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.
 * 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();
    }


    /**
     * 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());
    }

    // Have to override just to update the javadoc 

    /**
     * Adds the specified element to the tail of this queue.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     * @throws IllegalStateException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean add(E o) {
        return super.add(o);
    }

    /**
     * Adds all of the elements in the specified collection to this queue.
     * The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (This implies that the behavior of this call is undefined if
     * the specified collection is this queue, and this queue is nonempty.)
     * <p>
     * This implementation iterates over the specified collection, and adds
     * each object returned by the iterator to this queue's tail, in turn.
     * @throws IllegalStateException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }

    // 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 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.
    /**
     * 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();
    }

    /**
     * Adds the specified element to the tail of this queue, waiting if
     * necessary for space to become available.
     * @throws NullPointerException {@inheritDoc}
     */
    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();
    }

    /**
     * Adds 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 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.
    *
    * @throws NullPointerException {@inheritDoc}
    */
    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();
        }
    }

    /**
     * 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==null ? e==null :
     * o.equals(e))</tt>, if the queue contains one or more such
     * elements.  Returns <tt>true</tt> if the queue contained the
     * specified element (or equivalently, if the queue changed as a
     * result of the call).
     *
     * <p>This implementation iterates over the queue looking for the
     * specified element.  If it finds the element, it removes the element
     * from the queue using the iterator's remove method.<p>
     *
     */
    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();

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


Revision:	1.15
Committed:	Wed Aug 6 11:11:49 2003 UTC (20 years, 10 months ago) by dl
Branch:	MAIN
Changes since 1.14:	+5 -0 lines
Log Message:	Clarify iterator semantics
#	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.
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	* Creates 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	* Creates 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	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
170	* {@link Integer#MAX_VALUE}, initially containing the elements of the
171	* given collection,
172	* added in traversal order of the collection's iterator.
173	* @param c the collection of elements to initially contain
174	* @throws NullPointerException if <tt>c</tt> or any element within it
175	* is <tt>null</tt>
176	*/
177	public LinkedBlockingQueue(Collection<? extends E> c) {
178	this(Integer.MAX_VALUE);
179	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
180	add(it.next());
181	}
182
183	// Have to override just to update the javadoc
184
185	/**
186	* Adds the specified element to the tail of this queue.
187	* @return <tt>true</tt> (as per the general contract of
188	* <tt>Collection.add</tt>).
189	* @throws IllegalStateException {@inheritDoc}
190	* @throws NullPointerException {@inheritDoc}
191	*/
192	public boolean add(E o) {
193	return super.add(o);
194	}
195
196	/**
197	* Adds all of the elements in the specified collection to this queue.
198	* The behavior of this operation is undefined if
199	* the specified collection is modified while the operation is in
200	* progress. (This implies that the behavior of this call is undefined if
201	* the specified collection is this queue, and this queue is nonempty.)
202	* <p>
203	* This implementation iterates over the specified collection, and adds
204	* each object returned by the iterator to this queue's tail, in turn.
205	* @throws IllegalStateException {@inheritDoc}
206	* @throws NullPointerException {@inheritDoc}
207	*/
208	public boolean addAll(Collection<? extends E> c) {
209	return super.addAll(c);
210	}
211
212	// this doc comment is overridden to remove the reference to collections
213	// greater in size than Integer.MAX_VALUE
214	/**
215	* Returns the number of elements in this collection.
216	*/
217	public int size() {
218	return count.get();
219	}
220
221	// this doc comment is a modified copy of the inherited doc comment,
222	// without the reference to unlimited queues.
223	/**
224	* Returns the number of elements that this queue can ideally (in
225	* the absence of memory or resource constraints) accept without
226	* blocking. This is always equal to the initial capacity of this queue
227	* less the current <tt>size</tt> of this queue.
228	* <p>Note that you <em>cannot</em> always tell if
229	* an attempt to <tt>add</tt> an element will succeed by
230	* inspecting <tt>remainingCapacity</tt> because it may be the
231	* case that a waiting consumer is ready to <tt>take</tt> an
232	* element out of an otherwise full queue.
233	*/
234	public int remainingCapacity() {
235	return capacity - count.get();
236	}
237
238	/**
239	* Adds the specified element to the tail of this queue, waiting if
240	* necessary for space to become available.
241	* @throws NullPointerException {@inheritDoc}
242	*/
243	public void put(E o) throws InterruptedException {
244	if (o == null) throw new NullPointerException();
245	// Note: convention in all put/take/etc is to preset
246	// local var holding count negative to indicate failure unless set.
247	int c = -1;
248	putLock.lockInterruptibly();
249	try {
250	/*
251	* Note that count is used in wait guard even though it is
252	* not protected by lock. This works because count can
253	* only decrease at this point (all other puts are shut
254	* out by lock), and we (or some other waiting put) are
255	* signalled if it ever changes from
256	* capacity. Similarly for all other uses of count in
257	* other wait guards.
258	*/
259	try {
260	while (count.get() == capacity)
261	notFull.await();
262	}
263	catch (InterruptedException ie) {
264	notFull.signal(); // propagate to a non-interrupted thread
265	throw ie;
266	}
267	insert(o);
268	c = count.getAndIncrement();
269	if (c + 1 < capacity)
270	notFull.signal();
271	}
272	finally {
273	putLock.unlock();
274	}
275	if (c == 0)
276	signalNotEmpty();
277	}
278
279	/**
280	* Adds the specified element to the tail of this queue, waiting if
281	* necessary up to the specified wait time for space to become available.
282	* @throws NullPointerException {@inheritDoc}
283	*/
284	public boolean offer(E o, long timeout, TimeUnit unit)
285	throws InterruptedException {
286
287	if (o == null) throw new NullPointerException();
288	long nanos = unit.toNanos(timeout);
289	int c = -1;
290	putLock.lockInterruptibly();
291	try {
292	for (;;) {
293	if (count.get() < capacity) {
294	insert(o);
295	c = count.getAndIncrement();
296	if (c + 1 < capacity)
297	notFull.signal();
298	break;
299	}
300	if (nanos <= 0)
301	return false;
302	try {
303	nanos = notFull.awaitNanos(nanos);
304	}
305	catch (InterruptedException ie) {
306	notFull.signal(); // propagate to a non-interrupted thread
307	throw ie;
308	}
309	}
310	}
311	finally {
312	putLock.unlock();
313	}
314	if (c == 0)
315	signalNotEmpty();
316	return true;
317	}
318
319	/**
320	* Adds the specified element to the tail of this queue if possible,
321	* returning immediately if this queue is full.
322	*
323	* @throws NullPointerException {@inheritDoc}
324	*/
325	public boolean offer(E o) {
326	if (o == null) throw new NullPointerException();
327	if (count.get() == capacity)
328	return false;
329	int c = -1;
330	putLock.lock();
331	try {
332	if (count.get() < capacity) {
333	insert(o);
334	c = count.getAndIncrement();
335	if (c + 1 < capacity)
336	notFull.signal();
337	}
338	}
339	finally {
340	putLock.unlock();
341	}
342	if (c == 0)
343	signalNotEmpty();
344	return c >= 0;
345	}
346
347
348	public E take() throws InterruptedException {
349	E x;
350	int c = -1;
351	takeLock.lockInterruptibly();
352	try {
353	try {
354	while (count.get() == 0)
355	notEmpty.await();
356	}
357	catch (InterruptedException ie) {
358	notEmpty.signal(); // propagate to a non-interrupted thread
359	throw ie;
360	}
361
362	x = extract();
363	c = count.getAndDecrement();
364	if (c > 1)
365	notEmpty.signal();
366	}
367	finally {
368	takeLock.unlock();
369	}
370	if (c == capacity)
371	signalNotFull();
372	return x;
373	}
374
375	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
376	E x = null;
377	int c = -1;
378	long nanos = unit.toNanos(timeout);
379	takeLock.lockInterruptibly();
380	try {
381	for (;;) {
382	if (count.get() > 0) {
383	x = extract();
384	c = count.getAndDecrement();
385	if (c > 1)
386	notEmpty.signal();
387	break;
388	}
389	if (nanos <= 0)
390	return null;
391	try {
392	nanos = notEmpty.awaitNanos(nanos);
393	}
394	catch (InterruptedException ie) {
395	notEmpty.signal(); // propagate to a non-interrupted thread
396	throw ie;
397	}
398	}
399	}
400	finally {
401	takeLock.unlock();
402	}
403	if (c == capacity)
404	signalNotFull();
405	return x;
406	}
407
408	public E poll() {
409	if (count.get() == 0)
410	return null;
411	E x = null;
412	int c = -1;
413	takeLock.tryLock();
414	try {
415	if (count.get() > 0) {
416	x = extract();
417	c = count.getAndDecrement();
418	if (c > 1)
419	notEmpty.signal();
420	}
421	}
422	finally {
423	takeLock.unlock();
424	}
425	if (c == capacity)
426	signalNotFull();
427	return x;
428	}
429
430
431	public E peek() {
432	if (count.get() == 0)
433	return null;
434	takeLock.lock();
435	try {
436	Node<E> first = head.next;
437	if (first == null)
438	return null;
439	else
440	return first.item;
441	}
442	finally {
443	takeLock.unlock();
444	}
445	}
446
447	/**
448	* Removes a single instance of the specified element from this
449	* queue, if it is present. More formally,
450	* removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
451	* o.equals(e))</tt>, if the queue contains one or more such
452	* elements. Returns <tt>true</tt> if the queue contained the
453	* specified element (or equivalently, if the queue changed as a
454	* result of the call).
455	*
456	* <p>This implementation iterates over the queue looking for the
457	* specified element. If it finds the element, it removes the element
458	* from the queue using the iterator's remove method.<p>
459	*
460	*/
461	public boolean remove(Object o) {
462	if (o == null) return false;
463	boolean removed = false;
464	fullyLock();
465	try {
466	Node<E> trail = head;
467	Node<E> p = head.next;
468	while (p != null) {
469	if (o.equals(p.item)) {
470	removed = true;
471	break;
472	}
473	trail = p;
474	p = p.next;
475	}
476	if (removed) {
477	p.item = null;
478	trail.next = p.next;
479	if (count.getAndDecrement() == capacity)
480	notFull.signalAll();
481	}
482	}
483	finally {
484	fullyUnlock();
485	}
486	return removed;
487	}
488
489	public Object[] toArray() {
490	fullyLock();
491	try {
492	int size = count.get();
493	Object[] a = new Object[size];
494	int k = 0;
495	for (Node<E> p = head.next; p != null; p = p.next)
496	a[k++] = p.item;
497	return a;
498	}
499	finally {
500	fullyUnlock();
501	}
502	}
503
504	public <T> T[] toArray(T[] a) {
505	fullyLock();
506	try {
507	int size = count.get();
508	if (a.length < size)
509	a = (T[])java.lang.reflect.Array.newInstance
510	(a.getClass().getComponentType(), size);
511
512	int k = 0;
513	for (Node p = head.next; p != null; p = p.next)
514	a[k++] = (T)p.item;
515	return a;
516	}
517	finally {
518	fullyUnlock();
519	}
520	}
521
522	public String toString() {
523	fullyLock();
524	try {
525	return super.toString();
526	}
527	finally {
528	fullyUnlock();
529	}
530	}
531
532	/**
533	* Returns an iterator over the elements in this queue in proper sequence.
534	* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
535	* will never throw {@link java.util.ConcurrentModificationException},
536	* and guarantees to traverse elements as they existed upon
537	* construction of the iterator, and may (but is not guaranteed to)
538	* reflect any modifications subsequent to construction.
539	*
540	* @return an iterator over the elements in this queue in proper sequence.
541	*/
542	public Iterator<E> iterator() {
543	return new Itr();
544	}
545
546	private class Itr implements Iterator<E> {
547	/*
548	* Basic weak-consistent iterator. At all times hold the next
549	* item to hand out so that if hasNext() reports true, we will
550	* still have it to return even if lost race with a take etc.
551	*/
552	Node<E> current;
553	Node<E> lastRet;
554	E currentElement;
555
556	Itr() {
557	fullyLock();
558	try {
559	current = head.next;
560	if (current != null)
561	currentElement = current.item;
562	}
563	finally {
564	fullyUnlock();
565	}
566	}
567
568	public boolean hasNext() {
569	return current != null;
570	}
571
572	public E next() {
573	fullyLock();
574	try {
575	if (current == null)
576	throw new NoSuchElementException();
577	E x = currentElement;
578	lastRet = current;
579	current = current.next;
580	if (current != null)
581	currentElement = current.item;
582	return x;
583	}
584	finally {
585	fullyUnlock();
586	}
587
588	}
589
590	public void remove() {
591	if (lastRet == null)
592	throw new IllegalStateException();
593	fullyLock();
594	try {
595	Node<E> node = lastRet;
596	lastRet = null;
597	Node<E> trail = head;
598	Node<E> p = head.next;
599	while (p != null && p != node) {
600	trail = p;
601	p = p.next;
602	}
603	if (p == node) {
604	p.item = null;
605	trail.next = p.next;
606	int c = count.getAndDecrement();
607	if (c == capacity)
608	notFull.signalAll();
609	}
610	}
611	finally {
612	fullyUnlock();
613	}
614	}
615	}
616
617	/**
618	* Save the state to a stream (that is, serialize it).
619	*
620	* @serialData The capacity is emitted (int), followed by all of
621	* its elements (each an <tt>Object</tt>) in the proper order,
622	* followed by a null
623	* @param s the stream
624	*/
625	private void writeObject(java.io.ObjectOutputStream s)
626	throws java.io.IOException {
627
628	fullyLock();
629	try {
630	// Write out any hidden stuff, plus capacity
631	s.defaultWriteObject();
632
633	// Write out all elements in the proper order.
634	for (Node<E> p = head.next; p != null; p = p.next)
635	s.writeObject(p.item);
636
637	// Use trailing null as sentinel
638	s.writeObject(null);
639	}
640	finally {
641	fullyUnlock();
642	}
643	}
644
645	/**
646	* Reconstitute this queue instance from a stream (that is,
647	* deserialize it).
648	* @param s the stream
649	*/
650	private void readObject(java.io.ObjectInputStream s)
651	throws java.io.IOException, ClassNotFoundException {
652	// Read in capacity, and any hidden stuff
653	s.defaultReadObject();
654
655	// Read in all elements and place in queue
656	for (;;) {
657	E item = (E)s.readObject();
658	if (item == null)
659	break;
660	add(item);
661	}
662	}
663	}
664
665
666
667
668