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root/jsr166/jsr166/src/jsr166y/LinkedTransferQueue.java

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.5 by jsr166, Fri Jul 25 18:11:53 2008 UTC vs.
Revision 1.31 by jsr166, Wed Jul 29 02:17:02 2009 UTC

#	Line 5 | Line 5
5		*/
6
7		package jsr166y;
8	+
9		import java.util.concurrent.*;
10	<	import java.util.concurrent.locks.*;
11	<	import java.util.concurrent.atomic.*;
12	<	import java.util.*;
13	<	import java.io.*;
10	>
11	>	import java.util.AbstractQueue;
12	>	import java.util.Collection;
13	>	import java.util.Iterator;
14	>	import java.util.NoSuchElementException;
15	>	import java.util.concurrent.locks.LockSupport;
16	>	import java.util.concurrent.atomic.AtomicReference;
17
18		/**
19		* An unbounded {@linkplain TransferQueue} based on linked nodes.
#	Line 19 | Line 23 | import java.io.*;
23		* producer.  The <em>tail</em> of the queue is that element that has
24		* been on the queue the shortest time for some producer.
25		*
26	<	* <p>Beware that, unlike in most collections, the <tt>size</tt>
26	>	* <p>Beware that, unlike in most collections, the {@code size}
27		* method is <em>NOT</em> a constant-time operation. Because of the
28		* asynchronous nature of these queues, determining the current number
29		* of elements requires a traversal of the elements.
#	Line 42 | Line 46 | import java.io.*;
46		* @since 1.7
47		* @author Doug Lea
48		* @param <E> the type of elements held in this collection
45	–	*
49		*/
50		public class LinkedTransferQueue<E> extends AbstractQueue<E>
51		implements TransferQueue<E>, java.io.Serializable {
52		private static final long serialVersionUID = -3223113410248163686L;
53
54		/*
52	–	* This is still a work in progress...
53	–	*
55		* This class extends the approach used in FIFO-mode
56		* SynchronousQueues. See the internal documentation, as well as
57		* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
58		* Lea & Scott
59		* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf)
60		*
61	<	* The main extension is to provide different Wait modes
62	<	* for the main "xfer" method that puts or takes items.
63	<	* These don't impact the basic dual-queue logic, but instead
64	<	* control whether or how threads block upon insertion
65	<	* of request or data nodes into the dual queue.
61	>	* The main extension is to provide different Wait modes for the
62	>	* main "xfer" method that puts or takes items.  These don't
63	>	* impact the basic dual-queue logic, but instead control whether
64	>	* or how threads block upon insertion of request or data nodes
65	>	* into the dual queue. It also uses slightly different
66	>	* conventions for tracking whether nodes are off-list or
67	>	* cancelled.
68		*/
69
70		// Wait modes for xfer method
#	Line 79 | Line 82 | public class LinkedTransferQueue<E> exte
82		* seems not to vary with number of CPUs (beyond 2) so is just
83		* a constant.
84		*/
85	<	static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
85	>	static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
86
87		/**
88		* The number of times to spin before blocking in untimed waits.
#	Line 95 | Line 98 | public class LinkedTransferQueue<E> exte
98		static final long spinForTimeoutThreshold = 1000L;
99
100		/**
101	<	* Node class for LinkedTransferQueue. Opportunistically subclasses from
102	<	* AtomicReference to represent item. Uses Object, not E, to allow
103	<	* setting item to "this" after use, to avoid garbage
104	<	* retention. Similarly, setting the next field to this is used as
105	<	* sentinel that node is off list.
101	>	* Node class for LinkedTransferQueue. Opportunistically
102	>	* subclasses from AtomicReference to represent item. Uses Object,
103	>	* not E, to allow setting item to "this" after use, to avoid
104	>	* garbage retention. Similarly, setting the next field to this is
105	>	* used as sentinel that node is off list.
106		*/
107	<	static final class QNode extends AtomicReference<Object> {
108	<	volatile QNode next;
107	>	static final class Node<E> extends AtomicReference<Object> {
108	>	volatile Node<E> next;
109		volatile Thread waiter;       // to control park/unpark
110		final boolean isData;
111	<	QNode(Object item, boolean isData) {
111	>
112	>	Node(E item, boolean isData) {
113		super(item);
114		this.isData = isData;
115		}
116
117	<	static final AtomicReferenceFieldUpdater<QNode, QNode>
118	<	nextUpdater = AtomicReferenceFieldUpdater.newUpdater
119	<	(QNode.class, QNode.class, "next");
117	>	final boolean casNext(Node<E> cmp, Node<E> val) {
118	>	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
119	>	}
120
121	<	boolean casNext(QNode cmp, QNode val) {
122	<	return nextUpdater.compareAndSet(this, cmp, val);
121	>	final void clearNext() {
122	>	UNSAFE.putOrderedObject(this, nextOffset, this);
123		}
124	+
125	+	// Unsafe mechanics
126	+
127	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
128	+	private static final long nextOffset =
129	+	objectFieldOffset(UNSAFE, "next", Node.class);
130	+
131	+	/**
132	+	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
133	+	* Replace with a simple call to Unsafe.getUnsafe when integrating
134	+	* into a jdk.
135	+	*
136	+	* @return a sun.misc.Unsafe
137	+	*/
138	+	private static sun.misc.Unsafe getUnsafe() {
139	+	try {
140	+	return sun.misc.Unsafe.getUnsafe();
141	+	} catch (SecurityException se) {
142	+	try {
143	+	return java.security.AccessController.doPrivileged
144	+	(new java.security
145	+	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
146	+	public sun.misc.Unsafe run() throws Exception {
147	+	java.lang.reflect.Field f = sun.misc
148	+	.Unsafe.class.getDeclaredField("theUnsafe");
149	+	f.setAccessible(true);
150	+	return (sun.misc.Unsafe) f.get(null);
151	+	}});
152	+	} catch (java.security.PrivilegedActionException e) {
153	+	throw new RuntimeException("Could not initialize intrinsics",
154	+	e.getCause());
155	+	}
156	+	}
157	+	}
158	+
159	+	private static final long serialVersionUID = -3375979862319811754L;
160		}
161
162		/**
#	Line 128 | Line 168 | public class LinkedTransferQueue<E> exte
168		// enough padding for 64bytes with 4byte refs
169		Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
170		PaddedAtomicReference(T r) { super(r); }
171	+	private static final long serialVersionUID = 8170090609809740854L;
172		}
173
174
175	<	private final QNode dummy = new QNode(null, false);
176	<	private final PaddedAtomicReference<QNode> head =
177	<	new PaddedAtomicReference<QNode>(dummy);
178	<	private final PaddedAtomicReference<QNode> tail =
179	<	new PaddedAtomicReference<QNode>(dummy);
175	>	/** head of the queue */
176	>	private transient final PaddedAtomicReference<Node<E>> head;
177	>
178	>	/** tail of the queue */
179	>	private transient final PaddedAtomicReference<Node<E>> tail;
180
181		/**
182		* Reference to a cancelled node that might not yet have been
183		* unlinked from queue because it was the last inserted node
184		* when it cancelled.
185		*/
186	<	private final PaddedAtomicReference<QNode> cleanMe =
146	<	new PaddedAtomicReference<QNode>(null);
186	>	private transient final PaddedAtomicReference<Node<E>> cleanMe;
187
188		/**
189		* Tries to cas nh as new head; if successful, unlink
190		* old head's next node to avoid garbage retention.
191		*/
192	<	private boolean advanceHead(QNode h, QNode nh) {
192	>	private boolean advanceHead(Node<E> h, Node<E> nh) {
193		if (h == head.get() && head.compareAndSet(h, nh)) {
194	<	h.next = h; // forget old next
194	>	h.clearNext(); // forget old next
195		return true;
196		}
197		return false;
#	Line 159 | Line 199 | public class LinkedTransferQueue<E> exte
199
200		/**
201		* Puts or takes an item. Used for most queue operations (except
202	<	* poll() and tryTransfer())
202	>	* poll() and tryTransfer()). See the similar code in
203	>	* SynchronousQueue for detailed explanation.
204	>	*
205		* @param e the item or if null, signifies that this is a take
206		* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
207		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
208		* @return an item, or null on failure
209		*/
210	<	private Object xfer(Object e, int mode, long nanos) {
210	>	private E xfer(E e, int mode, long nanos) {
211		boolean isData = (e != null);
212	<	QNode s = null;
213	<	final PaddedAtomicReference<QNode> head = this.head;
214	<	final PaddedAtomicReference<QNode> tail = this.tail;
212	>	Node<E> s = null;
213	>	final PaddedAtomicReference<Node<E>> head = this.head;
214	>	final PaddedAtomicReference<Node<E>> tail = this.tail;
215
216		for (;;) {
217	<	QNode t = tail.get();
218	<	QNode h = head.get();
217	>	Node<E> t = tail.get();
218	>	Node<E> h = head.get();
219
220		if (t != null && (t == h || t.isData == isData)) {
221		if (s == null)
222	<	s = new QNode(e, isData);
223	<	QNode last = t.next;
222	>	s = new Node<E>(e, isData);
223	>	Node<E> last = t.next;
224		if (last != null) {
225		if (t == tail.get())
226		tail.compareAndSet(t, last);
#	Line 190 | Line 232 | public class LinkedTransferQueue<E> exte
232		}
233
234		else if (h != null) {
235	<	QNode first = h.next;
235	>	Node<E> first = h.next;
236		if (t == tail.get() && first != null &&
237		advanceHead(h, first)) {
238		Object x = first.get();
239		if (x != first && first.compareAndSet(x, e)) {
240		LockSupport.unpark(first.waiter);
241	<	return isData? e : x;
241	>	return isData ? e : (E) x;
242		}
243		}
244		}
#	Line 206 | Line 248 | public class LinkedTransferQueue<E> exte
248
249		/**
250		* Version of xfer for poll() and tryTransfer, which
251	<	* simplifies control paths both here and in xfer
251	>	* simplifies control paths both here and in xfer.
252		*/
253	<	private Object fulfill(Object e) {
253	>	private E fulfill(E e) {
254		boolean isData = (e != null);
255	<	final PaddedAtomicReference<QNode> head = this.head;
256	<	final PaddedAtomicReference<QNode> tail = this.tail;
255	>	final PaddedAtomicReference<Node<E>> head = this.head;
256	>	final PaddedAtomicReference<Node<E>> tail = this.tail;
257
258		for (;;) {
259	<	QNode t = tail.get();
260	<	QNode h = head.get();
259	>	Node<E> t = tail.get();
260	>	Node<E> h = head.get();
261
262		if (t != null && (t == h || t.isData == isData)) {
263	<	QNode last = t.next;
263	>	Node<E> last = t.next;
264		if (t == tail.get()) {
265		if (last != null)
266		tail.compareAndSet(t, last);
#	Line 227 | Line 269 | public class LinkedTransferQueue<E> exte
269		}
270		}
271		else if (h != null) {
272	<	QNode first = h.next;
272	>	Node<E> first = h.next;
273		if (t == tail.get() &&
274		first != null &&
275		advanceHead(h, first)) {
276		Object x = first.get();
277		if (x != first && first.compareAndSet(x, e)) {
278		LockSupport.unpark(first.waiter);
279	<	return isData? e : x;
279	>	return isData ? e : (E) x;
280		}
281		}
282		}
#	Line 252 | Line 294 | public class LinkedTransferQueue<E> exte
294		* @param nanos timeout value
295		* @return matched item, or s if cancelled
296		*/
297	<	private Object awaitFulfill(QNode pred, QNode s, Object e,
298	<	int mode, long nanos) {
297	>	private E awaitFulfill(Node<E> pred, Node<E> s, E e,
298	>	int mode, long nanos) {
299		if (mode == NOWAIT)
300		return null;
301
302	<	long lastTime = (mode == TIMEOUT)? System.nanoTime() : 0;
302	>	long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0;
303		Thread w = Thread.currentThread();
304		int spins = -1; // set to desired spin count below
305		for (;;) {
#	Line 266 | Line 308 | public class LinkedTransferQueue<E> exte
308		Object x = s.get();
309		if (x != e) {                 // Node was matched or cancelled
310		advanceHead(pred, s);     // unlink if head
311	<	if (x == s)               // was cancelled
312	<	return clean(pred, s);
311	>	if (x == s) {             // was cancelled
312	>	clean(pred, s);
313	>	return null;
314	>	}
315		else if (x != null) {
316		s.set(s);             // avoid garbage retention
317	<	return x;
317	>	return (E) x;
318		}
319		else
320		return e;
321		}
278	–
322		if (mode == TIMEOUT) {
323		long now = System.nanoTime();
324		nanos -= now - lastTime;
#	Line 286 | Line 329 | public class LinkedTransferQueue<E> exte
329		}
330		}
331		if (spins < 0) {
332	<	QNode h = head.get(); // only spin if at head
332	>	Node<E> h = head.get(); // only spin if at head
333		spins = ((h != null && h.next == s) ?
334	<	(mode == TIMEOUT?
334	>	((mode == TIMEOUT) ?
335		maxTimedSpins : maxUntimedSpins) : 0);
336		}
337		if (spins > 0)
#	Line 296 | Line 339 | public class LinkedTransferQueue<E> exte
339		else if (s.waiter == null)
340		s.waiter = w;
341		else if (mode != TIMEOUT) {
342	<	//                LockSupport.park(this);
300	<	LockSupport.park(); // allows run on java5
342	>	LockSupport.park(this);
343		s.waiter = null;
344		spins = -1;
345		}
346		else if (nanos > spinForTimeoutThreshold) {
347	<	//                LockSupport.parkNanos(this, nanos);
306	<	LockSupport.parkNanos(nanos);
347	>	LockSupport.parkNanos(this, nanos);
348		s.waiter = null;
349		spins = -1;
350		}
#	Line 311 | Line 352 | public class LinkedTransferQueue<E> exte
352		}
353
354		/**
355	+	* Returns validated tail for use in cleaning methods.
356	+	*/
357	+	private Node<E> getValidatedTail() {
358	+	for (;;) {
359	+	Node<E> h = head.get();
360	+	Node<E> first = h.next;
361	+	if (first != null && first.next == first) { // help advance
362	+	advanceHead(h, first);
363	+	continue;
364	+	}
365	+	Node<E> t = tail.get();
366	+	Node<E> last = t.next;
367	+	if (t == tail.get()) {
368	+	if (last != null)
369	+	tail.compareAndSet(t, last); // help advance
370	+	else
371	+	return t;
372	+	}
373	+	}
374	+	}
375	+
376	+	/**
377		* Gets rid of cancelled node s with original predecessor pred.
378	<	* @return null (to simplify use by callers)
378	>	*
379	>	* @param pred predecessor of cancelled node
380	>	* @param s the cancelled node
381		*/
382	<	private Object clean(QNode pred, QNode s) {
382	>	private void clean(Node<E> pred, Node<E> s) {
383		Thread w = s.waiter;
384		if (w != null) {             // Wake up thread
385		s.waiter = null;
#	Line 322 | Line 387 | public class LinkedTransferQueue<E> exte
387		LockSupport.unpark(w);
388		}
389
390	<	for (;;) {
391	<	if (pred.next != s) // already cleaned
392	<	return null;
393	<	QNode h = head.get();
394	<	QNode hn = h.next;   // Absorb cancelled first node as head
395	<	if (hn != null && hn.next == hn) {
396	<	advanceHead(h, hn);
397	<	continue;
398	<	}
399	<	QNode t = tail.get();      // Ensure consistent read for tail
400	<	if (t == h)
401	<	return null;
402	<	QNode tn = t.next;
403	<	if (t != tail.get())
404	<	continue;
405	<	if (tn != null) {          // Help advance tail
341	<	tail.compareAndSet(t, tn);
342	<	continue;
343	<	}
344	<	if (s != t) {             // If not tail, try to unsplice
345	<	QNode sn = s.next;
390	>	if (pred == null)
391	>	return;
392	>
393	>	/*
394	>	* At any given time, exactly one node on list cannot be
395	>	* deleted -- the last inserted node. To accommodate this, if
396	>	* we cannot delete s, we save its predecessor as "cleanMe",
397	>	* processing the previously saved version first. At least one
398	>	* of node s or the node previously saved can always be
399	>	* processed, so this always terminates.
400	>	*/
401	>	while (pred.next == s) {
402	>	Node<E> oldpred = reclean();  // First, help get rid of cleanMe
403	>	Node<E> t = getValidatedTail();
404	>	if (s != t) {               // If not tail, try to unsplice
405	>	Node<E> sn = s.next;      // s.next == s means s already off list
406		if (sn == s || pred.casNext(s, sn))
407	<	return null;
407	>	break;
408		}
409	<	QNode dp = cleanMe.get();
410	<	if (dp != null) {    // Try unlinking previous cancelled node
411	<	QNode d = dp.next;
412	<	QNode dn;
413	<	if (d == null ||               // d is gone or
414	<	d == dp ||                 // d is off list or
415	<	d.get() != d ||            // d not cancelled or
416	<	(d != t &&                 // d not tail and
417	<	(dn = d.next) != null &&  //   has successor
418	<	dn != d &&                //   that is on list
419	<	dp.casNext(d, dn)))       // d unspliced
420	<	cleanMe.compareAndSet(dp, null);
421	<	if (dp == pred)
422	<	return null;      // s is already saved node
409	>	else if (oldpred == pred || // Already saved
410	>	(oldpred == null && cleanMe.compareAndSet(null, pred)))
411	>	break;                  // Postpone cleaning
412	>	}
413	>	}
414	>
415	>	/**
416	>	* Tries to unsplice the cancelled node held in cleanMe that was
417	>	* previously uncleanable because it was at tail.
418	>	*
419	>	* @return current cleanMe node (or null)
420	>	*/
421	>	private Node<E> reclean() {
422	>	/*
423	>	* cleanMe is, or at one time was, predecessor of cancelled
424	>	* node s that was the tail so could not be unspliced.  If s
425	>	* is no longer the tail, try to unsplice if necessary and
426	>	* make cleanMe slot available.  This differs from similar
427	>	* code in clean() because we must check that pred still
428	>	* points to a cancelled node that must be unspliced -- if
429	>	* not, we can (must) clear cleanMe without unsplicing.
430	>	* This can loop only due to contention on casNext or
431	>	* clearing cleanMe.
432	>	*/
433	>	Node<E> pred;
434	>	while ((pred = cleanMe.get()) != null) {
435	>	Node<E> t = getValidatedTail();
436	>	Node<E> s = pred.next;
437	>	if (s != t) {
438	>	Node<E> sn;
439	>	if (s == null || s == pred || s.get() != s ||
440	>	(sn = s.next) == s || pred.casNext(s, sn))
441	>	cleanMe.compareAndSet(pred, null);
442		}
443	<	else if (cleanMe.compareAndSet(null, pred))
444	<	return null;          // Postpone cleaning s
443	>	else // s is still tail; cannot clean
444	>	break;
445		}
446	+	return pred;
447		}
448
449		/**
450	<	* Creates an initially empty <tt>LinkedTransferQueue</tt>.
450	>	* Creates an initially empty {@code LinkedTransferQueue}.
451		*/
452		public LinkedTransferQueue() {
453	+	Node<E> dummy = new Node<E>(null, false);
454	+	head = new PaddedAtomicReference<Node<E>>(dummy);
455	+	tail = new PaddedAtomicReference<Node<E>>(dummy);
456	+	cleanMe = new PaddedAtomicReference<Node<E>>(null);
457		}
458
459		/**
460	<	* Creates a <tt>LinkedTransferQueue</tt>
460	>	* Creates a {@code LinkedTransferQueue}
461		* initially containing the elements of the given collection,
462		* added in traversal order of the collection's iterator.
463	+	*
464		* @param c the collection of elements to initially contain
465		* @throws NullPointerException if the specified collection or any
466		*         of its elements are null
467		*/
468		public LinkedTransferQueue(Collection<? extends E> c) {
469	+	this();
470		addAll(c);
471		}
472
473	+	/**
474	+	* @throws InterruptedException {@inheritDoc}
475	+	* @throws NullPointerException {@inheritDoc}
476	+	*/
477		public void put(E e) throws InterruptedException {
478		if (e == null) throw new NullPointerException();
479		if (Thread.interrupted()) throw new InterruptedException();
480		xfer(e, NOWAIT, 0);
481		}
482
483	+	/**
484	+	* @throws InterruptedException {@inheritDoc}
485	+	* @throws NullPointerException {@inheritDoc}
486	+	*/
487		public boolean offer(E e, long timeout, TimeUnit unit)
488		throws InterruptedException {
489		if (e == null) throw new NullPointerException();
#	Line 398 | Line 492 | public class LinkedTransferQueue<E> exte
492		return true;
493		}
494
495	+	/**
496	+	* @throws NullPointerException {@inheritDoc}
497	+	*/
498		public boolean offer(E e) {
499		if (e == null) throw new NullPointerException();
500		xfer(e, NOWAIT, 0);
501		return true;
502		}
503
504	+	/**
505	+	* @throws NullPointerException {@inheritDoc}
506	+	*/
507	+	public boolean add(E e) {
508	+	if (e == null) throw new NullPointerException();
509	+	xfer(e, NOWAIT, 0);
510	+	return true;
511	+	}
512	+
513	+	/**
514	+	* @throws InterruptedException {@inheritDoc}
515	+	* @throws NullPointerException {@inheritDoc}
516	+	*/
517		public void transfer(E e) throws InterruptedException {
518		if (e == null) throw new NullPointerException();
519		if (xfer(e, WAIT, 0) == null) {
520	<	Thread.interrupted();
520	>	Thread.interrupted();
521		throw new InterruptedException();
522	<	}
522	>	}
523		}
524
525	+	/**
526	+	* @throws InterruptedException {@inheritDoc}
527	+	* @throws NullPointerException {@inheritDoc}
528	+	*/
529		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
530		throws InterruptedException {
531		if (e == null) throw new NullPointerException();
#	Line 422 | Line 536 | public class LinkedTransferQueue<E> exte
536		throw new InterruptedException();
537		}
538
539	+	/**
540	+	* @throws NullPointerException {@inheritDoc}
541	+	*/
542		public boolean tryTransfer(E e) {
543		if (e == null) throw new NullPointerException();
544		return fulfill(e) != null;
545		}
546
547	+	/**
548	+	* @throws InterruptedException {@inheritDoc}
549	+	*/
550		public E take() throws InterruptedException {
551		Object e = xfer(null, WAIT, 0);
552		if (e != null)
553	<	return (E)e;
554	<	Thread.interrupted();
553	>	return (E) e;
554	>	Thread.interrupted();
555		throw new InterruptedException();
556		}
557
558	+	/**
559	+	* @throws InterruptedException {@inheritDoc}
560	+	*/
561		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
562		Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
563		if (e != null || !Thread.interrupted())
564	<	return (E)e;
564	>	return (E) e;
565		throw new InterruptedException();
566		}
567
568		public E poll() {
569	<	return (E)fulfill(null);
569	>	return fulfill(null);
570		}
571
572	+	/**
573	+	* @throws NullPointerException     {@inheritDoc}
574	+	* @throws IllegalArgumentException {@inheritDoc}
575	+	*/
576		public int drainTo(Collection<? super E> c) {
577		if (c == null)
578		throw new NullPointerException();
#	Line 460 | Line 587 | public class LinkedTransferQueue<E> exte
587		return n;
588		}
589
590	+	/**
591	+	* @throws NullPointerException     {@inheritDoc}
592	+	* @throws IllegalArgumentException {@inheritDoc}
593	+	*/
594		public int drainTo(Collection<? super E> c, int maxElements) {
595		if (c == null)
596		throw new NullPointerException();
#	Line 477 | Line 608 | public class LinkedTransferQueue<E> exte
608		// Traversal-based methods
609
610		/**
611	<	* Return head after performing any outstanding helping steps
611	>	* Returns head after performing any outstanding helping steps.
612		*/
613	<	private QNode traversalHead() {
613	>	private Node<E> traversalHead() {
614		for (;;) {
615	<	QNode t = tail.get();
616	<	QNode h = head.get();
615	>	Node<E> t = tail.get();
616	>	Node<E> h = head.get();
617		if (h != null && t != null) {
618	<	QNode last = t.next;
619	<	QNode first = h.next;
618	>	Node<E> last = t.next;
619	>	Node<E> first = h.next;
620		if (t == tail.get()) {
621		if (last != null)
622		tail.compareAndSet(t, last);
#	Line 500 | Line 631 | public class LinkedTransferQueue<E> exte
631		return h;
632		}
633		}
634	+	reclean();
635		}
636		}
637
#	Line 516 | Line 648 | public class LinkedTransferQueue<E> exte
648		* if subsequently removed.
649		*/
650		class Itr implements Iterator<E> {
651	<	QNode nextNode;    // Next node to return next
652	<	QNode currentNode; // last returned node, for remove()
653	<	QNode prevNode;    // predecessor of last returned node
654	<	E nextItem;        // Cache of next item, once commited to in next
651	>	Node<E> next;        // node to return next
652	>	Node<E> pnext;       // predecessor of next
653	>	Node<E> snext;       // successor of next
654	>	Node<E> curr;        // last returned node, for remove()
655	>	Node<E> pcurr;       // predecessor of curr, for remove()
656	>	E nextItem;        // Cache of next item, once committed to in next
657
658		Itr() {
659	<	nextNode = traversalHead();
526	<	advance();
659	>	findNext();
660		}
661
662	<	E advance() {
663	<	prevNode = currentNode;
664	<	currentNode = nextNode;
665	<	E x = nextItem;
533	<
534	<	QNode p = nextNode.next;
662	>	/**
663	>	* Ensures next points to next valid node, or null if none.
664	>	*/
665	>	void findNext() {
666		for (;;) {
667	<	if (p == null || !p.isData) {
668	<	nextNode = null;
669	<	nextItem = null;
670	<	return x;
671	<	}
672	<	Object item = p.get();
673	<	if (item != p && item != null) {
674	<	nextNode = p;
675	<	nextItem = (E)item;
545	<	return x;
667	>	Node<E> pred = pnext;
668	>	Node<E> q = next;
669	>	if (pred == null || pred == q) {
670	>	pred = traversalHead();
671	>	q = pred.next;
672	>	}
673	>	if (q == null || !q.isData) {
674	>	next = null;
675	>	return;
676		}
677	<	prevNode = p;
678	<	p = p.next;
677	>	Object x = q.get();
678	>	Node<E> s = q.next;
679	>	if (x != null && q != x && q != s) {
680	>	nextItem = (E) x;
681	>	snext = s;
682	>	pnext = pred;
683	>	next = q;
684	>	return;
685	>	}
686	>	pnext = q;
687	>	next = s;
688		}
689		}
690
691		public boolean hasNext() {
692	<	return nextNode != null;
692	>	return next != null;
693		}
694
695		public E next() {
696	<	if (nextNode == null) throw new NoSuchElementException();
697	<	return advance();
696	>	if (next == null) throw new NoSuchElementException();
697	>	pcurr = pnext;
698	>	curr = next;
699	>	pnext = next;
700	>	next = snext;
701	>	E x = nextItem;
702	>	findNext();
703	>	return x;
704		}
705
706		public void remove() {
707	<	QNode p = currentNode;
708	<	QNode prev = prevNode;
564	<	if (prev == null || p == null)
707	>	Node<E> p = curr;
708	>	if (p == null)
709		throw new IllegalStateException();
710		Object x = p.get();
711		if (x != null && x != p && p.compareAndSet(x, p))
712	<	clean(prev, p);
712	>	clean(pcurr, p);
713		}
714		}
715
716		public E peek() {
717		for (;;) {
718	<	QNode h = traversalHead();
719	<	QNode p = h.next;
718	>	Node<E> h = traversalHead();
719	>	Node<E> p = h.next;
720		if (p == null)
721		return null;
722		Object x = p.get();
#	Line 580 | Line 724 | public class LinkedTransferQueue<E> exte
724		if (!p.isData)
725		return null;
726		if (x != null)
727	<	return (E)x;
727	>	return (E) x;
728		}
729		}
730		}
731
732		public boolean isEmpty() {
733		for (;;) {
734	<	QNode h = traversalHead();
735	<	QNode p = h.next;
734	>	Node<E> h = traversalHead();
735	>	Node<E> p = h.next;
736		if (p == null)
737		return true;
738		Object x = p.get();
#	Line 603 | Line 747 | public class LinkedTransferQueue<E> exte
747
748		public boolean hasWaitingConsumer() {
749		for (;;) {
750	<	QNode h = traversalHead();
751	<	QNode p = h.next;
750	>	Node<E> h = traversalHead();
751	>	Node<E> p = h.next;
752		if (p == null)
753		return false;
754		Object x = p.get();
#	Line 615 | Line 759 | public class LinkedTransferQueue<E> exte
759
760		/**
761		* Returns the number of elements in this queue.  If this queue
762	<	* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
763	<	* <tt>Integer.MAX_VALUE</tt>.
762	>	* contains more than {@code Integer.MAX_VALUE} elements, returns
763	>	* {@code Integer.MAX_VALUE}.
764		*
765		* <p>Beware that, unlike in most collections, this method is
766		* <em>NOT</em> a constant-time operation. Because of the
#	Line 627 | Line 771 | public class LinkedTransferQueue<E> exte
771		*/
772		public int size() {
773		int count = 0;
774	<	QNode h = traversalHead();
775	<	for (QNode p = h.next; p != null && p.isData; p = p.next) {
774	>	Node<E> h = traversalHead();
775	>	for (Node<E> p = h.next; p != null && p.isData; p = p.next) {
776		Object x = p.get();
777		if (x != null && x != p) {
778		if (++count == Integer.MAX_VALUE) // saturated
#	Line 640 | Line 784 | public class LinkedTransferQueue<E> exte
784
785		public int getWaitingConsumerCount() {
786		int count = 0;
787	<	QNode h = traversalHead();
788	<	for (QNode p = h.next; p != null && !p.isData; p = p.next) {
787	>	Node<E> h = traversalHead();
788	>	for (Node<E> p = h.next; p != null && !p.isData; p = p.next) {
789		if (p.get() == null) {
790		if (++count == Integer.MAX_VALUE)
791		break;
#	Line 654 | Line 798 | public class LinkedTransferQueue<E> exte
798		return Integer.MAX_VALUE;
799		}
800
801	+	public boolean remove(Object o) {
802	+	if (o == null)
803	+	return false;
804	+	for (;;) {
805	+	Node<E> pred = traversalHead();
806	+	for (;;) {
807	+	Node<E> q = pred.next;
808	+	if (q == null || !q.isData)
809	+	return false;
810	+	if (q == pred) // restart
811	+	break;
812	+	Object x = q.get();
813	+	if (x != null && x != q && o.equals(x) &&
814	+	q.compareAndSet(x, q)) {
815	+	clean(pred, q);
816	+	return true;
817	+	}
818	+	pred = q;
819	+	}
820	+	}
821	+	}
822	+
823		/**
824		* Save the state to a stream (that is, serialize it).
825		*
826	<	* @serialData All of the elements (each an <tt>E</tt>) in
826	>	* @serialData All of the elements (each an {@code E}) in
827		* the proper order, followed by a null
828		* @param s the stream
829		*/
830		private void writeObject(java.io.ObjectOutputStream s)
831		throws java.io.IOException {
832		s.defaultWriteObject();
833	<	for (Iterator<E> it = iterator(); it.hasNext(); )
834	<	s.writeObject(it.next());
833	>	for (E e : this)
834	>	s.writeObject(e);
835		// Use trailing null as sentinel
836		s.writeObject(null);
837		}
#	Line 673 | Line 839 | public class LinkedTransferQueue<E> exte
839		/**
840		* Reconstitute the Queue instance from a stream (that is,
841		* deserialize it).
842	+	*
843		* @param s the stream
844		*/
845		private void readObject(java.io.ObjectInputStream s)
846		throws java.io.IOException, ClassNotFoundException {
847		s.defaultReadObject();
848	+	resetHeadAndTail();
849		for (;;) {
850	<	E item = (E)s.readObject();
850	>	@SuppressWarnings("unchecked") E item = (E) s.readObject();
851		if (item == null)
852		break;
853		else
854		offer(item);
855		}
856		}
857	+
858	+	// Support for resetting head/tail while deserializing
859	+	private void resetHeadAndTail() {
860	+	Node<E> dummy = new Node<E>(null, false);
861	+	UNSAFE.putObjectVolatile(this, headOffset,
862	+	new PaddedAtomicReference<Node<E>>(dummy));
863	+	UNSAFE.putObjectVolatile(this, tailOffset,
864	+	new PaddedAtomicReference<Node<E>>(dummy));
865	+	UNSAFE.putObjectVolatile(this, cleanMeOffset,
866	+	new PaddedAtomicReference<Node<E>>(null));
867	+	}
868	+
869	+	// Unsafe mechanics
870	+
871	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
872	+	private static final long headOffset =
873	+	objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
874	+	private static final long tailOffset =
875	+	objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
876	+	private static final long cleanMeOffset =
877	+	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
878	+
879	+
880	+	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
881	+	String field, Class<?> klazz) {
882	+	try {
883	+	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
884	+	} catch (NoSuchFieldException e) {
885	+	// Convert Exception to corresponding Error
886	+	NoSuchFieldError error = new NoSuchFieldError(field);
887	+	error.initCause(e);
888	+	throw error;
889	+	}
890	+	}
891	+
892	+	/**
893	+	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
894	+	* Replace with a simple call to Unsafe.getUnsafe when integrating
895	+	* into a jdk.
896	+	*
897	+	* @return a sun.misc.Unsafe
898	+	*/
899	+	private static sun.misc.Unsafe getUnsafe() {
900	+	try {
901	+	return sun.misc.Unsafe.getUnsafe();
902	+	} catch (SecurityException se) {
903	+	try {
904	+	return java.security.AccessController.doPrivileged
905	+	(new java.security
906	+	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
907	+	public sun.misc.Unsafe run() throws Exception {
908	+	java.lang.reflect.Field f = sun.misc
909	+	.Unsafe.class.getDeclaredField("theUnsafe");
910	+	f.setAccessible(true);
911	+	return (sun.misc.Unsafe) f.get(null);
912	+	}});
913	+	} catch (java.security.PrivilegedActionException e) {
914	+	throw new RuntimeException("Could not initialize intrinsics",
915	+	e.getCause());
916	+	}
917	+	}
918	+	}
919		}

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