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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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|
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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 <tt>size</tt> |
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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 1.5 |
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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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*/ |
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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 is still a work in prgress... |
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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 |
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* for the main "xfer" method that puts or takes items. |
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* These don't impact the basic dual-queue logic, but instead |
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* control whether or how threads block upon insertion |
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* of request or data nodes into the dual queue. |
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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 subclasses from |
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* AtomicReference to represent item. Uses Object, not E, to allow |
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* setting item to "this" after use, to avoid garbage |
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* retention. Similarly, setting the next field to this is used as |
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* 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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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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|
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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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} |
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|
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|
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private final QNode dummy = new QNode(null, false); |
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private final PaddedAtomicReference<QNode> head = |
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new PaddedAtomicReference<QNode>(dummy); |
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private final PaddedAtomicReference<QNode> tail = |
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new PaddedAtomicReference<QNode>(dummy); |
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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 final PaddedAtomicReference<QNode> cleanMe = |
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new PaddedAtomicReference<QNode>(null); |
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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.next = h; // 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()) |
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* @param e the item or if null, signfies 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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* simpifies 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; |
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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, |
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* 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 |
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* @param mode mode |
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* @param nanos timeout value |
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* @return matched item, or s if cancelled |
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*/ |
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private Object awaitFulfill(QNode pred, QNode s, Object e, |
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int mode, long nanos) { |
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if (mode == NOWAIT) |
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return null; |
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|
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long lastTime = (mode == TIMEOUT)? System.nanoTime() : 0; |
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Thread w = Thread.currentThread(); |
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int spins = -1; // set to desired spin count below |
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for (;;) { |
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if (w.isInterrupted()) |
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s.compareAndSet(e, s); |
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Object x = s.get(); |
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if (x != e) { // Node was matched or cancelled |
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advanceHead(pred, s); // unlink if head |
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if (x == s) // was cancelled |
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return clean(pred, s); |
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else if (x != null) { |
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s.set(s); // avoid garbage retention |
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return x; |
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} |
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else |
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return e; |
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} |
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|
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if (mode == TIMEOUT) { |
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long now = System.nanoTime(); |
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nanos -= now - lastTime; |
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lastTime = now; |
283 |
if (nanos <= 0) { |
284 |
s.compareAndSet(e, s); // try to cancel |
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continue; |
286 |
} |
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} |
288 |
if (spins < 0) { |
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QNode h = head.get(); // only spin if at head |
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spins = ((h != null && h.next == s) ? |
291 |
(mode == TIMEOUT? |
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maxTimedSpins : maxUntimedSpins) : 0); |
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} |
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if (spins > 0) |
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--spins; |
296 |
else if (s.waiter == null) |
297 |
s.waiter = w; |
298 |
else if (mode != TIMEOUT) { |
299 |
// LockSupport.park(this); |
300 |
LockSupport.park(); // allows run on java5 |
301 |
s.waiter = null; |
302 |
spins = -1; |
303 |
} |
304 |
else if (nanos > spinForTimeoutThreshold) { |
305 |
// LockSupport.parkNanos(this, nanos); |
306 |
LockSupport.parkNanos(nanos); |
307 |
s.waiter = null; |
308 |
spins = -1; |
309 |
} |
310 |
} |
311 |
} |
312 |
|
313 |
/** |
314 |
* Gets rid of cancelled node s with original predecessor pred. |
315 |
* @return null (to simplify use by callers) |
316 |
*/ |
317 |
private Object clean(QNode pred, QNode s) { |
318 |
Thread w = s.waiter; |
319 |
if (w != null) { // Wake up thread |
320 |
s.waiter = null; |
321 |
if (w != Thread.currentThread()) |
322 |
LockSupport.unpark(w); |
323 |
} |
324 |
|
325 |
for (;;) { |
326 |
if (pred.next != s) // already cleaned |
327 |
return null; |
328 |
QNode h = head.get(); |
329 |
QNode hn = h.next; // Absorb cancelled first node as head |
330 |
if (hn != null && hn.next == hn) { |
331 |
advanceHead(h, hn); |
332 |
continue; |
333 |
} |
334 |
QNode t = tail.get(); // Ensure consistent read for tail |
335 |
if (t == h) |
336 |
return null; |
337 |
QNode tn = t.next; |
338 |
if (t != tail.get()) |
339 |
continue; |
340 |
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; |
346 |
if (sn == s || pred.casNext(s, sn)) |
347 |
return null; |
348 |
} |
349 |
QNode dp = cleanMe.get(); |
350 |
if (dp != null) { // Try unlinking previous cancelled node |
351 |
QNode d = dp.next; |
352 |
QNode dn; |
353 |
if (d == null || // d is gone or |
354 |
d == dp || // d is off list or |
355 |
d.get() != d || // d not cancelled or |
356 |
(d != t && // d not tail and |
357 |
(dn = d.next) != null && // has successor |
358 |
dn != d && // that is on list |
359 |
dp.casNext(d, dn))) // d unspliced |
360 |
cleanMe.compareAndSet(dp, null); |
361 |
if (dp == pred) |
362 |
return null; // s is already saved node |
363 |
} |
364 |
else if (cleanMe.compareAndSet(null, pred)) |
365 |
return null; // Postpone cleaning s |
366 |
} |
367 |
} |
368 |
|
369 |
/** |
370 |
* Creates an initially empty <tt>LinkedTransferQueue</tt>. |
371 |
*/ |
372 |
public LinkedTransferQueue() { |
373 |
} |
374 |
|
375 |
/** |
376 |
* Creates a <tt>LinkedTransferQueue</tt> |
377 |
* initially containing the elements of the given collection, |
378 |
* added in traversal order of the collection's iterator. |
379 |
* @param c the collection of elements to initially contain |
380 |
* @throws NullPointerException if the specified collection or any |
381 |
* of its elements are null |
382 |
*/ |
383 |
public LinkedTransferQueue(Collection<? extends E> c) { |
384 |
addAll(c); |
385 |
} |
386 |
|
387 |
public void put(E e) throws InterruptedException { |
388 |
if (e == null) throw new NullPointerException(); |
389 |
if (Thread.interrupted()) throw new InterruptedException(); |
390 |
xfer(e, NOWAIT, 0); |
391 |
} |
392 |
|
393 |
public boolean offer(E e, long timeout, TimeUnit unit) |
394 |
throws InterruptedException { |
395 |
if (e == null) throw new NullPointerException(); |
396 |
if (Thread.interrupted()) throw new InterruptedException(); |
397 |
xfer(e, NOWAIT, 0); |
398 |
return true; |
399 |
} |
400 |
|
401 |
public boolean offer(E e) { |
402 |
if (e == null) throw new NullPointerException(); |
403 |
xfer(e, NOWAIT, 0); |
404 |
return true; |
405 |
} |
406 |
|
407 |
public void transfer(E e) throws InterruptedException { |
408 |
if (e == null) throw new NullPointerException(); |
409 |
if (xfer(e, WAIT, 0) == null) { |
410 |
Thread.interrupted(); |
411 |
throw new InterruptedException(); |
412 |
} |
413 |
} |
414 |
|
415 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
416 |
throws InterruptedException { |
417 |
if (e == null) throw new NullPointerException(); |
418 |
if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null) |
419 |
return true; |
420 |
if (!Thread.interrupted()) |
421 |
return false; |
422 |
throw new InterruptedException(); |
423 |
} |
424 |
|
425 |
public boolean tryTransfer(E e) { |
426 |
if (e == null) throw new NullPointerException(); |
427 |
return fulfill(e) != null; |
428 |
} |
429 |
|
430 |
public E take() throws InterruptedException { |
431 |
Object e = xfer(null, WAIT, 0); |
432 |
if (e != null) |
433 |
return (E)e; |
434 |
Thread.interrupted(); |
435 |
throw new InterruptedException(); |
436 |
} |
437 |
|
438 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
439 |
Object e = xfer(null, TIMEOUT, unit.toNanos(timeout)); |
440 |
if (e != null || !Thread.interrupted()) |
441 |
return (E)e; |
442 |
throw new InterruptedException(); |
443 |
} |
444 |
|
445 |
public E poll() { |
446 |
return (E)fulfill(null); |
447 |
} |
448 |
|
449 |
public int drainTo(Collection<? super E> c) { |
450 |
if (c == null) |
451 |
throw new NullPointerException(); |
452 |
if (c == this) |
453 |
throw new IllegalArgumentException(); |
454 |
int n = 0; |
455 |
E e; |
456 |
while ( (e = poll()) != null) { |
457 |
c.add(e); |
458 |
++n; |
459 |
} |
460 |
return n; |
461 |
} |
462 |
|
463 |
public int drainTo(Collection<? super E> c, int maxElements) { |
464 |
if (c == null) |
465 |
throw new NullPointerException(); |
466 |
if (c == this) |
467 |
throw new IllegalArgumentException(); |
468 |
int n = 0; |
469 |
E e; |
470 |
while (n < maxElements && (e = poll()) != null) { |
471 |
c.add(e); |
472 |
++n; |
473 |
} |
474 |
return n; |
475 |
} |
476 |
|
477 |
// Traversal-based methods |
478 |
|
479 |
/** |
480 |
* Return head after performing any outstanding helping steps |
481 |
*/ |
482 |
private QNode traversalHead() { |
483 |
for (;;) { |
484 |
QNode t = tail.get(); |
485 |
QNode h = head.get(); |
486 |
if (h != null && t != null) { |
487 |
QNode last = t.next; |
488 |
QNode first = h.next; |
489 |
if (t == tail.get()) { |
490 |
if (last != null) |
491 |
tail.compareAndSet(t, last); |
492 |
else if (first != null) { |
493 |
Object x = first.get(); |
494 |
if (x == first) |
495 |
advanceHead(h, first); |
496 |
else |
497 |
return h; |
498 |
} |
499 |
else |
500 |
return h; |
501 |
} |
502 |
} |
503 |
} |
504 |
} |
505 |
|
506 |
|
507 |
public Iterator<E> iterator() { |
508 |
return new Itr(); |
509 |
} |
510 |
|
511 |
/** |
512 |
* Iterators. Basic strategy os to travers list, treating |
513 |
* non-data (i.e., request) nodes as terminating list. |
514 |
* Once a valid data node is found, the item is cached |
515 |
* so that the next call to next() will return it even |
516 |
* if subsequently removed. |
517 |
*/ |
518 |
class Itr implements Iterator<E> { |
519 |
QNode nextNode; // Next node to return next |
520 |
QNode currentNode; // last returned node, for remove() |
521 |
QNode prevNode; // predecessor of last returned node |
522 |
E nextItem; // Cache of next item, once commited to in next |
523 |
|
524 |
Itr() { |
525 |
nextNode = traversalHead(); |
526 |
advance(); |
527 |
} |
528 |
|
529 |
E advance() { |
530 |
prevNode = currentNode; |
531 |
currentNode = nextNode; |
532 |
E x = nextItem; |
533 |
|
534 |
QNode p = nextNode.next; |
535 |
for (;;) { |
536 |
if (p == null || !p.isData) { |
537 |
nextNode = null; |
538 |
nextItem = null; |
539 |
return x; |
540 |
} |
541 |
Object item = p.get(); |
542 |
if (item != p && item != null) { |
543 |
nextNode = p; |
544 |
nextItem = (E)item; |
545 |
return x; |
546 |
} |
547 |
prevNode = p; |
548 |
p = p.next; |
549 |
} |
550 |
} |
551 |
|
552 |
public boolean hasNext() { |
553 |
return nextNode != null; |
554 |
} |
555 |
|
556 |
public E next() { |
557 |
if (nextNode == null) throw new NoSuchElementException(); |
558 |
return advance(); |
559 |
} |
560 |
|
561 |
public void remove() { |
562 |
QNode p = currentNode; |
563 |
QNode prev = prevNode; |
564 |
if (prev == null || p == null) |
565 |
throw new IllegalStateException(); |
566 |
Object x = p.get(); |
567 |
if (x != null && x != p && p.compareAndSet(x, p)) |
568 |
clean(prev, p); |
569 |
} |
570 |
} |
571 |
|
572 |
public E peek() { |
573 |
for (;;) { |
574 |
QNode h = traversalHead(); |
575 |
QNode p = h.next; |
576 |
if (p == null) |
577 |
return null; |
578 |
Object x = p.get(); |
579 |
if (p != x) { |
580 |
if (!p.isData) |
581 |
return null; |
582 |
if (x != null) |
583 |
return (E)x; |
584 |
} |
585 |
} |
586 |
} |
587 |
|
588 |
public boolean hasWaitingConsumer() { |
589 |
for (;;) { |
590 |
QNode h = traversalHead(); |
591 |
QNode p = h.next; |
592 |
if (p == null) |
593 |
return false; |
594 |
Object x = p.get(); |
595 |
if (p != x) |
596 |
return !p.isData; |
597 |
} |
598 |
} |
599 |
|
600 |
/** |
601 |
* Returns the number of elements in this queue. If this queue |
602 |
* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns |
603 |
* <tt>Integer.MAX_VALUE</tt>. |
604 |
* |
605 |
* <p>Beware that, unlike in most collections, this method is |
606 |
* <em>NOT</em> a constant-time operation. Because of the |
607 |
* asynchronous nature of these queues, determining the current |
608 |
* number of elements requires an O(n) traversal. |
609 |
* |
610 |
* @return the number of elements in this queue |
611 |
*/ |
612 |
public int size() { |
613 |
int count = 0; |
614 |
QNode h = traversalHead(); |
615 |
for (QNode p = h.next; p != null && p.isData; p = p.next) { |
616 |
Object x = p.get(); |
617 |
if (x != null && x != p) { |
618 |
if (++count == Integer.MAX_VALUE) // saturated |
619 |
break; |
620 |
} |
621 |
} |
622 |
return count; |
623 |
} |
624 |
|
625 |
public int getWaitingConsumerCount() { |
626 |
int count = 0; |
627 |
QNode h = traversalHead(); |
628 |
for (QNode p = h.next; p != null && !p.isData; p = p.next) { |
629 |
if (p.get() == null) { |
630 |
if (++count == Integer.MAX_VALUE) |
631 |
break; |
632 |
} |
633 |
} |
634 |
return count; |
635 |
} |
636 |
|
637 |
public int remainingCapacity() { |
638 |
return Integer.MAX_VALUE; |
639 |
} |
640 |
|
641 |
/** |
642 |
* Save the state to a stream (that is, serialize it). |
643 |
* |
644 |
* @serialData All of the elements (each an <tt>E</tt>) in |
645 |
* the proper order, followed by a null |
646 |
* @param s the stream |
647 |
*/ |
648 |
private void writeObject(java.io.ObjectOutputStream s) |
649 |
throws java.io.IOException { |
650 |
s.defaultWriteObject(); |
651 |
for (Iterator<E> it = iterator(); it.hasNext(); ) |
652 |
s.writeObject(it.next()); |
653 |
// Use trailing null as sentinel |
654 |
s.writeObject(null); |
655 |
} |
656 |
|
657 |
/** |
658 |
* Reconstitute the Queue instance from a stream (that is, |
659 |
* deserialize it). |
660 |
* @param s the stream |
661 |
*/ |
662 |
private void readObject(java.io.ObjectInputStream s) |
663 |
throws java.io.IOException, ClassNotFoundException { |
664 |
s.defaultReadObject(); |
665 |
for (;;) { |
666 |
E item = (E)s.readObject(); |
667 |
if (item == null) |
668 |
break; |
669 |
else |
670 |
offer(item); |
671 |
} |
672 |
} |
673 |
} |