util/concurrent/ArrayBlockingQueue.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.locks.*;
import java.util.*;

/**
 * A bounded {@linkplain BlockingQueue blocking queue} backed by an array.
 * 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.
 *
 * <p>This is a classic &quot;bounded buffer&quot;, in which a fixed-sized
 * array holds
 * elements inserted by producers and extracted by consumers.  Once
 * created, the capacity can not be increased.  Attempts to offer an
 * element to a full queue will result in the offer operation
 * blocking; attempts to retrieve an element from an empty queue will
 * similarly block.
 *
 * <p> This class supports an optional fairness policy for ordering
 * threads blocked on an insertion or removal.  By default, this
 * ordering is not guaranteed. However, an <tt>ArrayBlockingQueue</tt>
 * constructed with fairness set to <tt>true</tt> grants blocked
 * threads access in FIFO order. Fairness generally substantially
 * decreases throughput but reduces variablility and avoids
 * starvation.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /** The queued items */
    private transient final E[] items;
    /** items index for next take, poll or remove */
    private transient int takeIndex;
    /** items index for next put, offer, or add. */
    private transient int putIndex;
    /** Number of items in the queue */
    private int count;

    /**
     * An array used only during deserialization, to hold
     * items read back in from the stream, and then used
     * as "items" by readResolve via the private constructor.
     */
    private transient E[] deserializedItems;

    /*
     * Concurrency control via the classic two-condition algorithm
     * found in any textbook.
     */

    /** Main lock guarding all access */
    private final ReentrantLock lock;
    /** Condition for waiting takes */
    private final Condition notEmpty;
    /** Condition for wiating puts */
    private final Condition notFull;

    // Internal helper methods

    /**
     * Circularly increment i.
     */
    int inc(int i) {
        return (++i == items.length)? 0 : i;
    }

    /**
     * Insert element at current put position, advance, and signal.
     * Call only when holding lock.
     */
    private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
        notEmpty.signal();
    }

    /**
     * Extract element at current take position, advance, and signal.
     * Call only when holding lock.
     */
    private E extract() {
        E x = items[takeIndex];
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
        --count;
        notFull.signal();
        return x;
    }

    /**
     * Utility for remove and iterator.remove: Delete item at position i.
     * Call only when holding lock.
     */
    void removeAt(int i) {
        // if removing front item, just advance
        if (i == takeIndex) {
            items[takeIndex] = null;
            takeIndex = inc(takeIndex);
        }
        else {
            // slide over all others up through putIndex.
            for (;;) {
                int nexti = inc(i);
                if (nexti != putIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                }
                else {
                    items[i] = null;
                    putIndex = i;
                    break;
                }
            }
        }
        --count;
        notFull.signal();
    }

    /**
     * Internal constructor also used by readResolve.
     * Sets all final fields, plus count.
     * @param cap the capacity
     * @param array the array to use or null if should create new one
     * @param count the number of items in the array, where indices 0
     * to count-1 hold items.
     * @param lk the lock to use with this queue
     */
    private ArrayBlockingQueue(int cap, E[] array, int count,
                               ReentrantLock lk) {
        if (cap <= 0)
            throw new IllegalArgumentException();
        if (array == null)
            this.items = (E[]) new Object[cap];
        else
            this.items = array;
        this.putIndex = count;
        this.count = count;
        lock = lk;
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

    /**
     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity and default access policy.
     * @param capacity the capacity of this queue
     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
     */
    public ArrayBlockingQueue(int capacity) {
        this(capacity, null, 0, new ReentrantLock());
    }

    /**
     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity and the specified access policy.
     * @param capacity the capacity of this queue
     * @param fair if <tt>true</tt> then queue accesses for threads blocked
     * on insertion or removal, are processed in FIFO order; if <tt>false</tt>
     * the access order is unspecified.
     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
     */
    public ArrayBlockingQueue(int capacity, boolean fair) {
        this(capacity, null, 0, new ReentrantLock(fair));
    }

    /**
     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity, the specified access policy and initially containing the
     * elements of the given collection,
     * added in traversal order of the collection's iterator.
     * @param capacity the capacity of this queue
     * @param fair if <tt>true</tt> then queue accesses for threads blocked
     * on insertion or removal, are processed in FIFO order; if <tt>false</tt>
     * the access order is unspecified.
     * @param c the collection of elements to initially contain
     * @throws IllegalArgumentException if <tt>capacity</tt> is less than
     * <tt>c.size()</tt>, or less than 1.
     * @throws NullPointerException if <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, null, 0, new ReentrantLock(fair));

        if (capacity < c.size())
            throw new IllegalArgumentException();

        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);
    }

   /**
    * 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();
        lock.lock();
        try {
            if (count == items.length)
                return false;
            else {
                insert(o);
                return true;
            }
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * 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();

        lock.lockInterruptibly();
        try {
            long nanos = unit.toNanos(timeout);
            for (;;) {
                if (count != items.length) {
                    insert(o);
                    return true;
                }
                if (nanos <= 0)
                    return false;
                try {
                    nanos = notFull.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notFull.signal(); // propagate to non-interrupted thread
                    throw ie;
                }
            }
        }
        finally {
            lock.unlock();
        }
    }


    public E poll() {
        lock.lock();
        try {
            if (count == 0)
                return null;
            E x = extract();
            return x;
        }
        finally {
            lock.unlock();
        }
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        lock.lockInterruptibly();
        try {
            long nanos = unit.toNanos(timeout);
            for (;;) {
                if (count != 0) {
                    E x = extract();
                    return x;
                }
                if (nanos <= 0)
                    return null;
                try {
                    nanos = notEmpty.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to non-interrupted thread
                    throw ie;
                }

            }
        }
        finally {
            lock.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;
        lock.lock();
        try {
            int i = takeIndex;
            int k = 0;
            for (;;) {
                if (k++ >= count)
                    return false;
                if (o.equals(items[i])) {
                    removeAt(i);
                    return true;
                }
                i = inc(i);
            }

        }
        finally {
            lock.unlock();
        }
    }

    public E peek() {
        lock.lock();
        try {
            return (count == 0) ? null : items[takeIndex];
        }
        finally {
            lock.unlock();
        }
    }

    public E take() throws InterruptedException {
        lock.lockInterruptibly();
        try {
            try {
                while (count == 0)
                    notEmpty.await();
            }
            catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            E x = extract();
            return x;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * 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();

        lock.lockInterruptibly();
        try {
            try {
                while (count == items.length)
                    notFull.await();
            }
            catch (InterruptedException ie) {
                notFull.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            insert(o);
        }
        finally {
            lock.unlock();
        }
    }

    // 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() {
        lock.lock();
        try {
            return count;
        }
        finally {
            lock.unlock();
        }
    }

    // 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() {
        lock.lock();
        try {
            return items.length - count;
        }
        finally {
            lock.unlock();
        }
    }


    public boolean contains(Object o) {
        if (o == null) return false;
        lock.lock();
        try {
            int i = takeIndex;
            int k = 0;
            while (k++ < count) {
                if (o.equals(items[i]))
                    return true;
                i = inc(i);
            }
            return false;
        }
        finally {
            lock.unlock();
        }
    }

    public Object[] toArray() {
        lock.lock();
        try {
            E[] a = (E[]) new Object[count];
            int k = 0;
            int i = takeIndex;
            while (k < count) {
                a[k++] = items[i];
                i = inc(i);
            }
            return a;
        }
        finally {
            lock.unlock();
        }
    }

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

            int k = 0;
            int i = takeIndex;
            while (k < count) {
                a[k++] = (T)items[i];
                i = inc(i);
            }
            if (a.length > count)
                a[count] = null;
            return a;
        }
        finally {
            lock.unlock();
        }
    }

    public String toString() {
        lock.lock();
        try {
            return super.toString();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     *
     * @return an iterator over the elements in this queue in proper sequence.
     */
    public Iterator<E> iterator() {
        lock.lock();
        try {
            return new Itr();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Iterator for ArrayBlockingQueue
     */
    private class Itr implements Iterator<E> {
        /**
         * Index of element to be returned by next,
         * or a negative number if no such.
         */
        private int nextIndex;

        /**
         * nextItem holds on to item fields because once we claim
         * that an element exists in hasNext(), we must return it in
         * the following next() call even if it was in the process of
         * being removed when hasNext() was called.
         **/
        private E nextItem;

        /**
         * Index of element returned by most recent call to next.
         * Reset to -1 if this element is deleted by a call to remove.
         */
        private int lastRet;

        Itr() {
            lastRet = -1;
            if (count == 0)
                nextIndex = -1;
            else {
                nextIndex = takeIndex;
                nextItem = items[takeIndex];
            }
        }

        public boolean hasNext() {
            /*
             * No sync. We can return true by mistake here
             * only if this iterator passed across threads,
             * which we don't support anyway.
             */
            return nextIndex >= 0;
        }

        /**
         * Check whether nextIndex is valid; if so setting nextItem.
         * Stops iterator when either hits putIndex or sees null item.
         */
        private void checkNext() {
            if (nextIndex == putIndex) {
                nextIndex = -1;
                nextItem = null;
            }
            else {
                nextItem = items[nextIndex];
                if (nextItem == null)
                    nextIndex = -1;
            }
        }

        public E next() {
            lock.lock();
            try {
                if (nextIndex < 0)
                    throw new NoSuchElementException();
                lastRet = nextIndex;
                E x = nextItem;
                nextIndex = inc(nextIndex);
                checkNext();
                return x;
            }
            finally {
                lock.unlock();
            }
        }

        public void remove() {
            lock.lock();
            try {
                int i = lastRet;
                if (i == -1)
                    throw new IllegalStateException();
                lastRet = -1;

                int ti = takeIndex;
                removeAt(i);
                // back up cursor (reset to front if was first element)
                nextIndex = (i == ti) ? takeIndex : i;
                checkNext();
            }
            finally {
                lock.unlock();
            }
        }
    }

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

        // Write out element count, and any hidden stuff
        s.defaultWriteObject();
        // Write out maximumSize == items length
        s.writeInt(items.length);

        // Write out all elements in the proper order.
        int i = takeIndex;
        int k = 0;
        while (k++ < count) {
            s.writeObject(items[i]);
            i = inc(i);
        }
    }

    /**
     * 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 size, and any hidden stuff
        s.defaultReadObject();
        int size = count;

        // Read in array length and allocate array
        int arrayLength = s.readInt();

        // We use deserializedItems here because "items" is final
        deserializedItems = (E[]) new Object[arrayLength];

        // Read in all elements in the proper order into deserializedItems
        for (int i = 0; i < size; i++)
            deserializedItems[i] = (E)s.readObject();
    }

    /**
     * Throw away the object created with readObject, and replace it
     * with a usable ArrayBlockingQueue.
     * @return the ArrayBlockingQueue
     */
    private Object readResolve() throws java.io.ObjectStreamException {
        E[] array = deserializedItems;
        deserializedItems = null;
        return new ArrayBlockingQueue<E>(array.length, array, count, lock);
    }
}
Revision:	1.21
Committed:	Wed Aug 6 01:57:53 2003 UTC (20 years, 10 months ago) by dholmes
Branch:	MAIN
Changes since 1.20:	+51 -26 lines
Log Message:	Final major updates to Collection related classes.
#	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.locks.*;
9	import java.util.*;
10
11	/**
12	* A bounded {@linkplain BlockingQueue blocking queue} backed by an array.
13	* This queue orders elements FIFO (first-in-first-out).
14	* The <em>head</em> of the queue is that element that has been on the
15	* queue the longest time.
16	* The <em>tail</em> of the queue is that element that has been on the
17	* queue the shortest time.
18	*
19	* <p>This is a classic "bounded buffer", in which a fixed-sized
20	* array holds
21	* elements inserted by producers and extracted by consumers. Once
22	* created, the capacity can not be increased. Attempts to offer an
23	* element to a full queue will result in the offer operation
24	* blocking; attempts to retrieve an element from an empty queue will
25	* similarly block.
26	*
27	* <p> This class supports an optional fairness policy for ordering
28	* threads blocked on an insertion or removal. By default, this
29	* ordering is not guaranteed. However, an <tt>ArrayBlockingQueue</tt>
30	* constructed with fairness set to <tt>true</tt> grants blocked
31	* threads access in FIFO order. Fairness generally substantially
32	* decreases throughput but reduces variablility and avoids
33	* starvation.
34	*
35	* @since 1.5
36	* @author Doug Lea
37	*/
38	public class ArrayBlockingQueue<E> extends AbstractQueue<E>
39	implements BlockingQueue<E>, java.io.Serializable {
40
41	/** The queued items */
42	private transient final E[] items;
43	/** items index for next take, poll or remove */
44	private transient int takeIndex;
45	/** items index for next put, offer, or add. */
46	private transient int putIndex;
47	/** Number of items in the queue */
48	private int count;
49
50	/**
51	* An array used only during deserialization, to hold
52	* items read back in from the stream, and then used
53	* as "items" by readResolve via the private constructor.
54	*/
55	private transient E[] deserializedItems;
56
57	/*
58	* Concurrency control via the classic two-condition algorithm
59	* found in any textbook.
60	*/
61
62	/** Main lock guarding all access */
63	private final ReentrantLock lock;
64	/** Condition for waiting takes */
65	private final Condition notEmpty;
66	/** Condition for wiating puts */
67	private final Condition notFull;
68
69	// Internal helper methods
70
71	/**
72	* Circularly increment i.
73	*/
74	int inc(int i) {
75	return (++i == items.length)? 0 : i;
76	}
77
78	/**
79	* Insert element at current put position, advance, and signal.
80	* Call only when holding lock.
81	*/
82	private void insert(E x) {
83	items[putIndex] = x;
84	putIndex = inc(putIndex);
85	++count;
86	notEmpty.signal();
87	}
88
89	/**
90	* Extract element at current take position, advance, and signal.
91	* Call only when holding lock.
92	*/
93	private E extract() {
94	E x = items[takeIndex];
95	items[takeIndex] = null;
96	takeIndex = inc(takeIndex);
97	--count;
98	notFull.signal();
99	return x;
100	}
101
102	/**
103	* Utility for remove and iterator.remove: Delete item at position i.
104	* Call only when holding lock.
105	*/
106	void removeAt(int i) {
107	// if removing front item, just advance
108	if (i == takeIndex) {
109	items[takeIndex] = null;
110	takeIndex = inc(takeIndex);
111	}
112	else {
113	// slide over all others up through putIndex.
114	for (;;) {
115	int nexti = inc(i);
116	if (nexti != putIndex) {
117	items[i] = items[nexti];
118	i = nexti;
119	}
120	else {
121	items[i] = null;
122	putIndex = i;
123	break;
124	}
125	}
126	}
127	--count;
128	notFull.signal();
129	}
130
131	/**
132	* Internal constructor also used by readResolve.
133	* Sets all final fields, plus count.
134	* @param cap the capacity
135	* @param array the array to use or null if should create new one
136	* @param count the number of items in the array, where indices 0
137	* to count-1 hold items.
138	* @param lk the lock to use with this queue
139	*/
140	private ArrayBlockingQueue(int cap, E[] array, int count,
141	ReentrantLock lk) {
142	if (cap <= 0)
143	throw new IllegalArgumentException();
144	if (array == null)
145	this.items = (E[]) new Object[cap];
146	else
147	this.items = array;
148	this.putIndex = count;
149	this.count = count;
150	lock = lk;
151	notEmpty = lock.newCondition();
152	notFull = lock.newCondition();
153	}
154
155	/**
156	* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
157	* capacity and default access policy.
158	* @param capacity the capacity of this queue
159	* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
160	*/
161	public ArrayBlockingQueue(int capacity) {
162	this(capacity, null, 0, new ReentrantLock());
163	}
164
165	/**
166	* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
167	* capacity and the specified access policy.
168	* @param capacity the capacity of this queue
169	* @param fair if <tt>true</tt> then queue accesses for threads blocked
170	* on insertion or removal, are processed in FIFO order; if <tt>false</tt>
171	* the access order is unspecified.
172	* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
173	*/
174	public ArrayBlockingQueue(int capacity, boolean fair) {
175	this(capacity, null, 0, new ReentrantLock(fair));
176	}
177
178	/**
179	* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
180	* capacity, the specified access policy and initially containing the
181	* elements of the given collection,
182	* added in traversal order of the collection's iterator.
183	* @param capacity the capacity of this queue
184	* @param fair if <tt>true</tt> then queue accesses for threads blocked
185	* on insertion or removal, are processed in FIFO order; if <tt>false</tt>
186	* the access order is unspecified.
187	* @param c the collection of elements to initially contain
188	* @throws IllegalArgumentException if <tt>capacity</tt> is less than
189	* <tt>c.size()</tt>, or less than 1.
190	* @throws NullPointerException if <tt>c</tt> or any element within it
191	* is <tt>null</tt>
192	*/
193	public ArrayBlockingQueue(int capacity, boolean fair,
194	Collection<? extends E> c) {
195	this(capacity, null, 0, new ReentrantLock(fair));
196
197	if (capacity < c.size())
198	throw new IllegalArgumentException();
199
200	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
201	add(it.next());
202	}
203
204	// Have to override just to update the javadoc
205
206	/**
207	* Adds the specified element to the tail of this queue.
208	* @return <tt>true</tt> (as per the general contract of
209	* <tt>Collection.add</tt>).
210	* @throws IllegalStateException {@inheritDoc}
211	* @throws NullPointerException {@inheritDoc}
212	*/
213	public boolean add(E o) {
214	return super.add(o);
215	}
216
217	/**
218	* Adds all of the elements in the specified collection to this queue.
219	* The behavior of this operation is undefined if
220	* the specified collection is modified while the operation is in
221	* progress. (This implies that the behavior of this call is undefined if
222	* the specified collection is this queue, and this queue is nonempty.)
223	* <p>
224	* This implementation iterates over the specified collection, and adds
225	* each object returned by the iterator to this queue's tail, in turn.
226	* @throws IllegalStateException {@inheritDoc}
227	* @throws NullPointerException {@inheritDoc}
228	*/
229	public boolean addAll(Collection<? extends E> c) {
230	return super.addAll(c);
231	}
232
233	/**
234	* Adds the specified element to the tail of this queue if possible,
235	* returning immediately if this queue is full.
236	*
237	* @throws NullPointerException {@inheritDoc}
238	*/
239	public boolean offer(E o) {
240	if (o == null) throw new NullPointerException();
241	lock.lock();
242	try {
243	if (count == items.length)
244	return false;
245	else {
246	insert(o);
247	return true;
248	}
249	}
250	finally {
251	lock.unlock();
252	}
253	}
254
255	/**
256	* Adds the specified element to the tail of this queue, waiting if
257	* necessary up to the specified wait time for space to become available.
258	* @throws NullPointerException {@inheritDoc}
259	*/
260	public boolean offer(E o, long timeout, TimeUnit unit)
261	throws InterruptedException {
262
263	if (o == null) throw new NullPointerException();
264
265	lock.lockInterruptibly();
266	try {
267	long nanos = unit.toNanos(timeout);
268	for (;;) {
269	if (count != items.length) {
270	insert(o);
271	return true;
272	}
273	if (nanos <= 0)
274	return false;
275	try {
276	nanos = notFull.awaitNanos(nanos);
277	}
278	catch (InterruptedException ie) {
279	notFull.signal(); // propagate to non-interrupted thread
280	throw ie;
281	}
282	}
283	}
284	finally {
285	lock.unlock();
286	}
287	}
288
289
290	public E poll() {
291	lock.lock();
292	try {
293	if (count == 0)
294	return null;
295	E x = extract();
296	return x;
297	}
298	finally {
299	lock.unlock();
300	}
301	}
302
303	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
304	lock.lockInterruptibly();
305	try {
306	long nanos = unit.toNanos(timeout);
307	for (;;) {
308	if (count != 0) {
309	E x = extract();
310	return x;
311	}
312	if (nanos <= 0)
313	return null;
314	try {
315	nanos = notEmpty.awaitNanos(nanos);
316	}
317	catch (InterruptedException ie) {
318	notEmpty.signal(); // propagate to non-interrupted thread
319	throw ie;
320	}
321
322	}
323	}
324	finally {
325	lock.unlock();
326	}
327	}
328
329
330	/**
331	* Removes a single instance of the specified element from this
332	* queue, if it is present. More formally,
333	* removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
334	* o.equals(e))</tt>, if the queue contains one or more such
335	* elements. Returns <tt>true</tt> if the queue contained the
336	* specified element (or equivalently, if the queue changed as a
337	* result of the call).
338	*
339	* <p>This implementation iterates over the queue looking for the
340	* specified element. If it finds the element, it removes the element
341	* from the queue using the iterator's remove method.<p>
342	*
343	*/
344	public boolean remove(Object o) {
345	if (o == null) return false;
346	lock.lock();
347	try {
348	int i = takeIndex;
349	int k = 0;
350	for (;;) {
351	if (k++ >= count)
352	return false;
353	if (o.equals(items[i])) {
354	removeAt(i);
355	return true;
356	}
357	i = inc(i);
358	}
359
360	}
361	finally {
362	lock.unlock();
363	}
364	}
365
366	public E peek() {
367	lock.lock();
368	try {
369	return (count == 0) ? null : items[takeIndex];
370	}
371	finally {
372	lock.unlock();
373	}
374	}
375
376	public E take() throws InterruptedException {
377	lock.lockInterruptibly();
378	try {
379	try {
380	while (count == 0)
381	notEmpty.await();
382	}
383	catch (InterruptedException ie) {
384	notEmpty.signal(); // propagate to non-interrupted thread
385	throw ie;
386	}
387	E x = extract();
388	return x;
389	}
390	finally {
391	lock.unlock();
392	}
393	}
394
395	/**
396	* Adds the specified element to the tail of this queue, waiting if
397	* necessary for space to become available.
398	* @throws NullPointerException {@inheritDoc}
399	*/
400	public void put(E o) throws InterruptedException {
401
402	if (o == null) throw new NullPointerException();
403
404	lock.lockInterruptibly();
405	try {
406	try {
407	while (count == items.length)
408	notFull.await();
409	}
410	catch (InterruptedException ie) {
411	notFull.signal(); // propagate to non-interrupted thread
412	throw ie;
413	}
414	insert(o);
415	}
416	finally {
417	lock.unlock();
418	}
419	}
420
421	// this doc comment is overridden to remove the reference to collections
422	// greater in size than Integer.MAX_VALUE
423	/**
424	* Returns the number of elements in this collection.
425	*/
426	public int size() {
427	lock.lock();
428	try {
429	return count;
430	}
431	finally {
432	lock.unlock();
433	}
434	}
435
436	// this doc comment is a modified copy of the inherited doc comment,
437	// without the reference to unlimited queues.
438	/**
439	* Returns the number of elements that this queue can ideally (in
440	* the absence of memory or resource constraints) accept without
441	* blocking. This is always equal to the initial capacity of this queue
442	* less the current <tt>size</tt> of this queue.
443	* <p>Note that you <em>cannot</em> always tell if
444	* an attempt to <tt>add</tt> an element will succeed by
445	* inspecting <tt>remainingCapacity</tt> because it may be the
446	* case that a waiting consumer is ready to <tt>take</tt> an
447	* element out of an otherwise full queue.
448	*/
449	public int remainingCapacity() {
450	lock.lock();
451	try {
452	return items.length - count;
453	}
454	finally {
455	lock.unlock();
456	}
457	}
458
459
460	public boolean contains(Object o) {
461	if (o == null) return false;
462	lock.lock();
463	try {
464	int i = takeIndex;
465	int k = 0;
466	while (k++ < count) {
467	if (o.equals(items[i]))
468	return true;
469	i = inc(i);
470	}
471	return false;
472	}
473	finally {
474	lock.unlock();
475	}
476	}
477
478	public Object[] toArray() {
479	lock.lock();
480	try {
481	E[] a = (E[]) new Object[count];
482	int k = 0;
483	int i = takeIndex;
484	while (k < count) {
485	a[k++] = items[i];
486	i = inc(i);
487	}
488	return a;
489	}
490	finally {
491	lock.unlock();
492	}
493	}
494
495	public <T> T[] toArray(T[] a) {
496	lock.lock();
497	try {
498	if (a.length < count)
499	a = (T[])java.lang.reflect.Array.newInstance(
500	a.getClass().getComponentType(),
501	count
502	);
503
504	int k = 0;
505	int i = takeIndex;
506	while (k < count) {
507	a[k++] = (T)items[i];
508	i = inc(i);
509	}
510	if (a.length > count)
511	a[count] = null;
512	return a;
513	}
514	finally {
515	lock.unlock();
516	}
517	}
518
519	public String toString() {
520	lock.lock();
521	try {
522	return super.toString();
523	}
524	finally {
525	lock.unlock();
526	}
527	}
528
529	/**
530	* Returns an iterator over the elements in this queue in proper sequence.
531	*
532	* @return an iterator over the elements in this queue in proper sequence.
533	*/
534	public Iterator<E> iterator() {
535	lock.lock();
536	try {
537	return new Itr();
538	}
539	finally {
540	lock.unlock();
541	}
542	}
543
544	/**
545	* Iterator for ArrayBlockingQueue
546	*/
547	private class Itr implements Iterator<E> {
548	/**
549	* Index of element to be returned by next,
550	* or a negative number if no such.
551	*/
552	private int nextIndex;
553
554	/**
555	* nextItem holds on to item fields because once we claim
556	* that an element exists in hasNext(), we must return it in
557	* the following next() call even if it was in the process of
558	* being removed when hasNext() was called.
559	**/
560	private E nextItem;
561
562	/**
563	* Index of element returned by most recent call to next.
564	* Reset to -1 if this element is deleted by a call to remove.
565	*/
566	private int lastRet;
567
568	Itr() {
569	lastRet = -1;
570	if (count == 0)
571	nextIndex = -1;
572	else {
573	nextIndex = takeIndex;
574	nextItem = items[takeIndex];
575	}
576	}
577
578	public boolean hasNext() {
579	/*
580	* No sync. We can return true by mistake here
581	* only if this iterator passed across threads,
582	* which we don't support anyway.
583	*/
584	return nextIndex >= 0;
585	}
586
587	/**
588	* Check whether nextIndex is valid; if so setting nextItem.
589	* Stops iterator when either hits putIndex or sees null item.
590	*/
591	private void checkNext() {
592	if (nextIndex == putIndex) {
593	nextIndex = -1;
594	nextItem = null;
595	}
596	else {
597	nextItem = items[nextIndex];
598	if (nextItem == null)
599	nextIndex = -1;
600	}
601	}
602
603	public E next() {
604	lock.lock();
605	try {
606	if (nextIndex < 0)
607	throw new NoSuchElementException();
608	lastRet = nextIndex;
609	E x = nextItem;
610	nextIndex = inc(nextIndex);
611	checkNext();
612	return x;
613	}
614	finally {
615	lock.unlock();
616	}
617	}
618
619	public void remove() {
620	lock.lock();
621	try {
622	int i = lastRet;
623	if (i == -1)
624	throw new IllegalStateException();
625	lastRet = -1;
626
627	int ti = takeIndex;
628	removeAt(i);
629	// back up cursor (reset to front if was first element)
630	nextIndex = (i == ti) ? takeIndex : i;
631	checkNext();
632	}
633	finally {
634	lock.unlock();
635	}
636	}
637	}
638
639	/**
640	* Save the state to a stream (that is, serialize it).
641	*
642	* @serialData The maximumSize is emitted (int), followed by all of
643	* its elements (each an <tt>E</tt>) in the proper order.
644	* @param s the stream
645	*/
646	private void writeObject(java.io.ObjectOutputStream s)
647	throws java.io.IOException {
648
649	// Write out element count, and any hidden stuff
650	s.defaultWriteObject();
651	// Write out maximumSize == items length
652	s.writeInt(items.length);
653
654	// Write out all elements in the proper order.
655	int i = takeIndex;
656	int k = 0;
657	while (k++ < count) {
658	s.writeObject(items[i]);
659	i = inc(i);
660	}
661	}
662
663	/**
664	* Reconstitute this queue instance from a stream (that is,
665	* deserialize it).
666	* @param s the stream
667	*/
668	private void readObject(java.io.ObjectInputStream s)
669	throws java.io.IOException, ClassNotFoundException {
670	// Read in size, and any hidden stuff
671	s.defaultReadObject();
672	int size = count;
673
674	// Read in array length and allocate array
675	int arrayLength = s.readInt();
676
677	// We use deserializedItems here because "items" is final
678	deserializedItems = (E[]) new Object[arrayLength];
679
680	// Read in all elements in the proper order into deserializedItems
681	for (int i = 0; i < size; i++)
682	deserializedItems[i] = (E)s.readObject();
683	}
684
685	/**
686	* Throw away the object created with readObject, and replace it
687	* with a usable ArrayBlockingQueue.
688	* @return the ArrayBlockingQueue
689	*/
690	private Object readResolve() throws java.io.ObjectStreamException {
691	E[] array = deserializedItems;
692	deserializedItems = null;
693	return new ArrayBlockingQueue<E>(array.length, array, count, lock);
694	}
695	}