BlockingQueue

接口分类

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BlockingQueue

问题：ArrayBlockingQueue和LinkedBlockingQueue到底该选哪个？
答案参见：https://stackoverflow.com/questions/35967792/when-to-prefer-linkedblockingqueue-over-arrayblockingqueue
array是有界的，linked既支持有界也支持无界。array的放和拿是加的同一把锁，linked是不同的锁。Executors使用的是linked。

1. ArrayBlockingQueue(环形缓冲区）;
2. LinkedBlockingQueue;
3. SynchronousQueue;
   不像ArrayBlockingQueue或LinkedListBlockingQueue，SynchronousQueue内部并没有数据缓存空间，你不能调用peek()方法来看队列中是否有数据元素，因为数据元素只有当你试着取走的时候才可能存在，不取走而只想偷窥一下是不行的，当然遍历这个队列的操作也是不允许的。队列头元素是第一个排队要插入数据的线程，而不是要交换的数据。数据是在配对的生产者和消费者线程之间直接传递的，并不会将数据缓冲数据到队列中。可以这样来理解：生产者和消费者互相等待对方，握手，然后一起离开。由于不维护Queue实体，所以没有插入、删除操作，效率很高。
   HSF的线程池用的就是SynchronousQueue，来一个请求开启一个线程。这样生产者和消费者都能够保持忙碌的状态。
4. PriorityBlockingQueue
   数组实现的极小堆，元素必须实现Comparable。

 /*
     * The implementation uses an array-based binary heap, with public
     * operations protected with a single lock. However, allocation
     * during resizing uses a simple spinlock (used only while not
     * holding main lock) in order to allow takes to operate
     * concurrently with allocation.  This avoids repeated
     * postponement of waiting consumers and consequent element
     * build-up. The need to back away from lock during allocation
     * makes it impossible to simply wrap delegated
     * java.util.PriorityQueue operations within a lock, as was done
     * in a previous version of this class. To maintain
     * interoperability, a plain PriorityQueue is still used during
     * serialization, which maintains compatibility at the expense of
     * transiently doubling overhead.
     */

5. DelayQueue
   元素必须实现Delayed。

public interface Delayed extends Comparable {

    /**
     * Returns the remaining delay associated with this object, in the
     * given time unit.
     *
     * @param unit the time unit
     * @return the remaining delay; zero or negative values indicate
     * that the delay has already elapsed
     */
    long getDelay(TimeUnit unit);
}

本质是对PriorityQueue的封装。在poll时，还是先调PriorityQueue的poll（poll的第一个），然后再调getDelay判断是否expire了，如果是再返回。该queue的顺序是Comparable决定的（极小堆），getDelay只是决定能否poll出去。

ScheduledThreadPoolExecutor内部依赖的Queue是DelayedWorkQueue，其基于极小堆,实际上功能和DelayQueue是相同的，compareTo比较的是要执行的时间，getDelay返回的是还要多久执行。

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take和poll

poll的源码：

 public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E first = q.peek();
            if (first == null || first.getDelay(NANOSECONDS) > 0)
                return null;
            else
                return q.poll();
        } finally {
            lock.unlock();
        }

take相对于poll是阻塞的，那么怎么delay到时之后通知正take的线程notify呢？源码里面是用leader线程做的notify，leader线程可能是正take的线程（await delay时间），也可能是别的线程（那就await无限时间）

   Delayed extends Comparable {

    /**
     * Returns the remaining delay associated with this object, in the
     * given time unit.
     *
     * @param unit the time unit
     * @return the remaining delay; zero or negative values indicate
     * that the delay has already elapsed
     */
    long getDelay(TimeUnit unit);
}