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808 lines
26 KiB
Java
808 lines
26 KiB
Java
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent;
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import java.util.concurrent.atomic.*;
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import java.util.concurrent.locks.*;
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import java.util.*;
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/**
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* An optionally-bounded {@linkplain BlockingQueue blocking queue} based on
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* linked nodes.
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* This queue orders elements FIFO (first-in-first-out).
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* The <em>head</em> of the queue is that element that has been on the
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* queue the longest time.
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* The <em>tail</em> of the queue is that element that has been on the
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* queue the shortest time. New elements
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* are inserted at the tail of the queue, and the queue retrieval
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* operations obtain elements at the head of the queue.
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* Linked queues typically have higher throughput than array-based queues but
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* less predictable performance in most concurrent applications.
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*
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* <p> The optional capacity bound constructor argument serves as a
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* way to prevent excessive queue expansion. The capacity, if unspecified,
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* is equal to {@link Integer#MAX_VALUE}. Linked nodes are
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* dynamically created upon each insertion unless this would bring the
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* queue above capacity.
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*
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* <p>This class and its iterator implement all of the
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* <em>optional</em> methods of the {@link Collection} and {@link
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* Iterator} interfaces.
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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 1.5
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* @author Doug Lea
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* @param <E> the type of elements held in this collection
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*
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*/
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public class LinkedBlockingQueue<E> extends AbstractQueue<E>
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implements BlockingQueue<E>, java.io.Serializable {
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private static final long serialVersionUID = -6903933977591709194L;
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/*
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* A variant of the "two lock queue" algorithm. The putLock gates
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* entry to put (and offer), and has an associated condition for
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* waiting puts. Similarly for the takeLock. The "count" field
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* that they both rely on is maintained as an atomic to avoid
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* needing to get both locks in most cases. Also, to minimize need
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* for puts to get takeLock and vice-versa, cascading notifies are
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* used. When a put notices that it has enabled at least one take,
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* it signals taker. That taker in turn signals others if more
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* items have been entered since the signal. And symmetrically for
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* takes signalling puts. Operations such as remove(Object) and
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* iterators acquire both locks.
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*/
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/**
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* Linked list node class
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*/
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static class Node<E> {
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/** The item, volatile to ensure barrier separating write and read */
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volatile E item;
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Node<E> next;
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Node(E x) { item = x; }
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}
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/** The capacity bound, or Integer.MAX_VALUE if none */
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private final int capacity;
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/** Current number of elements */
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private final AtomicInteger count = new AtomicInteger(0);
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/** Head of linked list */
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private transient Node<E> head;
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/** Tail of linked list */
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private transient Node<E> last;
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/** Lock held by take, poll, etc */
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private final ReentrantLock takeLock = new ReentrantLock();
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/** Wait queue for waiting takes */
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private final Condition notEmpty = takeLock.newCondition();
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/** Lock held by put, offer, etc */
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private final ReentrantLock putLock = new ReentrantLock();
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/** Wait queue for waiting puts */
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private final Condition notFull = putLock.newCondition();
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/**
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* Signals a waiting take. Called only from put/offer (which do not
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* otherwise ordinarily lock takeLock.)
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*/
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private void signalNotEmpty() {
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final ReentrantLock takeLock = this.takeLock;
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takeLock.lock();
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try {
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notEmpty.signal();
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} finally {
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takeLock.unlock();
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}
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}
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/**
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* Signals a waiting put. Called only from take/poll.
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*/
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private void signalNotFull() {
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final ReentrantLock putLock = this.putLock;
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putLock.lock();
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try {
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notFull.signal();
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} finally {
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putLock.unlock();
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}
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}
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/**
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* Creates a node and links it at end of queue.
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* @param x the item
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*/
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private void insert(E x) {
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last = last.next = new Node<E>(x);
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}
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/**
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* Removes a node from head of queue,
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* @return the node
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*/
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private E extract() {
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Node<E> first = head.next;
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head = first;
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E x = first.item;
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first.item = null;
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return x;
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}
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/**
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* Lock to prevent both puts and takes.
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*/
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private void fullyLock() {
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putLock.lock();
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takeLock.lock();
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}
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/**
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* Unlock to allow both puts and takes.
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*/
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private void fullyUnlock() {
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takeLock.unlock();
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putLock.unlock();
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}
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/**
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* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
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* {@link Integer#MAX_VALUE}.
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*/
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public LinkedBlockingQueue() {
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this(Integer.MAX_VALUE);
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}
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/**
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* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
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*
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* @param capacity the capacity of this queue
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* @throws IllegalArgumentException if <tt>capacity</tt> is not greater
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* than zero
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*/
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public LinkedBlockingQueue(int capacity) {
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if (capacity <= 0) throw new IllegalArgumentException();
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this.capacity = capacity;
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last = head = new Node<E>(null);
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}
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/**
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* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
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* {@link Integer#MAX_VALUE}, initially containing the elements of the
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* given collection,
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* added in traversal order of the collection's iterator.
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*
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* @param c the collection of elements to initially contain
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* @throws NullPointerException if the specified collection or any
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* of its elements are null
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*/
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public LinkedBlockingQueue(Collection<? extends E> c) {
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this(Integer.MAX_VALUE);
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for (E e : c)
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add(e);
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}
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// this doc comment is overridden to remove the reference to collections
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// greater in size than Integer.MAX_VALUE
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/**
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* Returns the number of elements in this queue.
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*
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* @return the number of elements in this queue
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*/
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public int size() {
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return count.get();
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}
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// this doc comment is a modified copy of the inherited doc comment,
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// without the reference to unlimited queues.
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/**
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* Returns the number of additional elements that this queue can ideally
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* (in the absence of memory or resource constraints) accept without
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* blocking. This is always equal to the initial capacity of this queue
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* less the current <tt>size</tt> of this queue.
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*
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* <p>Note that you <em>cannot</em> always tell if an attempt to insert
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* an element will succeed by inspecting <tt>remainingCapacity</tt>
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* because it may be the case that another thread is about to
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* insert or remove an element.
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*/
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public int remainingCapacity() {
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return capacity - count.get();
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}
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/**
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* Inserts the specified element at the tail of this queue, waiting if
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* necessary for space to become available.
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*
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* @throws InterruptedException {@inheritDoc}
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* @throws NullPointerException {@inheritDoc}
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*/
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public void put(E e) throws InterruptedException {
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if (e == null) throw new NullPointerException();
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// Note: convention in all put/take/etc is to preset
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// local var holding count negative to indicate failure unless set.
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int c = -1;
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final ReentrantLock putLock = this.putLock;
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final AtomicInteger count = this.count;
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putLock.lockInterruptibly();
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try {
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/*
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* Note that count is used in wait guard even though it is
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* not protected by lock. This works because count can
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* only decrease at this point (all other puts are shut
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* out by lock), and we (or some other waiting put) are
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* signalled if it ever changes from
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* capacity. Similarly for all other uses of count in
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* other wait guards.
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*/
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try {
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while (count.get() == capacity)
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notFull.await();
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} catch (InterruptedException ie) {
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notFull.signal(); // propagate to a non-interrupted thread
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throw ie;
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}
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insert(e);
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c = count.getAndIncrement();
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if (c + 1 < capacity)
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notFull.signal();
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} finally {
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putLock.unlock();
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}
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if (c == 0)
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signalNotEmpty();
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}
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/**
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* Inserts the specified element at the tail of this queue, waiting if
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* necessary up to the specified wait time for space to become available.
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*
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* @return <tt>true</tt> if successful, or <tt>false</tt> if
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* the specified waiting time elapses before space is available.
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* @throws InterruptedException {@inheritDoc}
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* @throws NullPointerException {@inheritDoc}
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*/
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public boolean offer(E e, long timeout, TimeUnit unit)
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throws InterruptedException {
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if (e == null) throw new NullPointerException();
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long nanos = unit.toNanos(timeout);
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int c = -1;
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final ReentrantLock putLock = this.putLock;
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final AtomicInteger count = this.count;
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putLock.lockInterruptibly();
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try {
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for (;;) {
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if (count.get() < capacity) {
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insert(e);
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c = count.getAndIncrement();
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if (c + 1 < capacity)
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notFull.signal();
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break;
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}
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if (nanos <= 0)
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return false;
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try {
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nanos = notFull.awaitNanos(nanos);
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} catch (InterruptedException ie) {
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notFull.signal(); // propagate to a non-interrupted thread
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throw ie;
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}
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}
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} finally {
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putLock.unlock();
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}
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if (c == 0)
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signalNotEmpty();
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return true;
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}
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/**
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* Inserts the specified element at the tail of this queue if it is
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* possible to do so immediately without exceeding the queue's capacity,
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* returning <tt>true</tt> upon success and <tt>false</tt> if this queue
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* is full.
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* When using a capacity-restricted queue, this method is generally
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* preferable to method {@link BlockingQueue#add add}, which can fail to
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* insert an element only by throwing an exception.
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*
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* @throws NullPointerException if the specified element is null
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*/
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public boolean offer(E e) {
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if (e == null) throw new NullPointerException();
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final AtomicInteger count = this.count;
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if (count.get() == capacity)
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return false;
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int c = -1;
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final ReentrantLock putLock = this.putLock;
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putLock.lock();
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try {
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if (count.get() < capacity) {
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insert(e);
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c = count.getAndIncrement();
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if (c + 1 < capacity)
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notFull.signal();
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}
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} finally {
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putLock.unlock();
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}
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if (c == 0)
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signalNotEmpty();
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return c >= 0;
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}
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public E take() throws InterruptedException {
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E x;
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int c = -1;
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final AtomicInteger count = this.count;
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final ReentrantLock takeLock = this.takeLock;
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takeLock.lockInterruptibly();
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try {
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try {
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while (count.get() == 0)
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notEmpty.await();
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} catch (InterruptedException ie) {
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notEmpty.signal(); // propagate to a non-interrupted thread
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throw ie;
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}
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x = extract();
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c = count.getAndDecrement();
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if (c > 1)
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notEmpty.signal();
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} finally {
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takeLock.unlock();
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}
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if (c == capacity)
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signalNotFull();
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return x;
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}
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public E poll(long timeout, TimeUnit unit) throws InterruptedException {
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E x = null;
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int c = -1;
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long nanos = unit.toNanos(timeout);
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final AtomicInteger count = this.count;
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final ReentrantLock takeLock = this.takeLock;
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takeLock.lockInterruptibly();
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try {
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for (;;) {
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if (count.get() > 0) {
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x = extract();
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c = count.getAndDecrement();
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if (c > 1)
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notEmpty.signal();
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break;
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}
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if (nanos <= 0)
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return null;
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try {
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nanos = notEmpty.awaitNanos(nanos);
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} catch (InterruptedException ie) {
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notEmpty.signal(); // propagate to a non-interrupted thread
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throw ie;
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}
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}
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} finally {
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takeLock.unlock();
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}
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if (c == capacity)
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signalNotFull();
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return x;
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}
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public E poll() {
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final AtomicInteger count = this.count;
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if (count.get() == 0)
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return null;
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E x = null;
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int c = -1;
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final ReentrantLock takeLock = this.takeLock;
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takeLock.lock();
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try {
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if (count.get() > 0) {
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x = extract();
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c = count.getAndDecrement();
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if (c > 1)
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notEmpty.signal();
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}
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} finally {
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takeLock.unlock();
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}
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if (c == capacity)
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signalNotFull();
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return x;
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}
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public E peek() {
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if (count.get() == 0)
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return null;
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final ReentrantLock takeLock = this.takeLock;
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takeLock.lock();
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try {
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Node<E> first = head.next;
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if (first == null)
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return null;
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else
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return first.item;
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} finally {
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takeLock.unlock();
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}
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}
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/**
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* Removes a single instance of the specified element from this queue,
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* if it is present. More formally, removes an element <tt>e</tt> such
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* that <tt>o.equals(e)</tt>, if this queue contains one or more such
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* elements.
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* Returns <tt>true</tt> if this queue contained the specified element
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* (or equivalently, if this queue changed as a result of the call).
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*
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* @param o element to be removed from this queue, if present
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* @return <tt>true</tt> if this queue changed as a result of the call
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*/
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public boolean remove(Object o) {
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if (o == null) return false;
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boolean removed = false;
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fullyLock();
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try {
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Node<E> trail = head;
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Node<E> p = head.next;
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while (p != null) {
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if (o.equals(p.item)) {
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removed = true;
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break;
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}
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trail = p;
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p = p.next;
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}
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if (removed) {
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p.item = null;
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trail.next = p.next;
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if (last == p)
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last = trail;
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if (count.getAndDecrement() == capacity)
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notFull.signalAll();
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}
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} finally {
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fullyUnlock();
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}
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return removed;
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}
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/**
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* Returns an array containing all of the elements in this queue, in
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* proper sequence.
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*
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* <p>The returned array will be "safe" in that no references to it are
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* maintained by this queue. (In other words, this method must allocate
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* a new array). The caller is thus free to modify the returned array.
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*
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* <p>This method acts as bridge between array-based and collection-based
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* APIs.
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*
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* @return an array containing all of the elements in this queue
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*/
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public Object[] toArray() {
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fullyLock();
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try {
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int size = count.get();
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Object[] a = new Object[size];
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int k = 0;
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for (Node<E> p = head.next; p != null; p = p.next)
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a[k++] = p.item;
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return a;
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} finally {
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fullyUnlock();
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}
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}
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/**
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* Returns an array containing all of the elements in this queue, in
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* proper sequence; the runtime type of the returned array is that of
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* the specified array. If the queue fits in the specified array, it
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* is returned therein. Otherwise, a new array is allocated with the
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* runtime type of the specified array and the size of this queue.
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*
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* <p>If this queue fits in the specified array with room to spare
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* (i.e., the array has more elements than this queue), the element in
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* the array immediately following the end of the queue is set to
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* <tt>null</tt>.
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*
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* <p>Like the {@link #toArray()} method, this method acts as bridge between
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* array-based and collection-based APIs. Further, this method allows
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* precise control over the runtime type of the output array, and may,
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* under certain circumstances, be used to save allocation costs.
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*
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* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
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* The following code can be used to dump the queue into a newly
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* allocated array of <tt>String</tt>:
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*
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* <pre>
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* String[] y = x.toArray(new String[0]);</pre>
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*
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* Note that <tt>toArray(new Object[0])</tt> is identical in function to
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* <tt>toArray()</tt>.
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*
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* @param a the array into which the elements of the queue are to
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* be stored, if it is big enough; otherwise, a new array of the
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* same runtime type is allocated for this purpose
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* @return an array containing all of the elements in this queue
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* @throws ArrayStoreException if the runtime type of the specified array
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* is not a supertype of the runtime type of every element in
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* this queue
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* @throws NullPointerException if the specified array is null
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*/
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public <T> T[] toArray(T[] a) {
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fullyLock();
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try {
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int size = count.get();
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if (a.length < size)
|
|
a = (T[])java.lang.reflect.Array.newInstance
|
|
(a.getClass().getComponentType(), size);
|
|
|
|
int k = 0;
|
|
for (Node p = head.next; p != null; p = p.next)
|
|
a[k++] = (T)p.item;
|
|
if (a.length > k)
|
|
a[k] = null;
|
|
return a;
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
}
|
|
|
|
public String toString() {
|
|
fullyLock();
|
|
try {
|
|
return super.toString();
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Atomically removes all of the elements from this queue.
|
|
* The queue will be empty after this call returns.
|
|
*/
|
|
public void clear() {
|
|
fullyLock();
|
|
try {
|
|
head.next = null;
|
|
assert head.item == null;
|
|
last = head;
|
|
if (count.getAndSet(0) == capacity)
|
|
notFull.signalAll();
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
Node<E> first;
|
|
fullyLock();
|
|
try {
|
|
first = head.next;
|
|
head.next = null;
|
|
assert head.item == null;
|
|
last = head;
|
|
if (count.getAndSet(0) == capacity)
|
|
notFull.signalAll();
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
// Transfer the elements outside of locks
|
|
int n = 0;
|
|
for (Node<E> p = first; p != null; p = p.next) {
|
|
c.add(p.item);
|
|
p.item = null;
|
|
++n;
|
|
}
|
|
return n;
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c, int maxElements) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
fullyLock();
|
|
try {
|
|
int n = 0;
|
|
Node<E> p = head.next;
|
|
while (p != null && n < maxElements) {
|
|
c.add(p.item);
|
|
p.item = null;
|
|
p = p.next;
|
|
++n;
|
|
}
|
|
if (n != 0) {
|
|
head.next = p;
|
|
assert head.item == null;
|
|
if (p == null)
|
|
last = head;
|
|
if (count.getAndAdd(-n) == capacity)
|
|
notFull.signalAll();
|
|
}
|
|
return n;
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Returns an iterator over the elements in this queue in proper sequence.
|
|
* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
|
|
* will never throw {@link 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<E> iterator() {
|
|
return new Itr();
|
|
}
|
|
|
|
private class Itr implements Iterator<E> {
|
|
/*
|
|
* Basic weak-consistent iterator. At all times hold the next
|
|
* item to hand out so that if hasNext() reports true, we will
|
|
* still have it to return even if lost race with a take etc.
|
|
*/
|
|
private Node<E> current;
|
|
private Node<E> lastRet;
|
|
private E currentElement;
|
|
|
|
Itr() {
|
|
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
|
|
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
|
|
putLock.lock();
|
|
takeLock.lock();
|
|
try {
|
|
current = head.next;
|
|
if (current != null)
|
|
currentElement = current.item;
|
|
} finally {
|
|
takeLock.unlock();
|
|
putLock.unlock();
|
|
}
|
|
}
|
|
|
|
public boolean hasNext() {
|
|
return current != null;
|
|
}
|
|
|
|
public E next() {
|
|
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
|
|
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
|
|
putLock.lock();
|
|
takeLock.lock();
|
|
try {
|
|
if (current == null)
|
|
throw new NoSuchElementException();
|
|
E x = currentElement;
|
|
lastRet = current;
|
|
current = current.next;
|
|
if (current != null)
|
|
currentElement = current.item;
|
|
return x;
|
|
} finally {
|
|
takeLock.unlock();
|
|
putLock.unlock();
|
|
}
|
|
}
|
|
|
|
public void remove() {
|
|
if (lastRet == null)
|
|
throw new IllegalStateException();
|
|
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
|
|
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
|
|
putLock.lock();
|
|
takeLock.lock();
|
|
try {
|
|
Node<E> node = lastRet;
|
|
lastRet = null;
|
|
Node<E> trail = head;
|
|
Node<E> p = head.next;
|
|
while (p != null && p != node) {
|
|
trail = p;
|
|
p = p.next;
|
|
}
|
|
if (p == node) {
|
|
p.item = null;
|
|
trail.next = p.next;
|
|
if (last == p)
|
|
last = trail;
|
|
int c = count.getAndDecrement();
|
|
if (c == capacity)
|
|
notFull.signalAll();
|
|
}
|
|
} finally {
|
|
takeLock.unlock();
|
|
putLock.unlock();
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Save the state to a stream (that is, serialize it).
|
|
*
|
|
* @serialData The capacity is emitted (int), followed by all of
|
|
* its elements (each an <tt>Object</tt>) in the proper order,
|
|
* followed by a null
|
|
* @param s the stream
|
|
*/
|
|
private void writeObject(java.io.ObjectOutputStream s)
|
|
throws java.io.IOException {
|
|
|
|
fullyLock();
|
|
try {
|
|
// Write out any hidden stuff, plus capacity
|
|
s.defaultWriteObject();
|
|
|
|
// Write out all elements in the proper order.
|
|
for (Node<E> p = head.next; p != null; p = p.next)
|
|
s.writeObject(p.item);
|
|
|
|
// Use trailing null as sentinel
|
|
s.writeObject(null);
|
|
} finally {
|
|
fullyUnlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Reconstitute this 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();
|
|
|
|
count.set(0);
|
|
last = head = new Node<E>(null);
|
|
|
|
// Read in all elements and place in queue
|
|
for (;;) {
|
|
E item = (E)s.readObject();
|
|
if (item == null)
|
|
break;
|
|
add(item);
|
|
}
|
|
}
|
|
}
|