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
 * @param <E> the type of elements held in this collection
 *
 **/
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 ReentrantLock.ConditionObject notEmpty = takeLock.newCondition();

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

    /** Wait queue for waiting puts */
    private final ReentrantLock.ConditionObject 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 = 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();
    }

    /**
     * Adds the specified element to the tail of this queue, waiting if
     * necessary for space to become available.
     * @param o the element to add
     * @throws InterruptedException if interrupted while waiting.
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     */
    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();
    }

    /**
     * Inserts the specified element at the tail of this queue, waiting if
     * necessary up to the specified wait time for space to become available.
     * @param o the element to add
     * @param timeout how long to wait before giving up, in units of
     * <tt>unit</tt>
     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
     * <tt>timeout</tt> parameter
     * @return <tt>true</tt> if successful, or <tt>false</tt> if
     * the specified waiting time elapses before space is available.
     * @throws InterruptedException if interrupted while waiting.
     * @throws NullPointerException if the specified element is <tt>null</tt>.
     */
    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;
    }

    /**
     * Inserts the specified element at 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();
        }
    }

    public void clear() {
        fullyLock();
        try {
            head.next = null;
            if (count.getAndSet(0) == capacity)
                notFull.signalAll();
        } finally {
            fullyUnlock();
        }
    }

    public int drainTo(Collection<? super E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        Node first;
        fullyLock();
        try {
            first = head.next;
            head.next = null;
            if (count.getAndSet(0) == capacity)
                notFull.signalAll();
        } finally {
            fullyUnlock();
        }
        // Transfer the elements outside of locks
        int n = 0;
        for (Node<E> p = first; p != null; p = p.next) {
            c.add(p.item);
            p.item = null;
            ++n;
        }
        return n;
    }
        
    public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        fullyLock();
        try {
            int n = 0;
            Node<E> p = head.next;
            while (p != null && n < maxElements) {
                c.add(p.item);
                p.item = null;
                p = p.next;
                ++n;
            }
            if (n != 0) {
                head.next = p;
                if (count.getAndAdd(-n) == capacity)
                    notFull.signalAll();
            }
            return n;
        } 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:	1.29
Committed:	Wed Nov 12 01:04:24 2003 UTC (20 years, 6 months ago) by dl
Branch:	MAIN
Changes since 1.28:	+4 -4 lines
Log Message:	fixed typos; avoided some casts
#	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	* @param <E> the type of elements held in this collection
37	*
38	**/
39	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
40	implements BlockingQueue<E>, java.io.Serializable {
41	private static final long serialVersionUID = -6903933977591709194L;
42
43	/*
44	* A variant of the "two lock queue" algorithm. The putLock gates
45	* entry to put (and offer), and has an associated condition for
46	* waiting puts. Similarly for the takeLock. The "count" field
47	* that they both rely on is maintained as an atomic to avoid
48	* needing to get both locks in most cases. Also, to minimize need
49	* for puts to get takeLock and vice-versa, cascading notifies are
50	* used. When a put notices that it has enabled at least one take,
51	* it signals taker. That taker in turn signals others if more
52	* items have been entered since the signal. And symmetrically for
53	* takes signalling puts. Operations such as remove(Object) and
54	* iterators acquire both locks.
55	*/
56
57	/**
58	* Linked list node class
59	*/
60	static class Node<E> {
61	/** The item, volatile to ensure barrier separating write and read */
62	volatile E item;
63	Node<E> next;
64	Node(E x) { item = x; }
65	}
66
67	/** The capacity bound, or Integer.MAX_VALUE if none */
68	private final int capacity;
69
70	/** Current number of elements */
71	private final AtomicInteger count = new AtomicInteger(0);
72
73	/** Head of linked list */
74	private transient Node<E> head;
75
76	/** Tail of linked list */
77	private transient Node<E> last;
78
79	/** Lock held by take, poll, etc */
80	private final ReentrantLock takeLock = new ReentrantLock();
81
82	/** Wait queue for waiting takes */
83	private final ReentrantLock.ConditionObject notEmpty = takeLock.newCondition();
84
85	/** Lock held by put, offer, etc */
86	private final ReentrantLock putLock = new ReentrantLock();
87
88	/** Wait queue for waiting puts */
89	private final ReentrantLock.ConditionObject notFull = putLock.newCondition();
90
91	/**
92	* Signal a waiting take. Called only from put/offer (which do not
93	* otherwise ordinarily lock takeLock.)
94	*/
95	private void signalNotEmpty() {
96	takeLock.lock();
97	try {
98	notEmpty.signal();
99	} finally {
100	takeLock.unlock();
101	}
102	}
103
104	/**
105	* Signal a waiting put. Called only from take/poll.
106	*/
107	private void signalNotFull() {
108	putLock.lock();
109	try {
110	notFull.signal();
111	} finally {
112	putLock.unlock();
113	}
114	}
115
116	/**
117	* Create a node and link it at end of queue
118	* @param x the item
119	*/
120	private void insert(E x) {
121	last = last.next = new Node<E>(x);
122	}
123
124	/**
125	* Remove a node from head of queue,
126	* @return the node
127	*/
128	private E extract() {
129	Node<E> first = head.next;
130	head = first;
131	E x = first.item;
132	first.item = null;
133	return x;
134	}
135
136	/**
137	* Lock to prevent both puts and takes.
138	*/
139	private void fullyLock() {
140	putLock.lock();
141	takeLock.lock();
142	}
143
144	/**
145	* Unlock to allow both puts and takes.
146	*/
147	private void fullyUnlock() {
148	takeLock.unlock();
149	putLock.unlock();
150	}
151
152
153	/**
154	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
155	* {@link Integer#MAX_VALUE}.
156	*/
157	public LinkedBlockingQueue() {
158	this(Integer.MAX_VALUE);
159	}
160
161	/**
162	* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
163	*
164	* @param capacity the capacity of this queue.
165	* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
166	* than zero.
167	*/
168	public LinkedBlockingQueue(int capacity) {
169	if (capacity <= 0) throw new IllegalArgumentException();
170	this.capacity = capacity;
171	last = head = new Node<E>(null);
172	}
173
174	/**
175	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
176	* {@link Integer#MAX_VALUE}, initially containing the elements of the
177	* given collection,
178	* added in traversal order of the collection's iterator.
179	* @param c the collection of elements to initially contain
180	* @throws NullPointerException if <tt>c</tt> or any element within it
181	* is <tt>null</tt>
182	*/
183	public LinkedBlockingQueue(Collection<? extends E> c) {
184	this(Integer.MAX_VALUE);
185	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
186	add(it.next());
187	}
188
189
190	// this doc comment is overridden to remove the reference to collections
191	// greater in size than Integer.MAX_VALUE
192	/**
193	* Returns the number of elements in this queue.
194	*
195	* @return the number of elements in this queue.
196	*/
197	public int size() {
198	return count.get();
199	}
200
201	// this doc comment is a modified copy of the inherited doc comment,
202	// without the reference to unlimited queues.
203	/**
204	* Returns the number of elements that this queue can ideally (in
205	* the absence of memory or resource constraints) accept without
206	* blocking. This is always equal to the initial capacity of this queue
207	* less the current <tt>size</tt> of this queue.
208	* <p>Note that you <em>cannot</em> always tell if
209	* an attempt to <tt>add</tt> an element will succeed by
210	* inspecting <tt>remainingCapacity</tt> because it may be the
211	* case that a waiting consumer is ready to <tt>take</tt> an
212	* element out of an otherwise full queue.
213	*/
214	public int remainingCapacity() {
215	return capacity - count.get();
216	}
217
218	/**
219	* Adds the specified element to the tail of this queue, waiting if
220	* necessary for space to become available.
221	* @param o the element to add
222	* @throws InterruptedException if interrupted while waiting.
223	* @throws NullPointerException if the specified element is <tt>null</tt>.
224	*/
225	public void put(E o) throws InterruptedException {
226	if (o == null) throw new NullPointerException();
227	// Note: convention in all put/take/etc is to preset
228	// local var holding count negative to indicate failure unless set.
229	int c = -1;
230	putLock.lockInterruptibly();
231	try {
232	/*
233	* Note that count is used in wait guard even though it is
234	* not protected by lock. This works because count can
235	* only decrease at this point (all other puts are shut
236	* out by lock), and we (or some other waiting put) are
237	* signalled if it ever changes from
238	* capacity. Similarly for all other uses of count in
239	* other wait guards.
240	*/
241	try {
242	while (count.get() == capacity)
243	notFull.await();
244	} catch (InterruptedException ie) {
245	notFull.signal(); // propagate to a non-interrupted thread
246	throw ie;
247	}
248	insert(o);
249	c = count.getAndIncrement();
250	if (c + 1 < capacity)
251	notFull.signal();
252	} finally {
253	putLock.unlock();
254	}
255	if (c == 0)
256	signalNotEmpty();
257	}
258
259	/**
260	* Inserts the specified element at the tail of this queue, waiting if
261	* necessary up to the specified wait time for space to become available.
262	* @param o the element to add
263	* @param timeout how long to wait before giving up, in units of
264	* <tt>unit</tt>
265	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
266	* <tt>timeout</tt> parameter
267	* @return <tt>true</tt> if successful, or <tt>false</tt> if
268	* the specified waiting time elapses before space is available.
269	* @throws InterruptedException if interrupted while waiting.
270	* @throws NullPointerException if the specified element is <tt>null</tt>.
271	*/
272	public boolean offer(E o, long timeout, TimeUnit unit)
273	throws InterruptedException {
274
275	if (o == null) throw new NullPointerException();
276	long nanos = unit.toNanos(timeout);
277	int c = -1;
278	putLock.lockInterruptibly();
279	try {
280	for (;;) {
281	if (count.get() < capacity) {
282	insert(o);
283	c = count.getAndIncrement();
284	if (c + 1 < capacity)
285	notFull.signal();
286	break;
287	}
288	if (nanos <= 0)
289	return false;
290	try {
291	nanos = notFull.awaitNanos(nanos);
292	} catch (InterruptedException ie) {
293	notFull.signal(); // propagate to a non-interrupted thread
294	throw ie;
295	}
296	}
297	} finally {
298	putLock.unlock();
299	}
300	if (c == 0)
301	signalNotEmpty();
302	return true;
303	}
304
305	/**
306	* Inserts the specified element at the tail of this queue if possible,
307	* returning immediately if this queue is full.
308	*
309	* @param o the element to add.
310	* @return <tt>true</tt> if it was possible to add the element to
311	* this queue, else <tt>false</tt>
312	* @throws NullPointerException if the specified element is <tt>null</tt>
313	*/
314	public boolean offer(E o) {
315	if (o == null) throw new NullPointerException();
316	if (count.get() == capacity)
317	return false;
318	int c = -1;
319	putLock.lock();
320	try {
321	if (count.get() < capacity) {
322	insert(o);
323	c = count.getAndIncrement();
324	if (c + 1 < capacity)
325	notFull.signal();
326	}
327	} finally {
328	putLock.unlock();
329	}
330	if (c == 0)
331	signalNotEmpty();
332	return c >= 0;
333	}
334
335
336	public E take() throws InterruptedException {
337	E x;
338	int c = -1;
339	takeLock.lockInterruptibly();
340	try {
341	try {
342	while (count.get() == 0)
343	notEmpty.await();
344	} catch (InterruptedException ie) {
345	notEmpty.signal(); // propagate to a non-interrupted thread
346	throw ie;
347	}
348
349	x = extract();
350	c = count.getAndDecrement();
351	if (c > 1)
352	notEmpty.signal();
353	} finally {
354	takeLock.unlock();
355	}
356	if (c == capacity)
357	signalNotFull();
358	return x;
359	}
360
361	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
362	E x = null;
363	int c = -1;
364	long nanos = unit.toNanos(timeout);
365	takeLock.lockInterruptibly();
366	try {
367	for (;;) {
368	if (count.get() > 0) {
369	x = extract();
370	c = count.getAndDecrement();
371	if (c > 1)
372	notEmpty.signal();
373	break;
374	}
375	if (nanos <= 0)
376	return null;
377	try {
378	nanos = notEmpty.awaitNanos(nanos);
379	} catch (InterruptedException ie) {
380	notEmpty.signal(); // propagate to a non-interrupted thread
381	throw ie;
382	}
383	}
384	} finally {
385	takeLock.unlock();
386	}
387	if (c == capacity)
388	signalNotFull();
389	return x;
390	}
391
392	public E poll() {
393	if (count.get() == 0)
394	return null;
395	E x = null;
396	int c = -1;
397	takeLock.tryLock();
398	try {
399	if (count.get() > 0) {
400	x = extract();
401	c = count.getAndDecrement();
402	if (c > 1)
403	notEmpty.signal();
404	}
405	} finally {
406	takeLock.unlock();
407	}
408	if (c == capacity)
409	signalNotFull();
410	return x;
411	}
412
413
414	public E peek() {
415	if (count.get() == 0)
416	return null;
417	takeLock.lock();
418	try {
419	Node<E> first = head.next;
420	if (first == null)
421	return null;
422	else
423	return first.item;
424	} finally {
425	takeLock.unlock();
426	}
427	}
428
429	public boolean remove(Object o) {
430	if (o == null) return false;
431	boolean removed = false;
432	fullyLock();
433	try {
434	Node<E> trail = head;
435	Node<E> p = head.next;
436	while (p != null) {
437	if (o.equals(p.item)) {
438	removed = true;
439	break;
440	}
441	trail = p;
442	p = p.next;
443	}
444	if (removed) {
445	p.item = null;
446	trail.next = p.next;
447	if (count.getAndDecrement() == capacity)
448	notFull.signalAll();
449	}
450	} finally {
451	fullyUnlock();
452	}
453	return removed;
454	}
455
456	public Object[] toArray() {
457	fullyLock();
458	try {
459	int size = count.get();
460	Object[] a = new Object[size];
461	int k = 0;
462	for (Node<E> p = head.next; p != null; p = p.next)
463	a[k++] = p.item;
464	return a;
465	} finally {
466	fullyUnlock();
467	}
468	}
469
470	public <T> T[] toArray(T[] a) {
471	fullyLock();
472	try {
473	int size = count.get();
474	if (a.length < size)
475	a = (T[])java.lang.reflect.Array.newInstance
476	(a.getClass().getComponentType(), size);
477
478	int k = 0;
479	for (Node p = head.next; p != null; p = p.next)
480	a[k++] = (T)p.item;
481	return a;
482	} finally {
483	fullyUnlock();
484	}
485	}
486
487	public String toString() {
488	fullyLock();
489	try {
490	return super.toString();
491	} finally {
492	fullyUnlock();
493	}
494	}
495
496	public void clear() {
497	fullyLock();
498	try {
499	head.next = null;
500	if (count.getAndSet(0) == capacity)
501	notFull.signalAll();
502	} finally {
503	fullyUnlock();
504	}
505	}
506
507	public int drainTo(Collection<? super E> c) {
508	if (c == null)
509	throw new NullPointerException();
510	if (c == this)
511	throw new IllegalArgumentException();
512	Node first;
513	fullyLock();
514	try {
515	first = head.next;
516	head.next = null;
517	if (count.getAndSet(0) == capacity)
518	notFull.signalAll();
519	} finally {
520	fullyUnlock();
521	}
522	// Transfer the elements outside of locks
523	int n = 0;
524	for (Node<E> p = first; p != null; p = p.next) {
525	c.add(p.item);
526	p.item = null;
527	++n;
528	}
529	return n;
530	}
531
532	public int drainTo(Collection<? super E> c, int maxElements) {
533	if (c == null)
534	throw new NullPointerException();
535	if (c == this)
536	throw new IllegalArgumentException();
537	if (maxElements <= 0)
538	return 0;
539	fullyLock();
540	try {
541	int n = 0;
542	Node<E> p = head.next;
543	while (p != null && n < maxElements) {
544	c.add(p.item);
545	p.item = null;
546	p = p.next;
547	++n;
548	}
549	if (n != 0) {
550	head.next = p;
551	if (count.getAndAdd(-n) == capacity)
552	notFull.signalAll();
553	}
554	return n;
555	} finally {
556	fullyUnlock();
557	}
558	}
559
560
561
562	/**
563	* Returns an iterator over the elements in this queue in proper sequence.
564	* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
565	* will never throw {@link java.util.ConcurrentModificationException},
566	* and guarantees to traverse elements as they existed upon
567	* construction of the iterator, and may (but is not guaranteed to)
568	* reflect any modifications subsequent to construction.
569	*
570	* @return an iterator over the elements in this queue in proper sequence.
571	*/
572	public Iterator<E> iterator() {
573	return new Itr();
574	}
575
576	private class Itr implements Iterator<E> {
577	/*
578	* Basic weak-consistent iterator. At all times hold the next
579	* item to hand out so that if hasNext() reports true, we will
580	* still have it to return even if lost race with a take etc.
581	*/
582	Node<E> current;
583	Node<E> lastRet;
584	E currentElement;
585
586	Itr() {
587	fullyLock();
588	try {
589	current = head.next;
590	if (current != null)
591	currentElement = current.item;
592	} finally {
593	fullyUnlock();
594	}
595	}
596
597	public boolean hasNext() {
598	return current != null;
599	}
600
601	public E next() {
602	fullyLock();
603	try {
604	if (current == null)
605	throw new NoSuchElementException();
606	E x = currentElement;
607	lastRet = current;
608	current = current.next;
609	if (current != null)
610	currentElement = current.item;
611	return x;
612	} finally {
613	fullyUnlock();
614	}
615
616	}
617
618	public void remove() {
619	if (lastRet == null)
620	throw new IllegalStateException();
621	fullyLock();
622	try {
623	Node<E> node = lastRet;
624	lastRet = null;
625	Node<E> trail = head;
626	Node<E> p = head.next;
627	while (p != null && p != node) {
628	trail = p;
629	p = p.next;
630	}
631	if (p == node) {
632	p.item = null;
633	trail.next = p.next;
634	int c = count.getAndDecrement();
635	if (c == capacity)
636	notFull.signalAll();
637	}
638	} finally {
639	fullyUnlock();
640	}
641	}
642	}
643
644	/**
645	* Save the state to a stream (that is, serialize it).
646	*
647	* @serialData The capacity is emitted (int), followed by all of
648	* its elements (each an <tt>Object</tt>) in the proper order,
649	* followed by a null
650	* @param s the stream
651	*/
652	private void writeObject(java.io.ObjectOutputStream s)
653	throws java.io.IOException {
654
655	fullyLock();
656	try {
657	// Write out any hidden stuff, plus capacity
658	s.defaultWriteObject();
659
660	// Write out all elements in the proper order.
661	for (Node<E> p = head.next; p != null; p = p.next)
662	s.writeObject(p.item);
663
664	// Use trailing null as sentinel
665	s.writeObject(null);
666	} finally {
667	fullyUnlock();
668	}
669	}
670
671	/**
672	* Reconstitute this queue instance from a stream (that is,
673	* deserialize it).
674	* @param s the stream
675	*/
676	private void readObject(java.io.ObjectInputStream s)
677	throws java.io.IOException, ClassNotFoundException {
678	// Read in capacity, and any hidden stuff
679	s.defaultReadObject();
680
681	count.set(0);
682	last = head = new Node<E>(null);
683
684	// Read in all elements and place in queue
685	for (;;) {
686	E item = (E)s.readObject();
687	if (item == null)
688	break;
689	add(item);
690	}
691	}
692	}