Class - java.util.concurrent.ArrayBlockingQueue
Created by : Mr Dk.
2020 / 10 / 21 15:21
Nanjing, Jiangsu, China
Definition
底层基于数组实现的 BlockQueue (与链表实现的阻塞队列形成对照)。FIFO 的逻辑类似，但数组是有边界的，所以基于数组实现的阻塞队列是一个经典的 有容量 队列。当队列容量已满时，插入线程将会被阻塞；当队列容量为空时，读取线程也会被阻塞。另外，这个类还支持可选的 公平性 参数，以牺牲吞吐量为代价，保证加入队列的 FIFO 顺序一定被满足。
/**
* 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;
}
Fields
该类显然会使用一个对象数组作为队列内存。数组被修饰为 final，表示一经初始化后 容量不可改变。为了避免元素的复制，基于数组的队列实现一般都会循环利用数组空间，因此队列的头尾直接体现为两个数组下标。count 用于维护队列中的元素个数。
在并发控制方面，这个类使用了一把可重入锁来锁定所有的存取操作。
/** 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;
Constructor
对于构造函数来说，必备的参数是队列容量，用于初始化队列内部的数组。可选的参数是 是否启用公平竞争机制，默认是非公平锁，以及在初始化数组后预先放入一个集合内的所有元素：
- 根据
capacity参数初始化内部数组 - 初始化可重入锁 (公平 / 非公平)，以及非空 / 非满的阻塞条件
- 如果需要加入初始元素
- 首先对队列加锁
- 依次放入集合元素
- 初始元素溢出将会产生异常
- 妥善设置队尾位置
- 解锁
/**
* 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();
}
}
Input Operations
Offer
非阻塞方法，向队尾插入元素。如果插入成功则返回 true，如果插入失败则返回 false。在操作前先要获取锁。
/**
* 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 {
enqueue(e);
return true;
}
} finally {
lock.unlock();
}
}
该函数还有定时阻塞版本：当队列已满时，等待指定长时间后如果队列还没有空，则返回 false：
/**
* 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);
}
enqueue(e);
return true;
} finally {
lock.unlock();
}
}
以下是具体的入队函数。在队尾放入元素后，将队尾指针自增 - 如果队尾已经到达数组尾部，则回到数组开头。增加计数器。释放非空的信号给可能存在的 take 线程。
/**
* Inserts element at current put position, advances, and signals.
* Call only when holding lock.
*/
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;
notEmpty.signal();
}
Add
默认使用了父类的 add()。在父类中可以看到，实际上使用 offer() 作为实现，进行非阻塞插入操作：
- 如果插入成功，则返回
true - 如果因队列已满而插入失败，则抛出异常
/**
* 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);
}
父类中的实现：
/**
* Inserts the specified element into this queue if it is possible to do so
* immediately without violating capacity restrictions, returning
* <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
* if no space is currently available.
*
* <p>This implementation returns <tt>true</tt> if <tt>offer</tt> succeeds,
* else throws an <tt>IllegalStateException</tt>.
*
* @param e the element to add
* @return <tt>true</tt> (as specified by {@link Collection#add})
* @throws IllegalStateException if the element cannot be added at this
* time due to capacity restrictions
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null and
* this queue does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this queue
*/
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
Put
向队列中插入元素，如果队列已满，则阻塞等待直到队列中有空位。
/**
* 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();
enqueue(e);
} finally {
lock.unlock();
}
}
Output Operations
Poll
非阻塞函数，从队列中取出元素。如果队列为空，则立刻返回 null：
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}
定时阻塞函数，从队列中取出元素。在等待指定长度的时间后，如果队列依旧为空，则返回 null：
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 dequeue();
} finally {
lock.unlock();
}
}
其中用到的出队函数：
- 从
takeIndex(队头) 处取出元素，将队头对应的引用设置为null - 队头向后移；如果队头超过数组边界，则绕回数组的最开头
count计数器自减- 向可能等待的 put 线程释放信号
/**
* Extracts element at current take position, advances, and signals.
* Call only when holding lock.
*/
private E dequeue() {
// assert lock.getHoldCount() == 1;
// assert items[takeIndex] != null;
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E x = (E) items[takeIndex];
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
notFull.signal();
return x;
}
Take
阻塞函数，从队列中取出元素。如果队列为空，则一直阻塞直至能够从队列中取出元素为止。
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
Peek
瞥一眼当前队头元素 (不进行出队操作)：
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return itemAt(takeIndex); // null when queue is empty
} finally {
lock.unlock();
}
}
Size
返回当前的队列长度。在对队列进行上锁后，直接返回 count 计数器的值：
// 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();
}
}
返回数组中剩余空间的长度。在上锁后，由数组容量减去当前队列长度就是剩余空间：
// 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();
}
}
Iteration
Remove
从队头开始遍历队列，删除 equals() 返回 true 的一个元素，并返回 true；如果没有对队列进行修改，则返回 false：
/**
* 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 {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i])) {
removeAt(i);
return true;
}
if (++i == items.length)
i = 0;
} while (i != putIndex);
}
return false;
} finally {
lock.unlock();
}
}
其中删除元素的具体逻辑：
- 如果被删除的元素恰好在队头，则直接出队
- 如果被删除的元素不在队头，需要把从该元素开始至队尾的每个元素向前移动
- 向可能存在的 put 线程释放信号
/**
* Deletes item at array index removeIndex.
* Utility for remove(Object) and iterator.remove.
* Call only when holding lock.
*/
void removeAt(final int removeIndex) {
// assert lock.getHoldCount() == 1;
// assert items[removeIndex] != null;
// assert removeIndex >= 0 && removeIndex < items.length;
final Object[] items = this.items;
if (removeIndex == takeIndex) {
// removing front item; just advance
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
} else {
// an "interior" remove
// slide over all others up through putIndex.
final int putIndex = this.putIndex;
for (int i = removeIndex;;) {
int next = i + 1;
if (next == items.length)
next = 0;
if (next != putIndex) {
items[i] = items[next];
i = next;
} else {
items[i] = null;
this.putIndex = i;
break;
}
}
count--;
if (itrs != null)
itrs.removedAt(removeIndex);
}
notFull.signal();
}
Contains
上锁并遍历数组，判断是否包含 equals() 返回 true 的元素。
/**
* 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 {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i]))
return true;
if (++i == items.length)
i = 0;
} while (i != putIndex);
}
return false;
} finally {
lock.unlock();
}
}
Iterator
以后再做补充，这里的迭代器实现还挺奇怪的。