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/* |
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* Written by Doug Lea and Martin Buchholz with assistance from members of |
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* JCP JSR-166 Expert Group and released to the public domain, as explained |
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* at http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.AbstractCollection; |
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import java.util.ArrayList; |
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import java.util.Collection; |
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import java.util.Deque; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.Spliterator; |
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import java.util.Spliterators; |
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import java.util.function.Consumer; |
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|
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/** |
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* An unbounded concurrent {@linkplain Deque deque} based on linked nodes. |
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* Concurrent insertion, removal, and access operations execute safely |
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* across multiple threads. |
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* A {@code ConcurrentLinkedDeque} is an appropriate choice when |
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* many threads will share access to a common collection. |
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* Like most other concurrent collection implementations, this class |
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* does not permit the use of {@code null} elements. |
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* |
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* <p>Iterators and spliterators are |
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* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} method |
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* is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these deques, determining the current number |
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* of elements requires a traversal of the elements, and so may report |
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* inaccurate results if this collection is modified during traversal. |
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* Additionally, the bulk operations {@code addAll}, |
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* {@code removeAll}, {@code retainAll}, {@code containsAll}, |
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* {@code equals}, and {@code toArray} are <em>not</em> guaranteed |
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* to be performed atomically. For example, an iterator operating |
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* concurrently with an {@code addAll} operation might view only some |
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* of the added elements. |
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* |
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* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Deque} and {@link Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent collections, |
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* actions in a thread prior to placing an object into a |
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* {@code ConcurrentLinkedDeque} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code ConcurrentLinkedDeque} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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* @author Martin Buchholz |
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* @param <E> the type of elements held in this deque |
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*/ |
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public class ConcurrentLinkedDeque<E> |
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extends AbstractCollection<E> |
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implements Deque<E>, java.io.Serializable { |
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|
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/* |
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* This is an implementation of a concurrent lock-free deque |
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* supporting interior removes but not interior insertions, as |
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* required to support the entire Deque interface. |
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* |
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* We extend the techniques developed for ConcurrentLinkedQueue and |
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* LinkedTransferQueue (see the internal docs for those classes). |
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* Understanding the ConcurrentLinkedQueue implementation is a |
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* prerequisite for understanding the implementation of this class. |
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* |
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* The data structure is a symmetrical doubly-linked "GC-robust" |
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* linked list of nodes. We minimize the number of volatile writes |
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* using two techniques: advancing multiple hops with a single CAS |
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* and mixing volatile and non-volatile writes of the same memory |
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* locations. |
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* |
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* A node contains the expected E ("item") and links to predecessor |
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* ("prev") and successor ("next") nodes: |
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* |
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* class Node<E> { volatile Node<E> prev, next; volatile E item; } |
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* |
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* A node p is considered "live" if it contains a non-null item |
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* (p.item != null). When an item is CASed to null, the item is |
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* atomically logically deleted from the collection. |
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* |
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* At any time, there is precisely one "first" node with a null |
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* prev reference that terminates any chain of prev references |
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* starting at a live node. Similarly there is precisely one |
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* "last" node terminating any chain of next references starting at |
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* a live node. The "first" and "last" nodes may or may not be live. |
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* The "first" and "last" nodes are always mutually reachable. |
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* |
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* A new element is added atomically by CASing the null prev or |
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* next reference in the first or last node to a fresh node |
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* containing the element. The element's node atomically becomes |
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* "live" at that point. |
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* |
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* A node is considered "active" if it is a live node, or the |
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* first or last node. Active nodes cannot be unlinked. |
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* |
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* A "self-link" is a next or prev reference that is the same node: |
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* p.prev == p or p.next == p |
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* Self-links are used in the node unlinking process. Active nodes |
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* never have self-links. |
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* |
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* A node p is active if and only if: |
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* |
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* p.item != null || |
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* (p.prev == null && p.next != p) || |
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* (p.next == null && p.prev != p) |
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* |
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* The deque object has two node references, "head" and "tail". |
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* The head and tail are only approximations to the first and last |
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* nodes of the deque. The first node can always be found by |
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* following prev pointers from head; likewise for tail. However, |
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* it is permissible for head and tail to be referring to deleted |
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* nodes that have been unlinked and so may not be reachable from |
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* any live node. |
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* |
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* There are 3 stages of node deletion; |
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* "logical deletion", "unlinking", and "gc-unlinking". |
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* |
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* 1. "logical deletion" by CASing item to null atomically removes |
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* the element from the collection, and makes the containing node |
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* eligible for unlinking. |
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* |
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* 2. "unlinking" makes a deleted node unreachable from active |
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* nodes, and thus eventually reclaimable by GC. Unlinked nodes |
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* may remain reachable indefinitely from an iterator. |
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* |
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* Physical node unlinking is merely an optimization (albeit a |
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* critical one), and so can be performed at our convenience. At |
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* any time, the set of live nodes maintained by prev and next |
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* links are identical, that is, the live nodes found via next |
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* links from the first node is equal to the elements found via |
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* prev links from the last node. However, this is not true for |
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* nodes that have already been logically deleted - such nodes may |
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* be reachable in one direction only. |
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* |
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* 3. "gc-unlinking" takes unlinking further by making active |
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* nodes unreachable from deleted nodes, making it easier for the |
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* GC to reclaim future deleted nodes. This step makes the data |
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* structure "gc-robust", as first described in detail by Boehm |
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* (http://portal.acm.org/citation.cfm?doid=503272.503282). |
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* |
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* GC-unlinked nodes may remain reachable indefinitely from an |
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* iterator, but unlike unlinked nodes, are never reachable from |
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* head or tail. |
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* |
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* Making the data structure GC-robust will eliminate the risk of |
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* unbounded memory retention with conservative GCs and is likely |
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* to improve performance with generational GCs. |
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* |
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* When a node is dequeued at either end, e.g. via poll(), we would |
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* like to break any references from the node to active nodes. We |
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* develop further the use of self-links that was very effective in |
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* other concurrent collection classes. The idea is to replace |
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* prev and next pointers with special values that are interpreted |
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* to mean off-the-list-at-one-end. These are approximations, but |
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* good enough to preserve the properties we want in our |
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* traversals, e.g. we guarantee that a traversal will never visit |
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* the same element twice, but we don't guarantee whether a |
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* traversal that runs out of elements will be able to see more |
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* elements later after enqueues at that end. Doing gc-unlinking |
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* safely is particularly tricky, since any node can be in use |
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* indefinitely (for example by an iterator). We must ensure that |
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* the nodes pointed at by head/tail never get gc-unlinked, since |
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* head/tail are needed to get "back on track" by other nodes that |
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* are gc-unlinked. gc-unlinking accounts for much of the |
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* implementation complexity. |
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* |
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* Since neither unlinking nor gc-unlinking are necessary for |
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* correctness, there are many implementation choices regarding |
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* frequency (eagerness) of these operations. Since volatile |
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* reads are likely to be much cheaper than CASes, saving CASes by |
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* unlinking multiple adjacent nodes at a time may be a win. |
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* gc-unlinking can be performed rarely and still be effective, |
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* since it is most important that long chains of deleted nodes |
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* are occasionally broken. |
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* |
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* The actual representation we use is that p.next == p means to |
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* goto the first node (which in turn is reached by following prev |
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* pointers from head), and p.next == null && p.prev == p means |
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* that the iteration is at an end and that p is a (static final) |
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* dummy node, NEXT_TERMINATOR, and not the last active node. |
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* Finishing the iteration when encountering such a TERMINATOR is |
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* good enough for read-only traversals, so such traversals can use |
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* p.next == null as the termination condition. When we need to |
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* find the last (active) node, for enqueueing a new node, we need |
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* to check whether we have reached a TERMINATOR node; if so, |
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* restart traversal from tail. |
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* |
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* The implementation is completely directionally symmetrical, |
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* except that most public methods that iterate through the list |
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* follow next pointers ("forward" direction). |
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* |
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* We believe (without full proof) that all single-element deque |
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* operations (e.g., addFirst, peekLast, pollLast) are linearizable |
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* (see Herlihy and Shavit's book). However, some combinations of |
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* operations are known not to be linearizable. In particular, |
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* when an addFirst(A) is racing with pollFirst() removing B, it is |
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* possible for an observer iterating over the elements to observe |
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* A B C and subsequently observe A C, even though no interior |
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* removes are ever performed. Nevertheless, iterators behave |
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* reasonably, providing the "weakly consistent" guarantees. |
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* |
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* Empirically, microbenchmarks suggest that this class adds about |
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* 40% overhead relative to ConcurrentLinkedQueue, which feels as |
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* good as we can hope for. |
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*/ |
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|
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private static final long serialVersionUID = 876323262645176354L; |
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|
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/** |
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* A node from which the first node on list (that is, the unique node p |
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* with p.prev == null && p.next != p) can be reached in O(1) time. |
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* Invariants: |
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* - the first node is always O(1) reachable from head via prev links |
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* - all live nodes are reachable from the first node via succ() |
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* - head != null |
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* - (tmp = head).next != tmp || tmp != head |
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* - head is never gc-unlinked (but may be unlinked) |
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* Non-invariants: |
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* - head.item may or may not be null |
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* - head may not be reachable from the first or last node, or from tail |
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*/ |
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private transient volatile Node<E> head; |
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|
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/** |
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* A node from which the last node on list (that is, the unique node p |
237 |
* with p.next == null && p.prev != p) can be reached in O(1) time. |
238 |
* Invariants: |
239 |
* - the last node is always O(1) reachable from tail via next links |
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* - all live nodes are reachable from the last node via pred() |
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* - tail != null |
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* - tail is never gc-unlinked (but may be unlinked) |
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* Non-invariants: |
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* - tail.item may or may not be null |
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* - tail may not be reachable from the first or last node, or from head |
246 |
*/ |
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private transient volatile Node<E> tail; |
248 |
|
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private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR; |
250 |
|
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@SuppressWarnings("unchecked") |
252 |
Node<E> prevTerminator() { |
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return (Node<E>) PREV_TERMINATOR; |
254 |
} |
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|
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@SuppressWarnings("unchecked") |
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Node<E> nextTerminator() { |
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return (Node<E>) NEXT_TERMINATOR; |
259 |
} |
260 |
|
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static final class Node<E> { |
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volatile Node<E> prev; |
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volatile E item; |
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volatile Node<E> next; |
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|
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Node() { // default constructor for NEXT_TERMINATOR, PREV_TERMINATOR |
267 |
} |
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|
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/** |
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* Constructs a new node. Uses relaxed write because item can |
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* only be seen after publication via casNext or casPrev. |
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*/ |
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Node(E item) { |
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UNSAFE.putObject(this, itemOffset, item); |
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} |
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|
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boolean casItem(E cmp, E val) { |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
279 |
} |
280 |
|
281 |
void lazySetNext(Node<E> val) { |
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UNSAFE.putOrderedObject(this, nextOffset, val); |
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} |
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|
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boolean casNext(Node<E> cmp, Node<E> val) { |
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return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
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} |
288 |
|
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void lazySetPrev(Node<E> val) { |
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UNSAFE.putOrderedObject(this, prevOffset, val); |
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} |
292 |
|
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boolean casPrev(Node<E> cmp, Node<E> val) { |
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return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val); |
295 |
} |
296 |
|
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// Unsafe mechanics |
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|
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private static final sun.misc.Unsafe UNSAFE; |
300 |
private static final long prevOffset; |
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private static final long itemOffset; |
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private static final long nextOffset; |
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|
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static { |
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try { |
306 |
UNSAFE = sun.misc.Unsafe.getUnsafe(); |
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Class<?> k = Node.class; |
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prevOffset = UNSAFE.objectFieldOffset |
309 |
(k.getDeclaredField("prev")); |
310 |
itemOffset = UNSAFE.objectFieldOffset |
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(k.getDeclaredField("item")); |
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nextOffset = UNSAFE.objectFieldOffset |
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(k.getDeclaredField("next")); |
314 |
} catch (Exception e) { |
315 |
throw new Error(e); |
316 |
} |
317 |
} |
318 |
} |
319 |
|
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/** |
321 |
* Links e as first element. |
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*/ |
323 |
private void linkFirst(E e) { |
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checkNotNull(e); |
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final Node<E> newNode = new Node<E>(e); |
326 |
|
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restartFromHead: |
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for (;;) |
329 |
for (Node<E> h = head, p = h, q;;) { |
330 |
if ((q = p.prev) != null && |
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(q = (p = q).prev) != null) |
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// Check for head updates every other hop. |
333 |
// If p == q, we are sure to follow head instead. |
334 |
p = (h != (h = head)) ? h : q; |
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else if (p.next == p) // PREV_TERMINATOR |
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continue restartFromHead; |
337 |
else { |
338 |
// p is first node |
339 |
newNode.lazySetNext(p); // CAS piggyback |
340 |
if (p.casPrev(null, newNode)) { |
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// Successful CAS is the linearization point |
342 |
// for e to become an element of this deque, |
343 |
// and for newNode to become "live". |
344 |
if (p != h) // hop two nodes at a time |
345 |
casHead(h, newNode); // Failure is OK. |
346 |
return; |
347 |
} |
348 |
// Lost CAS race to another thread; re-read prev |
349 |
} |
350 |
} |
351 |
} |
352 |
|
353 |
/** |
354 |
* Links e as last element. |
355 |
*/ |
356 |
private void linkLast(E e) { |
357 |
checkNotNull(e); |
358 |
final Node<E> newNode = new Node<E>(e); |
359 |
|
360 |
restartFromTail: |
361 |
for (;;) |
362 |
for (Node<E> t = tail, p = t, q;;) { |
363 |
if ((q = p.next) != null && |
364 |
(q = (p = q).next) != null) |
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// Check for tail updates every other hop. |
366 |
// If p == q, we are sure to follow tail instead. |
367 |
p = (t != (t = tail)) ? t : q; |
368 |
else if (p.prev == p) // NEXT_TERMINATOR |
369 |
continue restartFromTail; |
370 |
else { |
371 |
// p is last node |
372 |
newNode.lazySetPrev(p); // CAS piggyback |
373 |
if (p.casNext(null, newNode)) { |
374 |
// Successful CAS is the linearization point |
375 |
// for e to become an element of this deque, |
376 |
// and for newNode to become "live". |
377 |
if (p != t) // hop two nodes at a time |
378 |
casTail(t, newNode); // Failure is OK. |
379 |
return; |
380 |
} |
381 |
// Lost CAS race to another thread; re-read next |
382 |
} |
383 |
} |
384 |
} |
385 |
|
386 |
private static final int HOPS = 2; |
387 |
|
388 |
/** |
389 |
* Unlinks non-null node x. |
390 |
*/ |
391 |
void unlink(Node<E> x) { |
392 |
// assert x != null; |
393 |
// assert x.item == null; |
394 |
// assert x != PREV_TERMINATOR; |
395 |
// assert x != NEXT_TERMINATOR; |
396 |
|
397 |
final Node<E> prev = x.prev; |
398 |
final Node<E> next = x.next; |
399 |
if (prev == null) { |
400 |
unlinkFirst(x, next); |
401 |
} else if (next == null) { |
402 |
unlinkLast(x, prev); |
403 |
} else { |
404 |
// Unlink interior node. |
405 |
// |
406 |
// This is the common case, since a series of polls at the |
407 |
// same end will be "interior" removes, except perhaps for |
408 |
// the first one, since end nodes cannot be unlinked. |
409 |
// |
410 |
// At any time, all active nodes are mutually reachable by |
411 |
// following a sequence of either next or prev pointers. |
412 |
// |
413 |
// Our strategy is to find the unique active predecessor |
414 |
// and successor of x. Try to fix up their links so that |
415 |
// they point to each other, leaving x unreachable from |
416 |
// active nodes. If successful, and if x has no live |
417 |
// predecessor/successor, we additionally try to gc-unlink, |
418 |
// leaving active nodes unreachable from x, by rechecking |
419 |
// that the status of predecessor and successor are |
420 |
// unchanged and ensuring that x is not reachable from |
421 |
// tail/head, before setting x's prev/next links to their |
422 |
// logical approximate replacements, self/TERMINATOR. |
423 |
Node<E> activePred, activeSucc; |
424 |
boolean isFirst, isLast; |
425 |
int hops = 1; |
426 |
|
427 |
// Find active predecessor |
428 |
for (Node<E> p = prev; ; ++hops) { |
429 |
if (p.item != null) { |
430 |
activePred = p; |
431 |
isFirst = false; |
432 |
break; |
433 |
} |
434 |
Node<E> q = p.prev; |
435 |
if (q == null) { |
436 |
if (p.next == p) |
437 |
return; |
438 |
activePred = p; |
439 |
isFirst = true; |
440 |
break; |
441 |
} |
442 |
else if (p == q) |
443 |
return; |
444 |
else |
445 |
p = q; |
446 |
} |
447 |
|
448 |
// Find active successor |
449 |
for (Node<E> p = next; ; ++hops) { |
450 |
if (p.item != null) { |
451 |
activeSucc = p; |
452 |
isLast = false; |
453 |
break; |
454 |
} |
455 |
Node<E> q = p.next; |
456 |
if (q == null) { |
457 |
if (p.prev == p) |
458 |
return; |
459 |
activeSucc = p; |
460 |
isLast = true; |
461 |
break; |
462 |
} |
463 |
else if (p == q) |
464 |
return; |
465 |
else |
466 |
p = q; |
467 |
} |
468 |
|
469 |
// TODO: better HOP heuristics |
470 |
if (hops < HOPS |
471 |
// always squeeze out interior deleted nodes |
472 |
&& (isFirst | isLast)) |
473 |
return; |
474 |
|
475 |
// Squeeze out deleted nodes between activePred and |
476 |
// activeSucc, including x. |
477 |
skipDeletedSuccessors(activePred); |
478 |
skipDeletedPredecessors(activeSucc); |
479 |
|
480 |
// Try to gc-unlink, if possible |
481 |
if ((isFirst | isLast) && |
482 |
|
483 |
// Recheck expected state of predecessor and successor |
484 |
(activePred.next == activeSucc) && |
485 |
(activeSucc.prev == activePred) && |
486 |
(isFirst ? activePred.prev == null : activePred.item != null) && |
487 |
(isLast ? activeSucc.next == null : activeSucc.item != null)) { |
488 |
|
489 |
updateHead(); // Ensure x is not reachable from head |
490 |
updateTail(); // Ensure x is not reachable from tail |
491 |
|
492 |
// Finally, actually gc-unlink |
493 |
x.lazySetPrev(isFirst ? prevTerminator() : x); |
494 |
x.lazySetNext(isLast ? nextTerminator() : x); |
495 |
} |
496 |
} |
497 |
} |
498 |
|
499 |
/** |
500 |
* Unlinks non-null first node. |
501 |
*/ |
502 |
private void unlinkFirst(Node<E> first, Node<E> next) { |
503 |
// assert first != null; |
504 |
// assert next != null; |
505 |
// assert first.item == null; |
506 |
for (Node<E> o = null, p = next, q;;) { |
507 |
if (p.item != null || (q = p.next) == null) { |
508 |
if (o != null && p.prev != p && first.casNext(next, p)) { |
509 |
skipDeletedPredecessors(p); |
510 |
if (first.prev == null && |
511 |
(p.next == null || p.item != null) && |
512 |
p.prev == first) { |
513 |
|
514 |
updateHead(); // Ensure o is not reachable from head |
515 |
updateTail(); // Ensure o is not reachable from tail |
516 |
|
517 |
// Finally, actually gc-unlink |
518 |
o.lazySetNext(o); |
519 |
o.lazySetPrev(prevTerminator()); |
520 |
} |
521 |
} |
522 |
return; |
523 |
} |
524 |
else if (p == q) |
525 |
return; |
526 |
else { |
527 |
o = p; |
528 |
p = q; |
529 |
} |
530 |
} |
531 |
} |
532 |
|
533 |
/** |
534 |
* Unlinks non-null last node. |
535 |
*/ |
536 |
private void unlinkLast(Node<E> last, Node<E> prev) { |
537 |
// assert last != null; |
538 |
// assert prev != null; |
539 |
// assert last.item == null; |
540 |
for (Node<E> o = null, p = prev, q;;) { |
541 |
if (p.item != null || (q = p.prev) == null) { |
542 |
if (o != null && p.next != p && last.casPrev(prev, p)) { |
543 |
skipDeletedSuccessors(p); |
544 |
if (last.next == null && |
545 |
(p.prev == null || p.item != null) && |
546 |
p.next == last) { |
547 |
|
548 |
updateHead(); // Ensure o is not reachable from head |
549 |
updateTail(); // Ensure o is not reachable from tail |
550 |
|
551 |
// Finally, actually gc-unlink |
552 |
o.lazySetPrev(o); |
553 |
o.lazySetNext(nextTerminator()); |
554 |
} |
555 |
} |
556 |
return; |
557 |
} |
558 |
else if (p == q) |
559 |
return; |
560 |
else { |
561 |
o = p; |
562 |
p = q; |
563 |
} |
564 |
} |
565 |
} |
566 |
|
567 |
/** |
568 |
* Guarantees that any node which was unlinked before a call to |
569 |
* this method will be unreachable from head after it returns. |
570 |
* Does not guarantee to eliminate slack, only that head will |
571 |
* point to a node that was active while this method was running. |
572 |
*/ |
573 |
private final void updateHead() { |
574 |
// Either head already points to an active node, or we keep |
575 |
// trying to cas it to the first node until it does. |
576 |
Node<E> h, p, q; |
577 |
restartFromHead: |
578 |
while ((h = head).item == null && (p = h.prev) != null) { |
579 |
for (;;) { |
580 |
if ((q = p.prev) == null || |
581 |
(q = (p = q).prev) == null) { |
582 |
// It is possible that p is PREV_TERMINATOR, |
583 |
// but if so, the CAS is guaranteed to fail. |
584 |
if (casHead(h, p)) |
585 |
return; |
586 |
else |
587 |
continue restartFromHead; |
588 |
} |
589 |
else if (h != head) |
590 |
continue restartFromHead; |
591 |
else |
592 |
p = q; |
593 |
} |
594 |
} |
595 |
} |
596 |
|
597 |
/** |
598 |
* Guarantees that any node which was unlinked before a call to |
599 |
* this method will be unreachable from tail after it returns. |
600 |
* Does not guarantee to eliminate slack, only that tail will |
601 |
* point to a node that was active while this method was running. |
602 |
*/ |
603 |
private final void updateTail() { |
604 |
// Either tail already points to an active node, or we keep |
605 |
// trying to cas it to the last node until it does. |
606 |
Node<E> t, p, q; |
607 |
restartFromTail: |
608 |
while ((t = tail).item == null && (p = t.next) != null) { |
609 |
for (;;) { |
610 |
if ((q = p.next) == null || |
611 |
(q = (p = q).next) == null) { |
612 |
// It is possible that p is NEXT_TERMINATOR, |
613 |
// but if so, the CAS is guaranteed to fail. |
614 |
if (casTail(t, p)) |
615 |
return; |
616 |
else |
617 |
continue restartFromTail; |
618 |
} |
619 |
else if (t != tail) |
620 |
continue restartFromTail; |
621 |
else |
622 |
p = q; |
623 |
} |
624 |
} |
625 |
} |
626 |
|
627 |
private void skipDeletedPredecessors(Node<E> x) { |
628 |
whileActive: |
629 |
do { |
630 |
Node<E> prev = x.prev; |
631 |
// assert prev != null; |
632 |
// assert x != NEXT_TERMINATOR; |
633 |
// assert x != PREV_TERMINATOR; |
634 |
Node<E> p = prev; |
635 |
findActive: |
636 |
for (;;) { |
637 |
if (p.item != null) |
638 |
break findActive; |
639 |
Node<E> q = p.prev; |
640 |
if (q == null) { |
641 |
if (p.next == p) |
642 |
continue whileActive; |
643 |
break findActive; |
644 |
} |
645 |
else if (p == q) |
646 |
continue whileActive; |
647 |
else |
648 |
p = q; |
649 |
} |
650 |
|
651 |
// found active CAS target |
652 |
if (prev == p || x.casPrev(prev, p)) |
653 |
return; |
654 |
|
655 |
} while (x.item != null || x.next == null); |
656 |
} |
657 |
|
658 |
private void skipDeletedSuccessors(Node<E> x) { |
659 |
whileActive: |
660 |
do { |
661 |
Node<E> next = x.next; |
662 |
// assert next != null; |
663 |
// assert x != NEXT_TERMINATOR; |
664 |
// assert x != PREV_TERMINATOR; |
665 |
Node<E> p = next; |
666 |
findActive: |
667 |
for (;;) { |
668 |
if (p.item != null) |
669 |
break findActive; |
670 |
Node<E> q = p.next; |
671 |
if (q == null) { |
672 |
if (p.prev == p) |
673 |
continue whileActive; |
674 |
break findActive; |
675 |
} |
676 |
else if (p == q) |
677 |
continue whileActive; |
678 |
else |
679 |
p = q; |
680 |
} |
681 |
|
682 |
// found active CAS target |
683 |
if (next == p || x.casNext(next, p)) |
684 |
return; |
685 |
|
686 |
} while (x.item != null || x.prev == null); |
687 |
} |
688 |
|
689 |
/** |
690 |
* Returns the successor of p, or the first node if p.next has been |
691 |
* linked to self, which will only be true if traversing with a |
692 |
* stale pointer that is now off the list. |
693 |
*/ |
694 |
final Node<E> succ(Node<E> p) { |
695 |
// TODO: should we skip deleted nodes here? |
696 |
Node<E> q = p.next; |
697 |
return (p == q) ? first() : q; |
698 |
} |
699 |
|
700 |
/** |
701 |
* Returns the predecessor of p, or the last node if p.prev has been |
702 |
* linked to self, which will only be true if traversing with a |
703 |
* stale pointer that is now off the list. |
704 |
*/ |
705 |
final Node<E> pred(Node<E> p) { |
706 |
Node<E> q = p.prev; |
707 |
return (p == q) ? last() : q; |
708 |
} |
709 |
|
710 |
/** |
711 |
* Returns the first node, the unique node p for which: |
712 |
* p.prev == null && p.next != p |
713 |
* The returned node may or may not be logically deleted. |
714 |
* Guarantees that head is set to the returned node. |
715 |
*/ |
716 |
Node<E> first() { |
717 |
restartFromHead: |
718 |
for (;;) |
719 |
for (Node<E> h = head, p = h, q;;) { |
720 |
if ((q = p.prev) != null && |
721 |
(q = (p = q).prev) != null) |
722 |
// Check for head updates every other hop. |
723 |
// If p == q, we are sure to follow head instead. |
724 |
p = (h != (h = head)) ? h : q; |
725 |
else if (p == h |
726 |
// It is possible that p is PREV_TERMINATOR, |
727 |
// but if so, the CAS is guaranteed to fail. |
728 |
|| casHead(h, p)) |
729 |
return p; |
730 |
else |
731 |
continue restartFromHead; |
732 |
} |
733 |
} |
734 |
|
735 |
/** |
736 |
* Returns the last node, the unique node p for which: |
737 |
* p.next == null && p.prev != p |
738 |
* The returned node may or may not be logically deleted. |
739 |
* Guarantees that tail is set to the returned node. |
740 |
*/ |
741 |
Node<E> last() { |
742 |
restartFromTail: |
743 |
for (;;) |
744 |
for (Node<E> t = tail, p = t, q;;) { |
745 |
if ((q = p.next) != null && |
746 |
(q = (p = q).next) != null) |
747 |
// Check for tail updates every other hop. |
748 |
// If p == q, we are sure to follow tail instead. |
749 |
p = (t != (t = tail)) ? t : q; |
750 |
else if (p == t |
751 |
// It is possible that p is NEXT_TERMINATOR, |
752 |
// but if so, the CAS is guaranteed to fail. |
753 |
|| casTail(t, p)) |
754 |
return p; |
755 |
else |
756 |
continue restartFromTail; |
757 |
} |
758 |
} |
759 |
|
760 |
// Minor convenience utilities |
761 |
|
762 |
/** |
763 |
* Throws NullPointerException if argument is null. |
764 |
* |
765 |
* @param v the element |
766 |
*/ |
767 |
private static void checkNotNull(Object v) { |
768 |
if (v == null) |
769 |
throw new NullPointerException(); |
770 |
} |
771 |
|
772 |
/** |
773 |
* Returns element unless it is null, in which case throws |
774 |
* NoSuchElementException. |
775 |
* |
776 |
* @param v the element |
777 |
* @return the element |
778 |
*/ |
779 |
private E screenNullResult(E v) { |
780 |
if (v == null) |
781 |
throw new NoSuchElementException(); |
782 |
return v; |
783 |
} |
784 |
|
785 |
/** |
786 |
* Creates an array list and fills it with elements of this list. |
787 |
* Used by toArray. |
788 |
* |
789 |
* @return the array list |
790 |
*/ |
791 |
private ArrayList<E> toArrayList() { |
792 |
ArrayList<E> list = new ArrayList<E>(); |
793 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
794 |
E item = p.item; |
795 |
if (item != null) |
796 |
list.add(item); |
797 |
} |
798 |
return list; |
799 |
} |
800 |
|
801 |
/** |
802 |
* Constructs an empty deque. |
803 |
*/ |
804 |
public ConcurrentLinkedDeque() { |
805 |
head = tail = new Node<E>(null); |
806 |
} |
807 |
|
808 |
/** |
809 |
* Constructs a deque initially containing the elements of |
810 |
* the given collection, added in traversal order of the |
811 |
* collection's iterator. |
812 |
* |
813 |
* @param c the collection of elements to initially contain |
814 |
* @throws NullPointerException if the specified collection or any |
815 |
* of its elements are null |
816 |
*/ |
817 |
public ConcurrentLinkedDeque(Collection<? extends E> c) { |
818 |
// Copy c into a private chain of Nodes |
819 |
Node<E> h = null, t = null; |
820 |
for (E e : c) { |
821 |
checkNotNull(e); |
822 |
Node<E> newNode = new Node<E>(e); |
823 |
if (h == null) |
824 |
h = t = newNode; |
825 |
else { |
826 |
t.lazySetNext(newNode); |
827 |
newNode.lazySetPrev(t); |
828 |
t = newNode; |
829 |
} |
830 |
} |
831 |
initHeadTail(h, t); |
832 |
} |
833 |
|
834 |
/** |
835 |
* Initializes head and tail, ensuring invariants hold. |
836 |
*/ |
837 |
private void initHeadTail(Node<E> h, Node<E> t) { |
838 |
if (h == t) { |
839 |
if (h == null) |
840 |
h = t = new Node<E>(null); |
841 |
else { |
842 |
// Avoid edge case of a single Node with non-null item. |
843 |
Node<E> newNode = new Node<E>(null); |
844 |
t.lazySetNext(newNode); |
845 |
newNode.lazySetPrev(t); |
846 |
t = newNode; |
847 |
} |
848 |
} |
849 |
head = h; |
850 |
tail = t; |
851 |
} |
852 |
|
853 |
/** |
854 |
* Inserts the specified element at the front of this deque. |
855 |
* As the deque is unbounded, this method will never throw |
856 |
* {@link IllegalStateException}. |
857 |
* |
858 |
* @throws NullPointerException if the specified element is null |
859 |
*/ |
860 |
public void addFirst(E e) { |
861 |
linkFirst(e); |
862 |
} |
863 |
|
864 |
/** |
865 |
* Inserts the specified element at the end of this deque. |
866 |
* As the deque is unbounded, this method will never throw |
867 |
* {@link IllegalStateException}. |
868 |
* |
869 |
* <p>This method is equivalent to {@link #add}. |
870 |
* |
871 |
* @throws NullPointerException if the specified element is null |
872 |
*/ |
873 |
public void addLast(E e) { |
874 |
linkLast(e); |
875 |
} |
876 |
|
877 |
/** |
878 |
* Inserts the specified element at the front of this deque. |
879 |
* As the deque is unbounded, this method will never return {@code false}. |
880 |
* |
881 |
* @return {@code true} (as specified by {@link Deque#offerFirst}) |
882 |
* @throws NullPointerException if the specified element is null |
883 |
*/ |
884 |
public boolean offerFirst(E e) { |
885 |
linkFirst(e); |
886 |
return true; |
887 |
} |
888 |
|
889 |
/** |
890 |
* Inserts the specified element at the end of this deque. |
891 |
* As the deque is unbounded, this method will never return {@code false}. |
892 |
* |
893 |
* <p>This method is equivalent to {@link #add}. |
894 |
* |
895 |
* @return {@code true} (as specified by {@link Deque#offerLast}) |
896 |
* @throws NullPointerException if the specified element is null |
897 |
*/ |
898 |
public boolean offerLast(E e) { |
899 |
linkLast(e); |
900 |
return true; |
901 |
} |
902 |
|
903 |
public E peekFirst() { |
904 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
905 |
E item = p.item; |
906 |
if (item != null) |
907 |
return item; |
908 |
} |
909 |
return null; |
910 |
} |
911 |
|
912 |
public E peekLast() { |
913 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
914 |
E item = p.item; |
915 |
if (item != null) |
916 |
return item; |
917 |
} |
918 |
return null; |
919 |
} |
920 |
|
921 |
/** |
922 |
* @throws NoSuchElementException {@inheritDoc} |
923 |
*/ |
924 |
public E getFirst() { |
925 |
return screenNullResult(peekFirst()); |
926 |
} |
927 |
|
928 |
/** |
929 |
* @throws NoSuchElementException {@inheritDoc} |
930 |
*/ |
931 |
public E getLast() { |
932 |
return screenNullResult(peekLast()); |
933 |
} |
934 |
|
935 |
public E pollFirst() { |
936 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
937 |
E item = p.item; |
938 |
if (item != null && p.casItem(item, null)) { |
939 |
unlink(p); |
940 |
return item; |
941 |
} |
942 |
} |
943 |
return null; |
944 |
} |
945 |
|
946 |
public E pollLast() { |
947 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
948 |
E item = p.item; |
949 |
if (item != null && p.casItem(item, null)) { |
950 |
unlink(p); |
951 |
return item; |
952 |
} |
953 |
} |
954 |
return null; |
955 |
} |
956 |
|
957 |
/** |
958 |
* @throws NoSuchElementException {@inheritDoc} |
959 |
*/ |
960 |
public E removeFirst() { |
961 |
return screenNullResult(pollFirst()); |
962 |
} |
963 |
|
964 |
/** |
965 |
* @throws NoSuchElementException {@inheritDoc} |
966 |
*/ |
967 |
public E removeLast() { |
968 |
return screenNullResult(pollLast()); |
969 |
} |
970 |
|
971 |
// *** Queue and stack methods *** |
972 |
|
973 |
/** |
974 |
* Inserts the specified element at the tail of this deque. |
975 |
* As the deque is unbounded, this method will never return {@code false}. |
976 |
* |
977 |
* @return {@code true} (as specified by {@link Queue#offer}) |
978 |
* @throws NullPointerException if the specified element is null |
979 |
*/ |
980 |
public boolean offer(E e) { |
981 |
return offerLast(e); |
982 |
} |
983 |
|
984 |
/** |
985 |
* Inserts the specified element at the tail of this deque. |
986 |
* As the deque is unbounded, this method will never throw |
987 |
* {@link IllegalStateException} or return {@code false}. |
988 |
* |
989 |
* @return {@code true} (as specified by {@link Collection#add}) |
990 |
* @throws NullPointerException if the specified element is null |
991 |
*/ |
992 |
public boolean add(E e) { |
993 |
return offerLast(e); |
994 |
} |
995 |
|
996 |
public E poll() { return pollFirst(); } |
997 |
public E peek() { return peekFirst(); } |
998 |
|
999 |
/** |
1000 |
* @throws NoSuchElementException {@inheritDoc} |
1001 |
*/ |
1002 |
public E remove() { return removeFirst(); } |
1003 |
|
1004 |
/** |
1005 |
* @throws NoSuchElementException {@inheritDoc} |
1006 |
*/ |
1007 |
public E pop() { return removeFirst(); } |
1008 |
|
1009 |
/** |
1010 |
* @throws NoSuchElementException {@inheritDoc} |
1011 |
*/ |
1012 |
public E element() { return getFirst(); } |
1013 |
|
1014 |
/** |
1015 |
* @throws NullPointerException {@inheritDoc} |
1016 |
*/ |
1017 |
public void push(E e) { addFirst(e); } |
1018 |
|
1019 |
/** |
1020 |
* Removes the first occurrence of the specified element from this deque. |
1021 |
* If the deque does not contain the element, it is unchanged. |
1022 |
* More formally, removes the first element {@code e} such that |
1023 |
* {@code o.equals(e)} (if such an element exists). |
1024 |
* Returns {@code true} if this deque contained the specified element |
1025 |
* (or equivalently, if this deque changed as a result of the call). |
1026 |
* |
1027 |
* @param o element to be removed from this deque, if present |
1028 |
* @return {@code true} if the deque contained the specified element |
1029 |
* @throws NullPointerException if the specified element is null |
1030 |
*/ |
1031 |
public boolean removeFirstOccurrence(Object o) { |
1032 |
checkNotNull(o); |
1033 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1034 |
E item = p.item; |
1035 |
if (item != null && o.equals(item) && p.casItem(item, null)) { |
1036 |
unlink(p); |
1037 |
return true; |
1038 |
} |
1039 |
} |
1040 |
return false; |
1041 |
} |
1042 |
|
1043 |
/** |
1044 |
* Removes the last occurrence of the specified element from this deque. |
1045 |
* If the deque does not contain the element, it is unchanged. |
1046 |
* More formally, removes the last element {@code e} such that |
1047 |
* {@code o.equals(e)} (if such an element exists). |
1048 |
* Returns {@code true} if this deque contained the specified element |
1049 |
* (or equivalently, if this deque changed as a result of the call). |
1050 |
* |
1051 |
* @param o element to be removed from this deque, if present |
1052 |
* @return {@code true} if the deque contained the specified element |
1053 |
* @throws NullPointerException if the specified element is null |
1054 |
*/ |
1055 |
public boolean removeLastOccurrence(Object o) { |
1056 |
checkNotNull(o); |
1057 |
for (Node<E> p = last(); p != null; p = pred(p)) { |
1058 |
E item = p.item; |
1059 |
if (item != null && o.equals(item) && p.casItem(item, null)) { |
1060 |
unlink(p); |
1061 |
return true; |
1062 |
} |
1063 |
} |
1064 |
return false; |
1065 |
} |
1066 |
|
1067 |
/** |
1068 |
* Returns {@code true} if this deque contains the specified element. |
1069 |
* More formally, returns {@code true} if and only if this deque contains |
1070 |
* at least one element {@code e} such that {@code o.equals(e)}. |
1071 |
* |
1072 |
* @param o element whose presence in this deque is to be tested |
1073 |
* @return {@code true} if this deque contains the specified element |
1074 |
*/ |
1075 |
public boolean contains(Object o) { |
1076 |
if (o != null) { |
1077 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1078 |
E item = p.item; |
1079 |
if (item != null && o.equals(item)) |
1080 |
return true; |
1081 |
} |
1082 |
} |
1083 |
return false; |
1084 |
} |
1085 |
|
1086 |
/** |
1087 |
* Returns {@code true} if this collection contains no elements. |
1088 |
* |
1089 |
* @return {@code true} if this collection contains no elements |
1090 |
*/ |
1091 |
public boolean isEmpty() { |
1092 |
return peekFirst() == null; |
1093 |
} |
1094 |
|
1095 |
/** |
1096 |
* Returns the number of elements in this deque. If this deque |
1097 |
* contains more than {@code Integer.MAX_VALUE} elements, it |
1098 |
* returns {@code Integer.MAX_VALUE}. |
1099 |
* |
1100 |
* <p>Beware that, unlike in most collections, this method is |
1101 |
* <em>NOT</em> a constant-time operation. Because of the |
1102 |
* asynchronous nature of these deques, determining the current |
1103 |
* number of elements requires traversing them all to count them. |
1104 |
* Additionally, it is possible for the size to change during |
1105 |
* execution of this method, in which case the returned result |
1106 |
* will be inaccurate. Thus, this method is typically not very |
1107 |
* useful in concurrent applications. |
1108 |
* |
1109 |
* @return the number of elements in this deque |
1110 |
*/ |
1111 |
public int size() { |
1112 |
restartFromHead: for (;;) { |
1113 |
int count = 0; |
1114 |
for (Node<E> p = first(); p != null;) { |
1115 |
if (p.item != null) |
1116 |
if (++count == Integer.MAX_VALUE) |
1117 |
break; // @see Collection.size() |
1118 |
Node<E> next = p.next; |
1119 |
if (p == next) |
1120 |
continue restartFromHead; |
1121 |
p = next; |
1122 |
} |
1123 |
return count; |
1124 |
} |
1125 |
} |
1126 |
|
1127 |
/** |
1128 |
* Removes the first occurrence of the specified element from this deque. |
1129 |
* If the deque does not contain the element, it is unchanged. |
1130 |
* More formally, removes the first element {@code e} such that |
1131 |
* {@code o.equals(e)} (if such an element exists). |
1132 |
* Returns {@code true} if this deque contained the specified element |
1133 |
* (or equivalently, if this deque changed as a result of the call). |
1134 |
* |
1135 |
* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}. |
1136 |
* |
1137 |
* @param o element to be removed from this deque, if present |
1138 |
* @return {@code true} if the deque contained the specified element |
1139 |
* @throws NullPointerException if the specified element is null |
1140 |
*/ |
1141 |
public boolean remove(Object o) { |
1142 |
return removeFirstOccurrence(o); |
1143 |
} |
1144 |
|
1145 |
/** |
1146 |
* Appends all of the elements in the specified collection to the end of |
1147 |
* this deque, in the order that they are returned by the specified |
1148 |
* collection's iterator. Attempts to {@code addAll} of a deque to |
1149 |
* itself result in {@code IllegalArgumentException}. |
1150 |
* |
1151 |
* @param c the elements to be inserted into this deque |
1152 |
* @return {@code true} if this deque changed as a result of the call |
1153 |
* @throws NullPointerException if the specified collection or any |
1154 |
* of its elements are null |
1155 |
* @throws IllegalArgumentException if the collection is this deque |
1156 |
*/ |
1157 |
public boolean addAll(Collection<? extends E> c) { |
1158 |
if (c == this) |
1159 |
// As historically specified in AbstractQueue#addAll |
1160 |
throw new IllegalArgumentException(); |
1161 |
|
1162 |
// Copy c into a private chain of Nodes |
1163 |
Node<E> beginningOfTheEnd = null, last = null; |
1164 |
for (E e : c) { |
1165 |
checkNotNull(e); |
1166 |
Node<E> newNode = new Node<E>(e); |
1167 |
if (beginningOfTheEnd == null) |
1168 |
beginningOfTheEnd = last = newNode; |
1169 |
else { |
1170 |
last.lazySetNext(newNode); |
1171 |
newNode.lazySetPrev(last); |
1172 |
last = newNode; |
1173 |
} |
1174 |
} |
1175 |
if (beginningOfTheEnd == null) |
1176 |
return false; |
1177 |
|
1178 |
// Atomically append the chain at the tail of this collection |
1179 |
restartFromTail: |
1180 |
for (;;) |
1181 |
for (Node<E> t = tail, p = t, q;;) { |
1182 |
if ((q = p.next) != null && |
1183 |
(q = (p = q).next) != null) |
1184 |
// Check for tail updates every other hop. |
1185 |
// If p == q, we are sure to follow tail instead. |
1186 |
p = (t != (t = tail)) ? t : q; |
1187 |
else if (p.prev == p) // NEXT_TERMINATOR |
1188 |
continue restartFromTail; |
1189 |
else { |
1190 |
// p is last node |
1191 |
beginningOfTheEnd.lazySetPrev(p); // CAS piggyback |
1192 |
if (p.casNext(null, beginningOfTheEnd)) { |
1193 |
// Successful CAS is the linearization point |
1194 |
// for all elements to be added to this deque. |
1195 |
if (!casTail(t, last)) { |
1196 |
// Try a little harder to update tail, |
1197 |
// since we may be adding many elements. |
1198 |
t = tail; |
1199 |
if (last.next == null) |
1200 |
casTail(t, last); |
1201 |
} |
1202 |
return true; |
1203 |
} |
1204 |
// Lost CAS race to another thread; re-read next |
1205 |
} |
1206 |
} |
1207 |
} |
1208 |
|
1209 |
/** |
1210 |
* Removes all of the elements from this deque. |
1211 |
*/ |
1212 |
public void clear() { |
1213 |
while (pollFirst() != null) |
1214 |
; |
1215 |
} |
1216 |
|
1217 |
/** |
1218 |
* Returns an array containing all of the elements in this deque, in |
1219 |
* proper sequence (from first to last element). |
1220 |
* |
1221 |
* <p>The returned array will be "safe" in that no references to it are |
1222 |
* maintained by this deque. (In other words, this method must allocate |
1223 |
* a new array). The caller is thus free to modify the returned array. |
1224 |
* |
1225 |
* <p>This method acts as bridge between array-based and collection-based |
1226 |
* APIs. |
1227 |
* |
1228 |
* @return an array containing all of the elements in this deque |
1229 |
*/ |
1230 |
public Object[] toArray() { |
1231 |
return toArrayList().toArray(); |
1232 |
} |
1233 |
|
1234 |
/** |
1235 |
* Returns an array containing all of the elements in this deque, |
1236 |
* in proper sequence (from first to last element); the runtime |
1237 |
* type of the returned array is that of the specified array. If |
1238 |
* the deque fits in the specified array, it is returned therein. |
1239 |
* Otherwise, a new array is allocated with the runtime type of |
1240 |
* the specified array and the size of this deque. |
1241 |
* |
1242 |
* <p>If this deque fits in the specified array with room to spare |
1243 |
* (i.e., the array has more elements than this deque), the element in |
1244 |
* the array immediately following the end of the deque is set to |
1245 |
* {@code null}. |
1246 |
* |
1247 |
* <p>Like the {@link #toArray()} method, this method acts as |
1248 |
* bridge between array-based and collection-based APIs. Further, |
1249 |
* this method allows precise control over the runtime type of the |
1250 |
* output array, and may, under certain circumstances, be used to |
1251 |
* save allocation costs. |
1252 |
* |
1253 |
* <p>Suppose {@code x} is a deque known to contain only strings. |
1254 |
* The following code can be used to dump the deque into a newly |
1255 |
* allocated array of {@code String}: |
1256 |
* |
1257 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
1258 |
* |
1259 |
* Note that {@code toArray(new Object[0])} is identical in function to |
1260 |
* {@code toArray()}. |
1261 |
* |
1262 |
* @param a the array into which the elements of the deque are to |
1263 |
* be stored, if it is big enough; otherwise, a new array of the |
1264 |
* same runtime type is allocated for this purpose |
1265 |
* @return an array containing all of the elements in this deque |
1266 |
* @throws ArrayStoreException if the runtime type of the specified array |
1267 |
* is not a supertype of the runtime type of every element in |
1268 |
* this deque |
1269 |
* @throws NullPointerException if the specified array is null |
1270 |
*/ |
1271 |
public <T> T[] toArray(T[] a) { |
1272 |
return toArrayList().toArray(a); |
1273 |
} |
1274 |
|
1275 |
/** |
1276 |
* Returns an iterator over the elements in this deque in proper sequence. |
1277 |
* The elements will be returned in order from first (head) to last (tail). |
1278 |
* |
1279 |
* <p>The returned iterator is |
1280 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1281 |
* |
1282 |
* @return an iterator over the elements in this deque in proper sequence |
1283 |
*/ |
1284 |
public Iterator<E> iterator() { |
1285 |
return new Itr(); |
1286 |
} |
1287 |
|
1288 |
/** |
1289 |
* Returns an iterator over the elements in this deque in reverse |
1290 |
* sequential order. The elements will be returned in order from |
1291 |
* last (tail) to first (head). |
1292 |
* |
1293 |
* <p>The returned iterator is |
1294 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1295 |
* |
1296 |
* @return an iterator over the elements in this deque in reverse order |
1297 |
*/ |
1298 |
public Iterator<E> descendingIterator() { |
1299 |
return new DescendingItr(); |
1300 |
} |
1301 |
|
1302 |
private abstract class AbstractItr implements Iterator<E> { |
1303 |
/** |
1304 |
* Next node to return item for. |
1305 |
*/ |
1306 |
private Node<E> nextNode; |
1307 |
|
1308 |
/** |
1309 |
* nextItem holds on to item fields because once we claim |
1310 |
* that an element exists in hasNext(), we must return it in |
1311 |
* the following next() call even if it was in the process of |
1312 |
* being removed when hasNext() was called. |
1313 |
*/ |
1314 |
private E nextItem; |
1315 |
|
1316 |
/** |
1317 |
* Node returned by most recent call to next. Needed by remove. |
1318 |
* Reset to null if this element is deleted by a call to remove. |
1319 |
*/ |
1320 |
private Node<E> lastRet; |
1321 |
|
1322 |
abstract Node<E> startNode(); |
1323 |
abstract Node<E> nextNode(Node<E> p); |
1324 |
|
1325 |
AbstractItr() { |
1326 |
advance(); |
1327 |
} |
1328 |
|
1329 |
/** |
1330 |
* Sets nextNode and nextItem to next valid node, or to null |
1331 |
* if no such. |
1332 |
*/ |
1333 |
private void advance() { |
1334 |
lastRet = nextNode; |
1335 |
|
1336 |
Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode); |
1337 |
for (;; p = nextNode(p)) { |
1338 |
if (p == null) { |
1339 |
// might be at active end or TERMINATOR node; both are OK |
1340 |
nextNode = null; |
1341 |
nextItem = null; |
1342 |
break; |
1343 |
} |
1344 |
E item = p.item; |
1345 |
if (item != null) { |
1346 |
nextNode = p; |
1347 |
nextItem = item; |
1348 |
break; |
1349 |
} |
1350 |
} |
1351 |
} |
1352 |
|
1353 |
public boolean hasNext() { |
1354 |
return nextItem != null; |
1355 |
} |
1356 |
|
1357 |
public E next() { |
1358 |
E item = nextItem; |
1359 |
if (item == null) throw new NoSuchElementException(); |
1360 |
advance(); |
1361 |
return item; |
1362 |
} |
1363 |
|
1364 |
public void remove() { |
1365 |
Node<E> l = lastRet; |
1366 |
if (l == null) throw new IllegalStateException(); |
1367 |
l.item = null; |
1368 |
unlink(l); |
1369 |
lastRet = null; |
1370 |
} |
1371 |
} |
1372 |
|
1373 |
/** Forward iterator */ |
1374 |
private class Itr extends AbstractItr { |
1375 |
Node<E> startNode() { return first(); } |
1376 |
Node<E> nextNode(Node<E> p) { return succ(p); } |
1377 |
} |
1378 |
|
1379 |
/** Descending iterator */ |
1380 |
private class DescendingItr extends AbstractItr { |
1381 |
Node<E> startNode() { return last(); } |
1382 |
Node<E> nextNode(Node<E> p) { return pred(p); } |
1383 |
} |
1384 |
|
1385 |
/** A customized variant of Spliterators.IteratorSpliterator */ |
1386 |
static final class CLDSpliterator<E> implements Spliterator<E> { |
1387 |
static final int MAX_BATCH = 1 << 25; // max batch array size; |
1388 |
final ConcurrentLinkedDeque<E> queue; |
1389 |
Node<E> current; // current node; null until initialized |
1390 |
int batch; // batch size for splits |
1391 |
boolean exhausted; // true when no more nodes |
1392 |
CLDSpliterator(ConcurrentLinkedDeque<E> queue) { |
1393 |
this.queue = queue; |
1394 |
} |
1395 |
|
1396 |
public Spliterator<E> trySplit() { |
1397 |
Node<E> p; |
1398 |
final ConcurrentLinkedDeque<E> q = this.queue; |
1399 |
int b = batch; |
1400 |
int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1; |
1401 |
if (!exhausted && |
1402 |
((p = current) != null || (p = q.first()) != null)) { |
1403 |
if (p.item == null && p == (p = p.next)) |
1404 |
current = p = q.first(); |
1405 |
if (p != null && p.next != null) { |
1406 |
Object[] a = new Object[n]; |
1407 |
int i = 0; |
1408 |
do { |
1409 |
if ((a[i] = p.item) != null) |
1410 |
++i; |
1411 |
if (p == (p = p.next)) |
1412 |
p = q.first(); |
1413 |
} while (p != null && i < n); |
1414 |
if ((current = p) == null) |
1415 |
exhausted = true; |
1416 |
if (i > 0) { |
1417 |
batch = i; |
1418 |
return Spliterators.spliterator |
1419 |
(a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL | |
1420 |
Spliterator.CONCURRENT); |
1421 |
} |
1422 |
} |
1423 |
} |
1424 |
return null; |
1425 |
} |
1426 |
|
1427 |
public void forEachRemaining(Consumer<? super E> action) { |
1428 |
Node<E> p; |
1429 |
if (action == null) throw new NullPointerException(); |
1430 |
final ConcurrentLinkedDeque<E> q = this.queue; |
1431 |
if (!exhausted && |
1432 |
((p = current) != null || (p = q.first()) != null)) { |
1433 |
exhausted = true; |
1434 |
do { |
1435 |
E e = p.item; |
1436 |
if (p == (p = p.next)) |
1437 |
p = q.first(); |
1438 |
if (e != null) |
1439 |
action.accept(e); |
1440 |
} while (p != null); |
1441 |
} |
1442 |
} |
1443 |
|
1444 |
public boolean tryAdvance(Consumer<? super E> action) { |
1445 |
Node<E> p; |
1446 |
if (action == null) throw new NullPointerException(); |
1447 |
final ConcurrentLinkedDeque<E> q = this.queue; |
1448 |
if (!exhausted && |
1449 |
((p = current) != null || (p = q.first()) != null)) { |
1450 |
E e; |
1451 |
do { |
1452 |
e = p.item; |
1453 |
if (p == (p = p.next)) |
1454 |
p = q.first(); |
1455 |
} while (e == null && p != null); |
1456 |
if ((current = p) == null) |
1457 |
exhausted = true; |
1458 |
if (e != null) { |
1459 |
action.accept(e); |
1460 |
return true; |
1461 |
} |
1462 |
} |
1463 |
return false; |
1464 |
} |
1465 |
|
1466 |
public long estimateSize() { return Long.MAX_VALUE; } |
1467 |
|
1468 |
public int characteristics() { |
1469 |
return Spliterator.ORDERED | Spliterator.NONNULL | |
1470 |
Spliterator.CONCURRENT; |
1471 |
} |
1472 |
} |
1473 |
|
1474 |
/** |
1475 |
* Returns a {@link Spliterator} over the elements in this deque. |
1476 |
* |
1477 |
* <p>The returned spliterator is |
1478 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1479 |
* |
1480 |
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
1481 |
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
1482 |
* |
1483 |
* @implNote |
1484 |
* The {@code Spliterator} implements {@code trySplit} to permit limited |
1485 |
* parallelism. |
1486 |
* |
1487 |
* @return a {@code Spliterator} over the elements in this deque |
1488 |
* @since 1.8 |
1489 |
*/ |
1490 |
public Spliterator<E> spliterator() { |
1491 |
return new CLDSpliterator<E>(this); |
1492 |
} |
1493 |
|
1494 |
/** |
1495 |
* Saves this deque to a stream (that is, serializes it). |
1496 |
* |
1497 |
* @param s the stream |
1498 |
* @throws java.io.IOException if an I/O error occurs |
1499 |
* @serialData All of the elements (each an {@code E}) in |
1500 |
* the proper order, followed by a null |
1501 |
*/ |
1502 |
private void writeObject(java.io.ObjectOutputStream s) |
1503 |
throws java.io.IOException { |
1504 |
|
1505 |
// Write out any hidden stuff |
1506 |
s.defaultWriteObject(); |
1507 |
|
1508 |
// Write out all elements in the proper order. |
1509 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
1510 |
E item = p.item; |
1511 |
if (item != null) |
1512 |
s.writeObject(item); |
1513 |
} |
1514 |
|
1515 |
// Use trailing null as sentinel |
1516 |
s.writeObject(null); |
1517 |
} |
1518 |
|
1519 |
/** |
1520 |
* Reconstitutes this deque from a stream (that is, deserializes it). |
1521 |
* @param s the stream |
1522 |
* @throws ClassNotFoundException if the class of a serialized object |
1523 |
* could not be found |
1524 |
* @throws java.io.IOException if an I/O error occurs |
1525 |
*/ |
1526 |
private void readObject(java.io.ObjectInputStream s) |
1527 |
throws java.io.IOException, ClassNotFoundException { |
1528 |
s.defaultReadObject(); |
1529 |
|
1530 |
// Read in elements until trailing null sentinel found |
1531 |
Node<E> h = null, t = null; |
1532 |
for (Object item; (item = s.readObject()) != null; ) { |
1533 |
@SuppressWarnings("unchecked") |
1534 |
Node<E> newNode = new Node<E>((E) item); |
1535 |
if (h == null) |
1536 |
h = t = newNode; |
1537 |
else { |
1538 |
t.lazySetNext(newNode); |
1539 |
newNode.lazySetPrev(t); |
1540 |
t = newNode; |
1541 |
} |
1542 |
} |
1543 |
initHeadTail(h, t); |
1544 |
} |
1545 |
|
1546 |
private boolean casHead(Node<E> cmp, Node<E> val) { |
1547 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
1548 |
} |
1549 |
|
1550 |
private boolean casTail(Node<E> cmp, Node<E> val) { |
1551 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
1552 |
} |
1553 |
|
1554 |
// Unsafe mechanics |
1555 |
|
1556 |
private static final sun.misc.Unsafe UNSAFE; |
1557 |
private static final long headOffset; |
1558 |
private static final long tailOffset; |
1559 |
static { |
1560 |
PREV_TERMINATOR = new Node<Object>(); |
1561 |
PREV_TERMINATOR.next = PREV_TERMINATOR; |
1562 |
NEXT_TERMINATOR = new Node<Object>(); |
1563 |
NEXT_TERMINATOR.prev = NEXT_TERMINATOR; |
1564 |
try { |
1565 |
UNSAFE = sun.misc.Unsafe.getUnsafe(); |
1566 |
Class<?> k = ConcurrentLinkedDeque.class; |
1567 |
headOffset = UNSAFE.objectFieldOffset |
1568 |
(k.getDeclaredField("head")); |
1569 |
tailOffset = UNSAFE.objectFieldOffset |
1570 |
(k.getDeclaredField("tail")); |
1571 |
} catch (Exception e) { |
1572 |
throw new Error(e); |
1573 |
} |
1574 |
} |
1575 |
} |