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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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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166y; |
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– |
import java.util.concurrent.*; |
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|
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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– |
import java.util.ConcurrentModificationException; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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+ |
import java.util.concurrent.TimeUnit; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* An unbounded {@link TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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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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* <p>Beware that, unlike in most collections, the {@code size} method |
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* 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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* of elements requires a traversal of the elements, and so may report |
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* inaccurate results if this collection is modified during traversal. |
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> |
* Additionally, the bulk operations {@code addAll}, |
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* {@code removeAll}, {@code retainAll}, {@code containsAll}, |
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* {@code equals}, and {@code toArray} are <em>not</em> guaranteed |
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* to be performed atomically. For example, an iterator operating |
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* concurrently with an {@code addAll} operation might view only some |
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* of the added 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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* of less-contended queues. During spins threads check their |
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* interrupt status and generate a thread-local random number |
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* to decide to occasionally perform a Thread.yield. While |
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< |
* yield has underdefined specs, we assume that might it help, |
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> |
* yield has underdefined specs, we assume that it might help, |
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* and will not hurt in limiting impact of spinning on busy |
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* systems. We also use smaller (1/2) spins for nodes that are |
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* not known to be front but whose predecessors have not |
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* situations in which we cannot guarantee to make node s |
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* unreachable in this way: (1) If s is the trailing node of list |
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* (i.e., with null next), then it is pinned as the target node |
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* for appends, so can only be removed later when other nodes are |
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> |
* for appends, so can only be removed later after other nodes are |
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* appended. (2) We cannot necessarily unlink s given a |
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* predecessor node that is matched (including the case of being |
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* cancelled): the predecessor may already be unspliced, in which |
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* When these cases arise, rather than always retraversing the |
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* entire list to find an actual predecessor to unlink (which |
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* won't help for case (1) anyway), we record a conservative |
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* estimate of possible unsplice failures (in "sweepVotes"). We |
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* trigger a full sweep when the estimate exceeds a threshold |
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< |
* indicating the maximum number of estimated removal failures to |
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* tolerate before sweeping through, unlinking cancelled nodes |
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< |
* that were not unlinked upon initial removal. We perform sweeps |
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* by the thread hitting threshold (rather than background threads |
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< |
* or by spreading work to other threads) because in the main |
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< |
* contexts in which removal occurs, the caller is already |
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< |
* timed-out, cancelled, or performing a potentially O(n) |
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< |
* operation (i.e., remove(x)), none of which are time-critical |
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< |
* enough to warrant the overhead that alternatives would impose |
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< |
* on other threads. |
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> |
* estimate of possible unsplice failures (in "sweepVotes"). |
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* We trigger a full sweep when the estimate exceeds a threshold |
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> |
* ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
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* removal failures to tolerate before sweeping through, unlinking |
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> |
* cancelled nodes that were not unlinked upon initial removal. |
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> |
* We perform sweeps by the thread hitting threshold (rather than |
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> |
* background threads or by spreading work to other threads) |
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> |
* because in the main contexts in which removal occurs, the |
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* caller is already timed-out, cancelled, or performing a |
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> |
* potentially O(n) operation (e.g. remove(x)), none of which are |
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> |
* time-critical enough to warrant the overhead that alternatives |
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> |
* would impose on other threads. |
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* |
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* Because the sweepVotes estimate is conservative, and because |
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* nodes become unlinked "naturally" as they fall off the head of |
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} |
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|
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final boolean casItem(Object cmp, Object val) { |
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< |
assert cmp == null || cmp.getClass() != Node.class; |
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> |
// assert cmp == null || cmp.getClass() != Node.class; |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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|
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/** |
| 436 |
< |
* Creates a new node. Uses relaxed write because item can only |
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< |
* be seen if followed by CAS. |
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> |
* Constructs a new node. Uses relaxed write because item can |
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> |
* only be seen after publication via casNext. |
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*/ |
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Node(Object item, boolean isData) { |
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UNSAFE.putObject(this, itemOffset, item); // relaxed write |
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/** |
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* Sets item to self and waiter to null, to avoid garbage |
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* retention after matching or cancelling. Uses relaxed writes |
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< |
* bacause order is already constrained in the only calling |
| 455 |
> |
* because order is already constrained in the only calling |
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* contexts: item is forgotten only after volatile/atomic |
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* mechanics that extract items. Similarly, clearing waiter |
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* follows either CAS or return from park (if ever parked; |
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* Tries to artificially match a data node -- used by remove. |
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*/ |
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final boolean tryMatchData() { |
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< |
assert isData; |
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> |
// assert isData; |
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Object x = item; |
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if (x != null && x != this && casItem(x, null)) { |
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LockSupport.unpark(waiter); |
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return false; |
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} |
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|
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– |
// Unsafe mechanics |
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– |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
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– |
private static final long nextOffset = |
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– |
objectFieldOffset(UNSAFE, "next", Node.class); |
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– |
private static final long itemOffset = |
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– |
objectFieldOffset(UNSAFE, "item", Node.class); |
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– |
private static final long waiterOffset = |
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– |
objectFieldOffset(UNSAFE, "waiter", Node.class); |
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– |
|
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private static final long serialVersionUID = -3375979862319811754L; |
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+ |
|
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+ |
// Unsafe mechanics |
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+ |
private static final sun.misc.Unsafe UNSAFE; |
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+ |
private static final long itemOffset; |
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+ |
private static final long nextOffset; |
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+ |
private static final long waiterOffset; |
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+ |
static { |
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+ |
try { |
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+ |
UNSAFE = getUnsafe(); |
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+ |
Class<?> k = Node.class; |
| 517 |
+ |
itemOffset = UNSAFE.objectFieldOffset |
| 518 |
+ |
(k.getDeclaredField("item")); |
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+ |
nextOffset = UNSAFE.objectFieldOffset |
| 520 |
+ |
(k.getDeclaredField("next")); |
| 521 |
+ |
waiterOffset = UNSAFE.objectFieldOffset |
| 522 |
+ |
(k.getDeclaredField("waiter")); |
| 523 |
+ |
} catch (Exception e) { |
| 524 |
+ |
throw new Error(e); |
| 525 |
+ |
} |
| 526 |
+ |
} |
| 527 |
|
} |
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|
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/** head of the queue; null until first enqueue */ |
| 558 |
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|
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@SuppressWarnings("unchecked") |
| 560 |
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static <E> E cast(Object item) { |
| 561 |
< |
assert item == null || item.getClass() != Node.class; |
| 561 |
> |
// assert item == null || item.getClass() != Node.class; |
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return (E) item; |
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} |
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|
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throw new NullPointerException(); |
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Node s = null; // the node to append, if needed |
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|
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< |
retry: for (;;) { // restart on append race |
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> |
retry: |
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> |
for (;;) { // restart on append race |
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|
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for (Node h = head, p = h; p != null;) { // find & match first node |
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boolean isData = p.isData; |
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if (p.casItem(item, e)) { // match |
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for (Node q = p; q != h;) { |
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Node n = q.next; // update by 2 unless singleton |
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< |
if (head == h && casHead(h, n == null? q : n)) { |
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> |
if (head == h && casHead(h, n == null ? q : n)) { |
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h.forgetNext(); |
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break; |
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} // advance and retry |
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for (;;) { |
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Object item = s.item; |
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if (item != e) { // matched |
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< |
assert item != s; |
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> |
// assert item != s; |
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s.forgetContents(); // avoid garbage |
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return this.<E>cast(item); |
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} |
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* Moves to next node after prev, or first node if prev null. |
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*/ |
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private void advance(Node prev) { |
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< |
lastPred = lastRet; |
| 803 |
< |
lastRet = prev; |
| 804 |
< |
for (Node p = (prev == null) ? head : succ(prev); |
| 805 |
< |
p != null; p = succ(p)) { |
| 806 |
< |
Object item = p.item; |
| 807 |
< |
if (p.isData) { |
| 808 |
< |
if (item != null && item != p) { |
| 809 |
< |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
| 810 |
< |
nextNode = p; |
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> |
/* |
| 803 |
> |
* To track and avoid buildup of deleted nodes in the face |
| 804 |
> |
* of calls to both Queue.remove and Itr.remove, we must |
| 805 |
> |
* include variants of unsplice and sweep upon each |
| 806 |
> |
* advance: Upon Itr.remove, we may need to catch up links |
| 807 |
> |
* from lastPred, and upon other removes, we might need to |
| 808 |
> |
* skip ahead from stale nodes and unsplice deleted ones |
| 809 |
> |
* found while advancing. |
| 810 |
> |
*/ |
| 811 |
> |
|
| 812 |
> |
Node r, b; // reset lastPred upon possible deletion of lastRet |
| 813 |
> |
if ((r = lastRet) != null && !r.isMatched()) |
| 814 |
> |
lastPred = r; // next lastPred is old lastRet |
| 815 |
> |
else if ((b = lastPred) == null || b.isMatched()) |
| 816 |
> |
lastPred = null; // at start of list |
| 817 |
> |
else { |
| 818 |
> |
Node s, n; // help with removal of lastPred.next |
| 819 |
> |
while ((s = b.next) != null && |
| 820 |
> |
s != b && s.isMatched() && |
| 821 |
> |
(n = s.next) != null && n != s) |
| 822 |
> |
b.casNext(s, n); |
| 823 |
> |
} |
| 824 |
> |
|
| 825 |
> |
this.lastRet = prev; |
| 826 |
> |
|
| 827 |
> |
for (Node p = prev, s, n;;) { |
| 828 |
> |
s = (p == null) ? head : p.next; |
| 829 |
> |
if (s == null) |
| 830 |
> |
break; |
| 831 |
> |
else if (s == p) { |
| 832 |
> |
p = null; |
| 833 |
> |
continue; |
| 834 |
> |
} |
| 835 |
> |
Object item = s.item; |
| 836 |
> |
if (s.isData) { |
| 837 |
> |
if (item != null && item != s) { |
| 838 |
> |
nextItem = LinkedTransferQueue.<E>cast(item); |
| 839 |
> |
nextNode = s; |
| 840 |
|
return; |
| 841 |
|
} |
| 842 |
|
} |
| 843 |
|
else if (item == null) |
| 844 |
|
break; |
| 845 |
+ |
// assert s.isMatched(); |
| 846 |
+ |
if (p == null) |
| 847 |
+ |
p = s; |
| 848 |
+ |
else if ((n = s.next) == null) |
| 849 |
+ |
break; |
| 850 |
+ |
else if (s == n) |
| 851 |
+ |
p = null; |
| 852 |
+ |
else |
| 853 |
+ |
p.casNext(s, n); |
| 854 |
|
} |
| 855 |
|
nextNode = null; |
| 856 |
+ |
nextItem = null; |
| 857 |
|
} |
| 858 |
|
|
| 859 |
|
Itr() { |
| 873 |
|
} |
| 874 |
|
|
| 875 |
|
public final void remove() { |
| 876 |
< |
Node p = lastRet; |
| 877 |
< |
if (p == null) throw new IllegalStateException(); |
| 878 |
< |
if (p.tryMatchData()) |
| 879 |
< |
unsplice(lastPred, p); |
| 876 |
> |
final Node lastRet = this.lastRet; |
| 877 |
> |
if (lastRet == null) |
| 878 |
> |
throw new IllegalStateException(); |
| 879 |
> |
this.lastRet = null; |
| 880 |
> |
if (lastRet.tryMatchData()) |
| 881 |
> |
unsplice(lastPred, lastRet); |
| 882 |
|
} |
| 883 |
|
} |
| 884 |
|
|
| 935 |
|
} |
| 936 |
|
|
| 937 |
|
/** |
| 938 |
< |
* Unlinks matched nodes encountered in a traversal from head. |
| 938 |
> |
* Unlinks matched (typically cancelled) nodes encountered in a |
| 939 |
> |
* traversal from head. |
| 940 |
|
*/ |
| 941 |
|
private void sweep() { |
| 942 |
< |
Node p = head, s, n; |
| 943 |
< |
while (p != null && (s = p.next) != null && (n = s.next) != null) { |
| 944 |
< |
if (p == s || s == n) |
| 885 |
< |
p = head; // stale |
| 886 |
< |
else if (s.isMatched()) |
| 887 |
< |
p.casNext(s, n); |
| 888 |
< |
else |
| 942 |
> |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
| 943 |
> |
if (!s.isMatched()) |
| 944 |
> |
// Unmatched nodes are never self-linked |
| 945 |
|
p = s; |
| 946 |
+ |
else if ((n = s.next) == null) // trailing node is pinned |
| 947 |
+ |
break; |
| 948 |
+ |
else if (s == n) // stale |
| 949 |
+ |
// No need to also check for p == s, since that implies s == n |
| 950 |
+ |
p = head; |
| 951 |
+ |
else |
| 952 |
+ |
p.casNext(s, n); |
| 953 |
|
} |
| 954 |
|
} |
| 955 |
|
|
| 1016 |
|
* return {@code false}. |
| 1017 |
|
* |
| 1018 |
|
* @return {@code true} (as specified by |
| 1019 |
< |
* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) |
| 1019 |
> |
* {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit) |
| 1020 |
> |
* BlockingQueue.offer}) |
| 1021 |
|
* @throws NullPointerException if the specified element is null |
| 1022 |
|
*/ |
| 1023 |
|
public boolean offer(E e, long timeout, TimeUnit unit) { |
| 1029 |
|
* Inserts the specified element at the tail of this queue. |
| 1030 |
|
* As the queue is unbounded, this method will never return {@code false}. |
| 1031 |
|
* |
| 1032 |
< |
* @return {@code true} (as specified by |
| 969 |
< |
* {@link BlockingQueue#offer(Object) BlockingQueue.offer}) |
| 1032 |
> |
* @return {@code true} (as specified by {@link Queue#offer}) |
| 1033 |
|
* @throws NullPointerException if the specified element is null |
| 1034 |
|
*/ |
| 1035 |
|
public boolean offer(E e) { |
| 1161 |
|
} |
| 1162 |
|
|
| 1163 |
|
/** |
| 1164 |
< |
* Returns an iterator over the elements in this queue in proper |
| 1165 |
< |
* sequence, from head to tail. |
| 1164 |
> |
* Returns an iterator over the elements in this queue in proper sequence. |
| 1165 |
> |
* The elements will be returned in order from first (head) to last (tail). |
| 1166 |
|
* |
| 1167 |
|
* <p>The returned iterator is a "weakly consistent" iterator that |
| 1168 |
< |
* will never throw |
| 1169 |
< |
* {@link ConcurrentModificationException ConcurrentModificationException}, |
| 1170 |
< |
* and guarantees to traverse elements as they existed upon |
| 1171 |
< |
* construction of the iterator, and may (but is not guaranteed |
| 1172 |
< |
* to) reflect any modifications subsequent to construction. |
| 1168 |
> |
* will never throw {@link java.util.ConcurrentModificationException |
| 1169 |
> |
* ConcurrentModificationException}, and guarantees to traverse |
| 1170 |
> |
* elements as they existed upon construction of the iterator, and |
| 1171 |
> |
* may (but is not guaranteed to) reflect any modifications |
| 1172 |
> |
* subsequent to construction. |
| 1173 |
|
* |
| 1174 |
|
* @return an iterator over the elements in this queue in proper sequence |
| 1175 |
|
*/ |
| 1187 |
|
* @return {@code true} if this queue contains no elements |
| 1188 |
|
*/ |
| 1189 |
|
public boolean isEmpty() { |
| 1190 |
< |
return firstOfMode(true) == null; |
| 1190 |
> |
for (Node p = head; p != null; p = succ(p)) { |
| 1191 |
> |
if (!p.isMatched()) |
| 1192 |
> |
return !p.isData; |
| 1193 |
> |
} |
| 1194 |
> |
return true; |
| 1195 |
|
} |
| 1196 |
|
|
| 1197 |
|
public boolean hasWaitingConsumer() { |
| 1234 |
|
} |
| 1235 |
|
|
| 1236 |
|
/** |
| 1237 |
+ |
* Returns {@code true} if this queue contains the specified element. |
| 1238 |
+ |
* More formally, returns {@code true} if and only if this queue contains |
| 1239 |
+ |
* at least one element {@code e} such that {@code o.equals(e)}. |
| 1240 |
+ |
* |
| 1241 |
+ |
* @param o object to be checked for containment in this queue |
| 1242 |
+ |
* @return {@code true} if this queue contains the specified element |
| 1243 |
+ |
*/ |
| 1244 |
+ |
public boolean contains(Object o) { |
| 1245 |
+ |
if (o == null) return false; |
| 1246 |
+ |
for (Node p = head; p != null; p = succ(p)) { |
| 1247 |
+ |
Object item = p.item; |
| 1248 |
+ |
if (p.isData) { |
| 1249 |
+ |
if (item != null && item != p && o.equals(item)) |
| 1250 |
+ |
return true; |
| 1251 |
+ |
} |
| 1252 |
+ |
else if (item == null) |
| 1253 |
+ |
break; |
| 1254 |
+ |
} |
| 1255 |
+ |
return false; |
| 1256 |
+ |
} |
| 1257 |
+ |
|
| 1258 |
+ |
/** |
| 1259 |
|
* Always returns {@code Integer.MAX_VALUE} because a |
| 1260 |
|
* {@code LinkedTransferQueue} is not capacity constrained. |
| 1261 |
|
* |
| 1262 |
|
* @return {@code Integer.MAX_VALUE} (as specified by |
| 1263 |
< |
* {@link BlockingQueue#remainingCapacity()}) |
| 1263 |
> |
* {@link java.util.concurrent.BlockingQueue#remainingCapacity() |
| 1264 |
> |
* BlockingQueue.remainingCapacity}) |
| 1265 |
|
*/ |
| 1266 |
|
public int remainingCapacity() { |
| 1267 |
|
return Integer.MAX_VALUE; |
| 1303 |
|
|
| 1304 |
|
// Unsafe mechanics |
| 1305 |
|
|
| 1306 |
< |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
| 1307 |
< |
private static final long headOffset = |
| 1308 |
< |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
| 1309 |
< |
private static final long tailOffset = |
| 1310 |
< |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
| 1221 |
< |
private static final long sweepVotesOffset = |
| 1222 |
< |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
| 1223 |
< |
|
| 1224 |
< |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
| 1225 |
< |
String field, Class<?> klazz) { |
| 1306 |
> |
private static final sun.misc.Unsafe UNSAFE; |
| 1307 |
> |
private static final long headOffset; |
| 1308 |
> |
private static final long tailOffset; |
| 1309 |
> |
private static final long sweepVotesOffset; |
| 1310 |
> |
static { |
| 1311 |
|
try { |
| 1312 |
< |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
| 1313 |
< |
} catch (NoSuchFieldException e) { |
| 1314 |
< |
// Convert Exception to corresponding Error |
| 1315 |
< |
NoSuchFieldError error = new NoSuchFieldError(field); |
| 1316 |
< |
error.initCause(e); |
| 1317 |
< |
throw error; |
| 1312 |
> |
UNSAFE = getUnsafe(); |
| 1313 |
> |
Class<?> k = LinkedTransferQueue.class; |
| 1314 |
> |
headOffset = UNSAFE.objectFieldOffset |
| 1315 |
> |
(k.getDeclaredField("head")); |
| 1316 |
> |
tailOffset = UNSAFE.objectFieldOffset |
| 1317 |
> |
(k.getDeclaredField("tail")); |
| 1318 |
> |
sweepVotesOffset = UNSAFE.objectFieldOffset |
| 1319 |
> |
(k.getDeclaredField("sweepVotes")); |
| 1320 |
> |
} catch (Exception e) { |
| 1321 |
> |
throw new Error(e); |
| 1322 |
|
} |
| 1323 |
|
} |
| 1324 |
|
|
