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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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* |
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* A FIFO dual queue may be implemented using a variation of the |
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* Michael & Scott (M&S) lock-free queue algorithm |
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* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). |
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* (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf). |
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* It maintains two pointer fields, "head", pointing to a |
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* (matched) node that in turn points to the first actual |
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* (unmatched) queue node (or null if empty); and "tail" that |
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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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* and will not hurt in limiting impact of spinning on busy |
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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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* blocked -- these "chained" spins avoid artifacts of |
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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 |
352 |
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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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/** |
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* Creates a new node. Uses relaxed write because item can only |
437 |
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* be seen if followed by CAS. |
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* Constructs a new node. Uses relaxed write because item can |
437 |
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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 |
455 |
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* bacause order is already constrained in the only calling |
455 |
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* 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; |
497 |
> |
// 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); |
504 |
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private static final long itemOffset = |
505 |
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objectFieldOffset(UNSAFE, "item", Node.class); |
506 |
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private static final long waiterOffset = |
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objectFieldOffset(UNSAFE, "waiter", Node.class); |
508 |
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|
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private static final long serialVersionUID = -3375979862319811754L; |
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|
508 |
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// Unsafe mechanics |
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private static final sun.misc.Unsafe UNSAFE; |
510 |
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private static final long itemOffset; |
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private static final long nextOffset; |
512 |
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private static final long waiterOffset; |
513 |
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static { |
514 |
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try { |
515 |
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UNSAFE = getUnsafe(); |
516 |
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Class<?> k = Node.class; |
517 |
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itemOffset = UNSAFE.objectFieldOffset |
518 |
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(k.getDeclaredField("item")); |
519 |
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nextOffset = UNSAFE.objectFieldOffset |
520 |
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(k.getDeclaredField("next")); |
521 |
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waiterOffset = UNSAFE.objectFieldOffset |
522 |
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(k.getDeclaredField("waiter")); |
523 |
+ |
} catch (Exception e) { |
524 |
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throw new Error(e); |
525 |
+ |
} |
526 |
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} |
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} |
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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") |
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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; |
563 |
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} |
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|
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throw new NullPointerException(); |
578 |
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Node s = null; // the node to append, if needed |
579 |
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|
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retry: for (;;) { // restart on append race |
580 |
> |
retry: |
581 |
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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 |
592 |
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if (head == h && casHead(h, n == null? q : n)) { |
592 |
> |
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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break; // unless slack < 2 |
599 |
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} |
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LockSupport.unpark(p.waiter); |
601 |
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return this.<E>cast(item); |
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> |
return LinkedTransferQueue.<E>cast(item); |
602 |
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} |
603 |
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} |
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Node n = p.next; |
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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 |
679 |
< |
return this.<E>cast(item); |
679 |
> |
return LinkedTransferQueue.<E>cast(item); |
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} |
681 |
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if ((w.isInterrupted() || (timed && nanos <= 0)) && |
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s.casItem(e, s)) { // cancel |
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Object item = p.item; |
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if (p.isData) { |
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if (item != null && item != p) |
760 |
< |
return this.<E>cast(item); |
760 |
> |
return LinkedTransferQueue.<E>cast(item); |
761 |
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} |
762 |
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else if (item == null) |
763 |
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return null; |
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* Moves to next node after prev, or first node if prev null. |
800 |
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*/ |
801 |
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private void advance(Node prev) { |
802 |
< |
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; |
802 |
> |
/* |
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 |
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|
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 |
|
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
943 |
< |
if (p == s) // stale |
944 |
< |
p = head; |
885 |
< |
else if (!s.isMatched()) |
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 |
|
} |
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 |
970 |
< |
* {@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) { |
1134 |
|
if (c == this) |
1135 |
|
throw new IllegalArgumentException(); |
1136 |
|
int n = 0; |
1137 |
< |
E e; |
1076 |
< |
while ( (e = poll()) != null) { |
1137 |
> |
for (E e; (e = poll()) != null;) { |
1138 |
|
c.add(e); |
1139 |
|
++n; |
1140 |
|
} |
1151 |
|
if (c == this) |
1152 |
|
throw new IllegalArgumentException(); |
1153 |
|
int n = 0; |
1154 |
< |
E e; |
1094 |
< |
while (n < maxElements && (e = poll()) != null) { |
1154 |
> |
for (E e; n < maxElements && (e = poll()) != null;) { |
1155 |
|
c.add(e); |
1156 |
|
++n; |
1157 |
|
} |
1159 |
|
} |
1160 |
|
|
1161 |
|
/** |
1162 |
< |
* Returns an iterator over the elements in this queue in proper |
1163 |
< |
* sequence, from head to tail. |
1162 |
> |
* Returns an iterator over the elements in this queue in proper sequence. |
1163 |
> |
* The elements will be returned in order from first (head) to last (tail). |
1164 |
|
* |
1165 |
|
* <p>The returned iterator is a "weakly consistent" iterator that |
1166 |
< |
* will never throw |
1167 |
< |
* {@link ConcurrentModificationException ConcurrentModificationException}, |
1168 |
< |
* and guarantees to traverse elements as they existed upon |
1169 |
< |
* construction of the iterator, and may (but is not guaranteed |
1170 |
< |
* to) reflect any modifications subsequent to construction. |
1166 |
> |
* will never throw {@link java.util.ConcurrentModificationException |
1167 |
> |
* ConcurrentModificationException}, and guarantees to traverse |
1168 |
> |
* elements as they existed upon construction of the iterator, and |
1169 |
> |
* may (but is not guaranteed to) reflect any modifications |
1170 |
> |
* subsequent to construction. |
1171 |
|
* |
1172 |
|
* @return an iterator over the elements in this queue in proper sequence |
1173 |
|
*/ |
1232 |
|
} |
1233 |
|
|
1234 |
|
/** |
1235 |
+ |
* Returns {@code true} if this queue contains the specified element. |
1236 |
+ |
* More formally, returns {@code true} if and only if this queue contains |
1237 |
+ |
* at least one element {@code e} such that {@code o.equals(e)}. |
1238 |
+ |
* |
1239 |
+ |
* @param o object to be checked for containment in this queue |
1240 |
+ |
* @return {@code true} if this queue contains the specified element |
1241 |
+ |
*/ |
1242 |
+ |
public boolean contains(Object o) { |
1243 |
+ |
if (o == null) return false; |
1244 |
+ |
for (Node p = head; p != null; p = succ(p)) { |
1245 |
+ |
Object item = p.item; |
1246 |
+ |
if (p.isData) { |
1247 |
+ |
if (item != null && item != p && o.equals(item)) |
1248 |
+ |
return true; |
1249 |
+ |
} |
1250 |
+ |
else if (item == null) |
1251 |
+ |
break; |
1252 |
+ |
} |
1253 |
+ |
return false; |
1254 |
+ |
} |
1255 |
+ |
|
1256 |
+ |
/** |
1257 |
|
* Always returns {@code Integer.MAX_VALUE} because a |
1258 |
|
* {@code LinkedTransferQueue} is not capacity constrained. |
1259 |
|
* |
1260 |
|
* @return {@code Integer.MAX_VALUE} (as specified by |
1261 |
< |
* {@link BlockingQueue#remainingCapacity()}) |
1261 |
> |
* {@link java.util.concurrent.BlockingQueue#remainingCapacity() |
1262 |
> |
* BlockingQueue.remainingCapacity}) |
1263 |
|
*/ |
1264 |
|
public int remainingCapacity() { |
1265 |
|
return Integer.MAX_VALUE; |
1291 |
|
throws java.io.IOException, ClassNotFoundException { |
1292 |
|
s.defaultReadObject(); |
1293 |
|
for (;;) { |
1294 |
< |
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1294 |
> |
@SuppressWarnings("unchecked") |
1295 |
> |
E item = (E) s.readObject(); |
1296 |
|
if (item == null) |
1297 |
|
break; |
1298 |
|
else |
1302 |
|
|
1303 |
|
// Unsafe mechanics |
1304 |
|
|
1305 |
< |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1306 |
< |
private static final long headOffset = |
1307 |
< |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1308 |
< |
private static final long tailOffset = |
1309 |
< |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1226 |
< |
private static final long sweepVotesOffset = |
1227 |
< |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1228 |
< |
|
1229 |
< |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1230 |
< |
String field, Class<?> klazz) { |
1305 |
> |
private static final sun.misc.Unsafe UNSAFE; |
1306 |
> |
private static final long headOffset; |
1307 |
> |
private static final long tailOffset; |
1308 |
> |
private static final long sweepVotesOffset; |
1309 |
> |
static { |
1310 |
|
try { |
1311 |
< |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1312 |
< |
} catch (NoSuchFieldException e) { |
1313 |
< |
// Convert Exception to corresponding Error |
1314 |
< |
NoSuchFieldError error = new NoSuchFieldError(field); |
1315 |
< |
error.initCause(e); |
1316 |
< |
throw error; |
1311 |
> |
UNSAFE = getUnsafe(); |
1312 |
> |
Class<?> k = LinkedTransferQueue.class; |
1313 |
> |
headOffset = UNSAFE.objectFieldOffset |
1314 |
> |
(k.getDeclaredField("head")); |
1315 |
> |
tailOffset = UNSAFE.objectFieldOffset |
1316 |
> |
(k.getDeclaredField("tail")); |
1317 |
> |
sweepVotesOffset = UNSAFE.objectFieldOffset |
1318 |
> |
(k.getDeclaredField("sweepVotes")); |
1319 |
> |
} catch (Exception e) { |
1320 |
> |
throw new Error(e); |
1321 |
|
} |
1322 |
|
} |
1323 |
|
|
1331 |
|
static sun.misc.Unsafe getUnsafe() { |
1332 |
|
try { |
1333 |
|
return sun.misc.Unsafe.getUnsafe(); |
1334 |
< |
} catch (SecurityException se) { |
1335 |
< |
try { |
1336 |
< |
return java.security.AccessController.doPrivileged |
1337 |
< |
(new java.security |
1338 |
< |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1339 |
< |
public sun.misc.Unsafe run() throws Exception { |
1340 |
< |
java.lang.reflect.Field f = sun.misc |
1341 |
< |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1342 |
< |
f.setAccessible(true); |
1343 |
< |
return (sun.misc.Unsafe) f.get(null); |
1344 |
< |
}}); |
1345 |
< |
} catch (java.security.PrivilegedActionException e) { |
1346 |
< |
throw new RuntimeException("Could not initialize intrinsics", |
1347 |
< |
e.getCause()); |
1348 |
< |
} |
1334 |
> |
} catch (SecurityException tryReflectionInstead) {} |
1335 |
> |
try { |
1336 |
> |
return java.security.AccessController.doPrivileged |
1337 |
> |
(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1338 |
> |
public sun.misc.Unsafe run() throws Exception { |
1339 |
> |
Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; |
1340 |
> |
for (java.lang.reflect.Field f : k.getDeclaredFields()) { |
1341 |
> |
f.setAccessible(true); |
1342 |
> |
Object x = f.get(null); |
1343 |
> |
if (k.isInstance(x)) |
1344 |
> |
return k.cast(x); |
1345 |
> |
} |
1346 |
> |
throw new NoSuchFieldError("the Unsafe"); |
1347 |
> |
}}); |
1348 |
> |
} catch (java.security.PrivilegedActionException e) { |
1349 |
> |
throw new RuntimeException("Could not initialize intrinsics", |
1350 |
> |
e.getCause()); |
1351 |
|
} |
1352 |
|
} |
1268 |
– |
|
1353 |
|
} |