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