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 {@link 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.
 * 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.
 *
 * @since 1.5
 * @author Doug Lea
 * 
 **/
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /*
     * 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 transient 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();
    }


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

    /**
     * Create 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);
    }

    /**
     * Create a <tt>LinkedBlockingQueue</tt> with a capacity if
     * {@link Integer#MAX_VALUE}, initially holding the elements of the 
     * given collection, 
     * added in traversal order of the collection's iterator.
     * @param initialElements the elements to initially contain
     */
    public LinkedBlockingQueue(Collection<E> initialElements) {
        this(Integer.MAX_VALUE);
        for (Iterator<E> it = initialElements.iterator(); it.hasNext();) 
            add(it.next());
    }

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

    // this doc comment is a modified copy of the inherited doc comment,
    // without the reference to unlimited queues.
    /** 
     * Return 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();
    }

    /**
     * Add the specified element to the tail of this queue, waiting if 
     * necessary for space to become available.
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E x) throws InterruptedException {
        if (x == 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(x);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        }
        finally {
            putLock.unlock();
        }
        if (c == 0) 
            signalNotEmpty();
    }

    /**
     * Add the specified element to the tail of this queue, waiting if 
     * necessary up to the specified wait time for space to become available.
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E x, long timeout, TimeUnit unit) 
        throws InterruptedException {
       
        if (x == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        putLock.lockInterruptibly();
        try {
            for (;;) {
                if (count.get() < capacity) {
                    insert(x);
                    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;
    }

   /** 
    * Add the specified element to the tail of this queue if possible,
    * returning immediately if this queue is full.
    *
    * @throws NullPointerException {@inheritDoc}
    */
    public boolean offer(E x) {
        if (x == null) throw new NullPointerException();
        if (count.get() == capacity)
            return false;
        int c = -1; 
        putLock.lock();
        try {
            if (count.get() < capacity) {
                insert(x);
                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 x) {
        if (x == null) return false;
        boolean removed = false;
        fullyLock();
        try {
            Node<E> trail = head;
            Node<E> p = head.next;
            while (p != null) {
                if (x.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();
        }
    }

    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();

        // Read in all elements and place in queue
        for (;;) {
            E item = (E)s.readObject();
            if (item == null)
                break;
            add(item);
        }
    }
}


Revision:	1.8
Committed:	Mon Jul 28 04:11:54 2003 UTC (20 years, 10 months ago) by dholmes
Branch:	MAIN
Changes since 1.7:	+71 -19 lines
Log Message:	Significant doc updates: - inherit comments where appropriate - ensure runtime exception comments inherited (overriding as needed) - consistent descriptions - introduce head and tail terminology
#	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 {@link 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.
20	* Linked queues typically have higher throughput than array-based queues but
21	* less predictable performance in most concurrent applications.
22	*
23	* <p> The optional capacity bound constructor argument serves as a
24	* way to prevent excessive queue expansion. The capacity, if unspecified,
25	* is equal to {@link Integer#MAX_VALUE}. Linked nodes are
26	* dynamically created upon each insertion unless this would bring the
27	* queue above capacity.
28	*
29	* @since 1.5
30	* @author Doug Lea
31	*
32	**/
33	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
34	implements BlockingQueue<E>, java.io.Serializable {
35
36	/*
37	* A variant of the "two lock queue" algorithm. The putLock gates
38	* entry to put (and offer), and has an associated condition for
39	* waiting puts. Similarly for the takeLock. The "count" field
40	* that they both rely on is maintained as an atomic to avoid
41	* needing to get both locks in most cases. Also, to minimize need
42	* for puts to get takeLock and vice-versa, cascading notifies are
43	* used. When a put notices that it has enabled at least one take,
44	* it signals taker. That taker in turn signals others if more
45	* items have been entered since the signal. And symmetrically for
46	* takes signalling puts. Operations such as remove(Object) and
47	* iterators acquire both locks.
48	*/
49
50	/**
51	* Linked list node class
52	*/
53	static class Node<E> {
54	/** The item, volatile to ensure barrier separating write and read */
55	volatile E item;
56	Node<E> next;
57	Node(E x) { item = x; }
58	}
59
60	/** The capacity bound, or Integer.MAX_VALUE if none */
61	private final int capacity;
62
63	/** Current number of elements */
64	private transient final AtomicInteger count = new AtomicInteger(0);
65
66	/** Head of linked list */
67	private transient Node<E> head;
68
69	/** Tail of linked list */
70	private transient Node<E> last;
71
72	/** Lock held by take, poll, etc */
73	private final ReentrantLock takeLock = new ReentrantLock();
74
75	/** Wait queue for waiting takes */
76	private final Condition notEmpty = takeLock.newCondition();
77
78	/** Lock held by put, offer, etc */
79	private final ReentrantLock putLock = new ReentrantLock();
80
81	/** Wait queue for waiting puts */
82	private final Condition notFull = putLock.newCondition();
83
84	/**
85	* Signal a waiting take. Called only from put/offer (which do not
86	* otherwise ordinarily lock takeLock.)
87	*/
88	private void signalNotEmpty() {
89	takeLock.lock();
90	try {
91	notEmpty.signal();
92	}
93	finally {
94	takeLock.unlock();
95	}
96	}
97
98	/**
99	* Signal a waiting put. Called only from take/poll.
100	*/
101	private void signalNotFull() {
102	putLock.lock();
103	try {
104	notFull.signal();
105	}
106	finally {
107	putLock.unlock();
108	}
109	}
110
111	/**
112	* Create a node and link it at end of queue
113	* @param x the item
114	*/
115	private void insert(E x) {
116	last = last.next = new Node<E>(x);
117	}
118
119	/**
120	* Remove a node from head of queue,
121	* @return the node
122	*/
123	private E extract() {
124	Node<E> first = head.next;
125	head = first;
126	E x = (E)first.item;
127	first.item = null;
128	return x;
129	}
130
131	/**
132	* Lock to prevent both puts and takes.
133	*/
134	private void fullyLock() {
135	putLock.lock();
136	takeLock.lock();
137	}
138
139	/**
140	* Unlock to allow both puts and takes.
141	*/
142	private void fullyUnlock() {
143	takeLock.unlock();
144	putLock.unlock();
145	}
146
147
148	/**
149	* Create a <tt>LinkedBlockingQueue</tt> with a capacity of
150	* {@link Integer#MAX_VALUE}.
151	*/
152	public LinkedBlockingQueue() {
153	this(Integer.MAX_VALUE);
154	}
155
156	/**
157	* Create a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity
158	* @param capacity the capacity of this queue.
159	* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
160	* than zero.
161	*/
162	public LinkedBlockingQueue(int capacity) {
163	if (capacity <= 0) throw new IllegalArgumentException();
164	this.capacity = capacity;
165	last = head = new Node<E>(null);
166	}
167
168	/**
169	* Create a <tt>LinkedBlockingQueue</tt> with a capacity if
170	* {@link Integer#MAX_VALUE}, initially holding the elements of the
171	* given collection,
172	* added in traversal order of the collection's iterator.
173	* @param initialElements the elements to initially contain
174	*/
175	public LinkedBlockingQueue(Collection<E> initialElements) {
176	this(Integer.MAX_VALUE);
177	for (Iterator<E> it = initialElements.iterator(); it.hasNext();)
178	add(it.next());
179	}
180
181	// this doc comment is overridden to remove the reference to collections
182	// greater in size than Integer.MAX_VALUE
183	/**
184	* Return the number of elements in this collection.
185	*/
186	public int size() {
187	return count.get();
188	}
189
190	// this doc comment is a modified copy of the inherited doc comment,
191	// without the reference to unlimited queues.
192	/**
193	* Return the number of elements that this queue can ideally (in
194	* the absence of memory or resource constraints) accept without
195	* blocking. This is always equal to the initial capacity of this queue
196	* less the current <tt>size</tt> of this queue.
197	* <p>Note that you <em>cannot</em> always tell if
198	* an attempt to <tt>add</tt> an element will succeed by
199	* inspecting <tt>remainingCapacity</tt> because it may be the
200	* case that a waiting consumer is ready to <tt>take</tt> an
201	* element out of an otherwise full queue.
202	*/
203	public int remainingCapacity() {
204	return capacity - count.get();
205	}
206
207	/**
208	* Add the specified element to the tail of this queue, waiting if
209	* necessary for space to become available.
210	* @throws NullPointerException {@inheritDoc}
211	*/
212	public void put(E x) throws InterruptedException {
213	if (x == null) throw new NullPointerException();
214	// Note: convention in all put/take/etc is to preset
215	// local var holding count negative to indicate failure unless set.
216	int c = -1;
217	putLock.lockInterruptibly();
218	try {
219	/*
220	* Note that count is used in wait guard even though it is
221	* not protected by lock. This works because count can
222	* only decrease at this point (all other puts are shut
223	* out by lock), and we (or some other waiting put) are
224	* signalled if it ever changes from
225	* capacity. Similarly for all other uses of count in
226	* other wait guards.
227	*/
228	try {
229	while (count.get() == capacity)
230	notFull.await();
231	}
232	catch (InterruptedException ie) {
233	notFull.signal(); // propagate to a non-interrupted thread
234	throw ie;
235	}
236	insert(x);
237	c = count.getAndIncrement();
238	if (c + 1 < capacity)
239	notFull.signal();
240	}
241	finally {
242	putLock.unlock();
243	}
244	if (c == 0)
245	signalNotEmpty();
246	}
247
248	/**
249	* Add the specified element to the tail of this queue, waiting if
250	* necessary up to the specified wait time for space to become available.
251	* @throws NullPointerException {@inheritDoc}
252	*/
253	public boolean offer(E x, long timeout, TimeUnit unit)
254	throws InterruptedException {
255
256	if (x == null) throw new NullPointerException();
257	long nanos = unit.toNanos(timeout);
258	int c = -1;
259	putLock.lockInterruptibly();
260	try {
261	for (;;) {
262	if (count.get() < capacity) {
263	insert(x);
264	c = count.getAndIncrement();
265	if (c + 1 < capacity)
266	notFull.signal();
267	break;
268	}
269	if (nanos <= 0)
270	return false;
271	try {
272	nanos = notFull.awaitNanos(nanos);
273	}
274	catch (InterruptedException ie) {
275	notFull.signal(); // propagate to a non-interrupted thread
276	throw ie;
277	}
278	}
279	}
280	finally {
281	putLock.unlock();
282	}
283	if (c == 0)
284	signalNotEmpty();
285	return true;
286	}
287
288	/**
289	* Add the specified element to the tail of this queue if possible,
290	* returning immediately if this queue is full.
291	*
292	* @throws NullPointerException {@inheritDoc}
293	*/
294	public boolean offer(E x) {
295	if (x == null) throw new NullPointerException();
296	if (count.get() == capacity)
297	return false;
298	int c = -1;
299	putLock.lock();
300	try {
301	if (count.get() < capacity) {
302	insert(x);
303	c = count.getAndIncrement();
304	if (c + 1 < capacity)
305	notFull.signal();
306	}
307	}
308	finally {
309	putLock.unlock();
310	}
311	if (c == 0)
312	signalNotEmpty();
313	return c >= 0;
314	}
315
316
317	public E take() throws InterruptedException {
318	E x;
319	int c = -1;
320	takeLock.lockInterruptibly();
321	try {
322	try {
323	while (count.get() == 0)
324	notEmpty.await();
325	}
326	catch (InterruptedException ie) {
327	notEmpty.signal(); // propagate to a non-interrupted thread
328	throw ie;
329	}
330
331	x = extract();
332	c = count.getAndDecrement();
333	if (c > 1)
334	notEmpty.signal();
335	}
336	finally {
337	takeLock.unlock();
338	}
339	if (c == capacity)
340	signalNotFull();
341	return x;
342	}
343
344	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
345	E x = null;
346	int c = -1;
347	long nanos = unit.toNanos(timeout);
348	takeLock.lockInterruptibly();
349	try {
350	for (;;) {
351	if (count.get() > 0) {
352	x = extract();
353	c = count.getAndDecrement();
354	if (c > 1)
355	notEmpty.signal();
356	break;
357	}
358	if (nanos <= 0)
359	return null;
360	try {
361	nanos = notEmpty.awaitNanos(nanos);
362	}
363	catch (InterruptedException ie) {
364	notEmpty.signal(); // propagate to a non-interrupted thread
365	throw ie;
366	}
367	}
368	}
369	finally {
370	takeLock.unlock();
371	}
372	if (c == capacity)
373	signalNotFull();
374	return x;
375	}
376
377	public E poll() {
378	if (count.get() == 0)
379	return null;
380	E x = null;
381	int c = -1;
382	takeLock.tryLock();
383	try {
384	if (count.get() > 0) {
385	x = extract();
386	c = count.getAndDecrement();
387	if (c > 1)
388	notEmpty.signal();
389	}
390	}
391	finally {
392	takeLock.unlock();
393	}
394	if (c == capacity)
395	signalNotFull();
396	return x;
397	}
398
399
400	public E peek() {
401	if (count.get() == 0)
402	return null;
403	takeLock.lock();
404	try {
405	Node<E> first = head.next;
406	if (first == null)
407	return null;
408	else
409	return first.item;
410	}
411	finally {
412	takeLock.unlock();
413	}
414	}
415
416	public boolean remove(Object x) {
417	if (x == null) return false;
418	boolean removed = false;
419	fullyLock();
420	try {
421	Node<E> trail = head;
422	Node<E> p = head.next;
423	while (p != null) {
424	if (x.equals(p.item)) {
425	removed = true;
426	break;
427	}
428	trail = p;
429	p = p.next;
430	}
431	if (removed) {
432	p.item = null;
433	trail.next = p.next;
434	if (count.getAndDecrement() == capacity)
435	notFull.signalAll();
436	}
437	}
438	finally {
439	fullyUnlock();
440	}
441	return removed;
442	}
443
444	public Object[] toArray() {
445	fullyLock();
446	try {
447	int size = count.get();
448	Object[] a = new Object[size];
449	int k = 0;
450	for (Node<E> p = head.next; p != null; p = p.next)
451	a[k++] = p.item;
452	return a;
453	}
454	finally {
455	fullyUnlock();
456	}
457	}
458
459	public <T> T[] toArray(T[] a) {
460	fullyLock();
461	try {
462	int size = count.get();
463	if (a.length < size)
464	a = (T[])java.lang.reflect.Array.newInstance
465	(a.getClass().getComponentType(), size);
466
467	int k = 0;
468	for (Node p = head.next; p != null; p = p.next)
469	a[k++] = (T)p.item;
470	return a;
471	}
472	finally {
473	fullyUnlock();
474	}
475	}
476
477	public String toString() {
478	fullyLock();
479	try {
480	return super.toString();
481	}
482	finally {
483	fullyUnlock();
484	}
485	}
486
487	public Iterator<E> iterator() {
488	return new Itr();
489	}
490
491	private class Itr implements Iterator<E> {
492	/*
493	* Basic weak-consistent iterator. At all times hold the next
494	* item to hand out so that if hasNext() reports true, we will
495	* still have it to return even if lost race with a take etc.
496	*/
497	Node<E> current;
498	Node<E> lastRet;
499	E currentElement;
500
501	Itr() {
502	fullyLock();
503	try {
504	current = head.next;
505	if (current != null)
506	currentElement = current.item;
507	}
508	finally {
509	fullyUnlock();
510	}
511	}
512
513	public boolean hasNext() {
514	return current != null;
515	}
516
517	public E next() {
518	fullyLock();
519	try {
520	if (current == null)
521	throw new NoSuchElementException();
522	E x = currentElement;
523	lastRet = current;
524	current = current.next;
525	if (current != null)
526	currentElement = current.item;
527	return x;
528	}
529	finally {
530	fullyUnlock();
531	}
532
533	}
534
535	public void remove() {
536	if (lastRet == null)
537	throw new IllegalStateException();
538	fullyLock();
539	try {
540	Node<E> node = lastRet;
541	lastRet = null;
542	Node<E> trail = head;
543	Node<E> p = head.next;
544	while (p != null && p != node) {
545	trail = p;
546	p = p.next;
547	}
548	if (p == node) {
549	p.item = null;
550	trail.next = p.next;
551	int c = count.getAndDecrement();
552	if (c == capacity)
553	notFull.signalAll();
554	}
555	}
556	finally {
557	fullyUnlock();
558	}
559	}
560	}
561
562	/**
563	* Save the state to a stream (that is, serialize it).
564	*
565	* @serialData The capacity is emitted (int), followed by all of
566	* its elements (each an <tt>Object</tt>) in the proper order,
567	* followed by a null
568	* @param s the stream
569	*/
570	private void writeObject(java.io.ObjectOutputStream s)
571	throws java.io.IOException {
572
573	fullyLock();
574	try {
575	// Write out any hidden stuff, plus capacity
576	s.defaultWriteObject();
577
578	// Write out all elements in the proper order.
579	for (Node<E> p = head.next; p != null; p = p.next)
580	s.writeObject(p.item);
581
582	// Use trailing null as sentinel
583	s.writeObject(null);
584	}
585	finally {
586	fullyUnlock();
587	}
588	}
589
590	/**
591	* Reconstitute this queue instance from a stream (that is,
592	* deserialize it).
593	* @param s the stream
594	*/
595	private void readObject(java.io.ObjectInputStream s)
596	throws java.io.IOException, ClassNotFoundException {
597	// Read in capacity, and any hidden stuff
598	s.defaultReadObject();
599
600	// Read in all elements and place in queue
601	for (;;) {
602	E item = (E)s.readObject();
603	if (item == null)
604	break;
605	add(item);
606	}
607	}
608	}
609
610
611
612
613