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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package jsr166y; |
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import java.util.concurrent.*; |
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import java.util.concurrent.locks.*; |
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import java.util.concurrent.atomic.*; |
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import java.util.*; |
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import java.io.*; |
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import sun.misc.Unsafe; |
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import java.lang.reflect.*; |
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|
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/** |
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* An unbounded {@linkplain TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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* to any given producer. The <em>head</em> of the queue is that |
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* element that has been on the queue the longest time for some |
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* producer. The <em>tail</em> of the queue is that element that has |
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* been on the queue the shortest time for some producer. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} |
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* method is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these queues, determining the current number |
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* of elements requires a traversal of the elements. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code LinkedTransferQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code LinkedTransferQueue} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this collection |
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*/ |
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public class LinkedTransferQueue<E> extends AbstractQueue<E> |
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implements TransferQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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|
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/* |
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* This class extends the approach used in FIFO-mode |
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* SynchronousQueues. See the internal documentation, as well as |
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* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer, |
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* Lea & Scott |
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* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf) |
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* |
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* The main extension is to provide different Wait modes for the |
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* main "xfer" method that puts or takes items. These don't |
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* impact the basic dual-queue logic, but instead control whether |
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* or how threads block upon insertion of request or data nodes |
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* into the dual queue. It also uses slightly different |
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* conventions for tracking whether nodes are off-list or |
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* cancelled. |
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*/ |
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|
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// Wait modes for xfer method |
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static final int NOWAIT = 0; |
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static final int TIMEOUT = 1; |
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static final int WAIT = 2; |
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|
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/** The number of CPUs, for spin control */ |
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static final int NCPUS = Runtime.getRuntime().availableProcessors(); |
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|
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/** |
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* The number of times to spin before blocking in timed waits. |
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* The value is empirically derived -- it works well across a |
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* variety of processors and OSes. Empirically, the best value |
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* seems not to vary with number of CPUs (beyond 2) so is just |
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* a constant. |
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*/ |
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static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32; |
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|
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/** |
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* The number of times to spin before blocking in untimed waits. |
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* This is greater than timed value because untimed waits spin |
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* faster since they don't need to check times on each spin. |
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*/ |
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static final int maxUntimedSpins = maxTimedSpins * 16; |
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|
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/** |
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* The number of nanoseconds for which it is faster to spin |
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* rather than to use timed park. A rough estimate suffices. |
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*/ |
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static final long spinForTimeoutThreshold = 1000L; |
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|
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/** |
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* Node class for LinkedTransferQueue. Opportunistically |
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* subclasses from AtomicReference to represent item. Uses Object, |
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* not E, to allow setting item to "this" after use, to avoid |
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* garbage retention. Similarly, setting the next field to this is |
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* used as sentinel that node is off list. |
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*/ |
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static final class QNode extends AtomicReference<Object> { |
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volatile QNode next; |
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volatile Thread waiter; // to control park/unpark |
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final boolean isData; |
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QNode(Object item, boolean isData) { |
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super(item); |
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this.isData = isData; |
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} |
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|
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static final AtomicReferenceFieldUpdater<QNode, QNode> |
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nextUpdater = AtomicReferenceFieldUpdater.newUpdater |
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(QNode.class, QNode.class, "next"); |
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|
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final boolean casNext(QNode cmp, QNode val) { |
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return nextUpdater.compareAndSet(this, cmp, val); |
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} |
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|
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final void clearNext() { |
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nextUpdater.lazySet(this, this); |
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} |
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|
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private static final long serialVersionUID = -3375979862319811754L; |
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} |
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|
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/** |
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* Padded version of AtomicReference used for head, tail and |
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* cleanMe, to alleviate contention across threads CASing one vs |
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* the other. |
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*/ |
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static final class PaddedAtomicReference<T> extends AtomicReference<T> { |
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// enough padding for 64bytes with 4byte refs |
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Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe; |
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PaddedAtomicReference(T r) { super(r); } |
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private static final long serialVersionUID = 8170090609809740854L; |
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} |
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|
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|
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/** head of the queue */ |
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private transient final PaddedAtomicReference<QNode> head; |
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|
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/** tail of the queue */ |
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private transient final PaddedAtomicReference<QNode> tail; |
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|
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/** |
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* Reference to a cancelled node that might not yet have been |
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* unlinked from queue because it was the last inserted node |
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* when it cancelled. |
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*/ |
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private transient final PaddedAtomicReference<QNode> cleanMe; |
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|
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/** |
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* Tries to cas nh as new head; if successful, unlink |
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* old head's next node to avoid garbage retention. |
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*/ |
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private boolean advanceHead(QNode h, QNode nh) { |
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if (h == head.get() && head.compareAndSet(h, nh)) { |
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h.clearNext(); // forget old next |
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return true; |
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} |
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return false; |
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} |
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|
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/** |
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* Puts or takes an item. Used for most queue operations (except |
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* poll() and tryTransfer()). See the similar code in |
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* SynchronousQueue for detailed explanation. |
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* |
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* @param e the item or if null, signifies that this is a take |
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* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT |
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* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
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* @return an item, or null on failure |
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*/ |
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private Object xfer(Object e, int mode, long nanos) { |
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boolean isData = (e != null); |
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QNode s = null; |
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final PaddedAtomicReference<QNode> head = this.head; |
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final PaddedAtomicReference<QNode> tail = this.tail; |
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|
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for (;;) { |
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QNode t = tail.get(); |
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QNode h = head.get(); |
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|
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if (t != null && (t == h || t.isData == isData)) { |
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if (s == null) |
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s = new QNode(e, isData); |
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QNode last = t.next; |
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if (last != null) { |
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if (t == tail.get()) |
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tail.compareAndSet(t, last); |
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} |
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else if (t.casNext(null, s)) { |
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tail.compareAndSet(t, s); |
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return awaitFulfill(t, s, e, mode, nanos); |
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} |
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} |
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|
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else if (h != null) { |
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QNode first = h.next; |
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if (t == tail.get() && first != null && |
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advanceHead(h, first)) { |
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Object x = first.get(); |
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if (x != first && first.compareAndSet(x, e)) { |
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LockSupport.unpark(first.waiter); |
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return isData ? e : x; |
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} |
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} |
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} |
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} |
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} |
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|
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|
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/** |
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* Version of xfer for poll() and tryTransfer, which |
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* simplifies control paths both here and in xfer. |
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*/ |
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private Object fulfill(Object e) { |
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boolean isData = (e != null); |
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final PaddedAtomicReference<QNode> head = this.head; |
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final PaddedAtomicReference<QNode> tail = this.tail; |
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|
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for (;;) { |
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QNode t = tail.get(); |
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QNode h = head.get(); |
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|
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if (t != null && (t == h || t.isData == isData)) { |
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QNode last = t.next; |
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if (t == tail.get()) { |
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if (last != null) |
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tail.compareAndSet(t, last); |
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else |
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return null; |
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} |
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} |
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else if (h != null) { |
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QNode first = h.next; |
240 |
if (t == tail.get() && |
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first != null && |
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advanceHead(h, first)) { |
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Object x = first.get(); |
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if (x != first && first.compareAndSet(x, e)) { |
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LockSupport.unpark(first.waiter); |
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return isData ? e : x; |
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} |
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} |
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} |
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} |
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} |
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|
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/** |
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* Spins/blocks until node s is fulfilled or caller gives up, |
255 |
* depending on wait mode. |
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* |
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* @param pred the predecessor of waiting node |
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* @param s the waiting node |
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* @param e the comparison value for checking match |
260 |
* @param mode mode |
261 |
* @param nanos timeout value |
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* @return matched item, or s if cancelled |
263 |
*/ |
264 |
private Object awaitFulfill(QNode pred, QNode s, Object e, |
265 |
int mode, long nanos) { |
266 |
if (mode == NOWAIT) |
267 |
return null; |
268 |
|
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long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0; |
270 |
Thread w = Thread.currentThread(); |
271 |
int spins = -1; // set to desired spin count below |
272 |
for (;;) { |
273 |
if (w.isInterrupted()) |
274 |
s.compareAndSet(e, s); |
275 |
Object x = s.get(); |
276 |
if (x != e) { // Node was matched or cancelled |
277 |
advanceHead(pred, s); // unlink if head |
278 |
if (x == s) { // was cancelled |
279 |
clean(pred, s); |
280 |
return null; |
281 |
} |
282 |
else if (x != null) { |
283 |
s.set(s); // avoid garbage retention |
284 |
return x; |
285 |
} |
286 |
else |
287 |
return e; |
288 |
} |
289 |
if (mode == TIMEOUT) { |
290 |
long now = System.nanoTime(); |
291 |
nanos -= now - lastTime; |
292 |
lastTime = now; |
293 |
if (nanos <= 0) { |
294 |
s.compareAndSet(e, s); // try to cancel |
295 |
continue; |
296 |
} |
297 |
} |
298 |
if (spins < 0) { |
299 |
QNode h = head.get(); // only spin if at head |
300 |
spins = ((h != null && h.next == s) ? |
301 |
((mode == TIMEOUT) ? |
302 |
maxTimedSpins : maxUntimedSpins) : 0); |
303 |
} |
304 |
if (spins > 0) |
305 |
--spins; |
306 |
else if (s.waiter == null) |
307 |
s.waiter = w; |
308 |
else if (mode != TIMEOUT) { |
309 |
LockSupport.park(this); |
310 |
s.waiter = null; |
311 |
spins = -1; |
312 |
} |
313 |
else if (nanos > spinForTimeoutThreshold) { |
314 |
LockSupport.parkNanos(this, nanos); |
315 |
s.waiter = null; |
316 |
spins = -1; |
317 |
} |
318 |
} |
319 |
} |
320 |
|
321 |
/** |
322 |
* Returns validated tail for use in cleaning methods. |
323 |
*/ |
324 |
private QNode getValidatedTail() { |
325 |
for (;;) { |
326 |
QNode h = head.get(); |
327 |
QNode first = h.next; |
328 |
if (first != null && first.next == first) { // help advance |
329 |
advanceHead(h, first); |
330 |
continue; |
331 |
} |
332 |
QNode t = tail.get(); |
333 |
QNode last = t.next; |
334 |
if (t == tail.get()) { |
335 |
if (last != null) |
336 |
tail.compareAndSet(t, last); // help advance |
337 |
else |
338 |
return t; |
339 |
} |
340 |
} |
341 |
} |
342 |
|
343 |
/** |
344 |
* Gets rid of cancelled node s with original predecessor pred. |
345 |
* |
346 |
* @param pred predecessor of cancelled node |
347 |
* @param s the cancelled node |
348 |
*/ |
349 |
private void clean(QNode pred, QNode s) { |
350 |
Thread w = s.waiter; |
351 |
if (w != null) { // Wake up thread |
352 |
s.waiter = null; |
353 |
if (w != Thread.currentThread()) |
354 |
LockSupport.unpark(w); |
355 |
} |
356 |
|
357 |
if (pred == null) |
358 |
return; |
359 |
|
360 |
/* |
361 |
* At any given time, exactly one node on list cannot be |
362 |
* deleted -- the last inserted node. To accommodate this, if |
363 |
* we cannot delete s, we save its predecessor as "cleanMe", |
364 |
* processing the previously saved version first. At least one |
365 |
* of node s or the node previously saved can always be |
366 |
* processed, so this always terminates. |
367 |
*/ |
368 |
while (pred.next == s) { |
369 |
QNode oldpred = reclean(); // First, help get rid of cleanMe |
370 |
QNode t = getValidatedTail(); |
371 |
if (s != t) { // If not tail, try to unsplice |
372 |
QNode sn = s.next; // s.next == s means s already off list |
373 |
if (sn == s || pred.casNext(s, sn)) |
374 |
break; |
375 |
} |
376 |
else if (oldpred == pred || // Already saved |
377 |
(oldpred == null && cleanMe.compareAndSet(null, pred))) |
378 |
break; // Postpone cleaning |
379 |
} |
380 |
} |
381 |
|
382 |
/** |
383 |
* Tries to unsplice the cancelled node held in cleanMe that was |
384 |
* previously uncleanable because it was at tail. |
385 |
* |
386 |
* @return current cleanMe node (or null) |
387 |
*/ |
388 |
private QNode reclean() { |
389 |
/* |
390 |
* cleanMe is, or at one time was, predecessor of cancelled |
391 |
* node s that was the tail so could not be unspliced. If s |
392 |
* is no longer the tail, try to unsplice if necessary and |
393 |
* make cleanMe slot available. This differs from similar |
394 |
* code in clean() because we must check that pred still |
395 |
* points to a cancelled node that must be unspliced -- if |
396 |
* not, we can (must) clear cleanMe without unsplicing. |
397 |
* This can loop only due to contention on casNext or |
398 |
* clearing cleanMe. |
399 |
*/ |
400 |
QNode pred; |
401 |
while ((pred = cleanMe.get()) != null) { |
402 |
QNode t = getValidatedTail(); |
403 |
QNode s = pred.next; |
404 |
if (s != t) { |
405 |
QNode sn; |
406 |
if (s == null || s == pred || s.get() != s || |
407 |
(sn = s.next) == s || pred.casNext(s, sn)) |
408 |
cleanMe.compareAndSet(pred, null); |
409 |
} |
410 |
else // s is still tail; cannot clean |
411 |
break; |
412 |
} |
413 |
return pred; |
414 |
} |
415 |
|
416 |
/** |
417 |
* Creates an initially empty {@code LinkedTransferQueue}. |
418 |
*/ |
419 |
public LinkedTransferQueue() { |
420 |
QNode dummy = new QNode(null, false); |
421 |
head = new PaddedAtomicReference<QNode>(dummy); |
422 |
tail = new PaddedAtomicReference<QNode>(dummy); |
423 |
cleanMe = new PaddedAtomicReference<QNode>(null); |
424 |
} |
425 |
|
426 |
/** |
427 |
* Creates a {@code LinkedTransferQueue} |
428 |
* initially containing the elements of the given collection, |
429 |
* added in traversal order of the collection's iterator. |
430 |
* |
431 |
* @param c the collection of elements to initially contain |
432 |
* @throws NullPointerException if the specified collection or any |
433 |
* of its elements are null |
434 |
*/ |
435 |
public LinkedTransferQueue(Collection<? extends E> c) { |
436 |
this(); |
437 |
addAll(c); |
438 |
} |
439 |
|
440 |
public void put(E e) throws InterruptedException { |
441 |
if (e == null) throw new NullPointerException(); |
442 |
if (Thread.interrupted()) throw new InterruptedException(); |
443 |
xfer(e, NOWAIT, 0); |
444 |
} |
445 |
|
446 |
public boolean offer(E e, long timeout, TimeUnit unit) |
447 |
throws InterruptedException { |
448 |
if (e == null) throw new NullPointerException(); |
449 |
if (Thread.interrupted()) throw new InterruptedException(); |
450 |
xfer(e, NOWAIT, 0); |
451 |
return true; |
452 |
} |
453 |
|
454 |
public boolean offer(E e) { |
455 |
if (e == null) throw new NullPointerException(); |
456 |
xfer(e, NOWAIT, 0); |
457 |
return true; |
458 |
} |
459 |
|
460 |
public boolean add(E e) { |
461 |
if (e == null) throw new NullPointerException(); |
462 |
xfer(e, NOWAIT, 0); |
463 |
return true; |
464 |
} |
465 |
|
466 |
public void transfer(E e) throws InterruptedException { |
467 |
if (e == null) throw new NullPointerException(); |
468 |
if (xfer(e, WAIT, 0) == null) { |
469 |
Thread.interrupted(); |
470 |
throw new InterruptedException(); |
471 |
} |
472 |
} |
473 |
|
474 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
475 |
throws InterruptedException { |
476 |
if (e == null) throw new NullPointerException(); |
477 |
if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null) |
478 |
return true; |
479 |
if (!Thread.interrupted()) |
480 |
return false; |
481 |
throw new InterruptedException(); |
482 |
} |
483 |
|
484 |
public boolean tryTransfer(E e) { |
485 |
if (e == null) throw new NullPointerException(); |
486 |
return fulfill(e) != null; |
487 |
} |
488 |
|
489 |
public E take() throws InterruptedException { |
490 |
Object e = xfer(null, WAIT, 0); |
491 |
if (e != null) |
492 |
return (E) e; |
493 |
Thread.interrupted(); |
494 |
throw new InterruptedException(); |
495 |
} |
496 |
|
497 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
498 |
Object e = xfer(null, TIMEOUT, unit.toNanos(timeout)); |
499 |
if (e != null || !Thread.interrupted()) |
500 |
return (E) e; |
501 |
throw new InterruptedException(); |
502 |
} |
503 |
|
504 |
public E poll() { |
505 |
return (E) fulfill(null); |
506 |
} |
507 |
|
508 |
public int drainTo(Collection<? super E> c) { |
509 |
if (c == null) |
510 |
throw new NullPointerException(); |
511 |
if (c == this) |
512 |
throw new IllegalArgumentException(); |
513 |
int n = 0; |
514 |
E e; |
515 |
while ( (e = poll()) != null) { |
516 |
c.add(e); |
517 |
++n; |
518 |
} |
519 |
return n; |
520 |
} |
521 |
|
522 |
public int drainTo(Collection<? super E> c, int maxElements) { |
523 |
if (c == null) |
524 |
throw new NullPointerException(); |
525 |
if (c == this) |
526 |
throw new IllegalArgumentException(); |
527 |
int n = 0; |
528 |
E e; |
529 |
while (n < maxElements && (e = poll()) != null) { |
530 |
c.add(e); |
531 |
++n; |
532 |
} |
533 |
return n; |
534 |
} |
535 |
|
536 |
// Traversal-based methods |
537 |
|
538 |
/** |
539 |
* Returns head after performing any outstanding helping steps. |
540 |
*/ |
541 |
private QNode traversalHead() { |
542 |
for (;;) { |
543 |
QNode t = tail.get(); |
544 |
QNode h = head.get(); |
545 |
if (h != null && t != null) { |
546 |
QNode last = t.next; |
547 |
QNode first = h.next; |
548 |
if (t == tail.get()) { |
549 |
if (last != null) |
550 |
tail.compareAndSet(t, last); |
551 |
else if (first != null) { |
552 |
Object x = first.get(); |
553 |
if (x == first) |
554 |
advanceHead(h, first); |
555 |
else |
556 |
return h; |
557 |
} |
558 |
else |
559 |
return h; |
560 |
} |
561 |
} |
562 |
reclean(); |
563 |
} |
564 |
} |
565 |
|
566 |
|
567 |
public Iterator<E> iterator() { |
568 |
return new Itr(); |
569 |
} |
570 |
|
571 |
/** |
572 |
* Iterators. Basic strategy is to traverse list, treating |
573 |
* non-data (i.e., request) nodes as terminating list. |
574 |
* Once a valid data node is found, the item is cached |
575 |
* so that the next call to next() will return it even |
576 |
* if subsequently removed. |
577 |
*/ |
578 |
class Itr implements Iterator<E> { |
579 |
QNode next; // node to return next |
580 |
QNode pnext; // predecessor of next |
581 |
QNode snext; // successor of next |
582 |
QNode curr; // last returned node, for remove() |
583 |
QNode pcurr; // predecessor of curr, for remove() |
584 |
E nextItem; // Cache of next item, once committed to in next |
585 |
|
586 |
Itr() { |
587 |
findNext(); |
588 |
} |
589 |
|
590 |
/** |
591 |
* Ensures next points to next valid node, or null if none. |
592 |
*/ |
593 |
void findNext() { |
594 |
for (;;) { |
595 |
QNode pred = pnext; |
596 |
QNode q = next; |
597 |
if (pred == null || pred == q) { |
598 |
pred = traversalHead(); |
599 |
q = pred.next; |
600 |
} |
601 |
if (q == null || !q.isData) { |
602 |
next = null; |
603 |
return; |
604 |
} |
605 |
Object x = q.get(); |
606 |
QNode s = q.next; |
607 |
if (x != null && q != x && q != s) { |
608 |
nextItem = (E) x; |
609 |
snext = s; |
610 |
pnext = pred; |
611 |
next = q; |
612 |
return; |
613 |
} |
614 |
pnext = q; |
615 |
next = s; |
616 |
} |
617 |
} |
618 |
|
619 |
public boolean hasNext() { |
620 |
return next != null; |
621 |
} |
622 |
|
623 |
public E next() { |
624 |
if (next == null) throw new NoSuchElementException(); |
625 |
pcurr = pnext; |
626 |
curr = next; |
627 |
pnext = next; |
628 |
next = snext; |
629 |
E x = nextItem; |
630 |
findNext(); |
631 |
return x; |
632 |
} |
633 |
|
634 |
public void remove() { |
635 |
QNode p = curr; |
636 |
if (p == null) |
637 |
throw new IllegalStateException(); |
638 |
Object x = p.get(); |
639 |
if (x != null && x != p && p.compareAndSet(x, p)) |
640 |
clean(pcurr, p); |
641 |
} |
642 |
} |
643 |
|
644 |
public E peek() { |
645 |
for (;;) { |
646 |
QNode h = traversalHead(); |
647 |
QNode p = h.next; |
648 |
if (p == null) |
649 |
return null; |
650 |
Object x = p.get(); |
651 |
if (p != x) { |
652 |
if (!p.isData) |
653 |
return null; |
654 |
if (x != null) |
655 |
return (E) x; |
656 |
} |
657 |
} |
658 |
} |
659 |
|
660 |
public boolean isEmpty() { |
661 |
for (;;) { |
662 |
QNode h = traversalHead(); |
663 |
QNode p = h.next; |
664 |
if (p == null) |
665 |
return true; |
666 |
Object x = p.get(); |
667 |
if (p != x) { |
668 |
if (!p.isData) |
669 |
return true; |
670 |
if (x != null) |
671 |
return false; |
672 |
} |
673 |
} |
674 |
} |
675 |
|
676 |
public boolean hasWaitingConsumer() { |
677 |
for (;;) { |
678 |
QNode h = traversalHead(); |
679 |
QNode p = h.next; |
680 |
if (p == null) |
681 |
return false; |
682 |
Object x = p.get(); |
683 |
if (p != x) |
684 |
return !p.isData; |
685 |
} |
686 |
} |
687 |
|
688 |
/** |
689 |
* Returns the number of elements in this queue. If this queue |
690 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
691 |
* {@code Integer.MAX_VALUE}. |
692 |
* |
693 |
* <p>Beware that, unlike in most collections, this method is |
694 |
* <em>NOT</em> a constant-time operation. Because of the |
695 |
* asynchronous nature of these queues, determining the current |
696 |
* number of elements requires an O(n) traversal. |
697 |
* |
698 |
* @return the number of elements in this queue |
699 |
*/ |
700 |
public int size() { |
701 |
int count = 0; |
702 |
QNode h = traversalHead(); |
703 |
for (QNode p = h.next; p != null && p.isData; p = p.next) { |
704 |
Object x = p.get(); |
705 |
if (x != null && x != p) { |
706 |
if (++count == Integer.MAX_VALUE) // saturated |
707 |
break; |
708 |
} |
709 |
} |
710 |
return count; |
711 |
} |
712 |
|
713 |
public int getWaitingConsumerCount() { |
714 |
int count = 0; |
715 |
QNode h = traversalHead(); |
716 |
for (QNode p = h.next; p != null && !p.isData; p = p.next) { |
717 |
if (p.get() == null) { |
718 |
if (++count == Integer.MAX_VALUE) |
719 |
break; |
720 |
} |
721 |
} |
722 |
return count; |
723 |
} |
724 |
|
725 |
public int remainingCapacity() { |
726 |
return Integer.MAX_VALUE; |
727 |
} |
728 |
|
729 |
public boolean remove(Object o) { |
730 |
if (o == null) |
731 |
return false; |
732 |
for (;;) { |
733 |
QNode pred = traversalHead(); |
734 |
for (;;) { |
735 |
QNode q = pred.next; |
736 |
if (q == null || !q.isData) |
737 |
return false; |
738 |
if (q == pred) // restart |
739 |
break; |
740 |
Object x = q.get(); |
741 |
if (x != null && x != q && o.equals(x) && |
742 |
q.compareAndSet(x, q)) { |
743 |
clean(pred, q); |
744 |
return true; |
745 |
} |
746 |
pred = q; |
747 |
} |
748 |
} |
749 |
} |
750 |
|
751 |
/** |
752 |
* Save the state to a stream (that is, serialize it). |
753 |
* |
754 |
* @serialData All of the elements (each an {@code E}) in |
755 |
* the proper order, followed by a null |
756 |
* @param s the stream |
757 |
*/ |
758 |
private void writeObject(java.io.ObjectOutputStream s) |
759 |
throws java.io.IOException { |
760 |
s.defaultWriteObject(); |
761 |
for (E e : this) |
762 |
s.writeObject(e); |
763 |
// Use trailing null as sentinel |
764 |
s.writeObject(null); |
765 |
} |
766 |
|
767 |
/** |
768 |
* Reconstitute the Queue instance from a stream (that is, |
769 |
* deserialize it). |
770 |
* |
771 |
* @param s the stream |
772 |
*/ |
773 |
private void readObject(java.io.ObjectInputStream s) |
774 |
throws java.io.IOException, ClassNotFoundException { |
775 |
s.defaultReadObject(); |
776 |
resetHeadAndTail(); |
777 |
for (;;) { |
778 |
E item = (E) s.readObject(); |
779 |
if (item == null) |
780 |
break; |
781 |
else |
782 |
offer(item); |
783 |
} |
784 |
} |
785 |
|
786 |
|
787 |
// Support for resetting head/tail while deserializing |
788 |
private void resetHeadAndTail() { |
789 |
QNode dummy = new QNode(null, false); |
790 |
UNSAFE.putObjectVolatile(this, headOffset, |
791 |
new PaddedAtomicReference<QNode>(dummy)); |
792 |
UNSAFE.putObjectVolatile(this, tailOffset, |
793 |
new PaddedAtomicReference<QNode>(dummy)); |
794 |
UNSAFE.putObjectVolatile(this, cleanMeOffset, |
795 |
new PaddedAtomicReference<QNode>(null)); |
796 |
} |
797 |
|
798 |
// Temporary Unsafe mechanics for preliminary release |
799 |
private static Unsafe getUnsafe() throws Throwable { |
800 |
try { |
801 |
return Unsafe.getUnsafe(); |
802 |
} catch (SecurityException se) { |
803 |
try { |
804 |
return java.security.AccessController.doPrivileged |
805 |
(new java.security.PrivilegedExceptionAction<Unsafe>() { |
806 |
public Unsafe run() throws Exception { |
807 |
return getUnsafePrivileged(); |
808 |
}}); |
809 |
} catch (java.security.PrivilegedActionException e) { |
810 |
throw e.getCause(); |
811 |
} |
812 |
} |
813 |
} |
814 |
|
815 |
private static Unsafe getUnsafePrivileged() |
816 |
throws NoSuchFieldException, IllegalAccessException { |
817 |
Field f = Unsafe.class.getDeclaredField("theUnsafe"); |
818 |
f.setAccessible(true); |
819 |
return (Unsafe) f.get(null); |
820 |
} |
821 |
|
822 |
private static long fieldOffset(String fieldName) |
823 |
throws NoSuchFieldException { |
824 |
return UNSAFE.objectFieldOffset |
825 |
(LinkedTransferQueue.class.getDeclaredField(fieldName)); |
826 |
} |
827 |
|
828 |
private static final Unsafe UNSAFE; |
829 |
private static final long headOffset; |
830 |
private static final long tailOffset; |
831 |
private static final long cleanMeOffset; |
832 |
static { |
833 |
try { |
834 |
UNSAFE = getUnsafe(); |
835 |
headOffset = fieldOffset("head"); |
836 |
tailOffset = fieldOffset("tail"); |
837 |
cleanMeOffset = fieldOffset("cleanMe"); |
838 |
} catch (Throwable e) { |
839 |
throw new RuntimeException("Could not initialize intrinsics", e); |
840 |
} |
841 |
} |
842 |
|
843 |
} |