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

/**
 * A {@linkplain BlockingQueue blocking queue} in which each
 * <tt>put</tt> must wait for a <tt>take</tt>, and vice versa.  A
 * synchronous queue does not have any internal capacity - in
 * particular it does not have a capacity of one. You cannot
 * <tt>peek</tt> at a synchronous queue because an element is only
 * present when you try to take it; you cannot add an element (using
 * any method) unless another thread is trying to remove it; you
 * cannot iterate as there is nothing to iterate.  The <em>head</em>
 * of the queue is the element that the first queued thread is trying
 * to add to the queue; if there are no queued threads then no element
 * is being added and the head is <tt>null</tt>.  For purposes of
 * other <tt>Collection</tt> methods (for example <tt>contains</tt>),
 * a <tt>SynchronousQueue</tt> acts as an empty collection.  This
 * queue does not permit <tt>null</tt> elements.
 *
 * <p>Synchronous queues are similar to rendezvous channels used in
 * CSP and Ada. They are well suited for handoff designs, in which an
 * object running in one thread must sync up with an object running
 * in another thread in order to hand it some information, event, or
 * task.
 * <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 SynchronousQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -3223113410248163686L;

    /*
      This implementation divides actions into two cases for puts:

      * An arriving putter that does not already have a waiting taker
      creates a node holding item, and then waits for a taker to take it.
      * An arriving putter that does already have a waiting taker fills
      the slot node created by the taker, and notifies it to continue.

      And symmetrically, two for takes:

      * An arriving taker that does not already have a waiting putter
      creates an empty slot node, and then waits for a putter to fill it.
      * An arriving taker that does already have a waiting putter takes
      item from the node created by the putter, and notifies it to continue.

      This requires keeping two simple queues: waitingPuts and waitingTakes.

      When a put or take waiting for the actions of its counterpart
      aborts due to interruption or timeout, it marks the node
      it created as "CANCELLED", which causes its counterpart to retry
      the entire put or take sequence.
    */


    /*
     * Note that all fields are transient final, so there is
     * no explicit serialization code.
     */

    private transient final WaitQueue waitingPuts = new WaitQueue();
    private transient final WaitQueue waitingTakes = new WaitQueue();
    private transient final ReentrantLock qlock = new ReentrantLock();

    /**
     * Nodes each maintain an item and handle waits and signals for
     * getting and setting it. The class extends
     * AbstractQueuedSynchronizer to manage blocking, using AQS state
     *  0 for waiting, 1 for ack, -1 for cancelled.
     */
    private static final class Node extends AbstractQueuedSynchronizer {
        /** Synchronization state value representing that node acked */
        private static final int ACK    =  1;
        /** Synchronization state value representing that node cancelled */
        private static final int CANCEL = -1;

        /** The item being transferred */
        Object item;
        /** Next node in wait queue */
        Node next;

        /** Create node with initial item */
        Node(Object x) { item = x; }

        /**
         * Implements AQS base acquire to succeed if not in WAITING state
         */
        protected boolean tryAcquireExclusive(int ignore) {
            return getState() != 0;
        }

        /**
         * Implements AQS base release to signal if state changed
         */
        protected boolean tryReleaseExclusive(int newState) {
            return compareAndSetState(0, newState);
        }

        /**
         * Take item and null out field (for sake of GC)
         */
        private Object extract() {
            Object x = item;
            item = null;
            return x;
        }

        /**
         * Try to cancel on interrupt; if so rethrowing,
         * else setting interrupt state
         */
        private void checkCancellationOnInterrupt(InterruptedException ie) 
            throws InterruptedException {
            if (releaseExclusive(CANCEL)) 
                throw ie;
            Thread.currentThread().interrupt();
        }

        /**
         * Fill in the slot created by the taker and signal taker to
         * continue.
         */
        boolean setItem(Object x) {
            item = x; // can place in slot even if cancelled
            return releaseExclusive(ACK);
        }

        /**
         * Remove item from slot created by putter and signal putter
         * to continue.
         */
        Object getItem() {
            return (releaseExclusive(ACK))? extract() : null;
        }

        /**
         * Wait for a taker to take item placed by putter.
         */
        void waitForTake() throws InterruptedException {
            try {
                acquireExclusiveInterruptibly(0);
            } catch (InterruptedException ie) {
                checkCancellationOnInterrupt(ie);
            }
        }

        /**
         * Wait for a putter to put item placed by taker.
         */
        Object waitForPut() throws InterruptedException {
            try {
                acquireExclusiveInterruptibly(0);
            } catch (InterruptedException ie) {
                checkCancellationOnInterrupt(ie);
            }
            return extract();
        }

        /**
         * Wait for a taker to take item placed by putter or time out.
         */
        boolean waitForTake(long nanos) throws InterruptedException {
            try {
                if (!acquireExclusiveNanos(0, nanos) &&
                    releaseExclusive(CANCEL))
                    return false;
            } catch (InterruptedException ie) {
                checkCancellationOnInterrupt(ie);
            }
            return true;
        }

        /**
         * Wait for a putter to put item placed by taker, or time out.
         */
        Object waitForPut(long nanos) throws InterruptedException {
            try {
                if (!acquireExclusiveNanos(0, nanos) &&
                    releaseExclusive(CANCEL))
                    return null;
            } catch (InterruptedException ie) {
                checkCancellationOnInterrupt(ie);
            }
            return extract();
        }

    }

    /**
     * Simple FIFO queue class to hold waiting puts/takes.
     **/
    private static class WaitQueue<E> {
        Node head;
        Node last;

        Node enq(Object x) {
            Node p = new Node(x);
            if (last == null)
                last = head = p;
            else
                last = last.next = p;
            return p;
        }

        Node deq() {
            Node p = head;
            if (p != null) {
                if ((head = p.next) == null)
                    last = null;
                p.next = null;
            }
            return p;
        }
    }

    /**
     * Creates a <tt>SynchronousQueue</tt>.
     */
    public SynchronousQueue() {}


    /**
     * Adds the specified element to this queue, waiting if necessary for
     * another thread to receive it.
     * @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();
        final ReentrantLock qlock = this.qlock;

        for (;;) {
            Node node;
            boolean mustWait;
            qlock.lockInterruptibly();
            try {
                node = waitingTakes.deq();
                if ( (mustWait = (node == null)) )
                    node = waitingPuts.enq(o);
            } finally {
                qlock.unlock();
            }

            if (mustWait) {
                node.waitForTake();
                return;
            }

            else if (node.setItem(o))
                return;

            // else taker cancelled, so retry
        }
    }

    /**
     * Inserts the specified element into this queue, waiting if necessary
     * up to the specified wait time for another thread to receive it.
     * @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 a taker appears.
     * @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);
        final ReentrantLock qlock = this.qlock;
        for (;;) {
            Node node;
            boolean mustWait;
            qlock.lockInterruptibly();
            try {
                node = waitingTakes.deq();
                if ( (mustWait = (node == null)) )
                    node = waitingPuts.enq(o);
            } finally {
                qlock.unlock();
            }

            if (mustWait) 
                return node.waitForTake(nanos);

            else if (node.setItem(o))
                return true;

            // else taker cancelled, so retry
        }

    }


    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * for another thread to insert it.
     * @return the head of this queue
     */
    public E take() throws InterruptedException {
        final ReentrantLock qlock = this.qlock;
        for (;;) {
            Node node;
            boolean mustWait;

            qlock.lockInterruptibly();
            try {
                node = waitingPuts.deq();
                if ( (mustWait = (node == null)) )
                    node = waitingTakes.enq(null);
            } finally {
                qlock.unlock();
            }

            if (mustWait) {
                Object x = node.waitForPut();
                return (E)x;
            }
            else {
                Object x = node.getItem();
                if (x != null)
                    return (E)x;
                // else cancelled, so retry
            }
        }
    }

    /**
     * Retrieves and removes the head of this queue, waiting
     * if necessary up to the specified wait time, for another thread
     * to insert it.
     * @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 the head of this queue, or <tt>null</tt> if the
     * specified waiting time elapses before an element is present.
     * @throws InterruptedException if interrupted while waiting.
     */
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock qlock = this.qlock;

        for (;;) {
            Node node;
            boolean mustWait;

            qlock.lockInterruptibly();
            try {
                node = waitingPuts.deq();
                if ( (mustWait = (node == null)) )
                    node = waitingTakes.enq(null);
            } finally {
                qlock.unlock();
            }

            if (mustWait) {
                Object x = node.waitForPut(nanos);
                return (E)x;
            }
            else {
                Object x = node.getItem();
                if (x != null)
                    return (E)x;
                // else cancelled, so retry
            }
        }
    }

    // Untimed nonblocking versions

   /**
    * Inserts the specified element into this queue, if another thread is
    * waiting to receive it.
    *
    * @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 ReentrantLock qlock = this.qlock;

        for (;;) {
            Node node;
            qlock.lock();
            try {
                node = waitingTakes.deq();
            } finally {
                qlock.unlock();
            }
            if (node == null)
                return false;

            else if (node.setItem(o))
                return true;
            // else retry
        }
    }

    /**
     * Retrieves and removes the head of this queue, if another thread
     * is currently making an element available.
     *
     * @return the head of this queue, or <tt>null</tt> if no
     *         element is available.
     */
    public E poll() {
        final ReentrantLock qlock = this.qlock;
        for (;;) {
            Node node;
            qlock.lock();
            try {
                node = waitingPuts.deq();
            } finally {
                qlock.unlock();
            }
            if (node == null)
                return null;

            else {
                Object x = node.getItem();
                if (x != null)
                    return (E)x;
                // else retry
            }
        }
    }

    /**
     * Always returns <tt>true</tt>. 
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @return <tt>true</tt>
     */
    public boolean isEmpty() {
        return true;
    }

    /**
     * Always returns zero.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @return zero.
     */
    public int size() {
        return 0;
    }

    /**
     * Always returns zero.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @return zero.
     */
    public int remainingCapacity() {
        return 0;
    }

    /**
     * Does nothing.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     */
    public void clear() {}

    /**
     * Always returns <tt>false</tt>.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @param o the element
     * @return <tt>false</tt>
     */
    public boolean contains(Object o) {
        return false;
    }

    /**
     * Always returns <tt>false</tt>.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     *
     * @param o the element to remove
     * @return <tt>false</tt>
     */
    public boolean remove(Object o) {
        return false;
    }

    /**
     * Returns <tt>false</tt> unless given collection is empty.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @param c the collection
     * @return <tt>false</tt> unless given collection is empty
     */
    public boolean containsAll(Collection<?> c) {
        return c.isEmpty();
    }

    /**
     * Always returns <tt>false</tt>.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @param c the collection
     * @return <tt>false</tt>
     */
    public boolean removeAll(Collection<?> c) {
        return false;
    }

    /**
     * Always returns <tt>false</tt>.
     * A <tt>SynchronousQueue</tt> has no internal capacity.
     * @param c the collection
     * @return <tt>false</tt>
     */
    public boolean retainAll(Collection<?> c) {
        return false;
    }

    /**
     * Always returns <tt>null</tt>. 
     * A <tt>SynchronousQueue</tt> does not return elements
     * unless actively waited on.
     * @return <tt>null</tt>
     */
    public E peek() {
        return null;
    }


    static class EmptyIterator<E> implements Iterator<E> {
        public boolean hasNext() {
            return false;
        }
        public E next() {
            throw new NoSuchElementException();
        }
        public void remove() {
            throw new IllegalStateException();
        }
    }

    /**
     * Returns an empty iterator in which <tt>hasNext</tt> always returns
     * <tt>false</tt>.
     *
     * @return an empty iterator
     */
    public Iterator<E> iterator() {
        return new EmptyIterator<E>();
    }


    /**
     * Returns a zero-length array.
     * @return a zero-length array
     */
    public Object[] toArray() {
        return new Object[0];
    }

    /**
     * Sets the zeroeth element of the specified array to <tt>null</tt>
     * (if the array has non-zero length) and returns it.
     * @return the specified array
     */
    public <T> T[] toArray(T[] a) {
        if (a.length > 0)
            a[0] = null;
        return a;
    }


    public int drainTo(Collection<? super E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        int n = 0;
        E e;
        while ( (e = poll()) != null) {
            c.add(e);
            ++n;
        }
        return n;
    }

    public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        int n = 0;
        E e;
        while (n < maxElements && (e = poll()) != null) {
            c.add(e);
            ++n;
        }
        return n;
    }
}


Revision:	1.38
Committed:	Fri Jan 9 14:45:17 2004 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.37:	+2 -2 lines
Log Message:	Cosmetics
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.29	* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	dl	1.8	import java.util.concurrent.locks.*;
9	tim	1.1	import java.util.*;
10
11			/**
12	dl	1.18	* A {@linkplain BlockingQueue blocking queue} in which each
13			* <tt>put</tt> must wait for a <tt>take</tt>, and vice versa. A
14			* synchronous queue does not have any internal capacity - in
15			* particular it does not have a capacity of one. You cannot
16			* <tt>peek</tt> at a synchronous queue because an element is only
17			* present when you try to take it; you cannot add an element (using
18			* any method) unless another thread is trying to remove it; you
19			* cannot iterate as there is nothing to iterate. The <em>head</em>
20			* of the queue is the element that the first queued thread is trying
21			* to add to the queue; if there are no queued threads then no element
22			* is being added and the head is <tt>null</tt>. For purposes of
23			* other <tt>Collection</tt> methods (for example <tt>contains</tt>),
24	dl	1.19	* a <tt>SynchronousQueue</tt> acts as an empty collection. This
25	dl	1.18	* queue does not permit <tt>null</tt> elements.
26			*
27			* <p>Synchronous queues are similar to rendezvous channels used in
28			* CSP and Ada. They are well suited for handoff designs, in which an
29	dl	1.30	* object running in one thread must sync up with an object running
30	dl	1.18	* in another thread in order to hand it some information, event, or
31			* task.
32	dl	1.19	* <p>This class implements all of the <em>optional</em> methods
33			* of the {@link Collection} and {@link Iterator} interfaces.
34	dl	1.6	* @since 1.5
35			* @author Doug Lea
36	dl	1.24	* @param <E> the type of elements held in this collection
37	dl	1.23	*/
38	dl	1.2	public class SynchronousQueue<E> extends AbstractQueue<E>
39	tim	1.1	implements BlockingQueue<E>, java.io.Serializable {
40	dl	1.15	private static final long serialVersionUID = -3223113410248163686L;
41	tim	1.1
42	dl	1.2	/*
43			This implementation divides actions into two cases for puts:
44
45	tim	1.10	* An arriving putter that does not already have a waiting taker
46	dl	1.2	creates a node holding item, and then waits for a taker to take it.
47			* An arriving putter that does already have a waiting taker fills
48			the slot node created by the taker, and notifies it to continue.
49
50			And symmetrically, two for takes:
51
52			* An arriving taker that does not already have a waiting putter
53			creates an empty slot node, and then waits for a putter to fill it.
54			* An arriving taker that does already have a waiting putter takes
55			item from the node created by the putter, and notifies it to continue.
56
57			This requires keeping two simple queues: waitingPuts and waitingTakes.
58	tim	1.10
59	dl	1.2	When a put or take waiting for the actions of its counterpart
60			aborts due to interruption or timeout, it marks the node
61			it created as "CANCELLED", which causes its counterpart to retry
62			the entire put or take sequence.
63			*/
64
65
66			/*
67			* Note that all fields are transient final, so there is
68			* no explicit serialization code.
69			*/
70
71			private transient final WaitQueue waitingPuts = new WaitQueue();
72			private transient final WaitQueue waitingTakes = new WaitQueue();
73			private transient final ReentrantLock qlock = new ReentrantLock();
74
75			/**
76			* Nodes each maintain an item and handle waits and signals for
77	dl	1.31	* getting and setting it. The class extends
78			* AbstractQueuedSynchronizer to manage blocking, using AQS state
79			* 0 for waiting, 1 for ack, -1 for cancelled.
80	dl	1.2	*/
81	dl	1.31	private static final class Node extends AbstractQueuedSynchronizer {
82	dl	1.35	/** Synchronization state value representing that node acked */
83			private static final int ACK = 1;
84			/** Synchronization state value representing that node cancelled */
85			private static final int CANCEL = -1;
86
87	dl	1.6	/** The item being transferred */
88	dl	1.2	Object item;
89	dl	1.6	/** Next node in wait queue */
90	dl	1.2	Node next;
91	dl	1.35
92			/** Create node with initial item */
93	dl	1.31	Node(Object x) { item = x; }
94
95			/**
96			* Implements AQS base acquire to succeed if not in WAITING state
97			*/
98	dl	1.37	protected boolean tryAcquireExclusive(int ignore) {
99	dl	1.35	return getState() != 0;
100	dl	1.31	}
101
102			/**
103	dl	1.34	* Implements AQS base release to signal if state changed
104	dl	1.31	*/
105	dl	1.35	protected boolean tryReleaseExclusive(int newState) {
106			return compareAndSetState(0, newState);
107	dl	1.31	}
108
109			/**
110	dl	1.35	* Take item and null out field (for sake of GC)
111	dl	1.31	*/
112	dl	1.35	private Object extract() {
113			Object x = item;
114			item = null;
115			return x;
116	dl	1.31	}
117
118			/**
119	dl	1.35	* Try to cancel on interrupt; if so rethrowing,
120			* else setting interrupt state
121	dl	1.31	*/
122	dl	1.35	private void checkCancellationOnInterrupt(InterruptedException ie)
123			throws InterruptedException {
124			if (releaseExclusive(CANCEL))
125			throw ie;
126			Thread.currentThread().interrupt();
127	dl	1.31	}
128	dl	1.2
129			/**
130			* Fill in the slot created by the taker and signal taker to
131			* continue.
132			*/
133	dl	1.31	boolean setItem(Object x) {
134	dl	1.35	item = x; // can place in slot even if cancelled
135			return releaseExclusive(ACK);
136	dl	1.2	}
137
138			/**
139			* Remove item from slot created by putter and signal putter
140			* to continue.
141			*/
142	dl	1.31	Object getItem() {
143	dl	1.35	return (releaseExclusive(ACK))? extract() : null;
144			}
145
146			/**
147			* Wait for a taker to take item placed by putter.
148			*/
149			void waitForTake() throws InterruptedException {
150			try {
151			acquireExclusiveInterruptibly(0);
152			} catch (InterruptedException ie) {
153			checkCancellationOnInterrupt(ie);
154			}
155			}
156
157			/**
158			* Wait for a putter to put item placed by taker.
159			*/
160			Object waitForPut() throws InterruptedException {
161			try {
162			acquireExclusiveInterruptibly(0);
163			} catch (InterruptedException ie) {
164			checkCancellationOnInterrupt(ie);
165			}
166			return extract();
167	dl	1.31	}
168
169			/**
170	dl	1.33	* Wait for a taker to take item placed by putter or time out.
171	dl	1.31	*/
172	dl	1.35	boolean waitForTake(long nanos) throws InterruptedException {
173	dl	1.2	try {
174	dl	1.38	if (!acquireExclusiveNanos(0, nanos) &&
175	dl	1.35	releaseExclusive(CANCEL))
176	dl	1.33	return false;
177	dl	1.31	} catch (InterruptedException ie) {
178	dl	1.35	checkCancellationOnInterrupt(ie);
179	dl	1.2	}
180	dl	1.35	return true;
181	dl	1.2	}
182
183			/**
184	dl	1.33	* Wait for a putter to put item placed by taker, or time out.
185	dl	1.31	*/
186	dl	1.35	Object waitForPut(long nanos) throws InterruptedException {
187	dl	1.31	try {
188	dl	1.38	if (!acquireExclusiveNanos(0, nanos) &&
189	dl	1.35	releaseExclusive(CANCEL))
190	dl	1.33	return null;
191	dl	1.31	} catch (InterruptedException ie) {
192	dl	1.35	checkCancellationOnInterrupt(ie);
193	dl	1.2	}
194	dl	1.35	return extract();
195	dl	1.2	}
196
197			}
198
199			/**
200			* Simple FIFO queue class to hold waiting puts/takes.
201			**/
202			private static class WaitQueue<E> {
203			Node head;
204			Node last;
205
206	tim	1.10	Node enq(Object x) {
207	dl	1.2	Node p = new Node(x);
208	tim	1.10	if (last == null)
209	dl	1.2	last = head = p;
210	tim	1.10	else
211	dl	1.2	last = last.next = p;
212			return p;
213			}
214
215			Node deq() {
216			Node p = head;
217	dl	1.35	if (p != null) {
218			if ((head = p.next) == null)
219			last = null;
220			p.next = null;
221			}
222	dl	1.2	return p;
223			}
224			}
225
226			/**
227	dl	1.35	* Creates a <tt>SynchronousQueue</tt>.
228	tim	1.10	*/
229	dl	1.35	public SynchronousQueue() {}
230	dl	1.2
231
232			/**
233	dl	1.35	* Adds the specified element to this queue, waiting if necessary for
234			* another thread to receive it.
235			* @param o the element to add
236			* @throws InterruptedException if interrupted while waiting.
237			* @throws NullPointerException if the specified element is <tt>null</tt>.
238	tim	1.10	*/
239	dl	1.35	public void put(E o) throws InterruptedException {
240			if (o == null) throw new NullPointerException();
241			final ReentrantLock qlock = this.qlock;
242
243	dl	1.2	for (;;) {
244			Node node;
245			boolean mustWait;
246			qlock.lockInterruptibly();
247			try {
248	dl	1.35	node = waitingTakes.deq();
249	dl	1.2	if ( (mustWait = (node == null)) )
250	dl	1.35	node = waitingPuts.enq(o);
251	tim	1.14	} finally {
252	dl	1.2	qlock.unlock();
253			}
254
255	dl	1.31	if (mustWait) {
256	dl	1.35	node.waitForTake();
257			return;
258	dl	1.2	}
259
260	dl	1.35	else if (node.setItem(o))
261			return;
262	dl	1.2
263	dl	1.35	// else taker cancelled, so retry
264			}
265	tim	1.1	}
266
267	dholmes	1.11	/**
268	dl	1.20	* Inserts the specified element into this queue, waiting if necessary
269	dl	1.18	* up to the specified wait time for another thread to receive it.
270			* @param o the element to add
271			* @param timeout how long to wait before giving up, in units of
272			* <tt>unit</tt>
273			* @param unit a <tt>TimeUnit</tt> determining how to interpret the
274			* <tt>timeout</tt> parameter
275	dholmes	1.11	* @return <tt>true</tt> if successful, or <tt>false</tt> if
276			* the specified waiting time elapses before a taker appears.
277	dl	1.18	* @throws InterruptedException if interrupted while waiting.
278			* @throws NullPointerException if the specified element is <tt>null</tt>.
279	dholmes	1.11	*/
280	dl	1.18	public boolean offer(E o, long timeout, TimeUnit unit) throws InterruptedException {
281	dl	1.35	if (o == null) throw new NullPointerException();
282			long nanos = unit.toNanos(timeout);
283			final ReentrantLock qlock = this.qlock;
284			for (;;) {
285			Node node;
286			boolean mustWait;
287			qlock.lockInterruptibly();
288			try {
289			node = waitingTakes.deq();
290			if ( (mustWait = (node == null)) )
291			node = waitingPuts.enq(o);
292			} finally {
293			qlock.unlock();
294			}
295
296			if (mustWait)
297			return node.waitForTake(nanos);
298
299			else if (node.setItem(o))
300			return true;
301
302			// else taker cancelled, so retry
303			}
304
305	tim	1.1	}
306
307	dl	1.2
308	dholmes	1.11	/**
309			* Retrieves and removes the head of this queue, waiting if necessary
310			* for another thread to insert it.
311			* @return the head of this queue
312			*/
313	dl	1.2	public E take() throws InterruptedException {
314	dl	1.35	final ReentrantLock qlock = this.qlock;
315			for (;;) {
316			Node node;
317			boolean mustWait;
318
319			qlock.lockInterruptibly();
320			try {
321			node = waitingPuts.deq();
322			if ( (mustWait = (node == null)) )
323			node = waitingTakes.enq(null);
324			} finally {
325			qlock.unlock();
326			}
327
328	dl	1.36	if (mustWait) {
329			Object x = node.waitForPut();
330			return (E)x;
331			}
332	dl	1.35	else {
333			Object x = node.getItem();
334			if (x != null)
335			return (E)x;
336			// else cancelled, so retry
337			}
338			}
339	tim	1.1	}
340	dl	1.2
341	dholmes	1.11	/**
342			* Retrieves and removes the head of this queue, waiting
343			* if necessary up to the specified wait time, for another thread
344			* to insert it.
345	dl	1.18	* @param timeout how long to wait before giving up, in units of
346			* <tt>unit</tt>
347			* @param unit a <tt>TimeUnit</tt> determining how to interpret the
348			* <tt>timeout</tt> parameter
349			* @return the head of this queue, or <tt>null</tt> if the
350			* specified waiting time elapses before an element is present.
351			* @throws InterruptedException if interrupted while waiting.
352	dholmes	1.11	*/
353	dl	1.2	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
354	dl	1.35	long nanos = unit.toNanos(timeout);
355			final ReentrantLock qlock = this.qlock;
356
357			for (;;) {
358			Node node;
359			boolean mustWait;
360
361			qlock.lockInterruptibly();
362			try {
363			node = waitingPuts.deq();
364			if ( (mustWait = (node == null)) )
365			node = waitingTakes.enq(null);
366			} finally {
367			qlock.unlock();
368			}
369
370	dl	1.36	if (mustWait) {
371			Object x = node.waitForPut(nanos);
372			return (E)x;
373			}
374	dl	1.35	else {
375			Object x = node.getItem();
376			if (x != null)
377			return (E)x;
378			// else cancelled, so retry
379			}
380			}
381	tim	1.1	}
382	dl	1.2
383			// Untimed nonblocking versions
384
385	dl	1.18	/**
386	dl	1.20	* Inserts the specified element into this queue, if another thread is
387	dl	1.18	* waiting to receive it.
388			*
389			* @param o the element to add.
390			* @return <tt>true</tt> if it was possible to add the element to
391			* this queue, else <tt>false</tt>
392			* @throws NullPointerException if the specified element is <tt>null</tt>
393			*/
394	dholmes	1.11	public boolean offer(E o) {
395			if (o == null) throw new NullPointerException();
396	dl	1.27	final ReentrantLock qlock = this.qlock;
397	tim	1.10
398			for (;;) {
399	tim	1.26	Node node;
400	dl	1.2	qlock.lock();
401			try {
402			node = waitingTakes.deq();
403	tim	1.14	} finally {
404	dl	1.2	qlock.unlock();
405			}
406			if (node == null)
407			return false;
408	tim	1.10
409	dl	1.31	else if (node.setItem(o))
410	dl	1.2	return true;
411			// else retry
412			}
413	tim	1.1	}
414	dl	1.2
415	dl	1.18	/**
416			* Retrieves and removes the head of this queue, if another thread
417			* is currently making an element available.
418			*
419			* @return the head of this queue, or <tt>null</tt> if no
420			* element is available.
421			*/
422	dl	1.2	public E poll() {
423	dl	1.27	final ReentrantLock qlock = this.qlock;
424	dl	1.2	for (;;) {
425			Node node;
426			qlock.lock();
427			try {
428			node = waitingPuts.deq();
429	tim	1.14	} finally {
430	dl	1.2	qlock.unlock();
431			}
432			if (node == null)
433			return null;
434
435			else {
436	dl	1.31	Object x = node.getItem();
437	dl	1.2	if (x != null)
438			return (E)x;
439			// else retry
440			}
441			}
442	tim	1.1	}
443	dl	1.2
444	dl	1.5	/**
445	dholmes	1.11	* Always returns <tt>true</tt>.
446			* A <tt>SynchronousQueue</tt> has no internal capacity.
447			* @return <tt>true</tt>
448	dl	1.5	*/
449			public boolean isEmpty() {
450			return true;
451			}
452
453			/**
454	dholmes	1.11	* Always returns zero.
455			* A <tt>SynchronousQueue</tt> has no internal capacity.
456	dl	1.5	* @return zero.
457			*/
458			public int size() {
459			return 0;
460	tim	1.1	}
461	dl	1.2
462	dl	1.5	/**
463	dholmes	1.11	* Always returns zero.
464			* A <tt>SynchronousQueue</tt> has no internal capacity.
465	dl	1.5	* @return zero.
466			*/
467			public int remainingCapacity() {
468			return 0;
469			}
470
471			/**
472	dholmes	1.11	* Does nothing.
473			* A <tt>SynchronousQueue</tt> has no internal capacity.
474			*/
475			public void clear() {}
476
477			/**
478			* Always returns <tt>false</tt>.
479			* A <tt>SynchronousQueue</tt> has no internal capacity.
480	dl	1.18	* @param o the element
481	dholmes	1.11	* @return <tt>false</tt>
482			*/
483			public boolean contains(Object o) {
484			return false;
485			}
486
487			/**
488	dl	1.18	* Always returns <tt>false</tt>.
489			* A <tt>SynchronousQueue</tt> has no internal capacity.
490			*
491			* @param o the element to remove
492			* @return <tt>false</tt>
493			*/
494			public boolean remove(Object o) {
495			return false;
496			}
497
498			/**
499	dl	1.16	* Returns <tt>false</tt> unless given collection is empty.
500	dholmes	1.11	* A <tt>SynchronousQueue</tt> has no internal capacity.
501	dl	1.18	* @param c the collection
502	dl	1.16	* @return <tt>false</tt> unless given collection is empty
503	dholmes	1.11	*/
504	dl	1.12	public boolean containsAll(Collection<?> c) {
505	dl	1.16	return c.isEmpty();
506	dholmes	1.11	}
507
508			/**
509			* Always returns <tt>false</tt>.
510			* A <tt>SynchronousQueue</tt> has no internal capacity.
511	dl	1.18	* @param c the collection
512	dholmes	1.11	* @return <tt>false</tt>
513			*/
514	dl	1.12	public boolean removeAll(Collection<?> c) {
515	dholmes	1.11	return false;
516			}
517
518			/**
519			* Always returns <tt>false</tt>.
520			* A <tt>SynchronousQueue</tt> has no internal capacity.
521	dl	1.18	* @param c the collection
522	dholmes	1.11	* @return <tt>false</tt>
523			*/
524	dl	1.12	public boolean retainAll(Collection<?> c) {
525	dholmes	1.11	return false;
526			}
527
528			/**
529			* Always returns <tt>null</tt>.
530			* A <tt>SynchronousQueue</tt> does not return elements
531	dl	1.5	* unless actively waited on.
532	dholmes	1.11	* @return <tt>null</tt>
533	dl	1.5	*/
534			public E peek() {
535			return null;
536			}
537
538
539			static class EmptyIterator<E> implements Iterator<E> {
540	dl	1.2	public boolean hasNext() {
541			return false;
542			}
543			public E next() {
544			throw new NoSuchElementException();
545			}
546			public void remove() {
547	dl	1.17	throw new IllegalStateException();
548	dl	1.2	}
549	tim	1.1	}
550	dl	1.2
551	dl	1.5	/**
552	dl	1.18	* Returns an empty iterator in which <tt>hasNext</tt> always returns
553	tim	1.13	* <tt>false</tt>.
554			*
555	dholmes	1.11	* @return an empty iterator
556	dl	1.5	*/
557	dl	1.2	public Iterator<E> iterator() {
558	dl	1.5	return new EmptyIterator<E>();
559	tim	1.1	}
560
561	dl	1.2
562	dl	1.5	/**
563	dholmes	1.11	* Returns a zero-length array.
564			* @return a zero-length array
565	dl	1.5	*/
566	dl	1.3	public Object[] toArray() {
567	dl	1.25	return new Object[0];
568	tim	1.1	}
569
570	dholmes	1.11	/**
571			* Sets the zeroeth element of the specified array to <tt>null</tt>
572			* (if the array has non-zero length) and returns it.
573			* @return the specified array
574			*/
575	dl	1.2	public <T> T[] toArray(T[] a) {
576			if (a.length > 0)
577			a[0] = null;
578			return a;
579			}
580	dl	1.21
581
582			public int drainTo(Collection<? super E> c) {
583			if (c == null)
584			throw new NullPointerException();
585			if (c == this)
586			throw new IllegalArgumentException();
587			int n = 0;
588			E e;
589			while ( (e = poll()) != null) {
590			c.add(e);
591			++n;
592			}
593			return n;
594			}
595
596			public int drainTo(Collection<? super E> c, int maxElements) {
597			if (c == null)
598			throw new NullPointerException();
599			if (c == this)
600			throw new IllegalArgumentException();
601			int n = 0;
602			E e;
603			while (n < maxElements && (e = poll()) != null) {
604			c.add(e);
605			++n;
606			}
607			return n;
608			}
609	tim	1.1	}
610	dholmes	1.11
611
612
613
614