An unbounded TransferQueue based on linked nodes.
/*
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*
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*
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* General Public License Version 2 only ("GPL") or the Common Development
* and Distribution License("CDDL") (collectively, the "License"). You
* may not use this file except in compliance with the License. You can obtain
* a copy of the License at https://glassfish.dev.java.net/public/CDDL+GPL.html
* or glassfish/bootstrap/legal/LICENSE.txt. See the License for the specific
* language governing permissions and limitations under the License.
*
* When distributing the software, include this License Header Notice in each
* file and include the License file at glassfish/bootstrap/legal/LICENSE.txt.
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* as provided by Sun in the GPL Version 2 section of the License file that
* accompanied this code. If applicable, add the following below the License
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* recipient has the option to distribute your version of this file under
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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 com.google.code.yanf4j.util;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.util.concurrent.locks.LockSupport;
/**
* An unbounded <tt>TransferQueue</tt> 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 <tt>size</tt>
* 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.
*
* @author Doug Lea
* @author The Netty Project (netty-dev@lists.jboss.org)
* @author Trustin Lee (tlee@redhat.com)
*
* @param <E> the type of elements held in this collection
*
*/
public class LinkedTransferQueue<E> extends AbstractQueue<E> implements BlockingQueue<E> {
/*
* This class extends the approach used in FIFO-mode
* SynchronousQueues. See the internal documentation, as well as
* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
* Lea & Scott
* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf)
*
* The main extension is to provide different Wait modes for the
* main "xfer" method that puts or takes items. These don't
* impact the basic dual-queue logic, but instead control whether
* or how threads block upon insertion of request or data nodes
* into the dual queue. It also uses slightly different
* conventions for tracking whether nodes are off-list or
* cancelled.
*/
// Wait modes for xfer method
private static final int NOWAIT = 0;
private static final int TIMEOUT = 1;
private static final int WAIT = 2;
/** The number of CPUs, for spin control */
private static final int NCPUS = Runtime.getRuntime().availableProcessors();
/**
* The number of times to spin before blocking in timed waits.
* The value is empirically derived -- it works well across a
* variety of processors and OSes. Empirically, the best value
* seems not to vary with number of CPUs (beyond 2) so is just
* a constant.
*/
private static final int maxTimedSpins = NCPUS < 2? 0 : 32;
/**
* The number of times to spin before blocking in untimed waits.
* This is greater than timed value because untimed waits spin
* faster since they don't need to check times on each spin.
*/
private static final int maxUntimedSpins = maxTimedSpins * 16;
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices.
*/
private static final long spinForTimeoutThreshold = 1000L;
/**
* Node class for LinkedTransferQueue. Opportunistically
* subclasses from AtomicReference to represent item. Uses Object,
* not E, to allow setting item to "this" after use, to avoid
* garbage retention. Similarly, setting the next field to this is
* used as sentinel that node is off list.
*/
private static final class QNode extends AtomicReference<Object> {
private static final long serialVersionUID = 5925596372370723938L;
transient volatile QNode next;
transient volatile Thread waiter; // to control park/unpark
final boolean isData;
QNode(Object item, boolean isData) {
super(item);
this.isData = isData;
}
private static final AtomicReferenceFieldUpdater<QNode, QNode> nextUpdater;
static {
AtomicReferenceFieldUpdater<QNode, QNode> tmp = null;
try {
tmp = AtomicReferenceFieldUpdater.newUpdater(
QNode.class, QNode.class, "next");
// Test if AtomicReferenceFieldUpdater is really working.
QNode testNode = new QNode(null, false);
tmp.set(testNode, testNode);
if (testNode.next != testNode) {
// Not set as expected - fall back to the safe mode.
throw new Exception();
}
} catch (Throwable t) {
// Running in a restricted environment with a security manager.
tmp = null;
}
nextUpdater = tmp;
}
boolean casNext(QNode cmp, QNode val) {
if (nextUpdater != null) {
return nextUpdater.compareAndSet(this, cmp, val);
} else {
return alternativeCasNext(cmp, val);
}
}
private synchronized boolean alternativeCasNext(QNode cmp, QNode val) {
if (this.next == cmp) {
this.next = val;
return true;
} else {
return false;
}
}
}
/**
* Padded version of AtomicReference used for head, tail and
* cleanMe, to alleviate contention across threads CASing one vs
* the other.
*/
private static final class PaddedAtomicReference<T> extends AtomicReference<T> {
private static final long serialVersionUID = 4684288940772921317L;
// enough padding for 64bytes with 4byte refs
@SuppressWarnings("unused")
Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
PaddedAtomicReference(T r) { super(r); }
}
/** head of the queue */
private final PaddedAtomicReference<QNode> head;
/** tail of the queue */
private final PaddedAtomicReference<QNode> tail;
/**
* Reference to a cancelled node that might not yet have been
* unlinked from queue because it was the last inserted node
* when it cancelled.
*/
private final PaddedAtomicReference<QNode> cleanMe;
/**
* Tries to cas nh as new head; if successful, unlink
* old head's next node to avoid garbage retention.
*/
private boolean advanceHead(QNode h, QNode nh) {
if (h == this.head.get() && this.head.compareAndSet(h, nh)) {
h.next = h; // forget old next
return true;
}
return false;
}
/**
* Puts or takes an item. Used for most queue operations (except
* poll() and tryTransfer()). See the similar code in
* SynchronousQueue for detailed explanation.
* @param e the item or if null, signifies that this is a take
* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
* @return an item, or null on failure
*/
private Object xfer(Object e, int mode, long nanos) {
boolean isData = e != null;
QNode s = null;
final PaddedAtomicReference<QNode> head = this.head;
final PaddedAtomicReference<QNode> tail = this.tail;
for (;;) {
QNode t = tail.get();
QNode h = head.get();
if (t != null && (t == h || t.isData == isData)) {
if (s == null) {
s = new QNode(e, isData);
}
QNode last = t.next;
if (last != null) {
if (t == tail.get()) {
tail.compareAndSet(t, last);
}
}
else if (t.casNext(null, s)) {
tail.compareAndSet(t, s);
return awaitFulfill(t, s, e, mode, nanos);
}
}
else if (h != null) {
QNode first = h.next;
if (t == tail.get() && first != null &&
advanceHead(h, first)) {
Object x = first.get();
if (x != first && first.compareAndSet(x, e)) {
LockSupport.unpark(first.waiter);
return isData? e : x;
}
}
}
}
}
/**
* Version of xfer for poll() and tryTransfer, which
* simplifies control paths both here and in xfer
*/
private Object fulfill(Object e) {
boolean isData = e != null;
final PaddedAtomicReference<QNode> head = this.head;
final PaddedAtomicReference<QNode> tail = this.tail;
for (;;) {
QNode t = tail.get();
QNode h = head.get();
if (t != null && (t == h || t.isData == isData)) {
QNode last = t.next;
if (t == tail.get()) {
if (last != null) {
tail.compareAndSet(t, last);
} else {
return null;
}
}
}
else if (h != null) {
QNode first = h.next;
if (t == tail.get() &&
first != null &&
advanceHead(h, first)) {
Object x = first.get();
if (x != first && first.compareAndSet(x, e)) {
LockSupport.unpark(first.waiter);
return isData? e : x;
}
}
}
}
}
/**
* Spins/blocks until node s is fulfilled or caller gives up,
* depending on wait mode.
*
* @param pred the predecessor of waiting node
* @param s the waiting node
* @param e the comparison value for checking match
* @param mode mode
* @param nanos timeout value
* @return matched item, or s if cancelled
*/
private Object awaitFulfill(QNode pred, QNode s, Object e,
int mode, long nanos) {
if (mode == NOWAIT) {
return null;
}
long lastTime = mode == TIMEOUT? System.nanoTime() : 0;
Thread w = Thread.currentThread();
int spins = -1; // set to desired spin count below
for (;;) {
if (w.isInterrupted()) {
s.compareAndSet(e, s);
}
Object x = s.get();
if (x != e) { // Node was matched or cancelled
advanceHead(pred, s); // unlink if head
if (x == s) { // was cancelled
clean(pred, s);
return null;
}
else if (x != null) {
s.set(s); // avoid garbage retention
return x;
} else {
return e;
}
}
if (mode == TIMEOUT) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
s.compareAndSet(e, s); // try to cancel
continue;
}
}
if (spins < 0) {
QNode h = this.head.get(); // only spin if at head
spins = h != null && h.next == s ?
(mode == TIMEOUT?
maxTimedSpins : maxUntimedSpins) : 0;
}
if (spins > 0) {
--spins;
} else if (s.waiter == null) {
s.waiter = w;
} else if (mode != TIMEOUT) {
// LockSupport.park(this);
LockSupport.park(); // allows run on java5
s.waiter = null;
spins = -1;
}
else if (nanos > spinForTimeoutThreshold) {
// LockSupport.parkNanos(this, nanos);
LockSupport.parkNanos(nanos);
s.waiter = null;
spins = -1;
}
}
}
/**
* Returns validated tail for use in cleaning methods
*/
private QNode getValidatedTail() {
for (;;) {
QNode h = this.head.get();
QNode first = h.next;
if (first != null && first.next == first) { // help advance
advanceHead(h, first);
continue;
}
QNode t = this.tail.get();
QNode last = t.next;
if (t == this.tail.get()) {
if (last != null) {
this.tail.compareAndSet(t, last); // help advance
} else {
return t;
}
}
}
}
/**
* Gets rid of cancelled node s with original predecessor pred.
* @param pred predecessor of cancelled node
* @param s the cancelled node
*/
void clean(QNode pred, QNode s) {
Thread w = s.waiter;
if (w != null) { // Wake up thread
s.waiter = null;
if (w != Thread.currentThread()) {
LockSupport.unpark(w);
}
}
/*
* At any given time, exactly one node on list cannot be
* deleted -- the last inserted node. To accommodate this, if
* we cannot delete s, we save its predecessor as "cleanMe",
* processing the previously saved version first. At least one
* of node s or the node previously saved can always be
* processed, so this always terminates.
*/
while (pred.next == s) {
QNode oldpred = reclean(); // First, help get rid of cleanMe
QNode t = getValidatedTail();
if (s != t) { // If not tail, try to unsplice
QNode sn = s.next; // s.next == s means s already off list
if (sn == s || pred.casNext(s, sn)) {
break;
}
}
else if (oldpred == pred || // Already saved
oldpred == null && this.cleanMe.compareAndSet(null, pred)) {
break; // Postpone cleaning
}
}
}
/**
* Tries to unsplice the cancelled node held in cleanMe that was
* previously uncleanable because it was at tail.
* @return current cleanMe node (or null)
*/
private QNode reclean() {
/*
* cleanMe is, or at one time was, predecessor of cancelled
* node s that was the tail so could not be unspliced. If s
* is no longer the tail, try to unsplice if necessary and
* make cleanMe slot available. This differs from similar
* code in clean() because we must check that pred still
* points to a cancelled node that must be unspliced -- if
* not, we can (must) clear cleanMe without unsplicing.
* This can loop only due to contention on casNext or
* clearing cleanMe.
*/
QNode pred;
while ((pred = this.cleanMe.get()) != null) {
QNode t = getValidatedTail();
QNode s = pred.next;
if (s != t) {
QNode sn;
if (s == null || s == pred || s.get() != s ||
(sn = s.next) == s || pred.casNext(s, sn)) {
this.cleanMe.compareAndSet(pred, null);
}
} else {
break;
}
}
return pred;
}
@SuppressWarnings("unchecked")
E cast(Object e) {
return (E)e;
}
/**
* Creates an initially empty <tt>LinkedTransferQueue</tt>.
*/
public LinkedTransferQueue() {
QNode dummy = new QNode(null, false);
this.head = new PaddedAtomicReference<QNode>(dummy);
this.tail = new PaddedAtomicReference<QNode>(dummy);
this.cleanMe = new PaddedAtomicReference<QNode>(null);
}
/**
* Creates a <tt>LinkedTransferQueue</tt>
* 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);
}
public void put(E e) throws InterruptedException {
if (e == null) {
throw new NullPointerException();
}
if (Thread.interrupted()) {
throw new InterruptedException();
}
xfer(e, NOWAIT, 0);
}
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) {
throw new NullPointerException();
}
if (Thread.interrupted()) {
throw new InterruptedException();
}
xfer(e, NOWAIT, 0);
return true;
}
public boolean offer(E e) {
if (e == null) {
throw new NullPointerException();
}
xfer(e, NOWAIT, 0);
return true;
}
public void transfer(E e) throws InterruptedException {
if (e == null) {
throw new NullPointerException();
}
if (xfer(e, WAIT, 0) == null) {
Thread.interrupted();
throw new InterruptedException();
}
}
public boolean tryTransfer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) {
throw new NullPointerException();
}
if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null) {
return true;
}
if (!Thread.interrupted()) {
return false;
}
throw new InterruptedException();
}
public boolean tryTransfer(E e) {
if (e == null) {
throw new NullPointerException();
}
return fulfill(e) != null;
}
public E take() throws InterruptedException {
Object e = xfer(null, WAIT, 0);
if (e != null) {
return cast(e);
}
Thread.interrupted();
throw new InterruptedException();
}
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
if (e != null || !Thread.interrupted()) {
return cast(e);
}
throw new InterruptedException();
}
public E poll() {
return cast(fulfill(null));
}
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;
}
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;
}
// Traversal-based methods
/**
* Return head after performing any outstanding helping steps
*/
QNode traversalHead() {
for (;;) {
QNode t = this.tail.get();
QNode h = this.head.get();
if (h != null && t != null) {
QNode last = t.next;
QNode first = h.next;
if (t == this.tail.get()) {
if (last != null) {
this.tail.compareAndSet(t, last);
} else if (first != null) {
Object x = first.get();
if (x == first) {
advanceHead(h, first);
} else {
return h;
}
} else {
return h;
}
}
}
}
}
@Override
public Iterator<E> iterator() {
return new Itr();
}
/**
* Iterators. Basic strategy is to traverse list, treating
* non-data (i.e., request) nodes as terminating list.
* Once a valid data node is found, the item is cached
* so that the next call to next() will return it even
* if subsequently removed.
*/
class Itr implements Iterator<E> {
QNode nextNode; // Next node to return next
QNode currentNode; // last returned node, for remove()
QNode prevNode; // predecessor of last returned node
E nextItem; // Cache of next item, once commited to in next
Itr() {
this.nextNode = traversalHead();
advance();
}
E advance() {
this.prevNode = this.currentNode;
this.currentNode = this.nextNode;
E x = this.nextItem;
QNode p = this.nextNode.next;
for (;;) {
if (p == null || !p.isData) {
this.nextNode = null;
this.nextItem = null;
return x;
}
Object item = p.get();
if (item != p && item != null) {
this.nextNode = p;
this.nextItem = cast(item);
return x;
}
this.prevNode = p;
p = p.next;
}
}
public boolean hasNext() {
return this.nextNode != null;
}
public E next() {
if (this.nextNode == null) {
throw new NoSuchElementException();
}
return advance();
}
public void remove() {
QNode p = this.currentNode;
QNode prev = this.prevNode;
if (prev == null || p == null) {
throw new IllegalStateException();
}
Object x = p.get();
if (x != null && x != p && p.compareAndSet(x, p)) {
clean(prev, p);
}
}
}
public E peek() {
for (;;) {
QNode h = traversalHead();
QNode p = h.next;
if (p == null) {
return null;
}
Object x = p.get();
if (p != x) {
if (!p.isData) {
return null;
}
if (x != null) {
return cast(x);
}
}
}
}
@Override
public boolean isEmpty() {
for (;;) {
QNode h = traversalHead();
QNode p = h.next;
if (p == null) {
return true;
}
Object x = p.get();
if (p != x) {
if (!p.isData) {
return true;
}
if (x != null) {
return false;
}
}
}
}
public boolean hasWaitingConsumer() {
for (;;) {
QNode h = traversalHead();
QNode p = h.next;
if (p == null) {
return false;
}
Object x = p.get();
if (p != x) {
return !p.isData;
}
}
}
/**
* Returns the number of elements in this queue. If this queue
* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* <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
*/
@Override
public int size() {
int count = 0;
QNode h = traversalHead();
for (QNode p = h.next; p != null && p.isData; p = p.next) {
Object x = p.get();
if (x != null && x != p) {
if (++count == Integer.MAX_VALUE) {
break;
}
}
}
return count;
}
public int getWaitingConsumerCount() {
int count = 0;
QNode h = traversalHead();
for (QNode p = h.next; p != null && !p.isData; p = p.next) {
if (p.get() == null) {
if (++count == Integer.MAX_VALUE) {
break;
}
}
}
return count;
}
public int remainingCapacity() {
return Integer.MAX_VALUE;
}
}
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