jdk-sandbox: comparison jdk/src/share/classes/java/util/concurrent/ConcurrentLinkedQueue.java

equal deleted inserted replaced

-:c5fbc05d7347
+:f01ef94a63e7
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
-* Written by Doug Lea with assistance from members of JCP JSR-166
+* Written by Doug Lea and Martin Buchholz with assistance from members of
-* Expert Group and released to the public domain, as explained at
+* JCP JSR-166 Expert Group and released to the public domain, as explained
-* http://creativecommons.org/licenses/publicdomain
+* at http://creativecommons.org/licenses/publicdomain
 */
 package java.util.concurrent;
 import java.util.AbstractQueue;
 * queue the shortest time. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 * A {@code ConcurrentLinkedQueue} is an appropriate choice when
 * many threads will share access to a common collection.
-* This queue does not permit {@code null} elements.
+* Like most other concurrent collection implementations, this class
+* does not permit the use of {@code null} elements.
 *
 * <p>This implementation employs an efficient &quot;wait-free&quot;
 * algorithm based on one described in <a
 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 * Algorithms</a> by Maged M. Michael and Michael L. Scott.
 *
+* <p>Iterators are <i>weakly consistent</i>, returning elements
+* reflecting the state of the queue at some point at or since the
+* creation of the iterator.  They do <em>not</em> throw {@link
+* ConcurrentModificationException}, and may proceed concurrently with
+* other operations.  Elements contained in the queue since the creation
+* of the iterator will be returned exactly once.
+*
 * <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
+* <p>This class and its iterator implement all of the <em>optional</em>
-* <em>optional</em> methods of the {@link Collection} and {@link
+* methods of the {@link Queue} and {@link Iterator} interfaces.
-* Iterator} interfaces.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedQueue}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * linking a Node that has just been dequeued to itself.  Such a
 * self-link implicitly means to advance to head.
 *
 * Both head and tail are permitted to lag.  In fact, failing to
 * update them every time one could is a significant optimization
-* (fewer CASes). This is controlled by local "hops" variables
+* (fewer CASes). As with LinkedTransferQueue (see the internal
-* that only trigger helping-CASes after experiencing multiple
+* documentation for that class), we use a slack threshold of two;
-* lags.
+* that is, we update head/tail when the current pointer appears
+* to be two or more steps away from the first/last node.
 *
 * Since head and tail are updated concurrently and independently,
 * it is possible for tail to lag behind head (why not)?
 *
 * CASing a Node's item reference to null atomically removes the
 * to be successfully removed by two concurrent operations.  The
 * method remove(Object) also lazily unlinks deleted Nodes, but
 * this is merely an optimization.
 *
 * When constructing a Node (before enqueuing it) we avoid paying
-* for a volatile write to item by using lazySet instead of a
+* for a volatile write to item by using Unsafe.putObject instead
-* normal write.  This allows the cost of enqueue to be
+* of a normal write.  This allows the cost of enqueue to be
 * "one-and-a-half" CASes.
 *
 * Both head and tail may or may not point to a Node with a
 * non-null item.  If the queue is empty, all items must of course
 * be null.  Upon creation, both head and tail refer to a dummy
 * CAS, so they never regress, although again this is merely an
 * optimization.
 */
 private static class Node<E> {
-private volatile E item;
+volatile E item;
-private volatile Node<E> next;
+volatile Node<E> next;
+/**
+* Constructs a new node.  Uses relaxed write because item can
+* only be seen after publication via casNext.
+*/
 Node(E item) {
-// Piggyback on imminent casNext()
+UNSAFE.putObject(this, itemOffset, item);
-lazySetItem(item);
-}
-E getItem() {
-return item;
 }
 boolean casItem(E cmp, E val) {
 return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
 }
-void setItem(E val) {
-item = val;
-}
-void lazySetItem(E val) {
-UNSAFE.putOrderedObject(this, itemOffset, val);
-}
 void lazySetNext(Node<E> val) {
 UNSAFE.putOrderedObject(this, nextOffset, val);
-}
-Node<E> getNext() {
-return next;
 }
 boolean casNext(Node<E> cmp, Node<E> val) {
 return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
 }
 * Non-invariants:
 * - head.item may or may not be null.
 * - it is permitted for tail to lag behind head, that is, for tail
 *   to not be reachable from head!
 */
-private transient volatile Node<E> head = new Node<E>(null);
+private transient volatile Node<E> head;
 /**
 * A node from which the last node on list (that is, the unique
 * node with node.next == null) can be reached in O(1) time.
 * Invariants:
 * - tail.item may or may not be null.
 * - it is permitted for tail to lag behind head, that is, for tail
 *   to not be reachable from head!
 * - tail.next may or may not be self-pointing to tail.
 */
-private transient volatile Node<E> tail = head;
+private transient volatile Node<E> tail;
 /**
 * Creates a {@code ConcurrentLinkedQueue} that is initially empty.
 */
-public ConcurrentLinkedQueue() {}
+public ConcurrentLinkedQueue() {
+head = tail = new Node<E>(null);
+}
 /**
 * Creates a {@code ConcurrentLinkedQueue}
 * 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 ConcurrentLinkedQueue(Collection<? extends E> c) {
-for (E e : c)
+Node<E> h = null, t = null;
-add(e);
+for (E e : c) {
+checkNotNull(e);
+Node<E> newNode = new Node<E>(e);
+if (h == null)
+h = t = newNode;
+else {
+t.lazySetNext(newNode);
+t = newNode;
+}
+}
+if (h == null)
+h = t = new Node<E>(null);
+head = h;
+tail = t;
 }
 // Have to override just to update the javadoc
 /**
 * @throws NullPointerException if the specified element is null
 */
 public boolean add(E e) {
 return offer(e);
 }
-/**
-* We don't bother to update head or tail pointers if fewer than
-* HOPS links from "true" location.  We assume that volatile
-* writes are significantly more expensive than volatile reads.
-*/
-private static final int HOPS = 1;
 /**
 * Try to CAS head to p. If successful, repoint old head to itself
 * as sentinel for succ(), below.
 */
 * 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<E> succ(Node<E> p) {
-Node<E> next = p.getNext();
+Node<E> next = p.next;
 return (p == next) ? head : next;
 }
 /**
 * Inserts the specified element at the tail of this queue.
 *
 * @return {@code true} (as specified by {@link Queue#offer})
 * @throws NullPointerException if the specified element is null
 */
 public boolean offer(E e) {
-if (e == null) throw new NullPointerException();
+checkNotNull(e);
-Node<E> n = new Node<E>(e);
+final Node<E> newNode = new Node<E>(e);
-retry:
+for (Node<E> t = tail, p = t;;) {
+Node<E> q = p.next;
+if (q == null) {
+// p is last node
+if (p.casNext(null, newNode)) {
+// Successful CAS is the linearization point
+// for e to become an element of this queue,
+// and for newNode to become "live".
+if (p != t) // hop two nodes at a time
+casTail(t, newNode);  // Failure is OK.
+return true;
+}
+// Lost CAS race to another thread; re-read next
+}
+else if (p == q)
+// We have fallen off list.  If tail is unchanged, it
+// will also be off-list, in which case we need to
+// jump to head, from which all live nodes are always
+// reachable.  Else the new tail is a better bet.
+p = (t != (t = tail)) ? t : head;
+else
+// Check for tail updates after two hops.
+p = (p != t && t != (t = tail)) ? t : q;
+}
+}
+public E poll() {
+restartFromHead:
 for (;;) {
-Node<E> t = tail;
+for (Node<E> h = head, p = h, q;;) {
-Node<E> p = t;
+E item = p.item;
-for (int hops = 0; ; hops++) {
-Node<E> next = succ(p);
+if (item != null && p.casItem(item, null)) {
-if (next != null) {
+// Successful CAS is the linearization point
-if (hops > HOPS && t != tail)
+// for item to be removed from this queue.
-continue retry;
+if (p != h) // hop two nodes at a time
-p = next;
+updateHead(h, ((q = p.next) != null) ? q : p);
-} else if (p.casNext(null, n)) {
+return item;
-if (hops >= HOPS)
-casTail(t, n);  // Failure is OK.
-return true;
-} else {
-p = succ(p);
 }
-}
+else if ((q = p.next) == null) {
-}
+updateHead(h, p);
-}
+return null;
-public E poll() {
-Node<E> h = head;
-Node<E> p = h;
-for (int hops = 0; ; hops++) {
-E item = p.getItem();
-if (item != null && p.casItem(item, null)) {
-if (hops >= HOPS) {
-Node<E> q = p.getNext();
-updateHead(h, (q != null) ? q : p);
 }
-return item;
+else if (p == q)
-}
+continue restartFromHead;
-Node<E> next = succ(p);
+else
-if (next == null) {
+p = q;
-updateHead(h, p);
+}
-break;
+}
-}
-p = next;
-}
-return null;
 }
 public E peek() {
-Node<E> h = head;
+restartFromHead:
-Node<E> p = h;
-E item;
 for (;;) {
-item = p.getItem();
+for (Node<E> h = head, p = h, q;;) {
-if (item != null)
+E item = p.item;
-break;
+if (item != null || (q = p.next) == null) {
-Node<E> next = succ(p);
+updateHead(h, p);
-if (next == null) {
+return item;
-break;
+}
-}
+else if (p == q)
-p = next;
+continue restartFromHead;
-}
+else
-updateHead(h, p);
+p = q;
-return item;
+}
+}
 }
 /**
 * Returns the first live (non-deleted) node on list, or null if none.
 * This is yet another variant of poll/peek; here returning the
 * first(), but that would cost an extra volatile read of item,
 * and the need to add a retry loop to deal with the possibility
 * of losing a race to a concurrent poll().
 */
 Node<E> first() {
-Node<E> h = head;
+restartFromHead:
-Node<E> p = h;
-Node<E> result;
 for (;;) {
-E item = p.getItem();
+for (Node<E> h = head, p = h, q;;) {
-if (item != null) {
+boolean hasItem = (p.item != null);
-result = p;
+if (hasItem || (q = p.next) == null) {
-break;
+updateHead(h, p);
-}
+return hasItem ? p : null;
-Node<E> next = succ(p);
+}
-if (next == null) {
+else if (p == q)
-result = null;
+continue restartFromHead;
-break;
+else
-}
+p = q;
-p = next;
+}
 }
-updateHead(h, p);
-return result;
 }
 /**
 * Returns {@code true} if this queue contains no elements.
 *
 *
 * <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.
+* Additionally, if elements are added or removed during execution
+* of this method, the returned result may be inaccurate.  Thus,
+* this method is typically not very useful in concurrent
+* applications.
 *
 * @return the number of elements in this queue
 */
 public int size() {
 int count = 0;
-for (Node<E> p = first(); p != null; p = succ(p)) {
+for (Node<E> p = first(); p != null; p = succ(p))
-if (p.getItem() != null) {
+if (p.item != null)
-// Collections.size() spec says to max out
+// Collection.size() spec says to max out
 if (++count == Integer.MAX_VALUE)
 break;
-}
-}
 return count;
 }
 /**
 * Returns {@code true} if this queue contains the specified element.
 * @return {@code true} if this queue contains the specified element
 */
 public boolean contains(Object o) {
 if (o == null) return false;
 for (Node<E> p = first(); p != null; p = succ(p)) {
-E item = p.getItem();
+E item = p.item;
-if (item != null &&
+if (item != null && o.equals(item))
-o.equals(item))
 return true;
 }
 return false;
 }
 */
 public boolean remove(Object o) {
 if (o == null) return false;
 Node<E> pred = null;
 for (Node<E> p = first(); p != null; p = succ(p)) {
-E item = p.getItem();
+E item = p.item;
 if (item != null &&
 o.equals(item) &&
 p.casItem(item, null)) {
 Node<E> next = succ(p);
 if (pred != null && next != null)
 }
 return false;
 }
 /**
+* Appends all of the elements in the specified collection to the end of
+* this queue, in the order that they are returned by the specified
+* collection's iterator.  Attempts to {@code addAll} of a queue to
+* itself result in {@code IllegalArgumentException}.
+*
+* @param c the elements to be inserted into this queue
+* @return {@code true} if this queue changed as a result of the call
+* @throws NullPointerException if the specified collection or any
+*         of its elements are null
+* @throws IllegalArgumentException if the collection is this queue
+*/
+public boolean addAll(Collection<? extends E> c) {
+if (c == this)
+// As historically specified in AbstractQueue#addAll
+throw new IllegalArgumentException();
+// Copy c into a private chain of Nodes
+Node<E> beginningOfTheEnd = null, last = null;
+for (E e : c) {
+checkNotNull(e);
+Node<E> newNode = new Node<E>(e);
+if (beginningOfTheEnd == null)
+beginningOfTheEnd = last = newNode;
+else {
+last.lazySetNext(newNode);
+last = newNode;
+}
+}
+if (beginningOfTheEnd == null)
+return false;
+// Atomically append the chain at the tail of this collection
+for (Node<E> t = tail, p = t;;) {
+Node<E> q = p.next;
+if (q == null) {
+// p is last node
+if (p.casNext(null, beginningOfTheEnd)) {
+// Successful CAS is the linearization point
+// for all elements to be added to this queue.
+if (!casTail(t, last)) {
+// Try a little harder to update tail,
+// since we may be adding many elements.
+t = tail;
+if (last.next == null)
+casTail(t, last);
+}
+return true;
+}
+// Lost CAS race to another thread; re-read next
+}
+else if (p == q)
+// We have fallen off list.  If tail is unchanged, it
+// will also be off-list, in which case we need to
+// jump to head, from which all live nodes are always
+// reachable.  Else the new tail is a better bet.
+p = (t != (t = tail)) ? t : head;
+else
+// Check for tail updates after two hops.
+p = (p != t && t != (t = tail)) ? t : q;
+}
+}
+/**
 * Returns an array containing all of the elements in this queue, in
 * proper sequence.
 *
 * <p>The returned array will be "safe" in that no references to it are
 * maintained by this queue.  (In other words, this method must allocate
 */
 public Object[] toArray() {
 // Use ArrayList to deal with resizing.
 ArrayList<E> al = new ArrayList<E>();
 for (Node<E> p = first(); p != null; p = succ(p)) {
-E item = p.getItem();
+E item = p.item;
 if (item != null)
 al.add(item);
 }
 return al.toArray();
 }
 public <T> T[] toArray(T[] a) {
 // try to use sent-in array
 int k = 0;
 Node<E> p;
 for (p = first(); p != null && k < a.length; p = succ(p)) {
-E item = p.getItem();
+E item = p.item;
 if (item != null)
 a[k++] = (T)item;
 }
 if (p == null) {
 if (k < a.length)
 }
 // If won't fit, use ArrayList version
 ArrayList<E> al = new ArrayList<E>();
 for (Node<E> q = first(); q != null; q = succ(q)) {
-E item = q.getItem();
+E item = q.item;
 if (item != null)
 al.add(item);
 }
 return al.toArray(a);
 }
 /**
 * Returns an iterator over the elements in this queue in proper sequence.
-* The returned iterator is a "weakly consistent" iterator that
+* The elements will be returned in order from first (head) to last (tail).
+*
+* <p>The returned {@code Iterator} is a "weakly consistent" iterator that
 * will never throw {@link java.util.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.
 if (p == null) {
 nextNode = null;
 nextItem = null;
 return x;
 }
-E item = p.getItem();
+E item = p.item;
 if (item != null) {
 nextNode = p;
 nextItem = item;
 return x;
 } else {
 public void remove() {
 Node<E> l = lastRet;
 if (l == null) throw new IllegalStateException();
 // rely on a future traversal to relink.
-l.setItem(null);
+l.item = null;
 lastRet = null;
 }
 }
 /**
-* Save the state to a stream (that is, serialize it).
+* 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
 */
 // Write out any hidden stuff
 s.defaultWriteObject();
 // Write out all elements in the proper order.
 for (Node<E> p = first(); p != null; p = succ(p)) {
-Object item = p.getItem();
+Object item = p.item;
 if (item != null)
 s.writeObject(item);
 }
 // Use trailing null as sentinel
 s.writeObject(null);
 }
 /**
-* Reconstitute the Queue instance from a stream (that is,
+* Reconstitutes the instance from a stream (that is, deserializes it).
-* deserialize it).
 * @param s the stream
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
-// Read in capacity, and any hidden stuff
 s.defaultReadObject();
-head = new Node<E>(null);
-tail = head;
+// Read in elements until trailing null sentinel found
-// Read in all elements and place in queue
+Node<E> h = null, t = null;
-for (;;) {
+Object item;
+while ((item = s.readObject()) != null) {
 @SuppressWarnings("unchecked")
-E item = (E)s.readObject();
+Node<E> newNode = new Node<E>((E) item);
-if (item == null)
+if (h == null)
-break;
+h = t = newNode;
-else
+else {
-offer(item);
+t.lazySetNext(newNode);
-}
+t = newNode;
+}
+}
+if (h == null)
+h = t = new Node<E>(null);
+head = h;
+tail = t;
+}
+/**
+* Throws NullPointerException if argument is null.
+*
+* @param v the element
+*/
+private static void checkNotNull(Object v) {
+if (v == null)
+throw new NullPointerException();
 }
 // Unsafe mechanics
 private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
 return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
 }
 private boolean casHead(Node<E> cmp, Node<E> val) {
 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
-}
-private void lazySetHead(Node<E> val) {
-UNSAFE.putOrderedObject(this, headOffset, val);
 }
 static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
 String field, Class<?> klazz) {
 try {

changeset 6672	f01ef94a63e7
parent 5506	202f599c92aa
child 7518	0282db800fe1