/*
* 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 head of the queue is that
* element that has been on the queue the longest time for some
* producer. The tail of the queue is that element that has
* been on the queue the shortest time for some producer.
*
* Beware that, unlike in most collections, the {@code size}
* method is NOT a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current number
* of elements requires a traversal of the elements.
*
*
This class and its iterator implement all of the
* optional methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
*
Memory consistency effects: As with other concurrent
* collections, actions in a thread prior to placing an object into a
* {@code LinkedTransferQueue}
* happen-before
* actions subsequent to the access or removal of that element from
* the {@code LinkedTransferQueue} in another thread.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @since 1.7
* @author Doug Lea
* @param the type of elements held in this collection
*/
public class LinkedTransferQueue extends AbstractQueue
implements TransferQueue, 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) {
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 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() {
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 */
private transient volatile Node head;
/** predecessor of dangling unspliceable node */
private transient volatile Node cleanMe; // decl here to reduce 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. Beware that
* the order of assigned numerical values matters.
*/
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 TIMEOUT = 3; // for timed poll, tryTransfer
/**
* 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 TIMEOUT
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
* @return an item if matched, else e
* @throws NullPointerException if haveData mode but e is null
*/
private Object xfer(Object 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
Thread w = p.waiter;
while (p != h) { // update head
Node n = p.next; // by 2 unless singleton
if (n != null)
p = n;
if (head == h && casHead(h, p)) {
h.forgetNext();
break;
} // advance and retry
if ((h = head) == null ||
(p = h.next) == null || !p.isMatched())
break; // unless slack < 2
}
LockSupport.unpark(w);
return item;
}
}
Node n = p.next;
p = (p != n) ? n : (h = head); // Use head if p offlist
}
if (how >= ASYNC) { // 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 >= SYNC)
return awaitMatch(s, pred, e, how, 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 how either SYNC or TIMEOUT
* @param nanos timeout value
* @return matched item, or e if unmatched on interrupt or timeout
*/
private Object awaitMatch(Node s, Node pred, Object e,
int how, long nanos) {
long lastTime = (how == TIMEOUT) ? 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
s.forgetContents(); // avoid garbage
return item;
}
if ((w.isInterrupted() || (how == TIMEOUT && 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
}
else if (how == TIMEOUT) {
long now = System.nanoTime();
if ((nanos -= now - lastTime) > 0)
LockSupport.parkNanos(this, nanos);
lastTime = now;
}
else {
LockSupport.park(this);
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 first unmatched node of the given mode, or null if
* none. Used by methods isEmpty, hasWaitingConsumer.
*/
private Node firstOfMode(boolean data) {
for (Node p = head; p != null; ) {
if (!p.isMatched())
return (p.isData == data) ? p : null;
Node n = p.next;
p = (n != p) ? n : head;
}
return null;
}
/**
* Returns the item in the first unmatched node with isData; or
* null if none. Used by peek.
*/
private Object firstDataItem() {
for (Node p = head; p != null; ) {
boolean isData = p.isData;
Object item = p.item;
if (item != p && (item != null) == isData)
return isData ? item : null;
Node n = p.next;
p = (n != p) ? n : head;
}
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 {
private Node nextNode; // next node to return item for
private Object nextItem; // the corresponding item
private Node lastRet; // last returned node, to support remove
/**
* Moves to next node after prev, or first node if prev null.
*/
private void advance(Node prev) {
lastRet = prev;
Node p;
if (prev == null || (p = prev.next) == prev)
p = head;
while (p != null) {
Object item = p.item;
if (p.isData) {
if (item != null && item != p) {
nextItem = item;
nextNode = p;
return;
}
}
else if (item == null)
break;
Node n = p.next;
p = (n != p) ? n : head;
}
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();
Object e = nextItem;
advance(p);
return (E) e;
}
public final void remove() {
Node p = lastRet;
if (p == null) throw new IllegalStateException();
lastRet = null;
findAndRemoveNode(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 && casCleanMe(null, pred)))
break; // Postpone cleaning
}
}
}
/**
* 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(). Find
* and unsplice the given node.
*/
final void findAndRemoveNode(Node s) {
if (s.tryMatchData()) {
Node pred = null;
Node p = head;
while (p != null) {
if (p == s) {
unsplice(pred, p);
break;
}
if (!p.isData && !p.isMatched())
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) {
Node pred = null;
Node p = head;
while (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) {
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 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.
*
* 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.
*
*
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.
*
*
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, TIMEOUT, unit.toNanos(timeout)) == null)
return true;
if (!Thread.interrupted())
return false;
throw new InterruptedException();
}
public E take() throws InterruptedException {
Object e = xfer(null, false, SYNC, 0);
if (e != null)
return (E)e;
Thread.interrupted();
throw new InterruptedException();
}
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
if (e != null || !Thread.interrupted())
return (E)e;
throw new InterruptedException();
}
public E poll() {
return (E)xfer(null, false, NOW, 0);
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection 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 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.
*
*
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 iterator() {
return new Itr();
}
public E peek() {
return (E) 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}.
*
* Beware that, unlike in most collections, this method is
* NOT 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;
}
}
private static sun.misc.Unsafe getUnsafe() {
try {
return sun.misc.Unsafe.getUnsafe();
} catch (SecurityException se) {
try {
return java.security.AccessController.doPrivileged
(new java.security
.PrivilegedExceptionAction() {
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());
}
}
}
}