源码分析之Queue（二）ArrayBlockingQueue

　　ArrayBlockingQueue是数组实现的线程安全的有界的阻塞队列。线程安全是指，ArrayBlockingQueue内部通过“互斥锁”保护竞争资源，实现了多线程对竞争资源的互斥访问；有界，则是指ArrayBlockingQueue对应的数组是有界限的。 阻塞队列，是指多线程访问竞争资源时，当竞争资源已被某线程获取时，其它要获取该资源的线程需要阻塞等待；而且，ArrayBlockingQueue是按 FIFO（先进先出）原则对元素进行排序，元素都是从尾部插入到队列，从头部开始返回。

原理及数据结构

1. ArrayBlockingQueue继承于AbstractQueue，并且它实现了BlockingQueue接口。
2. ArrayBlockingQueue内部是通过Object[]数组保存数据的，也就是说ArrayBlockingQueue本质上是通过数组实现的。ArrayBlockingQueue的大小，即数组的容量是创建ArrayBlockingQueue时指定的。
3. ArrayBlockingQueue与ReentrantLock（一把锁）是组合关系，ArrayBlockingQueue中包含一个ReentrantLock对象(lock)。ReentrantLock是可重入的互斥锁，ArrayBlockingQueue就是根据该互斥锁实现“多线程对竞争资源的互斥访问”。而且，ReentrantLock分为公平锁和非公平锁，关于具体使用公平锁还是非公平锁，在创建ArrayBlockingQueue时可以指定；而且，ArrayBlockingQueue默认会使用非公平锁。
4.  ArrayBlockingQueue与Condition是组合关系，ArrayBlockingQueue中包含两个Condition对象(notEmpty和notFull)。而且，Condition又依赖于ArrayBlockingQueue而存在，通过Condition可以实现对ArrayBlockingQueue的更精确的访问 -- (01)若某线程(线程A)要取数据时，数组正好为空，则该线程会执行notEmpty.await()进行等待；当其它某个线程(线程B)向数组中插入了数据之后，会调用notEmpty.signal()唤醒“notEmpty上的等待线程”。此时，线程A会被唤醒从而得以继续运行。(02)若某线程(线程H)要插入数据时，数组已满，则该线程会它执行notFull.await()进行等待；当其它某个线程(线程I)取出数据之后，会调用notFull.signal()唤醒“notFull上的等待线程”。此时，线程H就会被唤醒从而得以继续运行。

源码解析：

public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, java.io.Serializable {

    /**
     * Serialization ID
     */
    private static final long serialVersionUID = -817911632652898426L;

    /** The queued items 使用数组存储队列元素*/
    final Object[] items;

    /** items index for next take, poll, peek or remove 取元素的index*/
    int takeIndex;

    /** items index for next put, offer, or add 存元素的index*/
    int putIndex;

    /** Number of elements in the queue 元素数量*/
    int count;

    /** Main lock guarding all access 保证并发访问的锁，俗称的“两把锁”*/
    final ReentrantLock lock;

    /** Condition for waiting takes 取元素的非空条件*/
    private final Condition notEmpty;

    /** Condition for waiting puts 存元素的非满条件*/
    private final Condition notFull;

    /**
     * Shared state for currently active iterators, or null if there are known not to be any.  Allows queue operations to update iterator state.
     */
    transient Itrs itrs = null;

    // Internal helper methods

    /**
     * Circularly decrement i.
     */
    final int dec(int i) {
        return ((i == 0) ? items.length : i) - 1;
    }

    /**
     * Returns item at index i.
     */
    @SuppressWarnings("unchecked")
    final E itemAt(int i) {
        return (E) 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. 元素入队
     */
    private void enqueue(E x) {
        final Object[] items = this.items;
        items[putIndex] = x;　　　　 // 如果数组空间满了，又从头开始put
        if (++putIndex == items.length)
            putIndex = 0;
        count++;　　　　 // 线程间通信。发出一个信号通知，说明队列不为空。还能取元素
        notEmpty.signal();
    }

    /**
     * Extracts element at current take position, advances, and signals.Call only when holding lock. 元素出队
     */
    private E dequeue() {
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        items[takeIndex] = null;　　　　 //如果taskIndex等于了数组空间的大小，说明队列元素个数已经取完，需要重置为0
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();　　　　 // 线程间通信，发出一个信号通知，说明队列不满，还能put元素
        notFull.signal();
        return x;
    }

    /**
     * Deletes item at array index removeIndex.Utility for remove(Object) and iterator.remove.
     * Call only when holding lock.
     */
    void removeAt(final int removeIndex) {
        final Object[] items = this.items;
        if (removeIndex == takeIndex) {// 同dequeue()方法
            items[takeIndex] = null;
            if (++takeIndex == items.length)
                takeIndex = 0;
            count--;
            if (itrs != null)
                itrs.elementDequeued();
        } else {
            
            final int putIndex = this.putIndex;
            for (int i = removeIndex;;) {
                int next = i + 1;
                if (next == items.length)
                    next = 0;
                if (next != putIndex) { //循环用后一个元素的值赋给前一个元素
                    items[i] = items[next];
                    i = next;
                } else {
                    items[i] = null;
                    this.putIndex = i;
                    break;
                }
            }
            count--;
            if (itrs != null)
                itrs.removedAt(removeIndex);
        }
        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. fair设置是否是公平锁
     * @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);//如果是公平锁，先到的线程会获得锁对象
        notEmpty = lock.newCondition();//为线程间通信做准备
        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.*/
    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);

        final ReentrantLock lock = this.lock;        //保证可见性 不是为了互斥 防止指令重排 保证item的安全
        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
     * 调用offer方法，增加了异常处理
     * @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 如果当前队列已满，则返回false
     * @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();
        }
    }

    /**
     * Inserts the specified element at the tail of this queue, waiting for space to become available if the queue is full.
     * 增加元素，队列满则阻塞等待
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();//和lock()的区别是在阻塞时也可抛异常跳出
        try {　　　　　　　//使用while而不是if，是因为可能多个线程阻塞在lock上，即使唤醒了可能其他线程先一步修改了队列又变成满的，因此必须重新判断
           while (count == items.length)
                notFull.await();//notFull等待表示现在队列满了，等待被唤醒
            enqueue(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. 如果在指定的等待时间内，还是等不到队列有空的位置，则返回false
     *
     * @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();
        }
    }

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

    public E take() throws InterruptedException {//取元素，队列为空时阻塞等待
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();
            return dequeue();
        } 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 dequeue();
        } finally {
            lock.unlock();
        }
    }

    public E peek() { //返回元素，但不删除
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // null when queue is empty
        } finally {
            lock.unlock();
        }
    }

    // this doc comment is overridden to remove the reference to collections
    // greater in size than Integer.MAX_VALUE
    /**
     * @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.
    /**
     * 剩余空间大小*/
    public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }

    /**
     * 将队列转移到集合中*/
    public int drainTo(Collection<? super E> c) {
        return drainTo(c, Integer.MAX_VALUE);
    }

    /**
     * @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 n = Math.min(maxElements, count);
            int take = takeIndex;
            int i = 0;
            try {
                while (i < n) {
                    @SuppressWarnings("unchecked")
                    E x = (E) items[take];
                    c.add(x);
                    items[take] = null;
                    if (++take == items.length)
                        take = 0;
                    i++;
                }
                return n;
            } finally {
                // Restore invariants even if c.add() threw
                if (i > 0) {
                    count -= i;
                    takeIndex = take;
                    if (itrs != null) {
                        if (count == 0)
                            itrs.queueIsEmpty();
                        else if (i > take)
                            itrs.takeIndexWrapped();
                    }
                    for (; i > 0 && lock.hasWaiters(notFull); i--)
                        notFull.signal();
                }
            }
        } finally {
            lock.unlock();
        }
    }
}

总结：

　　1.使用数组来实现队列，我们需要四个变量：Object[] array来存储队列中元素，headIndex和tailIndex分别记录队列头和队列尾，count记录队列的个数。

• 因为数组的长度是固定，所以当count==array.length时，表示队列已经满了，当count==0的时候，表示队列是空的。
• 当添加元素的时候，将array[tailIndex] = e将tailIndex位置设置成新元素，之后将tailIndex++自增，然后将count++自增。但是有两点需要注意，在添加之前必须先判断队列是否已满，不然会出现覆盖已有元素。当tailIndex的值等于数组最后一个位置的时候，需要将tailIndex=0，循环利用数组
• 当删除元素的时候，将先记录下array[headIndex] 元素，之后将headIndex++自增，然后将count--自减。但是有两点需要注意要注意，在删除之前，必须先判断队列是否为空，不然可能会删除已删除的元素。

　　2.线程间通信，基于Condition的await()和singal()方法来实现。

　　3.ArrayBlockingQueue 是有界的，所以我们在初始化是容量要设计好，因为它是不可以扩容的，