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/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/licenses/publicdomain */ package jsr166y; import java.util.concurrent.*; import java.util.AbstractQueue; import java.util.Collection; import java.util.ConcurrentModificationException; import java.util.Iterator; import java.util.NoSuchElementException; import java.util.Queue; import java.util.concurrent.locks.LockSupport; /** * An unbounded {@link TransferQueue} based on linked nodes. * This queue orders elements FIFO (first-in-first-out) with respect * to any given producer. The <em>head</em> of the queue is that * element that has been on the queue the longest time for some * producer. The <em>tail</em> of the queue is that element that has * been on the queue the shortest time for some producer. * * <p>Beware that, unlike in most collections, the {@code size} * method is <em>NOT</em> a constant-time operation. Because of the * asynchronous nature of these queues, determining the current number * of elements requires a traversal of the elements. * * <p>This class and its iterator implement all of the * <em>optional</em> methods of the {@link Collection} and {@link * Iterator} interfaces. * * <p>Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code LinkedTransferQueue} * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> * actions subsequent to the access or removal of that element from * the {@code LinkedTransferQueue} in another thread. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.7 * @author Doug Lea * @param <E> the type of elements held in this collection */ public class LinkedTransferQueue<E> extends AbstractQueue<E> implements TransferQueue<E>, java.io.Serializable { private static final long serialVersionUID = -3223113410248163686L; /* * *** Overview of Dual Queues with Slack *** * * Dual Queues, introduced by Scherer and Scott * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are * (linked) queues in which nodes may represent either data or * requests. When a thread tries to enqueue a data node, but * encounters a request node, it instead "matches" and removes it; * and vice versa for enqueuing requests. Blocking Dual Queues * arrange that threads enqueuing unmatched requests block until * other threads provide the match. Dual Synchronous Queues (see * Scherer, Lea, & Scott * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) * additionally arrange that threads enqueuing unmatched data also * block. Dual Transfer Queues support all of these modes, as * dictated by callers. * * A FIFO dual queue may be implemented using a variation of the * Michael & Scott (M&S) lock-free queue algorithm * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). * It maintains two pointer fields, "head", pointing to a * (matched) node that in turn points to the first actual * (unmatched) queue node (or null if empty); and "tail" that * points to the last node on the queue (or again null if * empty). For example, here is a possible queue with four data * elements: * * head tail * | | * v v * M -> U -> U -> U -> U * * The M&S queue algorithm is known to be prone to scalability and * overhead limitations when maintaining (via CAS) these head and * tail pointers. This has led to the development of * contention-reducing variants such as elimination arrays (see * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and * optimistic back pointers (see Ladan-Mozes & Shavit * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). * However, the nature of dual queues enables a simpler tactic for * improving M&S-style implementations when dual-ness is needed. * * In a dual queue, each node must atomically maintain its match * status. While there are other possible variants, we implement * this here as: for a data-mode node, matching entails CASing an * "item" field from a non-null data value to null upon match, and * vice-versa for request nodes, CASing from null to a data * value. (Note that the linearization properties of this style of * queue are easy to verify -- elements are made available by * linking, and unavailable by matching.) Compared to plain M&S * queues, this property of dual queues requires one additional * successful atomic operation per enq/deq pair. But it also * enables lower cost variants of queue maintenance mechanics. (A * variation of this idea applies even for non-dual queues that * support deletion of interior elements, such as * j.u.c.ConcurrentLinkedQueue.) * * Once a node is matched, its match status can never again * change. We may thus arrange that the linked list of them * contain a prefix of zero or more matched nodes, followed by a * suffix of zero or more unmatched nodes. (Note that we allow * both the prefix and suffix to be zero length, which in turn * means that we do not use a dummy header.) If we were not * concerned with either time or space efficiency, we could * correctly perform enqueue and dequeue operations by traversing * from a pointer to the initial node; CASing the item of the * first unmatched node on match and CASing the next field of the * trailing node on appends. (Plus some special-casing when * initially empty). While this would be a terrible idea in * itself, it does have the benefit of not requiring ANY atomic * updates on head/tail fields. * * We introduce here an approach that lies between the extremes of * never versus always updating queue (head and tail) pointers. * This offers a tradeoff between sometimes requiring extra * traversal steps to locate the first and/or last unmatched * nodes, versus the reduced overhead and contention of fewer * updates to queue pointers. For example, a possible snapshot of * a queue is: * * head tail * | | * v v * M -> M -> U -> U -> U -> U * * The best value for this "slack" (the targeted maximum distance * between the value of "head" and the first unmatched node, and * similarly for "tail") is an empirical matter. We have found * that using very small constants in the range of 1-3 work best * over a range of platforms. Larger values introduce increasing * costs of cache misses and risks of long traversal chains, while * smaller values increase CAS contention and overhead. * * Dual queues with slack differ from plain M&S dual queues by * virtue of only sometimes updating head or tail pointers when * matching, appending, or even traversing nodes; in order to * maintain a targeted slack. The idea of "sometimes" may be * operationalized in several ways. The simplest is to use a * per-operation counter incremented on each traversal step, and * to try (via CAS) to update the associated queue pointer * whenever the count exceeds a threshold. Another, that requires * more overhead, is to use random number generators to update * with a given probability per traversal step. * * In any strategy along these lines, because CASes updating * fields may fail, the actual slack may exceed targeted * slack. However, they may be retried at any time to maintain * targets. Even when using very small slack values, this * approach works well for dual queues because it allows all * operations up to the point of matching or appending an item * (hence potentially allowing progress by another thread) to be * read-only, thus not introducing any further contention. As * described below, we implement this by performing slack * maintenance retries only after these points. * * As an accompaniment to such techniques, traversal overhead can * be further reduced without increasing contention of head * pointer updates: Threads may sometimes shortcut the "next" link * path from the current "head" node to be closer to the currently * known first unmatched node, and similarly for tail. Again, this * may be triggered with using thresholds or randomization. * * These ideas must be further extended to avoid unbounded amounts * of costly-to-reclaim garbage caused by the sequential "next" * links of nodes starting at old forgotten head nodes: As first * described in detail by Boehm * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC * delays noticing that any arbitrarily old node has become * garbage, all newer dead nodes will also be unreclaimed. * (Similar issues arise in non-GC environments.) To cope with * this in our implementation, upon CASing to advance the head * pointer, we set the "next" link of the previous head to point * only to itself; thus limiting the length of connected dead lists. * (We also take similar care to wipe out possibly garbage * retaining values held in other Node fields.) However, doing so * adds some further complexity to traversal: If any "next" * pointer links to itself, it indicates that the current thread * has lagged behind a head-update, and so the traversal must * continue from the "head". Traversals trying to find the * current tail starting from "tail" may also encounter * self-links, in which case they also continue at "head". * * It is tempting in slack-based scheme to not even use CAS for * updates (similarly to Ladan-Mozes & Shavit). However, this * cannot be done for head updates under the above link-forgetting * mechanics because an update may leave head at a detached node. * And while direct writes are possible for tail updates, they * increase the risk of long retraversals, and hence long garbage * chains, which can be much more costly than is worthwhile * considering that the cost difference of performing a CAS vs * write is smaller when they are not triggered on each operation * (especially considering that writes and CASes equally require * additional GC bookkeeping ("write barriers") that are sometimes * more costly than the writes themselves because of contention). * * Removal of interior nodes (due to timed out or interrupted * waits, or calls to remove(x) or Iterator.remove) can use a * scheme roughly similar to that described in Scherer, Lea, and * Scott's SynchronousQueue. Given a predecessor, we can unsplice * any node except the (actual) tail of the queue. To avoid * build-up of cancelled trailing nodes, upon a request to remove * a trailing node, it is placed in field "cleanMe" to be * unspliced upon the next call to unsplice any other node. * Situations needing such mechanics are not common but do occur * in practice; for example when an unbounded series of short * timed calls to poll repeatedly time out but never otherwise * fall off the list because of an untimed call to take at the * front of the queue. Note that maintaining field cleanMe does * not otherwise much impact garbage retention even if never * cleared by some other call because the held node will * eventually either directly or indirectly lead to a self-link * once off the list. * * *** Overview of implementation *** * * We use a threshold-based approach to updates, with a slack * threshold of two -- that is, we update head/tail when the * current pointer appears to be two or more steps away from the * first/last node. The slack value is hard-wired: a path greater * than one is naturally implemented by checking equality of * traversal pointers except when the list has only one element, * in which case we keep slack threshold at one. Avoiding tracking * explicit counts across method calls slightly simplifies an * already-messy implementation. Using randomization would * probably work better if there were a low-quality dirt-cheap * per-thread one available, but even ThreadLocalRandom is too * heavy for these purposes. * * With such a small slack threshold value, it is rarely * worthwhile to augment this with path short-circuiting; i.e., * unsplicing nodes between head and the first unmatched node, or * similarly for tail, rather than advancing head or tail * proper. However, it is used (in awaitMatch) immediately before * a waiting thread starts to block, as a final bit of helping at * a point when contention with others is extremely unlikely * (since if other threads that could release it are operating, * then the current thread wouldn't be blocking). * * We allow both the head and tail fields to be null before any * nodes are enqueued; initializing upon first append. This * simplifies some other logic, as well as providing more * efficient explicit control paths instead of letting JVMs insert * implicit NullPointerExceptions when they are null. While not * currently fully implemented, we also leave open the possibility * of re-nulling these fields when empty (which is complicated to * arrange, for little benefit.) * * All enqueue/dequeue operations are handled by the single method * "xfer" with parameters indicating whether to act as some form * of offer, put, poll, take, or transfer (each possibly with * timeout). The relative complexity of using one monolithic * method outweighs the code bulk and maintenance problems of * using separate methods for each case. * * Operation consists of up to three phases. The first is * implemented within method xfer, the second in tryAppend, and * the third in method awaitMatch. * * 1. Try to match an existing node * * Starting at head, skip already-matched nodes until finding * an unmatched node of opposite mode, if one exists, in which * case matching it and returning, also if necessary updating * head to one past the matched node (or the node itself if the * list has no other unmatched nodes). If the CAS misses, then * a loop retries advancing head by two steps until either * success or the slack is at most two. By requiring that each * attempt advances head by two (if applicable), we ensure that * the slack does not grow without bound. Traversals also check * if the initial head is now off-list, in which case they * start at the new head. * * If no candidates are found and the call was untimed * poll/offer, (argument "how" is NOW) return. * * 2. Try to append a new node (method tryAppend) * * Starting at current tail pointer, find the actual last node * and try to append a new node (or if head was null, establish * the first node). Nodes can be appended only if their * predecessors are either already matched or are of the same * mode. If we detect otherwise, then a new node with opposite * mode must have been appended during traversal, so we must * restart at phase 1. The traversal and update steps are * otherwise similar to phase 1: Retrying upon CAS misses and * checking for staleness. In particular, if a self-link is * encountered, then we can safely jump to a node on the list * by continuing the traversal at current head. * * On successful append, if the call was ASYNC, return. * * 3. Await match or cancellation (method awaitMatch) * * Wait for another thread to match node; instead cancelling if * the current thread was interrupted or the wait timed out. On * multiprocessors, we use front-of-queue spinning: If a node * appears to be the first unmatched node in the queue, it * spins a bit before blocking. In either case, before blocking * it tries to unsplice any nodes between the current "head" * and the first unmatched node. * * Front-of-queue spinning vastly improves performance of * heavily contended queues. And so long as it is relatively * brief and "quiet", spinning does not much impact performance * of less-contended queues. During spins threads check their * interrupt status and generate a thread-local random number * to decide to occasionally perform a Thread.yield. While * yield has underdefined specs, we assume that might it help, * and will not hurt in limiting impact of spinning on busy * systems. We also use smaller (1/2) spins for nodes that are * not known to be front but whose predecessors have not * blocked -- these "chained" spins avoid artifacts of * front-of-queue rules which otherwise lead to alternating * nodes spinning vs blocking. Further, front threads that * represent phase changes (from data to request node or vice * versa) compared to their predecessors receive additional * chained spins, reflecting longer paths typically required to * unblock threads during phase changes. */ /** True if on multiprocessor */ private static final boolean MP = Runtime.getRuntime().availableProcessors() > 1; /** * The number of times to spin (with randomly interspersed calls * to Thread.yield) on multiprocessor before blocking when a node * is apparently the first waiter in the queue. See above for * explanation. Must be a power of two. The value is empirically * derived -- it works pretty well across a variety of processors, * numbers of CPUs, and OSes. */ private static final int FRONT_SPINS = 1 << 7; /** * The number of times to spin before blocking when a node is * preceded by another node that is apparently spinning. Also * serves as an increment to FRONT_SPINS on phase changes, and as * base average frequency for yielding during spins. Must be a * power of two. */ private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; /** * Queue nodes. Uses Object, not E, for items to allow forgetting * them after use. Relies heavily on Unsafe mechanics to minimize * unnecessary ordering constraints: Writes that intrinsically * precede or follow CASes use simple relaxed forms. Other * cleanups use releasing/lazy writes. */ static final class Node { final boolean isData; // false if this is a request node volatile Object item; // initially non-null if isData; CASed to match volatile Node next; volatile Thread waiter; // null until waiting // CAS methods for fields final boolean casNext(Node cmp, Node val) { return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); } final boolean casItem(Object cmp, Object val) { assert cmp == null || cmp.getClass() != Node.class; return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); } /** * Creates a new node. Uses relaxed write because item can only * be seen if followed by CAS. */ Node(Object item, boolean isData) { UNSAFE.putObject(this, itemOffset, item); // relaxed write this.isData = isData; } /** * Links node to itself to avoid garbage retention. Called * only after CASing head field, so uses relaxed write. */ final void forgetNext() { UNSAFE.putObject(this, nextOffset, this); } /** * Sets item to self (using a releasing/lazy write) and waiter * to null, to avoid garbage retention after extracting or * cancelling. */ final void forgetContents() { UNSAFE.putOrderedObject(this, itemOffset, this); UNSAFE.putOrderedObject(this, waiterOffset, null); } /** * Returns true if this node has been matched, including the * case of artificial matches due to cancellation. */ final boolean isMatched() { Object x = item; return (x == this) || ((x == null) == isData); } /** * Returns true if this is an unmatched request node. */ final boolean isUnmatchedRequest() { return !isData && item == null; } /** * Returns true if a node with the given mode cannot be * appended to this node because this node is unmatched and * has opposite data mode. */ final boolean cannotPrecede(boolean haveData) { boolean d = isData; Object x; return d != haveData && (x = item) != this && (x != null) == d; } /** * Tries to artificially match a data node -- used by remove. */ final boolean tryMatchData() { assert isData; Object x = item; if (x != null && x != this && casItem(x, null)) { LockSupport.unpark(waiter); return true; } return false; } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE = getUnsafe(); private static final long nextOffset = objectFieldOffset(UNSAFE, "next", Node.class); private static final long itemOffset = objectFieldOffset(UNSAFE, "item", Node.class); private static final long waiterOffset = objectFieldOffset(UNSAFE, "waiter", Node.class); private static final long serialVersionUID = -3375979862319811754L; } /** head of the queue; null until first enqueue */ transient volatile Node head; /** predecessor of dangling unspliceable node */ private transient volatile Node cleanMe; // decl here reduces contention /** tail of the queue; null until first append */ private transient volatile Node tail; // CAS methods for fields private boolean casTail(Node cmp, Node val) { return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); } private boolean casHead(Node cmp, Node val) { return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); } private boolean casCleanMe(Node cmp, Node val) { return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); } /* * Possible values for "how" argument in xfer method. */ private static final int NOW = 0; // for untimed poll, tryTransfer private static final int ASYNC = 1; // for offer, put, add private static final int SYNC = 2; // for transfer, take private static final int TIMED = 3; // for timed poll, tryTransfer @SuppressWarnings("unchecked") static <E> E cast(Object item) { assert item == null || item.getClass() != Node.class; return (E) item; } /** * Implements all queuing methods. See above for explanation. * * @param e the item or null for take * @param haveData true if this is a put, else a take * @param how NOW, ASYNC, SYNC, or TIMED * @param nanos timeout in nanosecs, used only if mode is TIMED * @return an item if matched, else e * @throws NullPointerException if haveData mode but e is null */ private E xfer(E e, boolean haveData, int how, long nanos) { if (haveData && (e == null)) throw new NullPointerException(); Node s = null; // the node to append, if needed retry: for (;;) { // restart on append race for (Node h = head, p = h; p != null;) { // find & match first node boolean isData = p.isData; Object item = p.item; if (item != p && (item != null) == isData) { // unmatched if (isData == haveData) // can't match break; if (p.casItem(item, e)) { // match for (Node q = p; q != h;) { Node n = q.next; // update head by 2 if (n != null) // unless singleton q = n; if (head == h && casHead(h, q)) { h.forgetNext(); break; } // advance and retry if ((h = head) == null || (q = h.next) == null || !q.isMatched()) break; // unless slack < 2 } LockSupport.unpark(p.waiter); return this.<E>cast(item); } } Node n = p.next; p = (p != n) ? n : (h = head); // Use head if p offlist } if (how != NOW) { // No matches available if (s == null) s = new Node(e, haveData); Node pred = tryAppend(s, haveData); if (pred == null) continue retry; // lost race vs opposite mode if (how != ASYNC) return awaitMatch(s, pred, e, (how == TIMED), nanos); } return e; // not waiting } } /** * Tries to append node s as tail. * * @param s the node to append * @param haveData true if appending in data mode * @return null on failure due to losing race with append in * different mode, else s's predecessor, or s itself if no * predecessor */ private Node tryAppend(Node s, boolean haveData) { for (Node t = tail, p = t;;) { // move p to last node and append Node n, u; // temps for reads of next & tail if (p == null && (p = head) == null) { if (casHead(null, s)) return s; // initialize } else if (p.cannotPrecede(haveData)) return null; // lost race vs opposite mode else if ((n = p.next) != null) // not last; keep traversing p = p != t && t != (u = tail) ? (t = u) : // stale tail (p != n) ? n : null; // restart if off list else if (!p.casNext(null, s)) p = p.next; // re-read on CAS failure else { if (p != t) { // update if slack now >= 2 while ((tail != t || !casTail(t, s)) && (t = tail) != null && (s = t.next) != null && // advance and retry (s = s.next) != null && s != t); } return p; } } } /** * Spins/yields/blocks until node s is matched or caller gives up. * * @param s the waiting node * @param pred the predecessor of s, or s itself if it has no * predecessor, or null if unknown (the null case does not occur * in any current calls but may in possible future extensions) * @param e the comparison value for checking match * @param timed if true, wait only until timeout elapses * @param nanos timeout in nanosecs, used only if timed is true * @return matched item, or e if unmatched on interrupt or timeout */ private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { long lastTime = timed ? System.nanoTime() : 0L; Thread w = Thread.currentThread(); int spins = -1; // initialized after first item and cancel checks ThreadLocalRandom randomYields = null; // bound if needed for (;;) { Object item = s.item; if (item != e) { // matched assert item != s; s.forgetContents(); // avoid garbage return this.<E>cast(item); } if ((w.isInterrupted() || (timed && nanos <= 0)) && s.casItem(e, s)) { // cancel unsplice(pred, s); return e; } if (spins < 0) { // establish spins at/near front if ((spins = spinsFor(pred, s.isData)) > 0) randomYields = ThreadLocalRandom.current(); } else if (spins > 0) { // spin if (--spins == 0) shortenHeadPath(); // reduce slack before blocking else if (randomYields.nextInt(CHAINED_SPINS) == 0) Thread.yield(); // occasionally yield } else if (s.waiter == null) { s.waiter = w; // request unpark then recheck } else if (timed) { long now = System.nanoTime(); if ((nanos -= now - lastTime) > 0) LockSupport.parkNanos(this, nanos); lastTime = now; } else { LockSupport.park(this); s.waiter = null; spins = -1; // spin if front upon wakeup } } } /** * Returns spin/yield value for a node with given predecessor and * data mode. See above for explanation. */ private static int spinsFor(Node pred, boolean haveData) { if (MP && pred != null) { if (pred.isData != haveData) // phase change return FRONT_SPINS + CHAINED_SPINS; if (pred.isMatched()) // probably at front return FRONT_SPINS; if (pred.waiter == null) // pred apparently spinning return CHAINED_SPINS; } return 0; } /** * Tries (once) to unsplice nodes between head and first unmatched * or trailing node; failing on contention. */ private void shortenHeadPath() { Node h, hn, p, q; if ((p = h = head) != null && h.isMatched() && (q = hn = h.next) != null) { Node n; while ((n = q.next) != q) { if (n == null || !q.isMatched()) { if (hn != q && h.next == hn) h.casNext(hn, q); break; } p = q; q = n; } } } /* -------------- Traversal methods -------------- */ /** * Returns the successor of p, or the head node if p.next has been * linked to self, which will only be true if traversing with a * stale pointer that is now off the list. */ final Node succ(Node p) { Node next = p.next; return (p == next) ? head : next; } /** * Returns the first unmatched node of the given mode, or null if * none. Used by methods isEmpty, hasWaitingConsumer. */ private Node firstOfMode(boolean isData) { for (Node p = head; p != null; p = succ(p)) { if (!p.isMatched()) return (p.isData == isData) ? p : null; } return null; } /** * Returns the item in the first unmatched node with isData; or * null if none. Used by peek. */ private E firstDataItem() { for (Node p = head; p != null; p = succ(p)) { Object item = p.item; if (p.isData) { if (item != null && item != p) return this.<E>cast(item); } else if (item == null) return null; } return null; } /** * Traverses and counts unmatched nodes of the given mode. * Used by methods size and getWaitingConsumerCount. */ private int countOfMode(boolean data) { int count = 0; for (Node p = head; p != null; ) { if (!p.isMatched()) { if (p.isData != data) return 0; if (++count == Integer.MAX_VALUE) // saturated break; } Node n = p.next; if (n != p) p = n; else { count = 0; p = head; } } return count; } final class Itr implements Iterator<E> { private Node nextNode; // next node to return item for private E nextItem; // the corresponding item private Node lastRet; // last returned node, to support remove private Node lastPred; // predecessor to unlink lastRet /** * Moves to next node after prev, or first node if prev null. */ private void advance(Node prev) { lastPred = lastRet; lastRet = prev; for (Node p = (prev == null) ? head : succ(prev); p != null; p = succ(p)) { Object item = p.item; if (p.isData) { if (item != null && item != p) { nextItem = LinkedTransferQueue.this.<E>cast(item); nextNode = p; return; } } else if (item == null) break; } nextNode = null; } Itr() { advance(null); } public final boolean hasNext() { return nextNode != null; } public final E next() { Node p = nextNode; if (p == null) throw new NoSuchElementException(); E e = nextItem; advance(p); return e; } public final void remove() { Node p = lastRet; if (p == null) throw new IllegalStateException(); findAndRemoveDataNode(lastPred, p); } } /* -------------- Removal methods -------------- */ /** * Unsplices (now or later) the given deleted/cancelled node with * the given predecessor. * * @param pred predecessor of node to be unspliced * @param s the node to be unspliced */ private void unsplice(Node pred, Node s) { s.forgetContents(); // clear unneeded fields /* * At any given time, exactly one node on list cannot be * unlinked -- the last inserted node. To accommodate this, if * we cannot unlink s, we save its predecessor as "cleanMe", * processing the previously saved version first. Because only * one node in the list can have a null next, at least one of * node s or the node previously saved can always be * processed, so this always terminates. */ if (pred != null && pred != s) { while (pred.next == s) { Node oldpred = (cleanMe == null) ? null : reclean(); Node n = s.next; if (n != null) { if (n != s) pred.casNext(s, n); break; } if (oldpred == pred || // Already saved ((oldpred == null || oldpred.next == s) && casCleanMe(oldpred, pred))) { break; } } } } /** * Tries to unsplice the deleted/cancelled node held in cleanMe * that was previously uncleanable because it was at tail. * * @return current cleanMe node (or null) */ private Node reclean() { /* * cleanMe is, or at one time was, predecessor of a cancelled * node s that was the tail so could not be unspliced. If it * is no longer the tail, try to unsplice if necessary and * make cleanMe slot available. This differs from similar * code in unsplice() because we must check that pred still * points to a matched node that can be unspliced -- if not, * we can (must) clear cleanMe without unsplicing. This can * loop only due to contention. */ Node pred; while ((pred = cleanMe) != null) { Node s = pred.next; Node n; if (s == null || s == pred || !s.isMatched()) casCleanMe(pred, null); // already gone else if ((n = s.next) != null) { if (n != s) pred.casNext(s, n); casCleanMe(pred, null); } else break; } return pred; } /** * Main implementation of Iterator.remove(). Finds * and unsplices the given data node. * * @param possiblePred possible predecessor of s * @param s the node to remove */ final void findAndRemoveDataNode(Node possiblePred, Node s) { assert s.isData; if (s.tryMatchData()) { if (possiblePred != null && possiblePred.next == s) unsplice(possiblePred, s); // was actual predecessor else { for (Node pred = null, p = head; p != null; ) { if (p == s) { unsplice(pred, p); break; } if (p.isUnmatchedRequest()) break; pred = p; if ((p = p.next) == pred) { // stale pred = null; p = head; } } } } } /** * Main implementation of remove(Object) */ private boolean findAndRemove(Object e) { if (e != null) { for (Node pred = null, p = head; p != null; ) { Object item = p.item; if (p.isData) { if (item != null && item != p && e.equals(item) && p.tryMatchData()) { unsplice(pred, p); return true; } } else if (item == null) break; pred = p; if ((p = p.next) == pred) { // stale pred = null; p = head; } } } return false; } /** * Creates an initially empty {@code LinkedTransferQueue}. */ public LinkedTransferQueue() { } /** * Creates a {@code LinkedTransferQueue} * initially containing the elements of the given collection, * added in traversal order of the collection's iterator. * * @param c the collection of elements to initially contain * @throws NullPointerException if the specified collection or any * of its elements are null */ public LinkedTransferQueue(Collection<? extends E> c) { this(); addAll(c); } /** * Inserts the specified element at the tail of this queue. * As the queue is unbounded, this method will never block. * * @throws NullPointerException if the specified element is null */ public void put(E e) { xfer(e, true, ASYNC, 0); } /** * Inserts the specified element at the tail of this queue. * As the queue is unbounded, this method will never block or * return {@code false}. * * @return {@code true} (as specified by * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) * @throws NullPointerException if the specified element is null */ public boolean offer(E e, long timeout, TimeUnit unit) { xfer(e, true, ASYNC, 0); return true; } /** * Inserts the specified element at the tail of this queue. * As the queue is unbounded, this method will never return {@code false}. * * @return {@code true} (as specified by * {@link BlockingQueue#offer(Object) BlockingQueue.offer}) * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { xfer(e, true, ASYNC, 0); return true; } /** * Inserts the specified element at the tail of this queue. * As the queue is unbounded, this method will never throw * {@link IllegalStateException} or return {@code false}. * * @return {@code true} (as specified by {@link Collection#add}) * @throws NullPointerException if the specified element is null */ public boolean add(E e) { xfer(e, true, ASYNC, 0); return true; } /** * Transfers the element to a waiting consumer immediately, if possible. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * otherwise returning {@code false} without enqueuing the element. * * @throws NullPointerException if the specified element is null */ public boolean tryTransfer(E e) { return xfer(e, true, NOW, 0) == null; } /** * Transfers the element to a consumer, waiting if necessary to do so. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else inserts the specified element at the tail of this queue * and waits until the element is received by a consumer. * * @throws NullPointerException if the specified element is null */ public void transfer(E e) throws InterruptedException { if (xfer(e, true, SYNC, 0) != null) { Thread.interrupted(); // failure possible only due to interrupt throw new InterruptedException(); } } /** * Transfers the element to a consumer if it is possible to do so * before the timeout elapses. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else inserts the specified element at the tail of this queue * and waits until the element is received by a consumer, * returning {@code false} if the specified wait time elapses * before the element can be transferred. * * @throws NullPointerException if the specified element is null */ public boolean tryTransfer(E e, long timeout, TimeUnit unit) throws InterruptedException { if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) return true; if (!Thread.interrupted()) return false; throw new InterruptedException(); } public E take() throws InterruptedException { E e = xfer(null, false, SYNC, 0); if (e != null) return e; Thread.interrupted(); throw new InterruptedException(); } public E poll(long timeout, TimeUnit unit) throws InterruptedException { E e = xfer(null, false, TIMED, unit.toNanos(timeout)); if (e != null || !Thread.interrupted()) return e; throw new InterruptedException(); } public E poll() { return xfer(null, false, NOW, 0); } /** * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c) { if (c == null) throw new NullPointerException(); if (c == this) throw new IllegalArgumentException(); int n = 0; E e; while ( (e = poll()) != null) { c.add(e); ++n; } return n; } /** * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c, int maxElements) { if (c == null) throw new NullPointerException(); if (c == this) throw new IllegalArgumentException(); int n = 0; E e; while (n < maxElements && (e = poll()) != null) { c.add(e); ++n; } return n; } /** * Returns an iterator over the elements in this queue in proper * sequence, from head to tail. * * <p>The returned iterator is a "weakly consistent" iterator that * will never throw * {@link ConcurrentModificationException ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed * to) reflect any modifications subsequent to construction. * * @return an iterator over the elements in this queue in proper sequence */ public Iterator<E> iterator() { return new Itr(); } public E peek() { return firstDataItem(); } /** * Returns {@code true} if this queue contains no elements. * * @return {@code true} if this queue contains no elements */ public boolean isEmpty() { return firstOfMode(true) == null; } public boolean hasWaitingConsumer() { return firstOfMode(false) != null; } /** * Returns the number of elements in this queue. If this queue * contains more than {@code Integer.MAX_VALUE} elements, returns * {@code Integer.MAX_VALUE}. * * <p>Beware that, unlike in most collections, this method is * <em>NOT</em> a constant-time operation. Because of the * asynchronous nature of these queues, determining the current * number of elements requires an O(n) traversal. * * @return the number of elements in this queue */ public int size() { return countOfMode(true); } public int getWaitingConsumerCount() { return countOfMode(false); } /** * Removes a single instance of the specified element from this queue, * if it is present. More formally, removes an element {@code e} such * that {@code o.equals(e)}, if this queue contains one or more such * elements. * Returns {@code true} if this queue contained the specified element * (or equivalently, if this queue changed as a result of the call). * * @param o element to be removed from this queue, if present * @return {@code true} if this queue changed as a result of the call */ public boolean remove(Object o) { return findAndRemove(o); } /** * Always returns {@code Integer.MAX_VALUE} because a * {@code LinkedTransferQueue} is not capacity constrained. * * @return {@code Integer.MAX_VALUE} (as specified by * {@link BlockingQueue#remainingCapacity()}) */ public int remainingCapacity() { return Integer.MAX_VALUE; } /** * Saves the state to a stream (that is, serializes it). * * @serialData All of the elements (each an {@code E}) in * the proper order, followed by a null * @param s the stream */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { s.defaultWriteObject(); for (E e : this) s.writeObject(e); // Use trailing null as sentinel s.writeObject(null); } /** * Reconstitutes the Queue instance from a stream (that is, * deserializes it). * * @param s the stream */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { s.defaultReadObject(); for (;;) { @SuppressWarnings("unchecked") E item = (E) s.readObject(); if (item == null) break; else offer(item); } } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE = getUnsafe(); private static final long headOffset = objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); private static final long tailOffset = objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); private static final long cleanMeOffset = objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); static long objectFieldOffset(sun.misc.Unsafe UNSAFE, String field, Class<?> klazz) { try { return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); } catch (NoSuchFieldException e) { // Convert Exception to corresponding Error NoSuchFieldError error = new NoSuchFieldError(field); error.initCause(e); throw error; } } /** * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. * Replace with a simple call to Unsafe.getUnsafe when integrating * into a jdk. * * @return a sun.misc.Unsafe */ static sun.misc.Unsafe getUnsafe() { try { return sun.misc.Unsafe.getUnsafe(); } catch (SecurityException se) { try { return java.security.AccessController.doPrivileged (new java.security .PrivilegedExceptionAction<sun.misc.Unsafe>() { public sun.misc.Unsafe run() throws Exception { java.lang.reflect.Field f = sun.misc .Unsafe.class.getDeclaredField("theUnsafe"); f.setAccessible(true); return (sun.misc.Unsafe) f.get(null); }}); } catch (java.security.PrivilegedActionException e) { throw new RuntimeException("Could not initialize intrinsics", e.getCause()); } } } }
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