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(boolean b, 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 (!acquireExclusiveTimed(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 (!acquireExclusiveTimed(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) 
                return (E)node.waitForPut();

            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) 
                return (E) node.waitForPut(nanos);

            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:	1.35
Committed:	Fri Jan 2 21:02:32 2004 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.34:	+157 -104 lines
Log Message:	Avoid timeout problems in fair modes; improve AQS method names
#	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.locks.*;
9	import java.util.*;
10
11	/**
12	* 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	* a <tt>SynchronousQueue</tt> acts as an empty collection. This
25	* 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	* object running in one thread must sync up with an object running
30	* in another thread in order to hand it some information, event, or
31	* task.
32	* <p>This class implements all of the <em>optional</em> methods
33	* of the {@link Collection} and {@link Iterator} interfaces.
34	* @since 1.5
35	* @author Doug Lea
36	* @param <E> the type of elements held in this collection
37	*/
38	public class SynchronousQueue<E> extends AbstractQueue<E>
39	implements BlockingQueue<E>, java.io.Serializable {
40	private static final long serialVersionUID = -3223113410248163686L;
41
42	/*
43	This implementation divides actions into two cases for puts:
44
45	* An arriving putter that does not already have a waiting taker
46	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
59	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	* 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	*/
81	private static final class Node extends AbstractQueuedSynchronizer {
82	/** 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	/** The item being transferred */
88	Object item;
89	/** Next node in wait queue */
90	Node next;
91
92	/** Create node with initial item */
93	Node(Object x) { item = x; }
94
95	/**
96	* Implements AQS base acquire to succeed if not in WAITING state
97	*/
98	protected boolean tryAcquireExclusive(boolean b, int ignore) {
99	return getState() != 0;
100	}
101
102	/**
103	* Implements AQS base release to signal if state changed
104	*/
105	protected boolean tryReleaseExclusive(int newState) {
106	return compareAndSetState(0, newState);
107	}
108
109	/**
110	* Take item and null out field (for sake of GC)
111	*/
112	private Object extract() {
113	Object x = item;
114	item = null;
115	return x;
116	}
117
118	/**
119	* Try to cancel on interrupt; if so rethrowing,
120	* else setting interrupt state
121	*/
122	private void checkCancellationOnInterrupt(InterruptedException ie)
123	throws InterruptedException {
124	if (releaseExclusive(CANCEL))
125	throw ie;
126	Thread.currentThread().interrupt();
127	}
128
129	/**
130	* Fill in the slot created by the taker and signal taker to
131	* continue.
132	*/
133	boolean setItem(Object x) {
134	item = x; // can place in slot even if cancelled
135	return releaseExclusive(ACK);
136	}
137
138	/**
139	* Remove item from slot created by putter and signal putter
140	* to continue.
141	*/
142	Object getItem() {
143	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	}
168
169	/**
170	* Wait for a taker to take item placed by putter or time out.
171	*/
172	boolean waitForTake(long nanos) throws InterruptedException {
173	try {
174	if (!acquireExclusiveTimed(0, nanos) &&
175	releaseExclusive(CANCEL))
176	return false;
177	} catch (InterruptedException ie) {
178	checkCancellationOnInterrupt(ie);
179	}
180	return true;
181	}
182
183	/**
184	* Wait for a putter to put item placed by taker, or time out.
185	*/
186	Object waitForPut(long nanos) throws InterruptedException {
187	try {
188	if (!acquireExclusiveTimed(0, nanos) &&
189	releaseExclusive(CANCEL))
190	return null;
191	} catch (InterruptedException ie) {
192	checkCancellationOnInterrupt(ie);
193	}
194	return extract();
195	}
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	Node enq(Object x) {
207	Node p = new Node(x);
208	if (last == null)
209	last = head = p;
210	else
211	last = last.next = p;
212	return p;
213	}
214
215	Node deq() {
216	Node p = head;
217	if (p != null) {
218	if ((head = p.next) == null)
219	last = null;
220	p.next = null;
221	}
222	return p;
223	}
224	}
225
226	/**
227	* Creates a <tt>SynchronousQueue</tt>.
228	*/
229	public SynchronousQueue() {}
230
231
232	/**
233	* 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	*/
239	public void put(E o) throws InterruptedException {
240	if (o == null) throw new NullPointerException();
241	final ReentrantLock qlock = this.qlock;
242
243	for (;;) {
244	Node node;
245	boolean mustWait;
246	qlock.lockInterruptibly();
247	try {
248	node = waitingTakes.deq();
249	if ( (mustWait = (node == null)) )
250	node = waitingPuts.enq(o);
251	} finally {
252	qlock.unlock();
253	}
254
255	if (mustWait) {
256	node.waitForTake();
257	return;
258	}
259
260	else if (node.setItem(o))
261	return;
262
263	// else taker cancelled, so retry
264	}
265	}
266
267	/**
268	* Inserts the specified element into this queue, waiting if necessary
269	* 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	* @return <tt>true</tt> if successful, or <tt>false</tt> if
276	* the specified waiting time elapses before a taker appears.
277	* @throws InterruptedException if interrupted while waiting.
278	* @throws NullPointerException if the specified element is <tt>null</tt>.
279	*/
280	public boolean offer(E o, long timeout, TimeUnit unit) throws InterruptedException {
281	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	}
306
307
308	/**
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	public E take() throws InterruptedException {
314	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	if (mustWait)
329	return (E)node.waitForPut();
330
331	else {
332	Object x = node.getItem();
333	if (x != null)
334	return (E)x;
335	// else cancelled, so retry
336	}
337	}
338	}
339
340	/**
341	* Retrieves and removes the head of this queue, waiting
342	* if necessary up to the specified wait time, for another thread
343	* to insert it.
344	* @param timeout how long to wait before giving up, in units of
345	* <tt>unit</tt>
346	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
347	* <tt>timeout</tt> parameter
348	* @return the head of this queue, or <tt>null</tt> if the
349	* specified waiting time elapses before an element is present.
350	* @throws InterruptedException if interrupted while waiting.
351	*/
352	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
353	long nanos = unit.toNanos(timeout);
354	final ReentrantLock qlock = this.qlock;
355
356	for (;;) {
357	Node node;
358	boolean mustWait;
359
360	qlock.lockInterruptibly();
361	try {
362	node = waitingPuts.deq();
363	if ( (mustWait = (node == null)) )
364	node = waitingTakes.enq(null);
365	} finally {
366	qlock.unlock();
367	}
368
369	if (mustWait)
370	return (E) node.waitForPut(nanos);
371
372	else {
373	Object x = node.getItem();
374	if (x != null)
375	return (E)x;
376	// else cancelled, so retry
377	}
378	}
379	}
380
381	// Untimed nonblocking versions
382
383	/**
384	* Inserts the specified element into this queue, if another thread is
385	* waiting to receive it.
386	*
387	* @param o the element to add.
388	* @return <tt>true</tt> if it was possible to add the element to
389	* this queue, else <tt>false</tt>
390	* @throws NullPointerException if the specified element is <tt>null</tt>
391	*/
392	public boolean offer(E o) {
393	if (o == null) throw new NullPointerException();
394	final ReentrantLock qlock = this.qlock;
395
396	for (;;) {
397	Node node;
398	qlock.lock();
399	try {
400	node = waitingTakes.deq();
401	} finally {
402	qlock.unlock();
403	}
404	if (node == null)
405	return false;
406
407	else if (node.setItem(o))
408	return true;
409	// else retry
410	}
411	}
412
413	/**
414	* Retrieves and removes the head of this queue, if another thread
415	* is currently making an element available.
416	*
417	* @return the head of this queue, or <tt>null</tt> if no
418	* element is available.
419	*/
420	public E poll() {
421	final ReentrantLock qlock = this.qlock;
422	for (;;) {
423	Node node;
424	qlock.lock();
425	try {
426	node = waitingPuts.deq();
427	} finally {
428	qlock.unlock();
429	}
430	if (node == null)
431	return null;
432
433	else {
434	Object x = node.getItem();
435	if (x != null)
436	return (E)x;
437	// else retry
438	}
439	}
440	}
441
442	/**
443	* Always returns <tt>true</tt>.
444	* A <tt>SynchronousQueue</tt> has no internal capacity.
445	* @return <tt>true</tt>
446	*/
447	public boolean isEmpty() {
448	return true;
449	}
450
451	/**
452	* Always returns zero.
453	* A <tt>SynchronousQueue</tt> has no internal capacity.
454	* @return zero.
455	*/
456	public int size() {
457	return 0;
458	}
459
460	/**
461	* Always returns zero.
462	* A <tt>SynchronousQueue</tt> has no internal capacity.
463	* @return zero.
464	*/
465	public int remainingCapacity() {
466	return 0;
467	}
468
469	/**
470	* Does nothing.
471	* A <tt>SynchronousQueue</tt> has no internal capacity.
472	*/
473	public void clear() {}
474
475	/**
476	* Always returns <tt>false</tt>.
477	* A <tt>SynchronousQueue</tt> has no internal capacity.
478	* @param o the element
479	* @return <tt>false</tt>
480	*/
481	public boolean contains(Object o) {
482	return false;
483	}
484
485	/**
486	* Always returns <tt>false</tt>.
487	* A <tt>SynchronousQueue</tt> has no internal capacity.
488	*
489	* @param o the element to remove
490	* @return <tt>false</tt>
491	*/
492	public boolean remove(Object o) {
493	return false;
494	}
495
496	/**
497	* Returns <tt>false</tt> unless given collection is empty.
498	* A <tt>SynchronousQueue</tt> has no internal capacity.
499	* @param c the collection
500	* @return <tt>false</tt> unless given collection is empty
501	*/
502	public boolean containsAll(Collection<?> c) {
503	return c.isEmpty();
504	}
505
506	/**
507	* Always returns <tt>false</tt>.
508	* A <tt>SynchronousQueue</tt> has no internal capacity.
509	* @param c the collection
510	* @return <tt>false</tt>
511	*/
512	public boolean removeAll(Collection<?> c) {
513	return false;
514	}
515
516	/**
517	* Always returns <tt>false</tt>.
518	* A <tt>SynchronousQueue</tt> has no internal capacity.
519	* @param c the collection
520	* @return <tt>false</tt>
521	*/
522	public boolean retainAll(Collection<?> c) {
523	return false;
524	}
525
526	/**
527	* Always returns <tt>null</tt>.
528	* A <tt>SynchronousQueue</tt> does not return elements
529	* unless actively waited on.
530	* @return <tt>null</tt>
531	*/
532	public E peek() {
533	return null;
534	}
535
536
537	static class EmptyIterator<E> implements Iterator<E> {
538	public boolean hasNext() {
539	return false;
540	}
541	public E next() {
542	throw new NoSuchElementException();
543	}
544	public void remove() {
545	throw new IllegalStateException();
546	}
547	}
548
549	/**
550	* Returns an empty iterator in which <tt>hasNext</tt> always returns
551	* <tt>false</tt>.
552	*
553	* @return an empty iterator
554	*/
555	public Iterator<E> iterator() {
556	return new EmptyIterator<E>();
557	}
558
559
560	/**
561	* Returns a zero-length array.
562	* @return a zero-length array
563	*/
564	public Object[] toArray() {
565	return new Object[0];
566	}
567
568	/**
569	* Sets the zeroeth element of the specified array to <tt>null</tt>
570	* (if the array has non-zero length) and returns it.
571	* @return the specified array
572	*/
573	public <T> T[] toArray(T[] a) {
574	if (a.length > 0)
575	a[0] = null;
576	return a;
577	}
578
579
580	public int drainTo(Collection<? super E> c) {
581	if (c == null)
582	throw new NullPointerException();
583	if (c == this)
584	throw new IllegalArgumentException();
585	int n = 0;
586	E e;
587	while ( (e = poll()) != null) {
588	c.add(e);
589	++n;
590	}
591	return n;
592	}
593
594	public int drainTo(Collection<? super E> c, int maxElements) {
595	if (c == null)
596	throw new NullPointerException();
597	if (c == this)
598	throw new IllegalArgumentException();
599	int n = 0;
600	E e;
601	while (n < maxElements && (e = poll()) != null) {
602	c.add(e);
603	++n;
604	}
605	return n;
606	}
607	}
608
609
610
611
612