Java底层类和源码分析系列-ArrayBlockingQueue底层架构和源码分析

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

//底层存储元素的数组。为final说明一旦初始化，容量不可变，所以是有界的。
final Object[] items;

//下一个take, poll, peek or remove操作的index位置
int takeIndex;

//下一个put, offer, or add操作的index位置
int putIndex;

// 元素数量
int count;

/**
* 用于并发控制：使用经典的双Condition算法
*/
final ReentrantLock lock;
/** 获取操作等待条件 */
private final Condition notEmpty;
/** 插入操作等待条件 */
private final Condition notFull;

//传入参数为队列的容量，传入后队列容量固定，不能扩容
 public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }
    
//传入队列容量和是否为公平锁
public ArrayBlockingQueue(int capacity, boolean fair) {
        //容量非空判断
        if (capacity <= 0)
            throw new IllegalArgumentException();
        //初始化存放队列元素的数组
        this.items = new Object[capacity];
        //初始化锁
        lock = new ReentrantLock(fair);
        //初始化用于阻塞的Condition
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

public boolean offer(E e) {
        //元素校验
        checkNotNull(e);
        //引用锁
        final ReentrantLock lock = this.lock;
        //上锁
        lock.lock();
        try {
            //当队列塞满后，不能再继续往队列中添加元素，返回false
            if (count == items.length)
                return false;
            else {
            //队列还未塞满，执行入列方法，入列成功返回true
                enqueue(e);
                return true;
            }
        } finally {
            //释放锁
            lock.unlock();
        }
    }

    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        //入列时间转化成纳秒
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        //上锁，如果线程在阻塞中中断，则抛出异常
        lock.lockInterruptibly();
        try {
              //当队列塞满的时候，进行超时等待
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            //入列
            enqueue(e);
            return true;
        } finally {
            //锁释放
            lock.unlock();
        }
    }

public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            //队列塞满了
            while (count == items.length)
                //线程进行无限期等待
                notFull.await();
            //执行入列方法
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

    private void enqueue(E x) {
        final Object[] items = this.items;
        //入列：往数组中存入元素
        items[putIndex] = x;
        /**判断元素是否存到了数组的最后一个位置上，如果是，
        就把下一个元素入列的索引置为1，防止索引越界，*/
        if (++putIndex == items.length)
            putIndex = 0;
        //入列成功，当前队列元素数量自增
        count++;
        /**通知还在等待的出列方法，队列中已有元素，
        可以进行出列了*/
        notEmpty.signal();
    }

//调用父类的add方法
public boolean add(E e) {
    return super.add(e);
}

//调用入列方法offer，offer入列失败，抛出异常，成功则返回true
public boolean add(E e) {
    if (offer(e))
        return true;
    else
        throw new IllegalStateException("Queue full");
}

public E poll() {
        final ReentrantLock lock = this.lock;
        //上锁
        lock.lock();
        try {
            //队列为空，返回null，否则执行出列方法
            return (count == 0) ? null : dequeue();
        } finally {
            //释放锁
            lock.unlock();
        }
    }
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        //超时时间转换成纳秒
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        //上锁，超时等待过程如果线程中断，则抛出异常
        lock.lockInterruptibly();
        try {
            //队列为空，超时等待，等待超时，返回null
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            //执行出列方法
            return dequeue();
        } finally {
            //释放锁
            lock.unlock();
        }
    }

public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // null when queue is empty
        } finally {
            lock.unlock();
        }
    }

   public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

private E dequeue() {
        // 引用队列存放的数组
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        //获取要出列的元素
        E x = (E) items[takeIndex];
        //移除出列后的元素
        items[takeIndex] = null;
        /**标记下次出列的元素的索引，并判断当前出列元素
        是否是数组中最后一条元素，如果是则，标记下次出列元素索引为0，从数组头部开始出列*/
        if (++takeIndex == items.length)
            takeIndex = 0;
        //出列成功，队列长度-1
        count--;
        if (itrs != null)
            itrs.elementDequeued();
        notFull.signal();
        return x;
    }

public E remove() {
        E x = poll();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }