ArrayBlockingQueue解析

于 2017-03-19 21:09:31 发布
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分类专栏： JUC 文章标签： java
本文深入介绍了ArrayBlockingQueue的实现原理及关键操作，包括put、offer、take、poll等方法的具体实现，并对比了ArrayBlockingQueue与LinkedBlockingQueue的区别。

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Queue接口定义：[url]http://donald-draper.iteye.com/blog/2363491[/url]
AbstractQueue简介：[url]http://donald-draper.iteye.com/blog/2363608[/url]
ConcurrentLinkedQueue解析：[url]http://donald-draper.iteye.com/blog/2363874[/url]
BlockingQueue接口的定义：[url]http://donald-draper.iteye.com/blog/2363942[/url]
LinkedBlockingQueue解析：[url]http://donald-draper.iteye.com/blog/2364007[/url]
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
 * [i]head[/i] of the queue is that element that has been on the
 * queue the longest time.  The [i]tail[/i] 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.
 *
 基于数组的有界阻塞FIFO队列。head是待在队列中，最久的元素；
 tail则是最短的元素。新元素插入时放在队尾，消费时，则从head获取。
 * <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.
 *
 这一个是个节点的有界缓存队列，生产者生产消息，消费者消费消息。
 队列创建后，容量不能被修改。尝试向一个已满队列生产元素，则会被阻塞，
 尝试从一个空队列消费元素，同样会被阻塞。
 * <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.
 *
ArrayBlockingQueue支持消费和生产线程等待条件时公平性与非公平性，默认为非公平锁。
这个我们可以通过构造公平性参数来设置。公平锁一般会降低吞吐量，但是可以减少不确定性，
避免锁饥饿情况的发生。
 * <p>This class and its iterator implement all of the
 * [i]optional[/i] methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 实现了所有Collection和Iterator接口 
 * <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;
    //存放元素的数组
    /** The queued items */
    final Object[] items;

    /** items index for next take, poll, peek or remove */
    //下一个take，poll，peek，remove元素的数组index
    int takeIndex;

    /** items index for next put, offer, or add */
    //下一个put, offer, or add元素的数组index
    int putIndex;

    /** Number of elements in the queue */
    //当前队列的元素数量
    int count;

    /*
     * Concurrency control uses the classic two-condition algorithm
     * found in any textbook.
     */
    //可重入锁
    /** Main lock guarding all access */
    final ReentrantLock lock;
    /** Condition for waiting takes ，队列非空条件*/
    private final Condition notEmpty;
    /** Condition for waiting puts 队列非满条件*/
    private final Condition notFull;
     /**
     * Creates an {@code ArrayBlockingQueue} with the given (fixed)
     * capacity and default access policy.
     *
     待容量参数的构造
     * @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.
     *
     待容量和公平性参数的构造
     * @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();
    }

    /**
     * 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;
    }
}

从上面来看，ArrayBlockingQueue是一个有界的线程安全FIFO队列，队列元素放在
一个元素数组中，一个takeIndex，表示下一个take，poll，peek，remove元素的数组index。
一个putIndex，表示下一个put, offer, or add元素的数组index，一个int Count，表示当前
队列元素数量，一把锁ReentrantLock用于访问控制，一个队列非空条件notEmpty，一个队列非满条件notFull，这两条件都是由ReentrantLock创建。
来看put操作:
/**
     * 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 {
        //检查元素是否为null
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
	//以可中断方式，获取锁
        lock.lockInterruptibly();
        try {
            while (count == items.length)
	        //如果队列已满，等待notFull条件
                notFull.await();
	    //否则插入元素
            insert(e);
        } finally {
            lock.unlock();
        }
    }

put操作首先检查元素是否为null，然后以可中断方式，获取锁，
如果队列已满，等待notFull条件，否则插入元素；
来看put操作的几个要点：
1.
//检查元素是否为null
checkNotNull(e);

/**
     * Throws NullPointerException if argument is null.
     *
     * @param v the element
     */
    private static void checkNotNull(Object v) {
       //为null，则抛出null指针异常
        if (v == null)
            throw new NullPointerException();
    }


2.
//否则插入元素
 insert(e);


/**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void insert(E x) {
        items[putIndex] = x;
	//put索引自增1
        putIndex = inc(putIndex);
	//容量自增
        ++count;
	//唤醒等待消费的线程
        notEmpty.signal();
    }

来看offer操作
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();
        }
    }

offer操作与put操作的区别，当队列满时，返回false；
再看超时offer:
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();
        }
    }

超时offer与put操作没有太大的区别，区别是当队列满时，
超时等待notFull条件，插入成功，则返回true。

再看add操作：
 public boolean add(E e) {
        return super.add(e);
    }

//AbstractQueue
 public boolean add(E e) {
        //委托给offer
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }


add操作实际上调用的offer操作。

再看take操作
public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
	//以可中断方式获取锁
        lock.lockInterruptibly();
        try {
            while (count == 0)
	        //当队列为空，等待非空条件
                notEmpty.await();
            //返回队列中takeIndex，索引对应的元素
            return extract();
        } finally {
            lock.unlock();
        }
    }

   /**
     * Extracts element at current take position, advances, and signals.
     * Call only when holding lock.
     */
    private E extract() {
        final Object[] items = this.items;
	//获取队列中takeIndex，索引对应的元素
        E x = this.<E>cast(items[takeIndex]);
	//设置takeIndex索引对应的元素为null
        items[takeIndex] = null;
	//takeIndex索引自增
        takeIndex = inc(takeIndex);
	//容量count自减
        --count;
	//唤醒等待notFull条件的线程
        notFull.signal();
        return x;
    }

从上来看，take操作首先以可中断方式获取锁，当队列为空，等待非空条件，
否则返回队列中takeIndex，索引对应的元素，akeIndex索引自增，容量count自减，
唤醒等待notFull条件的线程。
再看超时poll
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();
        }
    }

超时poll的唯一区别是当当队列为空，超时等待非空条件。

再看poll
 public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : extract();
        } finally {
            lock.unlock();
        }
    }

poll操作与take操作的最大的区别为当队列为空，返回null。


再看peek操作：
 public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : itemAt(takeIndex);
        } finally {
            lock.unlock();
        }
    }

peek操作首先获取锁，如果队列为空，则返回null，否则，返回takeIndex所对应的元素。
来看peek操作关键点itemAt，
/**
     * Returns item at index i.
     */
    final E itemAt(int i) {
        return this.<E>cast(items[i]);
    }

再看remove操作：
public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
	//获取锁
        lock.lock();
        try {
	    //遍历队列，找到元素相等，移除
            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();
        }
    }

remove操作，首先获取锁，遍历队列，找到元素相等，移除。

contains操作：
 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();
        }
    }

clear操作：
 /**
     * 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 {
	    //遍历队列，将队列清空
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
                items[i] = null;
            count = 0;
            putIndex = 0;
            takeIndex = 0;
	    //通知所有等待put的线程
            notFull.signalAll();
        } finally {
            lock.unlock();
        }
    }

再来看drainTo
drainTo(Collection<? super E> c)和drainTo(Collection<? super E> c, int maxElements) 基本上相同，我们来看
drainTo(Collection<? super E> c, int maxElements) 
/**
     * @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;
	    //取出所有可用元素，添加集合
            while (n < max) {
                c.add(this.<E>cast(items[i]));
                items[i] = null;
                i = inc(i);
                ++n;
            }
            if (n > 0) {
                count -= n;
                takeIndex = i;
		//唤醒所有等待put的线程
                notFull.signalAll();
            }
	    //返回取出的元素数量
            return n;
        } finally {
            lock.unlock();
        }
    }

//获取队列中的元素数量
 public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }

//获取队列剩余空间
public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }



总结：
[color=blue] ArrayBlockingQueue是一个有界的线程安全FIFO队列，队列元素放在
一个元素数组中，一个takeIndex，表示下一个take，poll，peek，remove元素的数组index。
一个putIndex，表示下一个put, offer, or add元素的数组index，一个int Count，表示当前
队列元素数量，一把锁ReentrantLock用于访问控制，一个队列非空条件notEmpty，一个队列非满条件notFull，这两个条件都是由ReentrantLock创建。
put操作首先检查元素是否为null，然后以可中断方式，获取锁，
如果队列已满，等待notFull条件，否则插入元素；putIndex索引自增，容量count自增，唤醒等待消费的线程。offer操作与put操作的区别，当队列满时，返回false；超时offer与put操作没有太大的区别，区别是当队列满时超时等待notFull条件，插入成功，则返回true。
具体选择何中操作，视具体的场景需求。
take操作首先以可中断方式获取锁，当队列为空，等待非空条件，否则返回队列中takeIndex，索引对应的元素，akeIndex索引自增，容量count自减，唤醒等待notFull条件的线程。超时poll的唯一区别是当当队列为空，超时等待非空条件。poll操作与take操作的区别为当队列为空，返回null。具体选择何中操作，视具体的场景需求。
peek操作首先获取锁，如果队列为空，则返回null，否则，返回takeIndex所对应的元素。
remove操作，首先获取锁，遍历队列，找到元素相等，移除。
ArrayBlockingQueue与LinkedBlockingQueue区别是LinkedBlockingQueue中元素以节点链表来存储，而ArrayBlockingQueue是放在数组中；LinkedBlockingQueue中有两把锁分别为put和take锁，读写分离，而ArrayBlockingQueue只有一把锁控制take和put。[/color]
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