数据共享通道—BlockingQueue源码分析

为什么BlockingQueue适合作为进行线程间安全的数据共享的通道?而高性能队列例如:ConcurrentLinkedQueue不适合呢?

  • 关键在于Blocking上面,即阻塞:
  • 当队列为空:进行取操作的线程,会被阻塞(相比循环监测,更省资源),并且当队列中有元素后,线程会被自动唤醒。
  • 当队列满时:和上面类似。

BlockingQueue作为接口,有一系列满足不同需求的实现类,下面以容量固定的ArrayBlockingQueue为例,进行源码分析,看看为什么这么神奇。

  • 主要体现在take()和put()方法上。

  • take():可以看到,当队列为空的时候,线程会进行等待,下面还有一个重要的dequeue()方法。

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

在dequeue()方法中,一旦有元素取出,就会通知等待在notFull上面的线程,让他们继续工作。

  • put()方法类似,不再赘述。。。
    /**
     * 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();
        }
    }

    /**
     * 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();
    }

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