been entered since the signal. And symmetrically for
52	* takes signalling puts. Operations such as remove(Object) and
53	* iterators acquire both locks.
54	*/
55
56	/**
57	* Linked list node class
58	*/
59	static class Node<E> {
60	/** The item, volatile to ensure barrier separating write and read */
61	volatile E item;
62	Node<E> next;
63	Node(E x) { item = x; }
64	}
65
66	/** The capacity bound, or Integer.MAX_VALUE if none */
67	private final int capacity;
68
69	/** Current number of elements */
70	private final AtomicInteger count = new AtomicInteger(0);
71
72	/** Head of linked list */
73	private transient Node<E> head;
74
75	/** Tail of linked list */
76	private transient Node<E> last;
77
78	/** Lock held by take, poll, etc */
79	private final ReentrantLock takeLock = new ReentrantLock();
80
81	/** Wait queue for waiting takes */
82	private final Condition notEmpty = takeLock.newCondition();
83
84	/** Lock held by put, offer, etc */
85	private final ReentrantLock putLock = new ReentrantLock();
86
87	/** Wait queue for waiting puts */
88	private final Condition notFull = putLock.newCondition();
89
90	/**
91	* Signal a waiting take. Called only from put/offer (which do not
92	* otherwise ordinarily lock takeLock.)
93	*/
94	private void signalNotEmpty() {
95	takeLock.lock();
96	try {
97	notEmpty.signal();
98	} finally {
99	takeLock.unlock();
100	}
101	}
102
103	/**
104	* Signal a waiting put. Called only from take/poll.
105	*/
106	private void signalNotFull() {
107	putLock.lock();
108	try {
109	notFull.signal();
110	} finally {
111	putLock.unlock();
112	}
113	}
114
115	/**
116	* Create a node and link it at end of queue
117	* @param x the item
118	*/
119	private void insert(E x) {
120	last = last.next = new Node<E>(x);
121	}
122
123	/**
124	* Remove a node from head of queue,
125	* @return the node
126	*/
127	private E extract() {
128	Node<E> first = head.next;
129	head = first;
130	E x = (E)first.item;
131	first.item = null;
132	return x;
133	}
134
135	/**
136	* Lock to prevent both puts and takes.
137	*/
138	private void fullyLock() {
139	putLock.lock();
140	takeLock.lock();
141	}
142
143	/**
144	* Unlock to allow both puts and takes.
145	*/
146	private void fullyUnlock() {
147	takeLock.unlock();
148	putLock.unlock();
149	}
150
151
152	/**
153	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
154	* {@link Integer#MAX_VALUE}.
155	*/
156	public LinkedBlockingQueue() {
157	this(Integer.MAX_VALUE);
158	}
159
160	/**
161	* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
162	*
163	* @param capacity the capacity of this queue.
164	* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
165	* than zero.
166	*/
167	public LinkedBlockingQueue(int capacity) {
168	if (capacity <= 0) throw new IllegalArgumentException();
169	this.capacity = capacity;
170	last = head = new Node<E>(null);
171	}
172
173	/**
174	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
175	* {@link Integer#MAX_VALUE}, initially containing the elements of the
176	* given collection,
177	* added in traversal order of the collection's iterator.
178	* @param c the collection of elements to initially contain
179	* @throws NullPointerException if <tt>c</tt> or any element within it
180	* is <tt>null</tt>
181	*/
182	public LinkedBlockingQueue(Collection<? extends E> c) {
183	this(Integer.MAX_VALUE);
184	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
185	add(it.next());
186	}
187
188
189	// this doc comment is overridden to remove the reference to collections
190	// greater in size than Integer.MAX_VALUE
191	/**
192	* Returns the number of elements in this queue.
193	*
194	* @return the number of elements in this queue.
195	*/
196	public int size() {
197	return count.get();
198	}
199
200	// this doc comment is a modified copy of the inherited doc comment,
201	// without the reference to unlimited queues.
202	/**
203	* Returns the number of elements that this queue can ideally (in
204	* the absence of memory or resource constraints) accept without
205	* blocking. This is always equal to the initial capacity of this queue
206	* less the current <tt>size</tt> of this queue.
207	* <p>Note that you <em>cannot</em> always tell if
208	* an attempt to <tt>add</tt> an element will succeed by
209	* inspecting <tt>remainingCapacity</tt> because it may be the
210	* case that a waiting consumer is ready to <tt>take</tt> an
211	* element out of an otherwise full queue.
212	*/
213	public int remainingCapacity() {
214	return capacity - count.get();
215	}
216
217	public void put(E o) throws InterruptedException {
218	if (o == null) throw new NullPointerException();
219	// Note: convention in all put/take/etc is to preset
220	// local var holding count negative to indicate failure unless set.
221	int c = -1;
222	putLock.lockInterruptibly();
223	try {
224	/*
225	* Note that count is used in wait guard even though it is
226	* not protected by lock. This works because count can
227	* only decrease at this point (all other puts are shut
228	* out by lock), and we (or some other waiting put) are
229	* signalled if it ever changes from
230	* capacity. Similarly for all other uses of count in
231	* other wait guards.
232	*/
233	try {
234	while (count.get() == capacity)
235	notFull.await();
236	} catch (InterruptedException ie) {
237	notFull.signal(); // propagate to a non-interrupted thread
238	throw ie;
239	}
240	insert(o);
241	c = count.getAndIncrement();
242	if (c + 1 < capacity)
243	notFull.signal();
244	} finally {
245	putLock.unlock();
246	}
247	if (c == 0)
248	signalNotEmpty();
249	}
250
251	public boolean offer(E o, long timeout, TimeUnit unit)
252	throws InterruptedException {
253
254	if (o == null) throw new NullPointerException();
255	long nanos = unit.toNanos(timeout);
256	int c = -1;
257	putLock.lockInterruptibly();
258	try {
259	for (;;) {
260	if (count.get() < capacity) {
261	insert(o);
262	c = count.getAndIncrement();
263	if (c + 1 < capacity)
264	notFull.signal();
265	break;
266	}
267	if (nanos <= 0)
268	return false;
269	try {
270	nanos = notFull.awaitNanos(nanos);
271	} catch (InterruptedException ie) {
272	notFull.signal(); // propagate to a non-interrupted thread
273	throw ie;
274	}
275	}
276	} finally {
277	putLock.unlock();
278	}
279	if (c == 0)
280	signalNotEmpty();
281	return true;
282	}
283
284	/**
285	* Adds the specified element to the tail of this queue if possible,
286	* returning immediately if this queue is full.
287	*
288	* @param o the element to add.
289	* @return <tt>true</tt> if it was possible to add the element to
290	* this queue, else <tt>false</tt>
291	* @throws NullPointerException if the specified element is <tt>null</tt>
292	*/
293	public boolean offer(E o) {
294	if (o == null) throw new NullPointerException();
295	if (count.get() == capacity)
296	return false;
297	int c = -1;
298	putLock.lock();
299	try {
300	if (count.get() < capacity) {
301	insert(o);
302	c = count.getAndIncrement();
303	if (c + 1 < capacity)
304	notFull.signal();
305	}
306	} finally {
307	putLock.unlock();
308	}
309	if (c == 0)
310	signalNotEmpty();
311	return c >= 0;
312	}
313
314
315	public E take() throws InterruptedException {
316	E x;
317	int c = -1;
318	takeLock.lockInterruptibly();
319	try {
320	try {
321	while (count.get() == 0)
322	notEmpty.await();
323	} catch (InterruptedException ie) {
324	notEmpty.signal(); // propagate to a non-interrupted thread
325	throw ie;
326	}
327
328	x = extract();
329	c = count.getAndDecrement();
330	if (c > 1)
331	notEmpty.signal();
332	} finally {
333	takeLock.unlock();
334	}
335	if (c == capacity)
336	signalNotFull();
337	return x;
338	}
339
340	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
341	E x = null;
342	int c = -1;
343	long nanos = unit.toNanos(timeout);
344	takeLock.lockInterruptibly();
345	try {
346	for (;;) {
347	if (count.get() > 0) {
348	x = extract();
349	c = count.getAndDecrement();
350	if (c > 1)
351	notEmpty.signal();
352	break;
353	}
354	if (nanos <= 0)
355	return null;
356	try {
357	nanos = notEmpty.awaitNanos(nanos);
358	} catch (InterruptedException ie) {
359	notEmpty.signal(); // propagate to a non-interrupted thread
360	throw ie;
361	}
362	}
363	} finally {
364	takeLock.unlock();
365	}
366	if (c == capacity)
367	signalNotFull();
368	return x;
369	}
370
371	public E poll() {
372	if (count.get() == 0)
373	return null;
374	E x = null;
375	int c = -1;
376	takeLock.tryLock();
377	try {
378	if (count.get() > 0) {
379	x = extract();
380	c = count.getAndDecrement();
381	if (c > 1)
382	notEmpty.signal();
383	}
384	} finally {
385	takeLock.unlock();
386	}
387	if (c == capacity)
388	signalNotFull();
389	return x;
390	}
391
392
393	public E peek() {
394	if (count.get() == 0)
395	return null;
396	takeLock.lock();
397	try {
398	Node<E> first = head.next;
399	if (first == null)
400	return null;
401	else
402	return first.item;
403	} finally {
404	takeLock.unlock();
405	}
406	}
407
408	public boolean remove(Object o) {
409	if (o == null) return false;
410	boolean removed = false;
411	fullyLock();
412	try {
413	Node<E> trail = head;
414	Node<E> p = head.next;
415	while (p != null) {
416	if (o.equals(p.item)) {
417	removed = true;
418	break;
419	}
420	trail = p;
421	p = p.next;
422	}
423	if (removed) {
424	p.item = null;
425	trail.next = p.next;
426	if (count.getAndDecrement() == capacity)
427	notFull.signalAll();
428	}
429	} finally {
430	fullyUnlock();
431	}
432	return removed;
433	}
434
435	public Object[] toArray() {
436	fullyLock();
437	try {
438	int size = count.get();
439	Object[] a = new Object[size];
440	int k = 0;
441	for (Node<E> p = head.next; p != null; p = p.next)
442	a[k++] = p.item;
443	return a;
444	} finally {
445	fullyUnlock();
446	}
447	}
448
449	public <T> T[] toArray(T[] a) {
450	fullyLock();
451	try {
452	int size = count.get();
453	if (a.length < size)
454	a = (T[])java.lang.reflect.Array.newInstance
455	(a.getClass().getComponentType(), size);
456
457	int k = 0;
458	for (Node p = head.next; p != null; p = p.next)
459	a[k++] = (T)p.item;
460	return a;
461	} finally {
462	fullyUnlock();
463	}
464	}
465
466	public String toString() {
467	fullyLock();
468	try {
469	return super.toString();
470	} finally {
471	fullyUnlock();
472	}
473	}
474
475	/**
476	* Returns an iterator over the elements in this queue in proper sequence.
477	* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
478	* will never throw {@link java.util.ConcurrentModificationException},
479	* and guarantees to traverse elements as they existed upon
480	* construction of the iterator, and may (but is not guaranteed to)
481	* reflect any modifications subsequent to construction.
482	*
483	* @return an iterator over the elements in this queue in proper sequence.
484	*/
485	public Iterator<E> iterator() {
486	return new Itr();
487	}
488
489	private class Itr implements Iterator<E> {
490	/*
491	* Basic weak-consistent iterator. At all times hold the next
492	* item to hand out so that if hasNext() reports true, we will
493	* still have it to return even if lost race with a take etc.
494	*/
495	Node<E> current;
496	Node<E> lastRet;
497	E currentElement;
498
499	Itr() {
500	fullyLock();
501	try {
502	current = head.next;
503	if (current != null)
504	currentElement = current.item;
505	} finally {
506	fullyUnlock();
507	}
508	}
509
510	public boolean hasNext() {
511	return current != null;
512	}
513
514	public E next() {
515	fullyLock();
516	try {
517	if (current == null)
518	throw new NoSuchElementException();
519	E x = currentElement;
520	lastRet = current;
521	current = current.next;
522	if (current != null)
523	currentElement = current.item;
524	return x;
525	} finally {
526	fullyUnlock();
527	}
528
529	}
530
531	public void remove() {
532	if (lastRet == null)
533	throw new IllegalStateException();
534	fullyLock();
535	try {
536	Node<E> node = lastRet;
537	lastRet = null;
538	Node<E> trail = head;
539	Node<E> p = head.next;
540	while (p != null && p != node) {
541	trail = p;
542	p = p.next;
543	}
544	if (p == node) {
545	p.item = null;
546	trail.next = p.next;
547	int c = count.getAndDecrement();
548	if (c == capacity)
549	notFull.signalAll();
550	}
551	} finally {
552	fullyUnlock();
553	}
554	}
555	}
556
557	/**
558	* Save the state to a stream (that is, serialize it).
559	*
560	* @serialData The capacity is emitted (int), followed by all of
561	* its elements (each an <tt>Object</tt>) in the proper order,
562	* followed by a null
563	* @param s the stream
564	*/
565	private void writeObject(java.io.ObjectOutputStream s)
566	throws java.io.IOException {
567
568	fullyLock();
569	try {
570	// Write out any hidden stuff, plus capacity
571	s.defaultWriteObject();
572
573	// Write out all elements in the proper order.
574	for (Node<E> p = head.next; p != null; p = p.next)
575	s.writeObject(p.item);
576
577	// Use trailing null as sentinel
578	s.writeObject(null);
579	} finally {
580	fullyUnlock();
581	}
582	}
583
584	/**
585	* Reconstitute this queue instance from a stream (that is,
586	* deserialize it).
587	* @param s the stream
588	*/
589	private void readObject(java.io.ObjectInputStream s)
590	throws java.io.IOException, ClassNotFoundException {
591	// Read in capacity, and any hidden stuff
592	s.defaultReadObject();
593
594	count.set(0);
595	last = head = new Node<E>(null);
596
597	// Read in all elements and place in queue
598	for (;;) {
599	E item = (E)s.readObject();
600	if (item == null)
601	break;
602	add(item);
603	}
604	}
605	}