Java tutorial
/* * 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. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.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 and Martin Buchholz with assistance from members of * JCP JSR-166 Expert Group and released to the public domain, as explained * at http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.lang.invoke.MethodHandles; import java.lang.invoke.VarHandle; import java.util.AbstractCollection; import java.util.Arrays; import java.util.Collection; import java.util.Deque; import java.util.Iterator; import java.util.NoSuchElementException; import java.util.Objects; import java.util.Queue; import java.util.Spliterator; import java.util.Spliterators; import java.util.function.Consumer; import java.util.function.Predicate; /** * An unbounded concurrent {@linkplain Deque deque} based on linked nodes. * Concurrent insertion, removal, and access operations execute safely * across multiple threads. * A {@code ConcurrentLinkedDeque} is an appropriate choice when * many threads will share access to a common collection. * Like most other concurrent collection implementations, this class * does not permit the use of {@code null} elements. * * <p>Iterators and spliterators are * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. * * <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 deques, determining the current number * of elements requires a traversal of the elements, and so may report * inaccurate results if this collection is modified during traversal. * * <p>Bulk operations that add, remove, or examine multiple elements, * such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, * are <em>not</em> guaranteed to be performed atomically. * For example, a {@code forEach} traversal concurrent with an {@code * addAll} operation might observe only some of the added elements. * * <p>This class and its iterator implement all of the <em>optional</em> * methods of the {@link Deque} 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 ConcurrentLinkedDeque} * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> * actions subsequent to the access or removal of that element from * the {@code ConcurrentLinkedDeque} in another thread. * * <p>This class is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.7 * @author Doug Lea * @author Martin Buchholz * @param <E> the type of elements held in this deque */ public class ConcurrentLinkedDeque<E> extends AbstractCollection<E> implements Deque<E>, java.io.Serializable { /* * This is an implementation of a concurrent lock-free deque * supporting interior removes but not interior insertions, as * required to support the entire Deque interface. * * We extend the techniques developed for ConcurrentLinkedQueue and * LinkedTransferQueue (see the internal docs for those classes). * Understanding the ConcurrentLinkedQueue implementation is a * prerequisite for understanding the implementation of this class. * * The data structure is a symmetrical doubly-linked "GC-robust" * linked list of nodes. We minimize the number of volatile writes * using two techniques: advancing multiple hops with a single CAS * and mixing volatile and non-volatile writes of the same memory * locations. * * A node contains the expected E ("item") and links to predecessor * ("prev") and successor ("next") nodes: * * class Node<E> { volatile Node<E> prev, next; volatile E item; } * * A node p is considered "live" if it contains a non-null item * (p.item != null). When an item is CASed to null, the item is * atomically logically deleted from the collection. * * At any time, there is precisely one "first" node with a null * prev reference that terminates any chain of prev references * starting at a live node. Similarly there is precisely one * "last" node terminating any chain of next references starting at * a live node. The "first" and "last" nodes may or may not be live. * The "first" and "last" nodes are always mutually reachable. * * A new element is added atomically by CASing the null prev or * next reference in the first or last node to a fresh node * containing the element. The element's node atomically becomes * "live" at that point. * * A node is considered "active" if it is a live node, or the * first or last node. Active nodes cannot be unlinked. * * A "self-link" is a next or prev reference that is the same node: * p.prev == p or p.next == p * Self-links are used in the node unlinking process. Active nodes * never have self-links. * * A node p is active if and only if: * * p.item != null || * (p.prev == null && p.next != p) || * (p.next == null && p.prev != p) * * The deque object has two node references, "head" and "tail". * The head and tail are only approximations to the first and last * nodes of the deque. The first node can always be found by * following prev pointers from head; likewise for tail. However, * it is permissible for head and tail to be referring to deleted * nodes that have been unlinked and so may not be reachable from * any live node. * * There are 3 stages of node deletion; * "logical deletion", "unlinking", and "gc-unlinking". * * 1. "logical deletion" by CASing item to null atomically removes * the element from the collection, and makes the containing node * eligible for unlinking. * * 2. "unlinking" makes a deleted node unreachable from active * nodes, and thus eventually reclaimable by GC. Unlinked nodes * may remain reachable indefinitely from an iterator. * * Physical node unlinking is merely an optimization (albeit a * critical one), and so can be performed at our convenience. At * any time, the set of live nodes maintained by prev and next * links are identical, that is, the live nodes found via next * links from the first node is equal to the elements found via * prev links from the last node. However, this is not true for * nodes that have already been logically deleted - such nodes may * be reachable in one direction only. * * 3. "gc-unlinking" takes unlinking further by making active * nodes unreachable from deleted nodes, making it easier for the * GC to reclaim future deleted nodes. This step makes the data * structure "gc-robust", as first described in detail by Boehm * (http://portal.acm.org/citation.cfm?doid=503272.503282). * * GC-unlinked nodes may remain reachable indefinitely from an * iterator, but unlike unlinked nodes, are never reachable from * head or tail. * * Making the data structure GC-robust will eliminate the risk of * unbounded memory retention with conservative GCs and is likely * to improve performance with generational GCs. * * When a node is dequeued at either end, e.g. via poll(), we would * like to break any references from the node to active nodes. We * develop further the use of self-links that was very effective in * other concurrent collection classes. The idea is to replace * prev and next pointers with special values that are interpreted * to mean off-the-list-at-one-end. These are approximations, but * good enough to preserve the properties we want in our * traversals, e.g. we guarantee that a traversal will never visit * the same element twice, but we don't guarantee whether a * traversal that runs out of elements will be able to see more * elements later after enqueues at that end. Doing gc-unlinking * safely is particularly tricky, since any node can be in use * indefinitely (for example by an iterator). We must ensure that * the nodes pointed at by head/tail never get gc-unlinked, since * head/tail are needed to get "back on track" by other nodes that * are gc-unlinked. gc-unlinking accounts for much of the * implementation complexity. * * Since neither unlinking nor gc-unlinking are necessary for * correctness, there are many implementation choices regarding * frequency (eagerness) of these operations. Since volatile * reads are likely to be much cheaper than CASes, saving CASes by * unlinking multiple adjacent nodes at a time may be a win. * gc-unlinking can be performed rarely and still be effective, * since it is most important that long chains of deleted nodes * are occasionally broken. * * The actual representation we use is that p.next == p means to * goto the first node (which in turn is reached by following prev * pointers from head), and p.next == null && p.prev == p means * that the iteration is at an end and that p is a (static final) * dummy node, NEXT_TERMINATOR, and not the last active node. * Finishing the iteration when encountering such a TERMINATOR is * good enough for read-only traversals, so such traversals can use * p.next == null as the termination condition. When we need to * find the last (active) node, for enqueueing a new node, we need * to check whether we have reached a TERMINATOR node; if so, * restart traversal from tail. * * The implementation is completely directionally symmetrical, * except that most public methods that iterate through the list * follow next pointers, in the "forward" direction. * * We believe (without full proof) that all single-element Deque * operations that operate directly at the two ends of the Deque * (e.g., addFirst, peekLast, pollLast) are linearizable (see * Herlihy and Shavit's book). However, some combinations of * operations are known not to be linearizable. In particular, * when an addFirst(A) is racing with pollFirst() removing B, it * is possible for an observer iterating over the elements to * observe first [A B C] and then [A C], even though no interior * removes are ever performed. Nevertheless, iterators behave * reasonably, providing the "weakly consistent" guarantees. * * Empirically, microbenchmarks suggest that this class adds about * 40% overhead relative to ConcurrentLinkedQueue, which feels as * good as we can hope for. */ private static final long serialVersionUID = 876323262645176354L; /** * A node from which the first node on list (that is, the unique node p * with p.prev == null && p.next != p) can be reached in O(1) time. * Invariants: * - the first node is always O(1) reachable from head via prev links * - all live nodes are reachable from the first node via succ() * - head != null * - (tmp = head).next != tmp || tmp != head * - head is never gc-unlinked (but may be unlinked) * Non-invariants: * - head.item may or may not be null * - head may not be reachable from the first or last node, or from tail */ private transient volatile Node<E> head; /** * A node from which the last node on list (that is, the unique node p * with p.next == null && p.prev != p) can be reached in O(1) time. * Invariants: * - the last node is always O(1) reachable from tail via next links * - all live nodes are reachable from the last node via pred() * - tail != null * - tail is never gc-unlinked (but may be unlinked) * Non-invariants: * - tail.item may or may not be null * - tail may not be reachable from the first or last node, or from head */ private transient volatile Node<E> tail; private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; @SuppressWarnings("unchecked") Node<E> prevTerminator() { return (Node<E>) PREV_TERMINATOR; } @SuppressWarnings("unchecked") Node<E> nextTerminator() { return (Node<E>) NEXT_TERMINATOR; } static final class Node<E> { volatile Node<E> prev; volatile E item; volatile Node<E> next; } /** * Returns a new node holding item. Uses relaxed write because item * can only be seen after piggy-backing publication via CAS. */ static <E> Node<E> newNode(E item) { Node<E> node = new Node<E>(); ITEM.set(node, item); return node; } /** * Links e as first element. */ private void linkFirst(E e) { final Node<E> newNode = newNode(Objects.requireNonNull(e)); restartFromHead: for (;;) for (Node<E> h = head, p = h, q;;) { if ((q = p.prev) != null && (q = (p = q).prev) != null) // Check for head updates every other hop. // If p == q, we are sure to follow head instead. p = (h != (h = head)) ? h : q; else if (p.next == p) // PREV_TERMINATOR continue restartFromHead; else { // p is first node NEXT.set(newNode, p); // CAS piggyback if (PREV.compareAndSet(p, null, newNode)) { // Successful CAS is the linearization point // for e to become an element of this deque, // and for newNode to become "live". if (p != h) // hop two nodes at a time; failure is OK HEAD.weakCompareAndSet(this, h, newNode); return; } // Lost CAS race to another thread; re-read prev } } } /** * Links e as last element. */ private void linkLast(E e) { final Node<E> newNode = newNode(Objects.requireNonNull(e)); restartFromTail: for (;;) for (Node<E> t = tail, p = t, q;;) { if ((q = p.next) != null && (q = (p = q).next) != null) // Check for tail updates every other hop. // If p == q, we are sure to follow tail instead. p = (t != (t = tail)) ? t : q; else if (p.prev == p) // NEXT_TERMINATOR continue restartFromTail; else { // p is last node PREV.set(newNode, p); // CAS piggyback if (NEXT.compareAndSet(p, null, newNode)) { // Successful CAS is the linearization point // for e to become an element of this deque, // and for newNode to become "live". if (p != t) // hop two nodes at a time; failure is OK TAIL.weakCompareAndSet(this, t, newNode); return; } // Lost CAS race to another thread; re-read next } } } private static final int HOPS = 2; /** * Unlinks non-null node x. */ void unlink(Node<E> x) { // assert x != null; // assert x.item == null; // assert x != PREV_TERMINATOR; // assert x != NEXT_TERMINATOR; final Node<E> prev = x.prev; final Node<E> next = x.next; if (prev == null) { unlinkFirst(x, next); } else if (next == null) { unlinkLast(x, prev); } else { // Unlink interior node. // // This is the common case, since a series of polls at the // same end will be "interior" removes, except perhaps for // the first one, since end nodes cannot be unlinked. // // At any time, all active nodes are mutually reachable by // following a sequence of either next or prev pointers. // // Our strategy is to find the unique active predecessor // and successor of x. Try to fix up their links so that // they point to each other, leaving x unreachable from // active nodes. If successful, and if x has no live // predecessor/successor, we additionally try to gc-unlink, // leaving active nodes unreachable from x, by rechecking // that the status of predecessor and successor are // unchanged and ensuring that x is not reachable from // tail/head, before setting x's prev/next links to their // logical approximate replacements, self/TERMINATOR. Node<E> activePred, activeSucc; boolean isFirst, isLast; int hops = 1; // Find active predecessor for (Node<E> p = prev;; ++hops) { if (p.item != null) { activePred = p; isFirst = false; break; } Node<E> q = p.prev; if (q == null) { if (p.next == p) return; activePred = p; isFirst = true; break; } else if (p == q) return; else p = q; } // Find active successor for (Node<E> p = next;; ++hops) { if (p.item != null) { activeSucc = p; isLast = false; break; } Node<E> q = p.next; if (q == null) { if (p.prev == p) return; activeSucc = p; isLast = true; break; } else if (p == q) return; else p = q; } // TODO: better HOP heuristics if (hops < HOPS // always squeeze out interior deleted nodes && (isFirst | isLast)) return; // Squeeze out deleted nodes between activePred and // activeSucc, including x. skipDeletedSuccessors(activePred); skipDeletedPredecessors(activeSucc); // Try to gc-unlink, if possible if ((isFirst | isLast) && // Recheck expected state of predecessor and successor (activePred.next == activeSucc) && (activeSucc.prev == activePred) && (isFirst ? activePred.prev == null : activePred.item != null) && (isLast ? activeSucc.next == null : activeSucc.item != null)) { updateHead(); // Ensure x is not reachable from head updateTail(); // Ensure x is not reachable from tail // Finally, actually gc-unlink PREV.setRelease(x, isFirst ? prevTerminator() : x); NEXT.setRelease(x, isLast ? nextTerminator() : x); } } } /** * Unlinks non-null first node. */ private void unlinkFirst(Node<E> first, Node<E> next) { // assert first != null; // assert next != null; // assert first.item == null; for (Node<E> o = null, p = next, q;;) { if (p.item != null || (q = p.next) == null) { if (o != null && p.prev != p && NEXT.compareAndSet(first, next, p)) { skipDeletedPredecessors(p); if (first.prev == null && (p.next == null || p.item != null) && p.prev == first) { updateHead(); // Ensure o is not reachable from head updateTail(); // Ensure o is not reachable from tail // Finally, actually gc-unlink NEXT.setRelease(o, o); PREV.setRelease(o, prevTerminator()); } } return; } else if (p == q) return; else { o = p; p = q; } } } /** * Unlinks non-null last node. */ private void unlinkLast(Node<E> last, Node<E> prev) { // assert last != null; // assert prev != null; // assert last.item == null; for (Node<E> o = null, p = prev, q;;) { if (p.item != null || (q = p.prev) == null) { if (o != null && p.next != p && PREV.compareAndSet(last, prev, p)) { skipDeletedSuccessors(p); if (last.next == null && (p.prev == null || p.item != null) && p.next == last) { updateHead(); // Ensure o is not reachable from head updateTail(); // Ensure o is not reachable from tail // Finally, actually gc-unlink PREV.setRelease(o, o); NEXT.setRelease(o, nextTerminator()); } } return; } else if (p == q) return; else { o = p; p = q; } } } /** * Guarantees that any node which was unlinked before a call to * this method will be unreachable from head after it returns. * Does not guarantee to eliminate slack, only that head will * point to a node that was active while this method was running. */ private final void updateHead() { // Either head already points to an active node, or we keep // trying to cas it to the first node until it does. Node<E> h, p, q; restartFromHead: while ((h = head).item == null && (p = h.prev) != null) { for (;;) { if ((q = p.prev) == null || (q = (p = q).prev) == null) { // It is possible that p is PREV_TERMINATOR, // but if so, the CAS is guaranteed to fail. if (HEAD.compareAndSet(this, h, p)) return; else continue restartFromHead; } else if (h != head) continue restartFromHead; else p = q; } } } /** * Guarantees that any node which was unlinked before a call to * this method will be unreachable from tail after it returns. * Does not guarantee to eliminate slack, only that tail will * point to a node that was active while this method was running. */ private final void updateTail() { // Either tail already points to an active node, or we keep // trying to cas it to the last node until it does. Node<E> t, p, q; restartFromTail: while ((t = tail).item == null && (p = t.next) != null) { for (;;) { if ((q = p.next) == null || (q = (p = q).next) == null) { // It is possible that p is NEXT_TERMINATOR, // but if so, the CAS is guaranteed to fail. if (TAIL.compareAndSet(this, t, p)) return; else continue restartFromTail; } else if (t != tail) continue restartFromTail; else p = q; } } } private void skipDeletedPredecessors(Node<E> x) { whileActive: do { Node<E> prev = x.prev; // assert prev != null; // assert x != NEXT_TERMINATOR; // assert x != PREV_TERMINATOR; Node<E> p = prev; findActive: for (;;) { if (p.item != null) break findActive; Node<E> q = p.prev; if (q == null) { if (p.next == p) continue whileActive; break findActive; } else if (p == q) continue whileActive; else p = q; } // found active CAS target if (prev == p || PREV.compareAndSet(x, prev, p)) return; } while (x.item != null || x.next == null); } private void skipDeletedSuccessors(Node<E> x) { whileActive: do { Node<E> next = x.next; // assert next != null; // assert x != NEXT_TERMINATOR; // assert x != PREV_TERMINATOR; Node<E> p = next; findActive: for (;;) { if (p.item != null) break findActive; Node<E> q = p.next; if (q == null) { if (p.prev == p) continue whileActive; break findActive; } else if (p == q) continue whileActive; else p = q; } // found active CAS target if (next == p || NEXT.compareAndSet(x, next, p)) return; } while (x.item != null || x.prev == null); } /** * Returns the successor of p, or the first 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) { // TODO: should we skip deleted nodes here? if (p == (p = p.next)) p = first(); return p; } /** * Returns the predecessor of p, or the last node if p.prev 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> pred(Node<E> p) { if (p == (p = p.prev)) p = last(); return p; } /** * Returns the first node, the unique node p for which: * p.prev == null && p.next != p * The returned node may or may not be logically deleted. * Guarantees that head is set to the returned node. */ Node<E> first() { restartFromHead: for (;;) for (Node<E> h = head, p = h, q;;) { if ((q = p.prev) != null && (q = (p = q).prev) != null) // Check for head updates every other hop. // If p == q, we are sure to follow head instead. p = (h != (h = head)) ? h : q; else if (p == h // It is possible that p is PREV_TERMINATOR, // but if so, the CAS is guaranteed to fail. || HEAD.compareAndSet(this, h, p)) return p; else continue restartFromHead; } } /** * Returns the last node, the unique node p for which: * p.next == null && p.prev != p * The returned node may or may not be logically deleted. * Guarantees that tail is set to the returned node. */ Node<E> last() { restartFromTail: for (;;) for (Node<E> t = tail, p = t, q;;) { if ((q = p.next) != null && (q = (p = q).next) != null) // Check for tail updates every other hop. // If p == q, we are sure to follow tail instead. p = (t != (t = tail)) ? t : q; else if (p == t // It is possible that p is NEXT_TERMINATOR, // but if so, the CAS is guaranteed to fail. || TAIL.compareAndSet(this, t, p)) return p; else continue restartFromTail; } } // Minor convenience utilities /** * Returns element unless it is null, in which case throws * NoSuchElementException. * * @param v the element * @return the element */ private E screenNullResult(E v) { if (v == null) throw new NoSuchElementException(); return v; } /** * Constructs an empty deque. */ public ConcurrentLinkedDeque() { head = tail = new Node<E>(); } /** * Constructs a deque 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 ConcurrentLinkedDeque(Collection<? extends E> c) { // Copy c into a private chain of Nodes Node<E> h = null, t = null; for (E e : c) { Node<E> newNode = newNode(Objects.requireNonNull(e)); if (h == null) h = t = newNode; else { NEXT.set(t, newNode); PREV.set(newNode, t); t = newNode; } } initHeadTail(h, t); } /** * Initializes head and tail, ensuring invariants hold. */ private void initHeadTail(Node<E> h, Node<E> t) { if (h == t) { if (h == null) h = t = new Node<E>(); else { // Avoid edge case of a single Node with non-null item. Node<E> newNode = new Node<E>(); NEXT.set(t, newNode); PREV.set(newNode, t); t = newNode; } } head = h; tail = t; } /** * Inserts the specified element at the front of this deque. * As the deque is unbounded, this method will never throw * {@link IllegalStateException}. * * @throws NullPointerException if the specified element is null */ public void addFirst(E e) { linkFirst(e); } /** * Inserts the specified element at the end of this deque. * As the deque is unbounded, this method will never throw * {@link IllegalStateException}. * * <p>This method is equivalent to {@link #add}. * * @throws NullPointerException if the specified element is null */ public void addLast(E e) { linkLast(e); } /** * Inserts the specified element at the front of this deque. * As the deque is unbounded, this method will never return {@code false}. * * @return {@code true} (as specified by {@link Deque#offerFirst}) * @throws NullPointerException if the specified element is null */ public boolean offerFirst(E e) { linkFirst(e); return true; } /** * Inserts the specified element at the end of this deque. * As the deque is unbounded, this method will never return {@code false}. * * <p>This method is equivalent to {@link #add}. * * @return {@code true} (as specified by {@link Deque#offerLast}) * @throws NullPointerException if the specified element is null */ public boolean offerLast(E e) { linkLast(e); return true; } public E peekFirst() { restart: for (;;) { E item; Node<E> first = first(), p = first; while ((item = p.item) == null) { if (p == (p = p.next)) continue restart; if (p == null) break; } // recheck for linearizability if (first.prev != null) continue restart; return item; } } public E peekLast() { restart: for (;;) { E item; Node<E> last = last(), p = last; while ((item = p.item) == null) { if (p == (p = p.prev)) continue restart; if (p == null) break; } // recheck for linearizability if (last.next != null) continue restart; return item; } } /** * @throws NoSuchElementException {@inheritDoc} */ public E getFirst() { return screenNullResult(peekFirst()); } /** * @throws NoSuchElementException {@inheritDoc} */ public E getLast() { return screenNullResult(peekLast()); } public E pollFirst() { restart: for (;;) { for (Node<E> first = first(), p = first;;) { final E item; if ((item = p.item) != null) { // recheck for linearizability if (first.prev != null) continue restart; if (ITEM.compareAndSet(p, item, null)) { unlink(p); return item; } } if (p == (p = p.next)) continue restart; if (p == null) { if (first.prev != null) continue restart; return null; } } } } public E pollLast() { restart: for (;;) { for (Node<E> last = last(), p = last;;) { final E item; if ((item = p.item) != null) { // recheck for linearizability if (last.next != null) continue restart; if (ITEM.compareAndSet(p, item, null)) { unlink(p); return item; } } if (p == (p = p.prev)) continue restart; if (p == null) { if (last.next != null) continue restart; return null; } } } } /** * @throws NoSuchElementException {@inheritDoc} */ public E removeFirst() { return screenNullResult(pollFirst()); } /** * @throws NoSuchElementException {@inheritDoc} */ public E removeLast() { return screenNullResult(pollLast()); } // *** Queue and stack methods *** /** * Inserts the specified element at the tail of this deque. * As the deque is unbounded, this method will never return {@code false}. * * @return {@code true} (as specified by {@link Queue#offer}) * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { return offerLast(e); } /** * Inserts the specified element at the tail of this deque. * As the deque 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) { return offerLast(e); } public E poll() { return pollFirst(); } public E peek() { return peekFirst(); } /** * @throws NoSuchElementException {@inheritDoc} */ public E remove() { return removeFirst(); } /** * @throws NoSuchElementException {@inheritDoc} */ public E pop() { return removeFirst(); } /** * @throws NoSuchElementException {@inheritDoc} */ public E element() { return getFirst(); } /** * @throws NullPointerException {@inheritDoc} */ public void push(E e) { addFirst(e); } /** * Removes the first occurrence of the specified element from this deque. * If the deque does not contain the element, it is unchanged. * More formally, removes the first element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element * @throws NullPointerException if the specified element is null */ public boolean removeFirstOccurrence(Object o) { Objects.requireNonNull(o); for (Node<E> p = first(); p != null; p = succ(p)) { final E item; if ((item = p.item) != null && o.equals(item) && ITEM.compareAndSet(p, item, null)) { unlink(p); return true; } } return false; } /** * Removes the last occurrence of the specified element from this deque. * If the deque does not contain the element, it is unchanged. * More formally, removes the last element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element * @throws NullPointerException if the specified element is null */ public boolean removeLastOccurrence(Object o) { Objects.requireNonNull(o); for (Node<E> p = last(); p != null; p = pred(p)) { final E item; if ((item = p.item) != null && o.equals(item) && ITEM.compareAndSet(p, item, null)) { unlink(p); return true; } } return false; } /** * Returns {@code true} if this deque contains the specified element. * More formally, returns {@code true} if and only if this deque contains * at least one element {@code e} such that {@code o.equals(e)}. * * @param o element whose presence in this deque is to be tested * @return {@code true} if this deque contains the specified element */ public boolean contains(Object o) { if (o != null) { for (Node<E> p = first(); p != null; p = succ(p)) { final E item; if ((item = p.item) != null && o.equals(item)) return true; } } return false; } /** * Returns {@code true} if this collection contains no elements. * * @return {@code true} if this collection contains no elements */ public boolean isEmpty() { return peekFirst() == null; } /** * Returns the number of elements in this deque. If this deque * contains more than {@code Integer.MAX_VALUE} elements, it * returns {@code Integer.MAX_VALUE}. * * <p>Beware that, unlike in most collections, this method is * <em>NOT</em> a constant-time operation. Because of the * asynchronous nature of these deques, determining the current * number of elements requires traversing them all to count them. * Additionally, it is possible for the size to change during * execution of this method, in which case the returned result * will be inaccurate. Thus, this method is typically not very * useful in concurrent applications. * * @return the number of elements in this deque */ public int size() { restart: for (;;) { int count = 0; for (Node<E> p = first(); p != null;) { if (p.item != null) if (++count == Integer.MAX_VALUE) break; // @see Collection.size() if (p == (p = p.next)) continue restart; } return count; } } /** * Removes the first occurrence of the specified element from this deque. * If the deque does not contain the element, it is unchanged. * More formally, removes the first element {@code e} such that * {@code o.equals(e)} (if such an element exists). * Returns {@code true} if this deque contained the specified element * (or equivalently, if this deque changed as a result of the call). * * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. * * @param o element to be removed from this deque, if present * @return {@code true} if the deque contained the specified element * @throws NullPointerException if the specified element is null */ public boolean remove(Object o) { return removeFirstOccurrence(o); } /** * Appends all of the elements in the specified collection to the end of * this deque, in the order that they are returned by the specified * collection's iterator. Attempts to {@code addAll} of a deque to * itself result in {@code IllegalArgumentException}. * * @param c the elements to be inserted into this deque * @return {@code true} if this deque 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 deque */ 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) { Node<E> newNode = newNode(Objects.requireNonNull(e)); if (beginningOfTheEnd == null) beginningOfTheEnd = last = newNode; else { NEXT.set(last, newNode); PREV.set(newNode, last); last = newNode; } } if (beginningOfTheEnd == null) return false; // Atomically append the chain at the tail of this collection restartFromTail: for (;;) for (Node<E> t = tail, p = t, q;;) { if ((q = p.next) != null && (q = (p = q).next) != null) // Check for tail updates every other hop. // If p == q, we are sure to follow tail instead. p = (t != (t = tail)) ? t : q; else if (p.prev == p) // NEXT_TERMINATOR continue restartFromTail; else { // p is last node PREV.set(beginningOfTheEnd, p); // CAS piggyback if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) { // Successful CAS is the linearization point // for all elements to be added to this deque. if (!TAIL.weakCompareAndSet(this, t, last)) { // Try a little harder to update tail, // since we may be adding many elements. t = tail; if (last.next == null) TAIL.weakCompareAndSet(this, t, last); } return true; } // Lost CAS race to another thread; re-read next } } } /** * Removes all of the elements from this deque. */ public void clear() { while (pollFirst() != null) ; } public String toString() { String[] a = null; restart: for (;;) { int charLength = 0; int size = 0; for (Node<E> p = first(); p != null;) { final E item; if ((item = p.item) != null) { if (a == null) a = new String[4]; else if (size == a.length) a = Arrays.copyOf(a, 2 * size); String s = item.toString(); a[size++] = s; charLength += s.length(); } if (p == (p = p.next)) continue restart; } if (size == 0) return "[]"; return Helpers.toString(a, size, charLength); } } private Object[] toArrayInternal(Object[] a) { Object[] x = a; restart: for (;;) { int size = 0; for (Node<E> p = first(); p != null;) { final E item; if ((item = p.item) != null) { if (x == null) x = new Object[4]; else if (size == x.length) x = Arrays.copyOf(x, 2 * (size + 4)); x[size++] = item; } if (p == (p = p.next)) continue restart; } if (x == null) return new Object[0]; else if (a != null && size <= a.length) { if (a != x) System.arraycopy(x, 0, a, 0, size); if (size < a.length) a[size] = null; return a; } return (size == x.length) ? x : Arrays.copyOf(x, size); } } /** * Returns an array containing all of the elements in this deque, in * proper sequence (from first to last element). * * <p>The returned array will be "safe" in that no references to it are * maintained by this deque. (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 deque */ public Object[] toArray() { return toArrayInternal(null); } /** * Returns an array containing all of the elements in this deque, * in proper sequence (from first to last element); the runtime * type of the returned array is that of the specified array. If * the deque 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 deque. * * <p>If this deque fits in the specified array with room to spare * (i.e., the array has more elements than this deque), the element in * the array immediately following the end of the deque is set to * {@code null}. * * <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 {@code x} is a deque known to contain only strings. * The following code can be used to dump the deque into a newly * allocated array of {@code String}: * * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> * * Note that {@code toArray(new Object[0])} is identical in function to * {@code toArray()}. * * @param a the array into which the elements of the deque 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 deque * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this deque * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a == null) throw new NullPointerException(); return (T[]) toArrayInternal(a); } /** * Returns an iterator over the elements in this deque in proper sequence. * The elements will be returned in order from first (head) to last (tail). * * <p>The returned iterator is * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. * * @return an iterator over the elements in this deque in proper sequence */ public Iterator<E> iterator() { return new Itr(); } /** * Returns an iterator over the elements in this deque in reverse * sequential order. The elements will be returned in order from * last (tail) to first (head). * * <p>The returned iterator is * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. * * @return an iterator over the elements in this deque in reverse order */ public Iterator<E> descendingIterator() { return new DescendingItr(); } private abstract class AbstractItr 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 returned by most recent call to next. Needed by remove. * Reset to null if this element is deleted by a call to remove. */ private Node<E> lastRet; abstract Node<E> startNode(); abstract Node<E> nextNode(Node<E> p); AbstractItr() { advance(); } /** * Sets nextNode and nextItem to next valid node, or to null * if no such. */ private void advance() { lastRet = nextNode; Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); for (;; p = nextNode(p)) { if (p == null) { // might be at active end or TERMINATOR node; both are OK nextNode = null; nextItem = null; break; } final E item; if ((item = p.item) != null) { nextNode = p; nextItem = item; break; } } } public boolean hasNext() { return nextItem != null; } public E next() { E item = nextItem; if (item == null) throw new NoSuchElementException(); advance(); return item; } public void remove() { Node<E> l = lastRet; if (l == null) throw new IllegalStateException(); l.item = null; unlink(l); lastRet = null; } } /** Forward iterator */ private class Itr extends AbstractItr { Itr() { } // prevent access constructor creation Node<E> startNode() { return first(); } Node<E> nextNode(Node<E> p) { return succ(p); } } /** Descending iterator */ private class DescendingItr extends AbstractItr { DescendingItr() { } // prevent access constructor creation Node<E> startNode() { return last(); } Node<E> nextNode(Node<E> p) { return pred(p); } } /** A customized variant of Spliterators.IteratorSpliterator */ final class CLDSpliterator implements Spliterator<E> { static final int MAX_BATCH = 1 << 25; // max batch array size; Node<E> current; // current node; null until initialized int batch; // batch size for splits boolean exhausted; // true when no more nodes public Spliterator<E> trySplit() { Node<E> p, q; if ((p = current()) == null || (q = p.next) == null) return null; int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH); Object[] a = null; do { final E e; if ((e = p.item) != null) { if (a == null) a = new Object[n]; a[i++] = e; } if (p == (p = q)) p = first(); } while (p != null && (q = p.next) != null && i < n); setCurrent(p); return (i == 0) ? null : Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED | Spliterator.NONNULL | Spliterator.CONCURRENT)); } public void forEachRemaining(Consumer<? super E> action) { Objects.requireNonNull(action); Node<E> p; if ((p = current()) != null) { current = null; exhausted = true; do { final E e; if ((e = p.item) != null) action.accept(e); if (p == (p = p.next)) p = first(); } while (p != null); } } public boolean tryAdvance(Consumer<? super E> action) { Objects.requireNonNull(action); Node<E> p; if ((p = current()) != null) { E e; do { e = p.item; if (p == (p = p.next)) p = first(); } while (e == null && p != null); setCurrent(p); if (e != null) { action.accept(e); return true; } } return false; } private void setCurrent(Node<E> p) { if ((current = p) == null) exhausted = true; } private Node<E> current() { Node<E> p; if ((p = current) == null && !exhausted) setCurrent(p = first()); return p; } public long estimateSize() { return Long.MAX_VALUE; } public int characteristics() { return (Spliterator.ORDERED | Spliterator.NONNULL | Spliterator.CONCURRENT); } } /** * Returns a {@link Spliterator} over the elements in this deque. * * <p>The returned spliterator is * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. * * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. * * @implNote * The {@code Spliterator} implements {@code trySplit} to permit limited * parallelism. * * @return a {@code Spliterator} over the elements in this deque * @since 1.8 */ public Spliterator<E> spliterator() { return new CLDSpliterator(); } /** * Saves this deque to a stream (that is, serializes it). * * @param s the stream * @throws java.io.IOException if an I/O error occurs * @serialData All of the elements (each an {@code E}) in * the proper order, followed by a null */ 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 = succ(p)) { final E item; if ((item = p.item) != null) s.writeObject(item); } // Use trailing null as sentinel s.writeObject(null); } /** * Reconstitutes this deque from a stream (that is, deserializes it). * @param s the stream * @throws ClassNotFoundException if the class of a serialized object * could not be found * @throws java.io.IOException if an I/O error occurs */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { s.defaultReadObject(); // Read in elements until trailing null sentinel found Node<E> h = null, t = null; for (Object item; (item = s.readObject()) != null;) { @SuppressWarnings("unchecked") Node<E> newNode = newNode((E) item); if (h == null) h = t = newNode; else { NEXT.set(t, newNode); PREV.set(newNode, t); t = newNode; } } initHeadTail(h, t); } /** * @throws NullPointerException {@inheritDoc} */ public boolean removeIf(Predicate<? super E> filter) { Objects.requireNonNull(filter); return bulkRemove(filter); } /** * @throws NullPointerException {@inheritDoc} */ public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return bulkRemove(e -> c.contains(e)); } /** * @throws NullPointerException {@inheritDoc} */ public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return bulkRemove(e -> !c.contains(e)); } /** Implementation of bulk remove methods. */ private boolean bulkRemove(Predicate<? super E> filter) { boolean removed = false; for (Node<E> p = first(), succ; p != null; p = succ) { succ = succ(p); final E item; if ((item = p.item) != null && filter.test(item) && ITEM.compareAndSet(p, item, null)) { unlink(p); removed = true; } } return removed; } /** * @throws NullPointerException {@inheritDoc} */ public void forEach(Consumer<? super E> action) { Objects.requireNonNull(action); E item; for (Node<E> p = first(); p != null; p = succ(p)) if ((item = p.item) != null) action.accept(item); } // VarHandle mechanics private static final VarHandle HEAD; private static final VarHandle TAIL; private static final VarHandle PREV; private static final VarHandle NEXT; private static final VarHandle ITEM; static { PREV_TERMINATOR = new Node<Object>(); PREV_TERMINATOR.next = PREV_TERMINATOR; NEXT_TERMINATOR = new Node<Object>(); NEXT_TERMINATOR.prev = NEXT_TERMINATOR; try { MethodHandles.Lookup l = MethodHandles.lookup(); HEAD = l.findVarHandle(ConcurrentLinkedDeque.class, "head", Node.class); TAIL = l.findVarHandle(ConcurrentLinkedDeque.class, "tail", Node.class); PREV = l.findVarHandle(Node.class, "prev", Node.class); NEXT = l.findVarHandle(Node.class, "next", Node.class); ITEM = l.findVarHandle(Node.class, "item", Object.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } }