ArrayBlockingQueue源码浅析

一个由数组结构组成的有界阻塞队列

 

构造方法

    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    // 初始化数组，实例化ReentrantLock和两个等待队列notEmpty、notFull
    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();
    }

    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection 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();
        }
    }

 

入队

    // 添加元素，通过多态调用offer方法
    public boolean add(E e) {
        return super.add(e);
    }

    // 添加元素(如果数组已满就取消添加)
    public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
	// 获取锁
        lock.lock();
        try {
	    // 如果数组已满，就取消添加元素
            if (count == items.length)
                return false;
            else {
		// 添加元素到数组
                enqueue(e);
                return true;
            }
        } finally {
	    // 释放锁
            lock.unlock();
        }
    }

    // 阻塞式添加元素，直至添加成功
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
		// 如果数组已满，就将当前线程加入到notFull的等待队列
                notFull.await();
	    // 添加元素到数组
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

    // 添加元素，超时后取消添加
    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 {
	    // 如果数组已满，就进入超时等待，如果超时了，就直接返回取消添加
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
	    // 添加元素
            enqueue(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

    // 添加元素
    private void enqueue(E x) {
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        // putIndex记录的是下一次将要添加的元素的索引位置
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;
        count++;
	// 通知notEmpty的等待队列的线程可以取出元素
        notEmpty.signal();
    }

 

出队

	// 取出元素，如果数组中没有元素，则返回null
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }

	// 阻塞式获取元素，直至取出元素
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try 
			// 如果当前数组里元素个数为0，则当前线程进入到notEmpty的等待队列
            while (count == 0)
                notEmpty.await();
			// 取出元素
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

    // 取出元素，超时后取消添加
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
	    // 如果数组元素个数为0，就进入超时等待，如果超时了，就直接返回
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
	    // 取出元素
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

    // 取出元素
    private E dequeue() {
        // assert lock.getHoldCount() == 1;
        // assert items[takeIndex] != null;
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        // takeIndex表示下一次取出元素时的索引位置
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();
	// 通知notFull的等待队列的线程可以添加元素
        notFull.signal();
        return x;
    }

 

包含

    // 包含
    public boolean contains(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
	// 遍历从takeIndex到putIndex，如果存在该元素，则返回true
        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();
        }
    }

 

删除

    // 删除
    public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
	// 从takeIndex到putIndex遍历，如果存在该元素，则删除
        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();
        }
    }