| /* |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Sun designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Sun in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| * CA 95054 USA or visit www.sun.com if you need additional information or |
| * have any questions. |
| */ |
| |
| /* |
| * 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 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/licenses/publicdomain |
| */ |
| |
| package java.util.concurrent; |
| import java.util.*; |
| import java.util.concurrent.atomic.*; |
| |
| |
| /** |
| * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. |
| * This queue orders elements FIFO (first-in-first-out). |
| * The <em>head</em> of the queue is that element that has been on the |
| * queue the longest time. |
| * The <em>tail</em> of the queue is that element that has been on the |
| * 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 <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when |
| * many threads will share access to a common collection. |
| * This queue does not permit <tt>null</tt> elements. |
| * |
| * <p>This implementation employs an efficient "wait-free" |
| * 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>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 ConcurrentLinkedQueue} |
| * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| * actions subsequent to the access or removal of that element from |
| * the {@code ConcurrentLinkedQueue} in another thread. |
| * |
| * <p>This class is a member of the |
| * <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
| * Java Collections Framework</a>. |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| * @param <E> the type of elements held in this collection |
| * |
| */ |
| public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> |
| implements Queue<E>, java.io.Serializable { |
| private static final long serialVersionUID = 196745693267521676L; |
| |
| /* |
| * This is a straight adaptation of Michael & Scott algorithm. |
| * For explanation, read the paper. The only (minor) algorithmic |
| * difference is that this version supports lazy deletion of |
| * internal nodes (method remove(Object)) -- remove CAS'es item |
| * fields to null. The normal queue operations unlink but then |
| * pass over nodes with null item fields. Similarly, iteration |
| * methods ignore those with nulls. |
| * |
| * Also note that like most non-blocking algorithms in this |
| * package, this implementation relies on the fact that in garbage |
| * collected systems, there is no possibility of ABA problems due |
| * to recycled nodes, so there is no need to use "counted |
| * pointers" or related techniques seen in versions used in |
| * non-GC'ed settings. |
| */ |
| |
| private static class Node<E> { |
| private volatile E item; |
| private volatile Node<E> next; |
| |
| private static final |
| AtomicReferenceFieldUpdater<Node, Node> |
| nextUpdater = |
| AtomicReferenceFieldUpdater.newUpdater |
| (Node.class, Node.class, "next"); |
| private static final |
| AtomicReferenceFieldUpdater<Node, Object> |
| itemUpdater = |
| AtomicReferenceFieldUpdater.newUpdater |
| (Node.class, Object.class, "item"); |
| |
| Node(E x) { item = x; } |
| |
| Node(E x, Node<E> n) { item = x; next = n; } |
| |
| E getItem() { |
| return item; |
| } |
| |
| boolean casItem(E cmp, E val) { |
| return itemUpdater.compareAndSet(this, cmp, val); |
| } |
| |
| void setItem(E val) { |
| itemUpdater.set(this, val); |
| } |
| |
| Node<E> getNext() { |
| return next; |
| } |
| |
| boolean casNext(Node<E> cmp, Node<E> val) { |
| return nextUpdater.compareAndSet(this, cmp, val); |
| } |
| |
| void setNext(Node<E> val) { |
| nextUpdater.set(this, val); |
| } |
| |
| } |
| |
| private static final |
| AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node> |
| tailUpdater = |
| AtomicReferenceFieldUpdater.newUpdater |
| (ConcurrentLinkedQueue.class, Node.class, "tail"); |
| private static final |
| AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node> |
| headUpdater = |
| AtomicReferenceFieldUpdater.newUpdater |
| (ConcurrentLinkedQueue.class, Node.class, "head"); |
| |
| private boolean casTail(Node<E> cmp, Node<E> val) { |
| return tailUpdater.compareAndSet(this, cmp, val); |
| } |
| |
| private boolean casHead(Node<E> cmp, Node<E> val) { |
| return headUpdater.compareAndSet(this, cmp, val); |
| } |
| |
| |
| /** |
| * Pointer to header node, initialized to a dummy node. The first |
| * actual node is at head.getNext(). |
| */ |
| private transient volatile Node<E> head = new Node<E>(null, null); |
| |
| /** Pointer to last node on list **/ |
| private transient volatile Node<E> tail = head; |
| |
| |
| /** |
| * Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty. |
| */ |
| public ConcurrentLinkedQueue() {} |
| |
| /** |
| * Creates a <tt>ConcurrentLinkedQueue</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 ConcurrentLinkedQueue(Collection<? extends E> c) { |
| for (Iterator<? extends E> it = c.iterator(); it.hasNext();) |
| add(it.next()); |
| } |
| |
| // Have to override just to update the javadoc |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * |
| * @return <tt>true</tt> (as specified by {@link Collection#add}) |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean add(E e) { |
| return offer(e); |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue. |
| * |
| * @return <tt>true</tt> (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(); |
| Node<E> n = new Node<E>(e, null); |
| for (;;) { |
| Node<E> t = tail; |
| Node<E> s = t.getNext(); |
| if (t == tail) { |
| if (s == null) { |
| if (t.casNext(s, n)) { |
| casTail(t, n); |
| return true; |
| } |
| } else { |
| casTail(t, s); |
| } |
| } |
| } |
| } |
| |
| public E poll() { |
| for (;;) { |
| Node<E> h = head; |
| Node<E> t = tail; |
| Node<E> first = h.getNext(); |
| if (h == head) { |
| if (h == t) { |
| if (first == null) |
| return null; |
| else |
| casTail(t, first); |
| } else if (casHead(h, first)) { |
| E item = first.getItem(); |
| if (item != null) { |
| first.setItem(null); |
| return item; |
| } |
| // else skip over deleted item, continue loop, |
| } |
| } |
| } |
| } |
| |
| public E peek() { // same as poll except don't remove item |
| for (;;) { |
| Node<E> h = head; |
| Node<E> t = tail; |
| Node<E> first = h.getNext(); |
| if (h == head) { |
| if (h == t) { |
| if (first == null) |
| return null; |
| else |
| casTail(t, first); |
| } else { |
| E item = first.getItem(); |
| if (item != null) |
| return item; |
| else // remove deleted node and continue |
| casHead(h, first); |
| } |
| } |
| } |
| } |
| |
| /** |
| * Returns the first actual (non-header) node on list. This is yet |
| * another variant of poll/peek; here returning out the first |
| * node, not element (so we cannot collapse with peek() without |
| * introducing race.) |
| */ |
| Node<E> first() { |
| for (;;) { |
| Node<E> h = head; |
| Node<E> t = tail; |
| Node<E> first = h.getNext(); |
| if (h == head) { |
| if (h == t) { |
| if (first == null) |
| return null; |
| else |
| casTail(t, first); |
| } else { |
| if (first.getItem() != null) |
| return first; |
| else // remove deleted node and continue |
| casHead(h, first); |
| } |
| } |
| } |
| } |
| |
| |
| /** |
| * Returns <tt>true</tt> if this queue contains no elements. |
| * |
| * @return <tt>true</tt> if this queue contains no elements |
| */ |
| public boolean isEmpty() { |
| return first() == null; |
| } |
| |
| /** |
| * 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 |
| */ |
| public int size() { |
| int count = 0; |
| for (Node<E> p = first(); p != null; p = p.getNext()) { |
| if (p.getItem() != null) { |
| // Collections.size() spec says to max out |
| if (++count == Integer.MAX_VALUE) |
| break; |
| } |
| } |
| return count; |
| } |
| |
| /** |
| * Returns <tt>true</tt> if this queue contains the specified element. |
| * More formally, returns <tt>true</tt> if and only if this queue contains |
| * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. |
| * |
| * @param o object to be checked for containment in this queue |
| * @return <tt>true</tt> 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 = p.getNext()) { |
| E item = p.getItem(); |
| if (item != null && |
| o.equals(item)) |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Removes a single instance of the specified element from this queue, |
| * if it is present. More formally, removes an element <tt>e</tt> such |
| * that <tt>o.equals(e)</tt>, if this queue contains one or more such |
| * elements. |
| * Returns <tt>true</tt> 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 <tt>true</tt> if this queue changed as a result of the call |
| */ |
| public boolean remove(Object o) { |
| if (o == null) return false; |
| for (Node<E> p = first(); p != null; p = p.getNext()) { |
| E item = p.getItem(); |
| if (item != null && |
| o.equals(item) && |
| p.casItem(item, null)) |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * 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 |
| * a new array). The caller is thus free to modify the returned array. |
| * |
| * <p>This method acts as bridge between array-based and collection-based |
| * APIs. |
| * |
| * @return an array containing all of the elements in this queue |
| */ |
| public Object[] toArray() { |
| // Use ArrayList to deal with resizing. |
| ArrayList<E> al = new ArrayList<E>(); |
| for (Node<E> p = first(); p != null; p = p.getNext()) { |
| E item = p.getItem(); |
| if (item != null) |
| al.add(item); |
| } |
| return al.toArray(); |
| } |
| |
| /** |
| * Returns an array containing all of the elements in this queue, in |
| * proper sequence; the runtime type of the returned array is that of |
| * the specified array. If the queue fits in the specified array, it |
| * is returned therein. Otherwise, a new array is allocated with the |
| * runtime type of the specified array and the size of this queue. |
| * |
| * <p>If this queue fits in the specified array with room to spare |
| * (i.e., the array has more elements than this queue), the element in |
| * the array immediately following the end of the queue is set to |
| * <tt>null</tt>. |
| * |
| * <p>Like the {@link #toArray()} method, this method acts as bridge between |
| * array-based and collection-based APIs. Further, this method allows |
| * precise control over the runtime type of the output array, and may, |
| * under certain circumstances, be used to save allocation costs. |
| * |
| * <p>Suppose <tt>x</tt> is a queue known to contain only strings. |
| * The following code can be used to dump the queue into a newly |
| * allocated array of <tt>String</tt>: |
| * |
| * <pre> |
| * String[] y = x.toArray(new String[0]);</pre> |
| * |
| * Note that <tt>toArray(new Object[0])</tt> is identical in function to |
| * <tt>toArray()</tt>. |
| * |
| * @param a the array into which the elements of the queue are to |
| * be stored, if it is big enough; otherwise, a new array of the |
| * same runtime type is allocated for this purpose |
| * @return an array containing all of the elements in this queue |
| * @throws ArrayStoreException if the runtime type of the specified array |
| * is not a supertype of the runtime type of every element in |
| * this queue |
| * @throws NullPointerException if the specified array is null |
| */ |
| 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 = p.getNext()) { |
| E item = p.getItem(); |
| if (item != null) |
| a[k++] = (T)item; |
| } |
| if (p == null) { |
| if (k < a.length) |
| a[k] = null; |
| return a; |
| } |
| |
| // If won't fit, use ArrayList version |
| ArrayList<E> al = new ArrayList<E>(); |
| for (Node<E> q = first(); q != null; q = q.getNext()) { |
| E item = q.getItem(); |
| 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 |
| * will never throw {@link 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(); |
| } |
| |
| private class Itr implements Iterator<E> { |
| /** |
| * Next node to return item for. |
| */ |
| private Node<E> nextNode; |
| |
| /** |
| * nextItem holds on to item fields because once we claim |
| * that an element exists in hasNext(), we must return it in |
| * the following next() call even if it was in the process of |
| * being removed when hasNext() was called. |
| */ |
| private E nextItem; |
| |
| /** |
| * Node of the last returned item, to support remove. |
| */ |
| private Node<E> lastRet; |
| |
| Itr() { |
| advance(); |
| } |
| |
| /** |
| * Moves to next valid node and returns item to return for |
| * next(), or null if no such. |
| */ |
| private E advance() { |
| lastRet = nextNode; |
| E x = nextItem; |
| |
| Node<E> p = (nextNode == null)? first() : nextNode.getNext(); |
| for (;;) { |
| if (p == null) { |
| nextNode = null; |
| nextItem = null; |
| return x; |
| } |
| E item = p.getItem(); |
| if (item != null) { |
| nextNode = p; |
| nextItem = item; |
| return x; |
| } else // skip over nulls |
| p = p.getNext(); |
| } |
| } |
| |
| public boolean hasNext() { |
| return nextNode != null; |
| } |
| |
| public E next() { |
| if (nextNode == null) throw new NoSuchElementException(); |
| return advance(); |
| } |
| |
| public void remove() { |
| Node<E> l = lastRet; |
| if (l == null) throw new IllegalStateException(); |
| // rely on a future traversal to relink. |
| l.setItem(null); |
| lastRet = null; |
| } |
| } |
| |
| /** |
| * Save the state to a stream (that is, serialize it). |
| * |
| * @serialData All of the elements (each an <tt>E</tt>) in |
| * the proper order, followed by a null |
| * @param s the stream |
| */ |
| private void writeObject(java.io.ObjectOutputStream s) |
| throws java.io.IOException { |
| |
| // Write out any hidden stuff |
| s.defaultWriteObject(); |
| |
| // Write out all elements in the proper order. |
| for (Node<E> p = first(); p != null; p = p.getNext()) { |
| Object item = p.getItem(); |
| if (item != null) |
| s.writeObject(item); |
| } |
| |
| // Use trailing null as sentinel |
| s.writeObject(null); |
| } |
| |
| /** |
| * Reconstitute the Queue instance from a stream (that is, |
| * 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, null); |
| tail = head; |
| // Read in all elements and place in queue |
| for (;;) { |
| E item = (E)s.readObject(); |
| if (item == null) |
| break; |
| else |
| offer(item); |
| } |
| } |
| |
| } |