util/concurrent/LinkedBlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.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 and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 *
 **/
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;

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

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

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

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

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

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

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

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

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

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

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

    /**
     * Signal a waiting put. Called only from take/poll.
     */
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        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 (E e : c)
            add(e);
    }


    // 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;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        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;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        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();
        final AtomicInteger count = this.count;
        if (count.get() == capacity)
            return false;
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        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;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        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);
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        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() {
        final AtomicInteger count = this.count;
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        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;
        final ReentrantLock takeLock = this.takeLock;
        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.
     */
    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();
        }
    }

    /**
     * Atomically removes all of the elements from this queue.
     * The queue will be empty after this call returns.
     */
    public void clear() {
        fullyLock();
        try {
            head.next = null;
            assert head.item == null;
            last = head;
            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;
            assert head.item == null;
            last = head;
            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();
        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;
                assert head.item == null;
                if (p == null)
                    last = head;
                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.
         */
        private Node<E> current;
        private Node<E> lastRet;
        private E currentElement;

        Itr() {
            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
            putLock.lock();
            takeLock.lock();
            try {
                current = head.next;
                if (current != null)
                    currentElement = current.item;
            } finally {
                takeLock.unlock();
                putLock.unlock();
            }
        }

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

        public E next() {
            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
            putLock.lock();
            takeLock.lock();
            try {
                if (current == null)
                    throw new NoSuchElementException();
                E x = currentElement;
                lastRet = current;
                current = current.next;
                if (current != null)
                    currentElement = current.item;
                return x;
            } finally {
                takeLock.unlock();
                putLock.unlock();
            }
        }

        public void remove() {
            if (lastRet == null)
                throw new IllegalStateException();
            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
            putLock.lock();
            takeLock.lock();
            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 {
                takeLock.unlock();
                putLock.unlock();
            }
        }
    }

    /**
     * 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:	1.38
Committed:	Thu Nov 4 19:48:19 2004 UTC (19 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.37:	+11 -13 lines
Log Message:	Simplify previous changes
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	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 and its iterator implement all of the
32	* <em>optional</em> methods of the {@link Collection} and {@link
33	* Iterator} interfaces.
34	*
35	* <p>This class is a member of the
36	* <a href="{@docRoot}/../guide/collections/index.html">
37	* Java Collections Framework</a>.
38	*
39	* @since 1.5
40	* @author Doug Lea
41	* @param <E> the type of elements held in this collection
42	*
43	**/
44	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
45	implements BlockingQueue<E>, java.io.Serializable {
46	private static final long serialVersionUID = -6903933977591709194L;
47
48	/*
49	* A variant of the "two lock queue" algorithm. The putLock gates
50	* entry to put (and offer), and has an associated condition for
51	* waiting puts. Similarly for the takeLock. The "count" field
52	* that they both rely on is maintained as an atomic to avoid
53	* needing to get both locks in most cases. Also, to minimize need
54	* for puts to get takeLock and vice-versa, cascading notifies are
55	* used. When a put notices that it has enabled at least one take,
56	* it signals taker. That taker in turn signals others if more
57	* items have been entered since the signal. And symmetrically for
58	* takes signalling puts. Operations such as remove(Object) and
59	* iterators acquire both locks.
60	*/
61
62	/**
63	* Linked list node class
64	*/
65	static class Node<E> {
66	/** The item, volatile to ensure barrier separating write and read */
67	volatile E item;
68	Node<E> next;
69	Node(E x) { item = x; }
70	}
71
72	/** The capacity bound, or Integer.MAX_VALUE if none */
73	private final int capacity;
74
75	/** Current number of elements */
76	private final AtomicInteger count = new AtomicInteger(0);
77
78	/** Head of linked list */
79	private transient Node<E> head;
80
81	/** Tail of linked list */
82	private transient Node<E> last;
83
84	/** Lock held by take, poll, etc */
85	private final ReentrantLock takeLock = new ReentrantLock();
86
87	/** Wait queue for waiting takes */
88	private final Condition notEmpty = takeLock.newCondition();
89
90	/** Lock held by put, offer, etc */
91	private final ReentrantLock putLock = new ReentrantLock();
92
93	/** Wait queue for waiting puts */
94	private final Condition notFull = putLock.newCondition();
95
96	/**
97	* Signal a waiting take. Called only from put/offer (which do not
98	* otherwise ordinarily lock takeLock.)
99	*/
100	private void signalNotEmpty() {
101	final ReentrantLock takeLock = this.takeLock;
102	takeLock.lock();
103	try {
104	notEmpty.signal();
105	} finally {
106	takeLock.unlock();
107	}
108	}
109
110	/**
111	* Signal a waiting put. Called only from take/poll.
112	*/
113	private void signalNotFull() {
114	final ReentrantLock putLock = this.putLock;
115	putLock.lock();
116	try {
117	notFull.signal();
118	} finally {
119	putLock.unlock();
120	}
121	}
122
123	/**
124	* Create a node and link it at end of queue
125	* @param x the item
126	*/
127	private void insert(E x) {
128	last = last.next = new Node<E>(x);
129	}
130
131	/**
132	* Remove a node from head of queue,
133	* @return the node
134	*/
135	private E extract() {
136	Node<E> first = head.next;
137	head = first;
138	E x = first.item;
139	first.item = null;
140	return x;
141	}
142
143	/**
144	* Lock to prevent both puts and takes.
145	*/
146	private void fullyLock() {
147	putLock.lock();
148	takeLock.lock();
149	}
150
151	/**
152	* Unlock to allow both puts and takes.
153	*/
154	private void fullyUnlock() {
155	takeLock.unlock();
156	putLock.unlock();
157	}
158
159
160	/**
161	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
162	* {@link Integer#MAX_VALUE}.
163	*/
164	public LinkedBlockingQueue() {
165	this(Integer.MAX_VALUE);
166	}
167
168	/**
169	* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
170	*
171	* @param capacity the capacity of this queue.
172	* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
173	* than zero.
174	*/
175	public LinkedBlockingQueue(int capacity) {
176	if (capacity <= 0) throw new IllegalArgumentException();
177	this.capacity = capacity;
178	last = head = new Node<E>(null);
179	}
180
181	/**
182	* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
183	* {@link Integer#MAX_VALUE}, initially containing the elements of the
184	* given collection,
185	* added in traversal order of the collection's iterator.
186	* @param c the collection of elements to initially contain
187	* @throws NullPointerException if <tt>c</tt> or any element within it
188	* is <tt>null</tt>
189	*/
190	public LinkedBlockingQueue(Collection<? extends E> c) {
191	this(Integer.MAX_VALUE);
192	for (E e : c)
193	add(e);
194	}
195
196
197	// this doc comment is overridden to remove the reference to collections
198	// greater in size than Integer.MAX_VALUE
199	/**
200	* Returns the number of elements in this queue.
201	*
202	* @return the number of elements in this queue.
203	*/
204	public int size() {
205	return count.get();
206	}
207
208	// this doc comment is a modified copy of the inherited doc comment,
209	// without the reference to unlimited queues.
210	/**
211	* Returns the number of elements that this queue can ideally (in
212	* the absence of memory or resource constraints) accept without
213	* blocking. This is always equal to the initial capacity of this queue
214	* less the current <tt>size</tt> of this queue.
215	* <p>Note that you <em>cannot</em> always tell if
216	* an attempt to <tt>add</tt> an element will succeed by
217	* inspecting <tt>remainingCapacity</tt> because it may be the
218	* case that a waiting consumer is ready to <tt>take</tt> an
219	* element out of an otherwise full queue.
220	*/
221	public int remainingCapacity() {
222	return capacity - count.get();
223	}
224
225	/**
226	* Adds the specified element to the tail of this queue, waiting if
227	* necessary for space to become available.
228	* @param o the element to add
229	* @throws InterruptedException if interrupted while waiting.
230	* @throws NullPointerException if the specified element is <tt>null</tt>.
231	*/
232	public void put(E o) throws InterruptedException {
233	if (o == null) throw new NullPointerException();
234	// Note: convention in all put/take/etc is to preset
235	// local var holding count negative to indicate failure unless set.
236	int c = -1;
237	final ReentrantLock putLock = this.putLock;
238	final AtomicInteger count = this.count;
239	putLock.lockInterruptibly();
240	try {
241	/*
242	* Note that count is used in wait guard even though it is
243	* not protected by lock. This works because count can
244	* only decrease at this point (all other puts are shut
245	* out by lock), and we (or some other waiting put) are
246	* signalled if it ever changes from
247	* capacity. Similarly for all other uses of count in
248	* other wait guards.
249	*/
250	try {
251	while (count.get() == capacity)
252	notFull.await();
253	} catch (InterruptedException ie) {
254	notFull.signal(); // propagate to a non-interrupted thread
255	throw ie;
256	}
257	insert(o);
258	c = count.getAndIncrement();
259	if (c + 1 < capacity)
260	notFull.signal();
261	} finally {
262	putLock.unlock();
263	}
264	if (c == 0)
265	signalNotEmpty();
266	}
267
268	/**
269	* Inserts the specified element at the tail of this queue, waiting if
270	* necessary up to the specified wait time for space to become available.
271	* @param o the element to add
272	* @param timeout how long to wait before giving up, in units of
273	* <tt>unit</tt>
274	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
275	* <tt>timeout</tt> parameter
276	* @return <tt>true</tt> if successful, or <tt>false</tt> if
277	* the specified waiting time elapses before space is available.
278	* @throws InterruptedException if interrupted while waiting.
279	* @throws NullPointerException if the specified element is <tt>null</tt>.
280	*/
281	public boolean offer(E o, long timeout, TimeUnit unit)
282	throws InterruptedException {
283
284	if (o == null) throw new NullPointerException();
285	long nanos = unit.toNanos(timeout);
286	int c = -1;
287	final ReentrantLock putLock = this.putLock;
288	final AtomicInteger count = this.count;
289	putLock.lockInterruptibly();
290	try {
291	for (;;) {
292	if (count.get() < capacity) {
293	insert(o);
294	c = count.getAndIncrement();
295	if (c + 1 < capacity)
296	notFull.signal();
297	break;
298	}
299	if (nanos <= 0)
300	return false;
301	try {
302	nanos = notFull.awaitNanos(nanos);
303	} catch (InterruptedException ie) {
304	notFull.signal(); // propagate to a non-interrupted thread
305	throw ie;
306	}
307	}
308	} finally {
309	putLock.unlock();
310	}
311	if (c == 0)
312	signalNotEmpty();
313	return true;
314	}
315
316	/**
317	* Inserts the specified element at the tail of this queue if possible,
318	* returning immediately if this queue is full.
319	*
320	* @param o the element to add.
321	* @return <tt>true</tt> if it was possible to add the element to
322	* this queue, else <tt>false</tt>
323	* @throws NullPointerException if the specified element is <tt>null</tt>
324	*/
325	public boolean offer(E o) {
326	if (o == null) throw new NullPointerException();
327	final AtomicInteger count = this.count;
328	if (count.get() == capacity)
329	return false;
330	int c = -1;
331	final ReentrantLock putLock = this.putLock;
332	putLock.lock();
333	try {
334	if (count.get() < capacity) {
335	insert(o);
336	c = count.getAndIncrement();
337	if (c + 1 < capacity)
338	notFull.signal();
339	}
340	} finally {
341	putLock.unlock();
342	}
343	if (c == 0)
344	signalNotEmpty();
345	return c >= 0;
346	}
347
348
349	public E take() throws InterruptedException {
350	E x;
351	int c = -1;
352	final AtomicInteger count = this.count;
353	final ReentrantLock takeLock = this.takeLock;
354	takeLock.lockInterruptibly();
355	try {
356	try {
357	while (count.get() == 0)
358	notEmpty.await();
359	} catch (InterruptedException ie) {
360	notEmpty.signal(); // propagate to a non-interrupted thread
361	throw ie;
362	}
363
364	x = extract();
365	c = count.getAndDecrement();
366	if (c > 1)
367	notEmpty.signal();
368	} finally {
369	takeLock.unlock();
370	}
371	if (c == capacity)
372	signalNotFull();
373	return x;
374	}
375
376	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
377	E x = null;
378	int c = -1;
379	long nanos = unit.toNanos(timeout);
380	final AtomicInteger count = this.count;
381	final ReentrantLock takeLock = this.takeLock;
382	takeLock.lockInterruptibly();
383	try {
384	for (;;) {
385	if (count.get() > 0) {
386	x = extract();
387	c = count.getAndDecrement();
388	if (c > 1)
389	notEmpty.signal();
390	break;
391	}
392	if (nanos <= 0)
393	return null;
394	try {
395	nanos = notEmpty.awaitNanos(nanos);
396	} catch (InterruptedException ie) {
397	notEmpty.signal(); // propagate to a non-interrupted thread
398	throw ie;
399	}
400	}
401	} finally {
402	takeLock.unlock();
403	}
404	if (c == capacity)
405	signalNotFull();
406	return x;
407	}
408
409	public E poll() {
410	final AtomicInteger count = this.count;
411	if (count.get() == 0)
412	return null;
413	E x = null;
414	int c = -1;
415	final ReentrantLock takeLock = this.takeLock;
416	takeLock.lock();
417	try {
418	if (count.get() > 0) {
419	x = extract();
420	c = count.getAndDecrement();
421	if (c > 1)
422	notEmpty.signal();
423	}
424	} finally {
425	takeLock.unlock();
426	}
427	if (c == capacity)
428	signalNotFull();
429	return x;
430	}
431
432
433	public E peek() {
434	if (count.get() == 0)
435	return null;
436	final ReentrantLock takeLock = this.takeLock;
437	takeLock.lock();
438	try {
439	Node<E> first = head.next;
440	if (first == null)
441	return null;
442	else
443	return first.item;
444	} finally {
445	takeLock.unlock();
446	}
447	}
448
449	/**
450	* Removes a single instance of the specified element from this
451	* queue, if it is present.
452	*/
453	public boolean remove(Object o) {
454	if (o == null) return false;
455	boolean removed = false;
456	fullyLock();
457	try {
458	Node<E> trail = head;
459	Node<E> p = head.next;
460	while (p != null) {
461	if (o.equals(p.item)) {
462	removed = true;
463	break;
464	}
465	trail = p;
466	p = p.next;
467	}
468	if (removed) {
469	p.item = null;
470	trail.next = p.next;
471	if (count.getAndDecrement() == capacity)
472	notFull.signalAll();
473	}
474	} finally {
475	fullyUnlock();
476	}
477	return removed;
478	}
479
480	public Object[] toArray() {
481	fullyLock();
482	try {
483	int size = count.get();
484	Object[] a = new Object[size];
485	int k = 0;
486	for (Node<E> p = head.next; p != null; p = p.next)
487	a[k++] = p.item;
488	return a;
489	} finally {
490	fullyUnlock();
491	}
492	}
493
494	public <T> T[] toArray(T[] a) {
495	fullyLock();
496	try {
497	int size = count.get();
498	if (a.length < size)
499	a = (T[])java.lang.reflect.Array.newInstance
500	(a.getClass().getComponentType(), size);
501
502	int k = 0;
503	for (Node p = head.next; p != null; p = p.next)
504	a[k++] = (T)p.item;
505	return a;
506	} finally {
507	fullyUnlock();
508	}
509	}
510
511	public String toString() {
512	fullyLock();
513	try {
514	return super.toString();
515	} finally {
516	fullyUnlock();
517	}
518	}
519
520	/**
521	* Atomically removes all of the elements from this queue.
522	* The queue will be empty after this call returns.
523	*/
524	public void clear() {
525	fullyLock();
526	try {
527	head.next = null;
528	assert head.item == null;
529	last = head;
530	if (count.getAndSet(0) == capacity)
531	notFull.signalAll();
532	} finally {
533	fullyUnlock();
534	}
535	}
536
537	public int drainTo(Collection<? super E> c) {
538	if (c == null)
539	throw new NullPointerException();
540	if (c == this)
541	throw new IllegalArgumentException();
542	Node first;
543	fullyLock();
544	try {
545	first = head.next;
546	head.next = null;
547	assert head.item == null;
548	last = head;
549	if (count.getAndSet(0) == capacity)
550	notFull.signalAll();
551	} finally {
552	fullyUnlock();
553	}
554	// Transfer the elements outside of locks
555	int n = 0;
556	for (Node<E> p = first; p != null; p = p.next) {
557	c.add(p.item);
558	p.item = null;
559	++n;
560	}
561	return n;
562	}
563
564	public int drainTo(Collection<? super E> c, int maxElements) {
565	if (c == null)
566	throw new NullPointerException();
567	if (c == this)
568	throw new IllegalArgumentException();
569	fullyLock();
570	try {
571	int n = 0;
572	Node<E> p = head.next;
573	while (p != null && n < maxElements) {
574	c.add(p.item);
575	p.item = null;
576	p = p.next;
577	++n;
578	}
579	if (n != 0) {
580	head.next = p;
581	assert head.item == null;
582	if (p == null)
583	last = head;
584	if (count.getAndAdd(-n) == capacity)
585	notFull.signalAll();
586	}
587	return n;
588	} finally {
589	fullyUnlock();
590	}
591	}
592
593	/**
594	* Returns an iterator over the elements in this queue in proper sequence.
595	* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
596	* will never throw {@link java.util.ConcurrentModificationException},
597	* and guarantees to traverse elements as they existed upon
598	* construction of the iterator, and may (but is not guaranteed to)
599	* reflect any modifications subsequent to construction.
600	*
601	* @return an iterator over the elements in this queue in proper sequence.
602	*/
603	public Iterator<E> iterator() {
604	return new Itr();
605	}
606
607	private class Itr implements Iterator<E> {
608	/*
609	* Basic weak-consistent iterator. At all times hold the next
610	* item to hand out so that if hasNext() reports true, we will
611	* still have it to return even if lost race with a take etc.
612	*/
613	private Node<E> current;
614	private Node<E> lastRet;
615	private E currentElement;
616
617	Itr() {
618	final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
619	final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
620	putLock.lock();
621	takeLock.lock();
622	try {
623	current = head.next;
624	if (current != null)
625	currentElement = current.item;
626	} finally {
627	takeLock.unlock();
628	putLock.unlock();
629	}
630	}
631
632	public boolean hasNext() {
633	return current != null;
634	}
635
636	public E next() {
637	final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
638	final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
639	putLock.lock();
640	takeLock.lock();
641	try {
642	if (current == null)
643	throw new NoSuchElementException();
644	E x = currentElement;
645	lastRet = current;
646	current = current.next;
647	if (current != null)
648	currentElement = current.item;
649	return x;
650	} finally {
651	takeLock.unlock();
652	putLock.unlock();
653	}
654	}
655
656	public void remove() {
657	if (lastRet == null)
658	throw new IllegalStateException();
659	final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
660	final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
661	putLock.lock();
662	takeLock.lock();
663	try {
664	Node<E> node = lastRet;
665	lastRet = null;
666	Node<E> trail = head;
667	Node<E> p = head.next;
668	while (p != null && p != node) {
669	trail = p;
670	p = p.next;
671	}
672	if (p == node) {
673	p.item = null;
674	trail.next = p.next;
675	int c = count.getAndDecrement();
676	if (c == capacity)
677	notFull.signalAll();
678	}
679	} finally {
680	takeLock.unlock();
681	putLock.unlock();
682	}
683	}
684	}
685
686	/**
687	* Save the state to a stream (that is, serialize it).
688	*
689	* @serialData The capacity is emitted (int), followed by all of
690	* its elements (each an <tt>Object</tt>) in the proper order,
691	* followed by a null
692	* @param s the stream
693	*/
694	private void writeObject(java.io.ObjectOutputStream s)
695	throws java.io.IOException {
696
697	fullyLock();
698	try {
699	// Write out any hidden stuff, plus capacity
700	s.defaultWriteObject();
701
702	// Write out all elements in the proper order.
703	for (Node<E> p = head.next; p != null; p = p.next)
704	s.writeObject(p.item);
705
706	// Use trailing null as sentinel
707	s.writeObject(null);
708	} finally {
709	fullyUnlock();
710	}
711	}
712
713	/**
714	* Reconstitute this queue instance from a stream (that is,
715	* deserialize it).
716	* @param s the stream
717	*/
718	private void readObject(java.io.ObjectInputStream s)
719	throws java.io.IOException, ClassNotFoundException {
720	// Read in capacity, and any hidden stuff
721	s.defaultReadObject();
722
723	count.set(0);
724	last = head = new Node<E>(null);
725
726	// Read in all elements and place in queue
727	for (;;) {
728	E item = (E)s.readObject();
729	if (item == null)
730	break;
731	add(item);
732	}
733	}
734	}