edu.emory.mathcs.backport.java.util.concurrent.ConcurrentLinkedQueue Maven / Gradle / Ivy
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/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package edu.emory.mathcs.backport.java.util.concurrent;
import edu.emory.mathcs.backport.java.util.*;
import java.io.*;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
* This queue orders elements FIFO (first-in-first-out).
* The head of the queue is that element that has been on the
* queue the longest time.
* The tail of the queue is that element that has been on the
* queue the shortest time. New elements
* are inserted at the tail of the queue, and the queue retrieval
* operations obtain elements at the head of the queue.
* A ConcurrentLinkedQueue is an appropriate choice when
* many threads will share access to a common collection.
* This queue does not permit null elements.
*
* This implementation employs an efficient "wait-free"
* algorithm based on one described in Simple,
* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
* Algorithms by Maged M. Michael and Michael L. Scott.
*
*
Beware that, unlike in most collections, the size method
* is NOT a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current number
* of elements requires a traversal of the elements.
*
*
This class and its iterator implement all of the
* optional methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
*
Memory consistency effects: As with other concurrent
* collections, actions in a thread prior to placing an object into a
* {@code ConcurrentLinkedQueue}
* happen-before
* actions subsequent to the access or removal of that element from
* the {@code ConcurrentLinkedQueue} in another thread.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @since 1.5
* @author Doug Lea
*
*/
public class ConcurrentLinkedQueue extends AbstractQueue
implements Queue, java.io.Serializable {
private static final long serialVersionUID = 196745693267521676L;
private final Object headLock = new SerializableLock();
private final Object tailLock = new SerializableLock();
/*
* This is a straight adaptation of Michael & Scott algorithm.
* For explanation, read the paper. The only (minor) algorithmic
* difference is that this version supports lazy deletion of
* internal nodes (method remove(Object)) -- remove CAS'es item
* fields to null. The normal queue operations unlink but then
* pass over nodes with null item fields. Similarly, iteration
* methods ignore those with nulls.
*
* Also note that like most non-blocking algorithms in this
* package, this implementation relies on the fact that in garbage
* collected systems, there is no possibility of ABA problems due
* to recycled nodes, so there is no need to use "counted
* pointers" or related techniques seen in versions used in
* non-GC'ed settings.
*/
private static class Node {
private volatile Object item;
private volatile Node next;
Node(Object x) { item = x; }
Node(Object x, Node n) { item = x; next = n; }
Object getItem() {
return item;
}
synchronized boolean casItem(Object cmp, Object val) {
if (item == cmp) {
item = val;
return true;
}
else {
return false;
}
}
synchronized void setItem(Object val) {
item = val;
}
Node getNext() {
return next;
}
synchronized boolean casNext(Node cmp, Node val) {
if (next == cmp) {
next = val;
return true;
}
else {
return false;
}
}
synchronized void setNext(Node val) {
next = val;
}
}
private boolean casTail(Node cmp, Node val) {
synchronized (tailLock) {
if (tail == cmp) {
tail = val;
return true;
}
else {
return false;
}
}
}
private boolean casHead(Node cmp, Node val) {
synchronized (headLock) {
if (head == cmp) {
head = val;
return true;
}
else {
return false;
}
}
}
/**
* Pointer to header node, initialized to a dummy node. The first
* actual node is at head.getNext().
*/
private transient volatile Node head = new Node(null, null);
/** Pointer to last node on list **/
private transient volatile Node tail = head;
/**
* Creates a ConcurrentLinkedQueue that is initially empty.
*/
public ConcurrentLinkedQueue() {}
/**
* Creates a ConcurrentLinkedQueue
* initially containing the elements of the given collection,
* added in traversal order of the collection's iterator.
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public ConcurrentLinkedQueue(Collection c) {
for (Iterator it = c.iterator(); it.hasNext();)
add(it.next());
}
// Have to override just to update the javadoc
/**
* Inserts the specified element at the tail of this queue.
*
* @return true (as specified by {@link Collection#add})
* @throws NullPointerException if the specified element is null
*/
public boolean add(Object e) {
return offer(e);
}
/**
* Inserts the specified element at the tail of this queue.
*
* @return true (as specified by {@link Queue#offer})
* @throws NullPointerException if the specified element is null
*/
public boolean offer(Object e) {
if (e == null) throw new NullPointerException();
Node n = new Node(e, null);
for(;;) {
Node t = tail;
Node s = t.getNext();
if (t == tail) {
if (s == null) {
if (t.casNext(s, n)) {
casTail(t, n);
return true;
}
} else {
casTail(t, s);
}
}
}
}
public Object poll() {
for (;;) {
Node h = head;
Node t = tail;
Node first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else if (casHead(h, first)) {
Object item = first.getItem();
if (item != null) {
first.setItem(null);
return item;
}
// else skip over deleted item, continue loop,
}
}
}
}
public Object peek() { // same as poll except don't remove item
for (;;) {
Node h = head;
Node t = tail;
Node first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else {
Object item = first.getItem();
if (item != null)
return item;
else // remove deleted node and continue
casHead(h, first);
}
}
}
}
/**
* Returns the first actual (non-header) node on list. This is yet
* another variant of poll/peek; here returning out the first
* node, not element (so we cannot collapse with peek() without
* introducing race.)
*/
Node first() {
for (;;) {
Node h = head;
Node t = tail;
Node first = h.getNext();
if (h == head) {
if (h == t) {
if (first == null)
return null;
else
casTail(t, first);
} else {
if (first.getItem() != null)
return first;
else // remove deleted node and continue
casHead(h, first);
}
}
}
}
/**
* Returns true if this queue contains no elements.
*
* @return true if this queue contains no elements
*/
public boolean isEmpty() {
return first() == null;
}
/**
* Returns the number of elements in this queue. If this queue
* contains more than Integer.MAX_VALUE elements, returns
* Integer.MAX_VALUE.
*
*
Beware that, unlike in most collections, this method is
* NOT a constant-time operation. Because of the
* asynchronous nature of these queues, determining the current
* number of elements requires an O(n) traversal.
*
* @return the number of elements in this queue
*/
public int size() {
int count = 0;
for (Node p = first(); p != null; p = p.getNext()) {
if (p.getItem() != null) {
// Collections.size() spec says to max out
if (++count == Integer.MAX_VALUE)
break;
}
}
return count;
}
/**
* Returns true if this queue contains the specified element.
* More formally, returns true if and only if this queue contains
* at least one element e such that o.equals(e).
*
* @param o object to be checked for containment in this queue
* @return true if this queue contains the specified element
*/
public boolean contains(Object o) {
if (o == null) return false;
for (Node p = first(); p != null; p = p.getNext()) {
Object item = p.getItem();
if (item != null &&
o.equals(item))
return true;
}
return false;
}
/**
* Removes a single instance of the specified element from this queue,
* if it is present. More formally, removes an element e such
* that o.equals(e), if this queue contains one or more such
* elements.
* Returns true if this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
*
* @param o element to be removed from this queue, if present
* @return true if this queue changed as a result of the call
*/
public boolean remove(Object o) {
if (o == null) return false;
for (Node p = first(); p != null; p = p.getNext()) {
Object item = p.getItem();
if (item != null &&
o.equals(item) &&
p.casItem(item, null))
return true;
}
return false;
}
/**
* Returns an iterator over the elements in this queue in proper sequence.
* The returned iterator is a "weakly consistent" iterator that
* will never throw {@link java.util.ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator iterator() {
return new Itr();
}
private class Itr implements Iterator {
/**
* Next node to return item for.
*/
private Node 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 Object nextItem;
/**
* Node of the last returned item, to support remove.
*/
private Node lastRet;
Itr() {
advance();
}
/**
* Moves to next valid node and returns item to return for
* next(), or null if no such.
*/
private Object advance() {
lastRet = nextNode;
Object x = nextItem;
Node p = (nextNode == null) ? first() : nextNode.getNext();
for (;;) {
if (p == null) {
nextNode = null;
nextItem = null;
return x;
}
Object item = p.getItem();
if (item != null) {
nextNode = p;
nextItem = item;
return x;
} else // skip over nulls
p = p.getNext();
}
}
public boolean hasNext() {
return nextNode != null;
}
public Object next() {
if (nextNode == null) throw new NoSuchElementException();
return advance();
}
public void remove() {
Node l = lastRet;
if (l == null) throw new IllegalStateException();
// rely on a future traversal to relink.
l.setItem(null);
lastRet = null;
}
}
/**
* Save the state to a stream (that is, serialize it).
*
* @serialData All of the elements (each an E) in
* the proper order, followed by a null
* @param s the stream
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden stuff
s.defaultWriteObject();
// Write out all elements in the proper order.
for (Node p = first(); p != null; p = p.getNext()) {
Object item = p.getItem();
if (item != null)
s.writeObject(item);
}
// Use trailing null as sentinel
s.writeObject(null);
}
/**
* Reconstitute the Queue instance from a stream (that is,
* deserialize it).
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in capacity, and any hidden stuff
s.defaultReadObject();
head = new Node(null, null);
tail = head;
// Read in all elements and place in queue
for (;;) {
Object item = s.readObject();
if (item == null)
break;
else
offer(item);
}
}
private static class SerializableLock implements Serializable {}
}