J.U.C并发框架源码阅读（八）ArrayBlockingQueue
基于版本jdk1.7.0_80
java.util.concurrent.ArrayBlockingQueue
代码如下
/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * 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/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.concurrent.locks.*; import java.util.*; /** * A bounded {@linkplain BlockingQueue blocking queue} backed by an * array. This queue orders elements FIFO (first-in-first-out). The * <em>head</em> of the queue is that element that has been on the * queue the longest time. The <em>tail</em> 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. * * <p>This is a classic "bounded buffer", in which a * fixed-sized array holds elements inserted by producers and * extracted by consumers. Once created, the capacity cannot be * changed. Attempts to {@code put} an element into a full queue * will result in the operation blocking; attempts to {@code take} an * element from an empty queue will similarly block. * * <p>This class supports an optional fairness policy for ordering * waiting producer and consumer threads. By default, this ordering * is not guaranteed. However, a queue constructed with fairness set * to {@code true} grants threads access in FIFO order. Fairness * generally decreases throughput but reduces variability and avoids * starvation. * * <p>This class and its iterator implement all of the * <em>optional</em> methods of the {@link Collection} and {@link * Iterator} interfaces. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this collection */ public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, java.io.Serializable { /** * Serialization ID. This class relies on default serialization * even for the items array, which is default-serialized, even if * it is empty. Otherwise it could not be declared final, which is * necessary here. */ private static final long serialVersionUID = -817911632652898426L; /** The queued items */ final Object[] items; /** items index for next take, poll, peek or remove */ int takeIndex; /** items index for next put, offer, or add */ int putIndex; /** Number of elements in the queue */ int count; /* * Concurrency control uses the classic two-condition algorithm * found in any textbook. */ /** Main lock guarding all access */ final ReentrantLock lock; /** Condition for waiting takes */ private final Condition notEmpty; /** Condition for waiting puts */ private final Condition notFull; // Internal helper methods /** * Circularly increment i. */ final int inc(int i) { return (++i == items.length) ? 0 : i; } /** * Circularly decrement i. */ final int dec(int i) { return ((i == 0) ? items.length : i) - 1; } @SuppressWarnings("unchecked") static <E> E cast(Object item) { return (E) item; } /** * Returns item at index i. */ final E itemAt(int i) { return this.<E>cast(items[i]); } /** * Throws NullPointerException if argument is null. * * @param v the element */ private static void checkNotNull(Object v) { if (v == null) throw new NullPointerException(); } /** * Inserts element at current put position, advances, and signals. * Call only when holding lock. */ private void insert(E x) { items[putIndex] = x; putIndex = inc(putIndex); ++count; notEmpty.signal(); } /** * Extracts element at current take position, advances, and signals. * Call only when holding lock. */ private E extract() { final Object[] items = this.items; E x = this.<E>cast(items[takeIndex]); items[takeIndex] = null; takeIndex = inc(takeIndex); --count; notFull.signal(); return x; } /** * Deletes item at position i. * Utility for remove and iterator.remove. * Call only when holding lock. */ void removeAt(int i) { final Object[] items = this.items; // if removing front item, just advance if (i == takeIndex) { items[takeIndex] = null; takeIndex = inc(takeIndex); } else { // slide over all others up through putIndex. for (;;) { int nexti = inc(i); if (nexti != putIndex) { items[i] = items[nexti]; i = nexti; } else { items[i] = null; putIndex = i; break; } } } --count; notFull.signal(); } /** * Creates an {@code ArrayBlockingQueue} with the given (fixed) * capacity and default access policy. * * @param capacity the capacity of this queue * @throws IllegalArgumentException if {@code capacity < 1} */ public ArrayBlockingQueue(int capacity) { this(capacity, false); } /** * Creates an {@code ArrayBlockingQueue} with the given (fixed) * capacity and the specified access policy. * * @param capacity the capacity of this queue * @param fair if {@code true} then queue accesses for threads blocked * on insertion or removal, are processed in FIFO order; * if {@code false} the access order is unspecified. * @throws IllegalArgumentException if {@code capacity < 1} */ public ArrayBlockingQueue(int capacity, boolean fair) { if (capacity <= 0) throw new IllegalArgumentException(); this.items = new Object[capacity]; lock = new ReentrantLock(fair); notEmpty = lock.newCondition(); notFull = lock.newCondition(); } /** * Creates an {@code ArrayBlockingQueue} with the given (fixed) * capacity, the specified access policy and initially containing the * elements of the given collection, * added in traversal order of the collection's iterator. * * @param capacity the capacity of this queue * @param fair if {@code true} then queue accesses for threads blocked * on insertion or removal, are processed in FIFO order; * if {@code false} the access order is unspecified. * @param c the collection of elements to initially contain * @throws IllegalArgumentException if {@code capacity} is less than * {@code c.size()}, or less than 1. * @throws NullPointerException if the specified collection or any * of its elements are null */ public ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> c) { this(capacity, fair); final ReentrantLock lock = this.lock; lock.lock(); // Lock only for visibility, not mutual exclusion try { int i = 0; try { for (E e : c) { checkNotNull(e); items[i++] = e; } } catch (ArrayIndexOutOfBoundsException ex) { throw new IllegalArgumentException(); } count = i; putIndex = (i == capacity) ? 0 : i; } finally { lock.unlock(); } } /** * Inserts the specified element at the tail of this queue if it is * possible to do so immediately without exceeding the queue's capacity, * returning {@code true} upon success and throwing an * {@code IllegalStateException} if this queue is full. * * @param e the element to add * @return {@code true} (as specified by {@link Collection#add}) * @throws IllegalStateException if this queue is full * @throws NullPointerException if the specified element is null */ public boolean add(E e) { return super.add(e); } /** * Inserts the specified element at the tail of this queue if it is * possible to do so immediately without exceeding the queue's capacity, * returning {@code true} upon success and {@code false} if this queue * is full. This method is generally preferable to method {@link #add}, * which can fail to insert an element only by throwing an exception. * * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { checkNotNull(e); final ReentrantLock lock = this.lock; lock.lock(); try { if (count == items.length) return false; else { insert(e); return true; } } finally { lock.unlock(); } } /** * Inserts the specified element at the tail of this queue, waiting * for space to become available if the queue is full. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public void put(E e) throws InterruptedException { checkNotNull(e); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == items.length) notFull.await(); insert(e); } finally { lock.unlock(); } } /** * Inserts the specified element at the tail of this queue, waiting * up to the specified wait time for space to become available if * the queue is full. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException { checkNotNull(e); long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == items.length) { if (nanos <= 0) return false; nanos = notFull.awaitNanos(nanos); } insert(e); return true; } finally { lock.unlock(); } } public E poll() { final ReentrantLock lock = this.lock; lock.lock(); try { return (count == 0) ? null : extract(); } finally { lock.unlock(); } } public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == 0) notEmpty.await(); return extract(); } finally { lock.unlock(); } } public E poll(long timeout, TimeUnit unit) throws InterruptedException { long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == 0) { if (nanos <= 0) return null; nanos = notEmpty.awaitNanos(nanos); } return extract(); } finally { lock.unlock(); } } public E peek() { final ReentrantLock lock = this.lock; lock.lock(); try { return (count == 0) ? null : itemAt(takeIndex); } finally { lock.unlock(); } } // this doc comment is overridden to remove the reference to collections // greater in size than Integer.MAX_VALUE /** * Returns the number of elements in this queue. * * @return the number of elements in this queue */ public int size() { final ReentrantLock lock = this.lock; lock.lock(); try { return count; } finally { lock.unlock(); } } // this doc comment is a modified copy of the inherited doc comment, // without the reference to unlimited queues. /** * Returns the number of additional elements that this queue can ideally * (in the absence of memory or resource constraints) accept without * blocking. This is always equal to the initial capacity of this queue * less the current {@code size} of this queue. * * <p>Note that you <em>cannot</em> always tell if an attempt to insert * an element will succeed by inspecting {@code remainingCapacity} * because it may be the case that another thread is about to * insert or remove an element. */ public int remainingCapacity() { final ReentrantLock lock = this.lock; lock.lock(); try { return items.length - count; } finally { lock.unlock(); } } /** * Removes a single instance of the specified element from this queue, * if it is present. More formally, removes an element {@code e} such * that {@code o.equals(e)}, if this queue contains one or more such * elements. * Returns {@code true} if this queue contained the specified element * (or equivalently, if this queue changed as a result of the call). * * <p>Removal of interior elements in circular array based queues * is an intrinsically slow and disruptive operation, so should * be undertaken only in exceptional circumstances, ideally * only when the queue is known not to be accessible by other * threads. * * @param o element to be removed from this queue, if present * @return {@code true} if this queue changed as a result of the call */ public boolean remove(Object o) { if (o == null) return false; final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) { if (o.equals(items[i])) { removeAt(i); return true; } } return false; } finally { lock.unlock(); } } /** * Returns {@code true} if this queue contains the specified element. * More formally, returns {@code true} if and only if this queue contains * at least one element {@code e} such that {@code o.equals(e)}. * * @param o object to be checked for containment in this queue * @return {@code true} if this queue contains the specified element */ public boolean contains(Object o) { if (o == null) return false; final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) if (o.equals(items[i])) return true; return false; } finally { lock.unlock(); } } /** * Returns an array containing all of the elements in this queue, in * proper sequence. * * <p>The returned array will be "safe" in that no references to it are * maintained by this queue. (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 queue */ public Object[] toArray() { final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { final int count = this.count; Object[] a = new Object[count]; for (int i = takeIndex, k = 0; k < count; i = inc(i), k++) a[k] = items[i]; return a; } finally { lock.unlock(); } } /** * Returns an array containing all of the elements in this queue, in * proper sequence; the runtime type of the returned array is that of * the specified array. If the queue 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 queue. * * <p>If this queue fits in the specified array with room to spare * (i.e., the array has more elements than this queue), the element in * the array immediately following the end of the queue 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 queue known to contain only strings. * The following code can be used to dump the queue into a newly * allocated array of {@code String}: * * <pre> * String[] y = x.toArray(new String[0]);</pre> * * Note that {@code toArray(new Object[0])} is identical in function to * {@code toArray()}. * * @param a the array into which the elements of the queue 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 queue * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this queue * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { final int count = this.count; final int len = a.length; if (len < count) a = (T[])java.lang.reflect.Array.newInstance( a.getClass().getComponentType(), count); for (int i = takeIndex, k = 0; k < count; i = inc(i), k++) a[k] = (T) items[i]; if (len > count) a[count] = null; return a; } finally { lock.unlock(); } } public String toString() { final ReentrantLock lock = this.lock; lock.lock(); try { int k = count; if (k == 0) return "[]"; StringBuilder sb = new StringBuilder(); sb.append('['); for (int i = takeIndex; ; i = inc(i)) { Object e = items[i]; sb.append(e == this ? "(this Collection)" : e); if (--k == 0) return sb.append(']').toString(); sb.append(',').append(' '); } } finally { lock.unlock(); } } /** * Atomically removes all of the elements from this queue. * The queue will be empty after this call returns. */ public void clear() { final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) items[i] = null; count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c) { checkNotNull(c); if (c == this) throw new IllegalArgumentException(); final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int n = 0; int max = count; while (n < max) { c.add(this.<E>cast(items[i])); items[i] = null; i = inc(i); ++n; } if (n > 0) { count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } return n; } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c, int maxElements) { checkNotNull(c); if (c == this) throw new IllegalArgumentException(); if (maxElements <= 0) return 0; final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int n = 0; int max = (maxElements < count) ? maxElements : count; while (n < max) { c.add(this.<E>cast(items[i])); items[i] = null; i = inc(i); ++n; } if (n > 0) { count -= n; takeIndex = i; notFull.signalAll(); } return n; } finally { lock.unlock(); } } /** * Returns an iterator over the elements in this queue in proper sequence. * The elements will be returned in order from first (head) to last (tail). * * <p>The returned {@code Iterator} is a "weakly consistent" iterator that * will never throw {@link java.util.ConcurrentModificationException * ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. * * @return an iterator over the elements in this queue in proper sequence */ public Iterator<E> iterator() { return new Itr(); } /** * Iterator for ArrayBlockingQueue. To maintain weak consistency * with respect to puts and takes, we (1) read ahead one slot, so * as to not report hasNext true but then not have an element to * return -- however we later recheck this slot to use the most * current value; (2) ensure that each array slot is traversed at * most once (by tracking "remaining" elements); (3) skip over * null slots, which can occur if takes race ahead of iterators. * However, for circular array-based queues, we cannot rely on any * well established definition of what it means to be weakly * consistent with respect to interior removes since these may * require slot overwrites in the process of sliding elements to * cover gaps. So we settle for resiliency, operating on * established apparent nexts, which may miss some elements that * have moved between calls to next. */ private class Itr implements Iterator<E> { private int remaining; // Number of elements yet to be returned private int nextIndex; // Index of element to be returned by next private E nextItem; // Element to be returned by next call to next private E lastItem; // Element returned by last call to next private int lastRet; // Index of last element returned, or -1 if none Itr() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { lastRet = -1; if ((remaining = count) > 0) nextItem = itemAt(nextIndex = takeIndex); } finally { lock.unlock(); } } public boolean hasNext() { return remaining > 0; } public E next() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { if (remaining <= 0) throw new NoSuchElementException(); lastRet = nextIndex; E x = itemAt(nextIndex); // check for fresher value if (x == null) { x = nextItem; // we are forced to report old value lastItem = null; // but ensure remove fails } else lastItem = x; while (--remaining > 0 && // skip over nulls (nextItem = itemAt(nextIndex = inc(nextIndex))) == null) ; return x; } finally { lock.unlock(); } } public void remove() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { int i = lastRet; if (i == -1) throw new IllegalStateException(); lastRet = -1; E x = lastItem; lastItem = null; // only remove if item still at index if (x != null && x == items[i]) { boolean removingHead = (i == takeIndex); removeAt(i); if (!removingHead) nextIndex = dec(nextIndex); } } finally { lock.unlock(); } } } }
0. ArrayBlockingQueue简介
用循环数组实现的有界阻塞队列，线程安全。初始化时要求设定容量，在队列满时继续put元素会被阻塞，在队列为空时继续poll元素也会被阻塞。
ArrayBlockingQueue也提供了非阻塞以及可中断的插入/提取元素的方法。
1. 接口分析
ArrayBlockingQueue继承于AbstractQueue抽象类
BlockingQueue<E>（阻塞队列语义）, java.io.Serializable接口
2. ArrayBlockingQueue原理概述
ArrayBlockingQueue内部维护了一个ReentrantLock对象lock，lock有两个Condition：notEmpty与notFull，所有对ArrayBlockingQueue的操作都会用lock加锁，所以ArrayBlockingQueue是线程安全的。
而在调用ArrayBlockingQueue的put方法时，会检查队列长度，如果队列已满，则调用notFull.await等待。在调用ArrayBlockingQueue的poll方法时，则会直接调用notFull.signal，如果有线程在notFull.await上等待，这个线程就会被唤醒，从而实现了队列满时继续put元素会被阻塞的语义。
在队列为空时继续poll元素也会被阻塞的语义的实现原理也是类似的。
3. ArrayBlockingQueue的关键方法解析
/** The queued items */ final Object[] items;//底层存储容器 /** items index for next take, poll, peek or remove */ int takeIndex; /** items index for next put, offer, or add */ int putIndex; /** Number of elements in the queue */ int count; /* * Concurrency control uses the classic two-condition algorithm * found in any textbook. */ /** Main lock guarding all access */ final ReentrantLock lock;//全局锁 /** Condition for waiting takes */ private final Condition notEmpty; /** Condition for waiting puts */ private final Condition notFull; // Internal helper methods /** * Circularly increment i. */ final int inc(int i) { return (++i == items.length) ? 0 : i; } /** * Circularly decrement i. */ final int dec(int i) { return ((i == 0) ? items.length : i) - 1; } /** * Inserts element at current put position, advances, and signals. * Call only when holding lock. */ private void insert(E x) {//向队列中插入元素 items[putIndex] = x;//把元素放到putIndex对应的位置上 putIndex = inc(putIndex);//更新putIndex ++count;//计数器自加 notEmpty.signal();//唤醒可能在notEmpty条件上等待的线程 } /** * Extracts element at current take position, advances, and signals. * Call only when holding lock. */ private E extract() {//从队列中提取元素 final Object[] items = this.items; E x = this.<E>cast(items[takeIndex]);//获取takeIndex位置上的元素 items[takeIndex] = null;//takeIndex位置上的元素清空，便于gc takeIndex = inc(takeIndex);//更新takeIndex --count;//计数器自减 notFull.signal();//唤醒可能在notFull条件上等待的线程 return x; } /** * Deletes item at position i. * Utility for remove and iterator.remove. * Call only when holding lock. */ void removeAt(int i) { final Object[] items = this.items; // if removing front item, just advance if (i == takeIndex) { items[takeIndex] = null; takeIndex = inc(takeIndex); } else { // slide over all others up through putIndex. for (;;) { int nexti = inc(i); if (nexti != putIndex) { items[i] = items[nexti]; i = nexti; } else { items[i] = null; putIndex = i; break; } } } --count;//计数器自减 notFull.signal();//唤醒可能在notFull条件上等待的线程 } /** * Inserts the specified element at the tail of this queue if it is * possible to do so immediately without exceeding the queue's capacity, * returning {@code true} upon success and {@code false} if this queue * is full. This method is generally preferable to method {@link #add}, * which can fail to insert an element only by throwing an exception. * * @throws NullPointerException if the specified element is null */ public boolean offer(E e) {//不阻塞的插入方法 checkNotNull(e); final ReentrantLock lock = this.lock;//加上全局锁，保证只有当前线程在操作 lock.lock(); try { if (count == items.length)//如果队列已满，直接返回false，不阻塞 return false; else { insert(e); return true; } } finally { lock.unlock();//解锁 } } /** * Inserts the specified element at the tail of this queue, waiting * for space to become available if the queue is full. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public void put(E e) throws InterruptedException {//可中断的阻塞插入方法 checkNotNull(e); final ReentrantLock lock = this.lock; lock.lockInterruptibly();//加上可中断的全局锁 try { while (count == items.length)//如果队列已满，在notFull信号量上等待 notFull.await(); insert(e);//此时队列必然不满，可以安全插入元素 } finally { lock.unlock();//解锁 } } /** * Inserts the specified element at the tail of this queue, waiting * up to the specified wait time for space to become available if * the queue is full. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public boolean offer(E e, long timeout, TimeUnit unit)//可中断可超时的阻塞插入方法 throws InterruptedException { checkNotNull(e); long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly();//加上可中断的全局锁 try { while (count == items.length) {//如果队列已满，在notFull条件上等待指定的时间 if (nanos <= 0)//如果已超时，返回false return false; nanos = notFull.awaitNanos(nanos); } insert(e);//此时队列必然不满，可以安全插入元素 return true; } finally { lock.unlock();//解锁 } } public E poll() {//不阻塞的提取方法 final ReentrantLock lock = this.lock; lock.lock();//全局锁 try { return (count == 0) ? null : extract();//如果队列为空，直接返回null，否则正常提取元素 } finally { lock.unlock();//解锁 } } public E take() throws InterruptedException {//可中断的阻塞提取方法 final ReentrantLock lock = this.lock; lock.lockInterruptibly();//加上可中断的全局锁 try { while (count == 0)//如果队列为空，在notEmpty条件上等待 notEmpty.await(); return extract();//此时队列必然不为空，提取元素并返回 } finally { lock.unlock();//解锁 } } public E poll(long timeout, TimeUnit unit) throws InterruptedException {//可中断可超时的阻塞提取方法 long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly();//加上可中断的全局锁 try { while (count == 0) {//如果队列为空，在notEmptyl条件上等待指定的时间 if (nanos <= 0)//如果已超时，返回null return null; nanos = notEmpty.awaitNanos(nanos); } return extract();//此时队列必然不空，可以安全提取元素 } finally { lock.unlock(); } } public E peek() {//获取队首元素 final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try { return (count == 0) ? null : itemAt(takeIndex);//获取队首元素 } finally { lock.unlock(); } } // this doc comment is overridden to remove the reference to collections // greater in size than Integer.MAX_VALUE /** * Returns the number of elements in this queue. * * @return the number of elements in this queue */ public int size() { final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try { return count; } finally { lock.unlock(); } } // this doc comment is a modified copy of the inherited doc comment, // without the reference to unlimited queues. /** * Returns the number of additional elements that this queue can ideally * (in the absence of memory or resource constraints) accept without * blocking. This is always equal to the initial capacity of this queue * less the current {@code size} of this queue. * * <p>Note that you <em>cannot</em> always tell if an attempt to insert * an element will succeed by inspecting {@code remainingCapacity} * because it may be the case that another thread is about to * insert or remove an element. */ public int remainingCapacity() { final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try { return items.length - count; } finally { lock.unlock(); } } /** * Removes a single instance of the specified element from this queue, * if it is present. More formally, removes an element {@code e} such * that {@code o.equals(e)}, if this queue contains one or more such * elements. * Returns {@code true} if this queue contained the specified element * (or equivalently, if this queue changed as a result of the call). * * <p>Removal of interior elements in circular array based queues * is an intrinsically slow and disruptive operation, so should * be undertaken only in exceptional circumstances, ideally * only when the queue is known not to be accessible by other * threads. * * @param o element to be removed from this queue, if present * @return {@code true} if this queue changed as a result of the call */ public boolean remove(Object o) { if (o == null) return false; final Object[] items = this.items; final ReentrantLock lock = this.lock;//加上全局锁，确保线程安全 lock.lock(); try { for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) { if (o.equals(items[i])) { removeAt(i); return true; } } return false; } finally { lock.unlock(); } } /** * Returns {@code true} if this queue contains the specified element. * More formally, returns {@code true} if and only if this queue contains * at least one element {@code e} such that {@code o.equals(e)}. * * @param o object to be checked for containment in this queue * @return {@code true} if this queue contains the specified element */ public boolean contains(Object o) { if (o == null) return false; final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try { for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) if (o.equals(items[i])) return true; return false; } finally { lock.unlock(); } } /** * Atomically removes all of the elements from this queue. * The queue will be empty after this call returns. */ public void clear() { final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try {//清空所有元素，并唤醒在notFull条件上等待的所有线程 for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) items[i] = null; count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c) { checkNotNull(c); if (c == this) throw new IllegalArgumentException(); final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try {//将所有元素转移到c中，并唤醒在notFull条件上等待的所有线程 int i = takeIndex; int n = 0; int max = count; while (n < max) { c.add(this.<E>cast(items[i])); items[i] = null; i = inc(i); ++n; } if (n > 0) { count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } return n; } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c, int maxElements) { checkNotNull(c); if (c == this) throw new IllegalArgumentException(); if (maxElements <= 0) return 0; final Object[] items = this.items; final ReentrantLock lock = this.lock; lock.lock();//加上全局锁，确保线程安全 try {//将n个元素转移到c中，并唤醒在notFull条件上等待的所有线程 int i = takeIndex; int n = 0; int max = (maxElements < count) ? maxElements : count; while (n < max) { c.add(this.<E>cast(items[i])); items[i] = null; i = inc(i); ++n; } if (n > 0) { count -= n; takeIndex = i; notFull.signalAll(); } return n; } finally { lock.unlock(); } }
代码逻辑很简单，看看注释就明了其原理了。所有涉及到对items数组操作的方法，都加上了全局锁，所以ArrayBlockingQueue是线程安全的。
ps. 注释里写的非阻塞，是指不会被两个Condition所阻塞，这些方法如果被多线程并发调用，涉及到对lock的争用，那肯定是会在lock上阻塞住的。
4. ArrayBlockingQueue的迭代器
ArrayBlockingQueue的迭代器是弱一致的，它不会抛出ConcurrentModificationException。
设计思路很奇怪，创建迭代器的瞬间，记下了ArrayBlockingQueue的size，与takeIndex的位置（也就是确定了对底层数组的遍历区间），还会缓存住调用next方法即将返回的元素。
然后在调用next方法时，如果对应的元素已经被删除，返回之前缓存的元素（弱一致性），然后向后遍历，跳过null，直到找到下一个非空元素或者把size个元素都遍历完为止。
相关源码如下：
/** * Iterator for ArrayBlockingQueue. To maintain weak consistency * with respect to puts and takes, we (1) read ahead one slot, so * as to not report hasNext true but then not have an element to * return -- however we later recheck this slot to use the most * current value; (2) ensure that each array slot is traversed at * most once (by tracking "remaining" elements); (3) skip over * null slots, which can occur if takes race ahead of iterators. * However, for circular array-based queues, we cannot rely on any * well established definition of what it means to be weakly * consistent with respect to interior removes since these may * require slot overwrites in the process of sliding elements to * cover gaps. So we settle for resiliency, operating on * established apparent nexts, which may miss some elements that * have moved between calls to next. */ private class Itr implements Iterator<E> { private int remaining; // Number of elements yet to be returned private int nextIndex; // Index of element to be returned by next private E nextItem; // Element to be returned by next call to next private E lastItem; // Element returned by last call to next private int lastRet; // Index of last element returned, or -1 if none Itr() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { lastRet = -1; if ((remaining = count) > 0) nextItem = itemAt(nextIndex = takeIndex); } finally { lock.unlock(); } } public boolean hasNext() { return remaining > 0; } public E next() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { if (remaining <= 0) throw new NoSuchElementException(); lastRet = nextIndex; E x = itemAt(nextIndex); // check for fresher value if (x == null) { x = nextItem; // we are forced to report old value lastItem = null; // but ensure remove fails } else lastItem = x; while (--remaining > 0 && // skip over nulls (nextItem = itemAt(nextIndex = inc(nextIndex))) == null) ; return x; } finally { lock.unlock(); } } public void remove() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { int i = lastRet; if (i == -1) throw new IllegalStateException(); lastRet = -1; E x = lastItem; lastItem = null; // only remove if item still at index if (x != null && x == items[i]) { boolean removingHead = (i == takeIndex); removeAt(i); if (!removingHead) nextIndex = dec(nextIndex); } } finally { lock.unlock(); } } }
