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.
     */
    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 tryAcquire(int ignore) {
            return getState() != 0;
        }

        /**
         * Implements AQS base release to signal if state changed
         */
        protected boolean tryRelease(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 (release(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 release(ACK);
        }

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

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

        /**
         * Wait for a putter to put item placed by taker.
         */
        Object waitForPut() throws InterruptedException {
            try {
                acquireInterruptibly(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 (!tryAcquireNanos(0, nanos) &&
                    release(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 (!tryAcquireNanos(0, nanos) &&
                    release(CANCEL))
                    return null;
            } catch (InterruptedException ie) {
                checkCancellationOnInterrupt(ie);
            }
            return extract();
        }

    }

    /**
     * Simple FIFO queue class to hold waiting puts/takes.
     **/
    static final 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.
     * @throws InterruptedException if interrupted while waiting.
     * @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.
     * @param a the array
     * @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:	1.41
Committed:	Wed Jan 21 15:20:35 2004 UTC (20 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.40:	+2 -2 lines
Log Message:	doc improvements; consistent conventions for nested 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, 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	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 tryAcquire(int ignore) {
99	return getState() != 0;
100	}
101
102	/**
103	* Implements AQS base release to signal if state changed
104	*/
105	protected boolean tryRelease(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 (release(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 release(ACK);
136	}
137
138	/**
139	* Remove item from slot created by putter and signal putter
140	* to continue.
141	*/
142	Object getItem() {
143	return (release(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	acquireInterruptibly(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	acquireInterruptibly(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 (!tryAcquireNanos(0, nanos) &&
175	release(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 (!tryAcquireNanos(0, nanos) &&
189	release(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	static final 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	* @throws InterruptedException if interrupted while waiting.
312	* @return the head of this queue
313	*/
314	public E take() throws InterruptedException {
315	final ReentrantLock qlock = this.qlock;
316	for (;;) {
317	Node node;
318	boolean mustWait;
319
320	qlock.lockInterruptibly();
321	try {
322	node = waitingPuts.deq();
323	if ( (mustWait = (node == null)) )
324	node = waitingTakes.enq(null);
325	} finally {
326	qlock.unlock();
327	}
328
329	if (mustWait) {
330	Object x = node.waitForPut();
331	return (E)x;
332	}
333	else {
334	Object x = node.getItem();
335	if (x != null)
336	return (E)x;
337	// else cancelled, so retry
338	}
339	}
340	}
341
342	/**
343	* Retrieves and removes the head of this queue, waiting
344	* if necessary up to the specified wait time, for another thread
345	* to insert it.
346	* @param timeout how long to wait before giving up, in units of
347	* <tt>unit</tt>
348	* @param unit a <tt>TimeUnit</tt> determining how to interpret the
349	* <tt>timeout</tt> parameter
350	* @return the head of this queue, or <tt>null</tt> if the
351	* specified waiting time elapses before an element is present.
352	* @throws InterruptedException if interrupted while waiting.
353	*/
354	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
355	long nanos = unit.toNanos(timeout);
356	final ReentrantLock qlock = this.qlock;
357
358	for (;;) {
359	Node node;
360	boolean mustWait;
361
362	qlock.lockInterruptibly();
363	try {
364	node = waitingPuts.deq();
365	if ( (mustWait = (node == null)) )
366	node = waitingTakes.enq(null);
367	} finally {
368	qlock.unlock();
369	}
370
371	if (mustWait) {
372	Object x = node.waitForPut(nanos);
373	return (E)x;
374	}
375	else {
376	Object x = node.getItem();
377	if (x != null)
378	return (E)x;
379	// else cancelled, so retry
380	}
381	}
382	}
383
384	// Untimed nonblocking versions
385
386	/**
387	* Inserts the specified element into this queue, if another thread is
388	* waiting to receive it.
389	*
390	* @param o the element to add.
391	* @return <tt>true</tt> if it was possible to add the element to
392	* this queue, else <tt>false</tt>
393	* @throws NullPointerException if the specified element is <tt>null</tt>
394	*/
395	public boolean offer(E o) {
396	if (o == null) throw new NullPointerException();
397	final ReentrantLock qlock = this.qlock;
398
399	for (;;) {
400	Node node;
401	qlock.lock();
402	try {
403	node = waitingTakes.deq();
404	} finally {
405	qlock.unlock();
406	}
407	if (node == null)
408	return false;
409
410	else if (node.setItem(o))
411	return true;
412	// else retry
413	}
414	}
415
416	/**
417	* Retrieves and removes the head of this queue, if another thread
418	* is currently making an element available.
419	*
420	* @return the head of this queue, or <tt>null</tt> if no
421	* element is available.
422	*/
423	public E poll() {
424	final ReentrantLock qlock = this.qlock;
425	for (;;) {
426	Node node;
427	qlock.lock();
428	try {
429	node = waitingPuts.deq();
430	} finally {
431	qlock.unlock();
432	}
433	if (node == null)
434	return null;
435
436	else {
437	Object x = node.getItem();
438	if (x != null)
439	return (E)x;
440	// else retry
441	}
442	}
443	}
444
445	/**
446	* Always returns <tt>true</tt>.
447	* A <tt>SynchronousQueue</tt> has no internal capacity.
448	* @return <tt>true</tt>
449	*/
450	public boolean isEmpty() {
451	return true;
452	}
453
454	/**
455	* Always returns zero.
456	* A <tt>SynchronousQueue</tt> has no internal capacity.
457	* @return zero.
458	*/
459	public int size() {
460	return 0;
461	}
462
463	/**
464	* Always returns zero.
465	* A <tt>SynchronousQueue</tt> has no internal capacity.
466	* @return zero.
467	*/
468	public int remainingCapacity() {
469	return 0;
470	}
471
472	/**
473	* Does nothing.
474	* A <tt>SynchronousQueue</tt> has no internal capacity.
475	*/
476	public void clear() {}
477
478	/**
479	* Always returns <tt>false</tt>.
480	* A <tt>SynchronousQueue</tt> has no internal capacity.
481	* @param o the element
482	* @return <tt>false</tt>
483	*/
484	public boolean contains(Object o) {
485	return false;
486	}
487
488	/**
489	* Always returns <tt>false</tt>.
490	* A <tt>SynchronousQueue</tt> has no internal capacity.
491	*
492	* @param o the element to remove
493	* @return <tt>false</tt>
494	*/
495	public boolean remove(Object o) {
496	return false;
497	}
498
499	/**
500	* Returns <tt>false</tt> unless given collection is empty.
501	* A <tt>SynchronousQueue</tt> has no internal capacity.
502	* @param c the collection
503	* @return <tt>false</tt> unless given collection is empty
504	*/
505	public boolean containsAll(Collection<?> c) {
506	return c.isEmpty();
507	}
508
509	/**
510	* Always returns <tt>false</tt>.
511	* A <tt>SynchronousQueue</tt> has no internal capacity.
512	* @param c the collection
513	* @return <tt>false</tt>
514	*/
515	public boolean removeAll(Collection<?> c) {
516	return false;
517	}
518
519	/**
520	* Always returns <tt>false</tt>.
521	* A <tt>SynchronousQueue</tt> has no internal capacity.
522	* @param c the collection
523	* @return <tt>false</tt>
524	*/
525	public boolean retainAll(Collection<?> c) {
526	return false;
527	}
528
529	/**
530	* Always returns <tt>null</tt>.
531	* A <tt>SynchronousQueue</tt> does not return elements
532	* unless actively waited on.
533	* @return <tt>null</tt>
534	*/
535	public E peek() {
536	return null;
537	}
538
539
540	static class EmptyIterator<E> implements Iterator<E> {
541	public boolean hasNext() {
542	return false;
543	}
544	public E next() {
545	throw new NoSuchElementException();
546	}
547	public void remove() {
548	throw new IllegalStateException();
549	}
550	}
551
552	/**
553	* Returns an empty iterator in which <tt>hasNext</tt> always returns
554	* <tt>false</tt>.
555	*
556	* @return an empty iterator
557	*/
558	public Iterator<E> iterator() {
559	return new EmptyIterator<E>();
560	}
561
562
563	/**
564	* Returns a zero-length array.
565	* @return a zero-length array
566	*/
567	public Object[] toArray() {
568	return new Object[0];
569	}
570
571	/**
572	* Sets the zeroeth element of the specified array to <tt>null</tt>
573	* (if the array has non-zero length) and returns it.
574	* @param a the array
575	* @return the specified array
576	*/
577	public <T> T[] toArray(T[] a) {
578	if (a.length > 0)
579	a[0] = null;
580	return a;
581	}
582
583
584	public int drainTo(Collection<? super E> c) {
585	if (c == null)
586	throw new NullPointerException();
587	if (c == this)
588	throw new IllegalArgumentException();
589	int n = 0;
590	E e;
591	while ( (e = poll()) != null) {
592	c.add(e);
593	++n;
594	}
595	return n;
596	}
597
598	public int drainTo(Collection<? super E> c, int maxElements) {
599	if (c == null)
600	throw new NullPointerException();
601	if (c == this)
602	throw new IllegalArgumentException();
603	int n = 0;
604	E e;
605	while (n < maxElements && (e = poll()) != null) {
606	c.add(e);
607	++n;
608	}
609	return n;
610	}
611	}
612
613
614
615
616