| /* |
| * 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/licenses/publicdomain |
| */ |
| |
| package java.util.concurrent; |
| |
| import java.util.AbstractCollection; |
| import java.util.ArrayList; |
| import java.util.Collection; |
| import java.util.ConcurrentModificationException; |
| import java.util.Deque; |
| import java.util.Iterator; |
| import java.util.NoSuchElementException; |
| |
| /** |
| * A concurrent linked-list implementation of a {@link Deque} |
| * (double-ended queue). Concurrent insertion, removal, and access |
| * operations execute safely across multiple threads. Iterators are |
| * <i>weakly consistent</i>, returning elements reflecting the state |
| * of the deque 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. |
| * |
| * <p>This class and its iterators implement all of the |
| * <em>optional</em> methods of the {@link Collection} and {@link |
| * Iterator} interfaces. Like most other concurrent collection |
| * implementations, this class does not permit the use of |
| * {@code null} elements. because some null arguments and return |
| * values cannot be reliably distinguished from the absence of |
| * elements. Arbitrarily, the {@link Collection#remove} method is |
| * mapped to {@code removeFirstOccurrence}, and {@link |
| * Collection#add} is mapped to {@code addLast}. |
| * |
| * <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. |
| * |
| * <p>This class is {@code Serializable}, but relies on default |
| * serialization mechanisms. Usually, it is a better idea for any |
| * serializable class using a {@code ConcurrentLinkedDeque} to instead |
| * serialize a snapshot of the elements obtained by method |
| * {@code toArray}. |
| * |
| * @hide |
| * |
| * @author Doug Lea |
| * @author Martin Buchholz |
| * @param <E> the type of elements held in this collection |
| */ |
| 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 fully support the Deque interface. |
| * |
| * We extend the techniques developed for |
| * ConcurrentLinkedQueue and LinkedTransferQueue |
| * (see the internal docs for those classes). |
| * |
| * At any time, there is precisely one "first" active node with a |
| * null prev pointer. Similarly there is one "last" active node |
| * with a null next pointer. New nodes are simply enqueued by |
| * null-CASing. |
| * |
| * A node p is considered "active" if it either contains an |
| * element, or is an end node and neither next nor prev pointers |
| * are self-links: |
| * |
| * p.item != null || |
| * (p.prev == null && p.next != p) || |
| * (p.next == null && p.prev != p) |
| * |
| * The head and tail pointers are only approximations to the start |
| * and end of the deque. The first node can always be found by |
| * following prev pointers from head; likewise for tail. However, |
| * head and tail may be pointing at deleted nodes that have been |
| * unlinked and so may not be reachable from any live node. |
| * |
| * There are 3 levels of node deletion: |
| * - logical deletion atomically removes the element |
| * - "unlinking" makes a deleted node unreachable from active |
| * nodes, and thus eventually reclaimable by GC |
| * - "gc-unlinking" further does the reverse of making active |
| * nodes unreachable from deleted nodes, making it easier for |
| * the GC to reclaim future deleted nodes |
| * |
| * TODO: find a better name for "gc-unlinked" |
| * |
| * Logical deletion of a node simply involves CASing its element |
| * to null. Physical deletion is merely an optimization (albeit a |
| * critical one), and can be performed at our convenience. At any |
| * time, the set of non-logically-deleted nodes maintained by prev |
| * and next links are identical, that is the live elements 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 only be reachable in one direction. |
| * |
| * When a node is dequeued at either end, e.g. via poll(), we |
| * would like to break any references from the node to live nodes, |
| * to stop old garbage from causing retention of new garbage with |
| * a generational or conservative GC. We develop further the |
| * self-linking trick that was very effective in other concurrent |
| * collection classes. The idea is to replace prev and next |
| * pointers to active nodes 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 hit 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 more elements are added 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 make sure that the nodes pointed at by |
| * head/tail do not 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, and p.next == null && p.prev == p means |
| * that the iteration is at an end and that p is a (final static) |
| * 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. When the last active |
| * node is desired, for example when enqueueing, goto tail and |
| * continue traversal. |
| * |
| * The implementation is completely directionally symmetrical, |
| * except that most public methods that iterate through the list |
| * follow next pointers ("forward" direction). |
| * |
| * There is one desirable property we would like to have, but |
| * don't: it is possible, when an addFirst(A) is racing with |
| * pollFirst() removing B, for an iterating observer to see A B C |
| * and subsequently see A C, even though no interior removes are |
| * ever performed. I believe this wart can only be removed at |
| * significant runtime cost. |
| * |
| * Empirically, microbenchmarks suggest that this class adds about |
| * 40% overhead relative to ConcurrentLinkedQueue, which feels as |
| * good as we can hope for. |
| */ |
| |
| /** |
| * A node from which the first node on list (that is, the unique |
| * node with node.prev == null) 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 |
| * 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 = new Node<E>(null); |
| |
| private final static Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; |
| |
| static { |
| PREV_TERMINATOR = new Node<Object>(null); |
| PREV_TERMINATOR.next = PREV_TERMINATOR; |
| NEXT_TERMINATOR = new Node<Object>(null); |
| NEXT_TERMINATOR.prev = NEXT_TERMINATOR; |
| } |
| |
| @SuppressWarnings("unchecked") |
| Node<E> prevTerminator() { |
| return (Node<E>) PREV_TERMINATOR; |
| } |
| |
| @SuppressWarnings("unchecked") |
| Node<E> nextTerminator() { |
| return (Node<E>) NEXT_TERMINATOR; |
| } |
| |
| /** |
| * 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: |
| * - 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 |
| * 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 = head; |
| |
| static final class Node<E> { |
| volatile Node<E> prev; |
| volatile E item; |
| volatile Node<E> next; |
| |
| Node(E item) { |
| // Piggyback on imminent casNext() or casPrev() |
| lazySetItem(item); |
| } |
| |
| boolean casItem(E cmp, E val) { |
| return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
| } |
| |
| void lazySetItem(E val) { |
| UNSAFE.putOrderedObject(this, itemOffset, val); |
| } |
| |
| void lazySetNext(Node<E> val) { |
| UNSAFE.putOrderedObject(this, nextOffset, val); |
| } |
| |
| boolean casNext(Node<E> cmp, Node<E> val) { |
| return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
| } |
| |
| void lazySetPrev(Node<E> val) { |
| UNSAFE.putOrderedObject(this, prevOffset, val); |
| } |
| |
| boolean casPrev(Node<E> cmp, Node<E> val) { |
| return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val); |
| } |
| |
| // Unsafe mechanics |
| |
| private static final sun.misc.Unsafe UNSAFE = |
| sun.misc.Unsafe.getUnsafe(); |
| private static final long prevOffset = |
| objectFieldOffset(UNSAFE, "prev", Node.class); |
| private static final long itemOffset = |
| objectFieldOffset(UNSAFE, "item", Node.class); |
| private static final long nextOffset = |
| objectFieldOffset(UNSAFE, "next", Node.class); |
| } |
| |
| /** |
| * Links e as first element. |
| */ |
| private void linkFirst(E e) { |
| checkNotNull(e); |
| final Node<E> newNode = new Node<E>(e); |
| |
| retry: |
| for (;;) { |
| for (Node<E> h = head, p = h;;) { |
| Node<E> q = p.prev; |
| if (q == null) { |
| if (p.next == p) |
| continue retry; |
| newNode.lazySetNext(p); // CAS piggyback |
| if (p.casPrev(null, newNode)) { |
| if (p != h) // hop two nodes at a time |
| casHead(h, newNode); |
| return; |
| } else { |
| p = p.prev; // lost CAS race to another thread |
| } |
| } |
| else if (p == q) |
| continue retry; |
| else |
| p = q; |
| } |
| } |
| } |
| |
| /** |
| * Links e as last element. |
| */ |
| private void linkLast(E e) { |
| checkNotNull(e); |
| final Node<E> newNode = new Node<E>(e); |
| |
| retry: |
| for (;;) { |
| for (Node<E> t = tail, p = t;;) { |
| Node<E> q = p.next; |
| if (q == null) { |
| if (p.prev == p) |
| continue retry; |
| newNode.lazySetPrev(p); // CAS piggyback |
| if (p.casNext(null, newNode)) { |
| if (p != t) // hop two nodes at a time |
| casTail(t, newNode); |
| return; |
| } else { |
| p = p.next; // lost CAS race to another thread |
| } |
| } |
| else if (p == q) |
| continue retry; |
| else |
| p = q; |
| } |
| } |
| } |
| |
| // TODO: Is there a better cheap way of performing some cleanup |
| // operation "occasionally"? |
| static class Count { |
| int count = 0; |
| } |
| private final static ThreadLocal<Count> tlc = |
| new ThreadLocal<Count>() { |
| protected Count initialValue() { return new Count(); } |
| }; |
| private static boolean shouldGCUnlinkOccasionally() { |
| return (tlc.get().count++ & 0x3) == 0; |
| } |
| |
| private final static 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 physically removed. |
| // |
| // 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 leave |
| // 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 == p.next) |
| 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 == p.prev) |
| 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) && |
| //shouldGCUnlinkOccasionally() && |
| |
| // 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)) { |
| |
| // Ensure x is not reachable from head or tail |
| updateHead(); |
| updateTail(); |
| x.lazySetPrev(isFirst ? prevTerminator() : x); |
| x.lazySetNext(isLast ? nextTerminator() : x); |
| } |
| } |
| } |
| |
| /** |
| * Unlinks non-null first node. |
| */ |
| private void unlinkFirst(Node<E> first, Node<E> next) { |
| assert first != null && next != null && first.item == null; |
| Node<E> o = null, p = next; |
| for (int hops = 0;; ++hops) { |
| Node<E> q; |
| if (p.item != null || (q = p.next) == null) { |
| if (hops >= HOPS) { |
| if (p == p.prev) |
| return; |
| if (first.casNext(next, p)) { |
| skipDeletedPredecessors(p); |
| if (//shouldGCUnlinkOccasionally() && |
| first.prev == null && |
| (p.next == null || p.item != null) && |
| p.prev == first) { |
| |
| updateHead(); |
| updateTail(); |
| o.lazySetNext(o); |
| o.lazySetPrev(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 && prev != null && last.item == null; |
| Node<E> o = null, p = prev; |
| for (int hops = 0;; ++hops) { |
| Node<E> q; |
| if (p.item != null || (q = p.prev) == null) { |
| if (hops >= HOPS) { |
| if (p == p.next) |
| return; |
| if (last.casPrev(prev, p)) { |
| skipDeletedSuccessors(p); |
| if (//shouldGCUnlinkOccasionally() && |
| last.next == null && |
| (p.prev == null || p.item != null) && |
| p.next == last) { |
| |
| updateHead(); |
| updateTail(); |
| o.lazySetPrev(o); |
| o.lazySetNext(nextTerminator()); |
| } |
| } |
| } |
| return; |
| } |
| else if (p == q) |
| return; |
| else { |
| o = p; |
| p = q; |
| } |
| } |
| } |
| |
| private final void updateHead() { |
| first(); |
| } |
| |
| private final void updateTail() { |
| last(); |
| } |
| |
| 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 || x.casPrev(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 || x.casNext(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? |
| Node<E> q = p.next; |
| return (p == q) ? first() : q; |
| } |
| |
| /** |
| * 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) { |
| Node<E> q = p.prev; |
| return (p == q) ? last() : q; |
| } |
| |
| /** |
| * Returns the first node, the unique node which has a null prev link. |
| * The returned node may or may not be logically deleted. |
| * Guarantees that head is set to the returned node. |
| */ |
| Node<E> first() { |
| retry: |
| for (;;) { |
| for (Node<E> h = head, p = h;;) { |
| Node<E> q = p.prev; |
| if (q == null) { |
| if (p == h |
| // It is possible that p is PREV_TERMINATOR, |
| // but if so, the CAS will fail. |
| || casHead(h, p)) |
| return p; |
| else |
| continue retry; |
| } else if (p == q) { |
| continue retry; |
| } else { |
| p = q; |
| } |
| } |
| } |
| } |
| |
| /** |
| * Returns the last node, the unique node which has a null next link. |
| * The returned node may or may not be logically deleted. |
| * Guarantees that tail is set to the returned node. |
| */ |
| Node<E> last() { |
| retry: |
| for (;;) { |
| for (Node<E> t = tail, p = t;;) { |
| Node<E> q = p.next; |
| if (q == null) { |
| if (p == t |
| // It is possible that p is NEXT_TERMINATOR, |
| // but if so, the CAS will fail. |
| || casTail(t, p)) |
| return p; |
| else |
| continue retry; |
| } else if (p == q) { |
| continue retry; |
| } else { |
| p = q; |
| } |
| } |
| } |
| } |
| |
| // Minor convenience utilities |
| |
| /** |
| * Throws NullPointerException if argument is null. |
| * |
| * @param v the element |
| */ |
| private static void checkNotNull(Object v) { |
| if (v == null) |
| throw new NullPointerException(); |
| } |
| |
| /** |
| * 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; |
| } |
| |
| /** |
| * Creates an array list and fills it with elements of this list. |
| * Used by toArray. |
| * |
| * @return the arrayList |
| */ |
| private ArrayList<E> toArrayList() { |
| ArrayList<E> c = new ArrayList<E>(); |
| for (Node<E> p = first(); p != null; p = succ(p)) { |
| E item = p.item; |
| if (item != null) |
| c.add(item); |
| } |
| return c; |
| } |
| |
| // Fields and constructors |
| |
| private static final long serialVersionUID = 876323262645176354L; |
| |
| /** |
| * Constructs an empty deque. |
| */ |
| public ConcurrentLinkedDeque() {} |
| |
| /** |
| * 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) { |
| this(); |
| addAll(c); |
| } |
| |
| /** |
| * Inserts the specified element at the front of this deque. |
| * |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public void addFirst(E e) { |
| linkFirst(e); |
| } |
| |
| /** |
| * Inserts the specified element at the end of this deque. |
| * This is identical in function to the {@code add} method. |
| * |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public void addLast(E e) { |
| linkLast(e); |
| } |
| |
| /** |
| * Inserts the specified element at the front of this deque. |
| * |
| * @return {@code true} always |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean offerFirst(E e) { |
| linkFirst(e); |
| return true; |
| } |
| |
| /** |
| * Inserts the specified element at the end of this deque. |
| * |
| * <p>This method is equivalent to {@link #add}. |
| * |
| * @return {@code true} always |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean offerLast(E e) { |
| linkLast(e); |
| return true; |
| } |
| |
| public E peekFirst() { |
| for (Node<E> p = first(); p != null; p = succ(p)) { |
| E item = p.item; |
| if (item != null) |
| return item; |
| } |
| return null; |
| } |
| |
| public E peekLast() { |
| for (Node<E> p = last(); p != null; p = pred(p)) { |
| E item = p.item; |
| if (item != null) |
| return item; |
| } |
| return null; |
| } |
| |
| /** |
| * @throws NoSuchElementException {@inheritDoc} |
| */ |
| public E getFirst() { |
| return screenNullResult(peekFirst()); |
| } |
| |
| /** |
| * @throws NoSuchElementException {@inheritDoc} |
| */ |
| public E getLast() { |
| return screenNullResult(peekLast()); |
| } |
| |
| public E pollFirst() { |
| for (Node<E> p = first(); p != null; p = succ(p)) { |
| E item = p.item; |
| if (item != null && p.casItem(item, null)) { |
| unlink(p); |
| return item; |
| } |
| } |
| return null; |
| } |
| |
| public E pollLast() { |
| for (Node<E> p = last(); p != null; p = pred(p)) { |
| E item = p.item; |
| if (item != null && p.casItem(item, null)) { |
| unlink(p); |
| return item; |
| } |
| } |
| 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. |
| * |
| * @return {@code true} (as specified by {@link java.util.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. |
| * |
| * @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 remove() { return removeFirst(); } |
| public E peek() { return peekFirst(); } |
| public E element() { return getFirst(); } |
| public void push(E e) { addFirst(e); } |
| public E pop() { return removeFirst(); } |
| |
| /** |
| * Removes the first element {@code e} such that |
| * {@code o.equals(e)}, if such an element exists in this deque. |
| * If the deque does not contain the element, it is unchanged. |
| * |
| * @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 {@code null} |
| */ |
| public boolean removeFirstOccurrence(Object o) { |
| checkNotNull(o); |
| for (Node<E> p = first(); p != null; p = succ(p)) { |
| E item = p.item; |
| if (item != null && o.equals(item) && p.casItem(item, null)) { |
| unlink(p); |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| /** |
| * Removes the last element {@code e} such that |
| * {@code o.equals(e)}, if such an element exists in this deque. |
| * If the deque does not contain the element, it is unchanged. |
| * |
| * @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 {@code null} |
| */ |
| public boolean removeLastOccurrence(Object o) { |
| checkNotNull(o); |
| for (Node<E> p = last(); p != null; p = pred(p)) { |
| E item = p.item; |
| if (item != null && o.equals(item) && p.casItem(item, null)) { |
| unlink(p); |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| /** |
| * Returns {@code true} 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) return false; |
| for (Node<E> p = first(); p != null; p = succ(p)) { |
| E item = p.item; |
| if (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() { |
| long count = 0; |
| for (Node<E> p = first(); p != null; p = succ(p)) |
| if (p.item != null) |
| ++count; |
| return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count; |
| } |
| |
| /** |
| * Removes the first element {@code e} such that |
| * {@code o.equals(e)}, if such an element exists in this deque. |
| * If the deque does not contain the element, it is unchanged. |
| * |
| * @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 {@code 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. The behavior of this operation is undefined if |
| * the specified collection is modified while the operation is in |
| * progress. (This implies that the behavior of this call is undefined if |
| * the specified Collection is this deque, and this deque is nonempty.) |
| * |
| * @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 {@code c} or any element within it |
| * is {@code null} |
| */ |
| public boolean addAll(Collection<? extends E> c) { |
| Iterator<? extends E> it = c.iterator(); |
| if (!it.hasNext()) |
| return false; |
| do { |
| addLast(it.next()); |
| } while (it.hasNext()); |
| return true; |
| } |
| |
| /** |
| * Removes all of the elements from this deque. |
| */ |
| public void clear() { |
| while (pollFirst() != null) |
| ; |
| } |
| |
| /** |
| * 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 toArrayList().toArray(); |
| } |
| |
| /** |
| * 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> |
| * 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 |
| */ |
| public <T> T[] toArray(T[] a) { |
| return toArrayList().toArray(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 {@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. |
| * |
| * @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 {@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. |
| */ |
| 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) { |
| // p might be active end or TERMINATOR node; both are OK |
| nextNode = null; |
| nextItem = null; |
| break; |
| } |
| E item = p.item; |
| if (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 { |
| Node<E> startNode() { return first(); } |
| Node<E> nextNode(Node<E> p) { return succ(p); } |
| } |
| |
| /** Descending iterator */ |
| private class DescendingItr extends AbstractItr { |
| Node<E> startNode() { return last(); } |
| Node<E> nextNode(Node<E> p) { return pred(p); } |
| } |
| |
| /** |
| * Save the state to a stream (that is, serialize it). |
| * |
| * @serialData All of the elements (each an {@code E}) in |
| * the proper order, followed by a null |
| * @param s the stream |
| */ |
| private void writeObject(java.io.ObjectOutputStream s) |
| throws java.io.IOException { |
| |
| // 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.item; |
| 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(); |
| tail = head = new Node<E>(null); |
| // Read in all elements and place in queue |
| for (;;) { |
| @SuppressWarnings("unchecked") |
| E item = (E)s.readObject(); |
| if (item == null) |
| break; |
| else |
| offer(item); |
| } |
| } |
| |
| // Unsafe mechanics |
| |
| private static final sun.misc.Unsafe UNSAFE = |
| sun.misc.Unsafe.getUnsafe(); |
| private static final long headOffset = |
| objectFieldOffset(UNSAFE, "head", ConcurrentLinkedDeque.class); |
| private static final long tailOffset = |
| objectFieldOffset(UNSAFE, "tail", ConcurrentLinkedDeque.class); |
| |
| private boolean casHead(Node<E> cmp, Node<E> val) { |
| return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
| } |
| |
| private boolean casTail(Node<E> cmp, Node<E> val) { |
| return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
| } |
| |
| static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
| String field, Class<?> klazz) { |
| try { |
| return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
| } catch (NoSuchFieldException e) { |
| // Convert Exception to corresponding Error |
| NoSuchFieldError error = new NoSuchFieldError(field); |
| error.initCause(e); |
| throw error; |
| } |
| } |
| } |
