java并发中DelayQueue延迟队列原理剖析

 public interface Delayed extends Comparable<Delayed> {
 
     /**
      * 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);
}

 // 锁。
 private final transient ReentrantLock lock = new ReentrantLock();
 // 优先队列。
 private final PriorityQueue<E> q = new PriorityQueue<E>();
 
 /**
  * Leader-Follower的变种。
  * Thread designated to wait for the element at the head of
  * the queue.  This variant of the Leader-Follower pattern
 * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
 * minimize unnecessary timed waiting.  When a thread becomes
 * the leader, it waits only for the next delay to elapse, but
 * other threads await indefinitely.  The leader thread must
 * signal some other thread before returning from take() or
 * poll(...), unless some other thread becomes leader in the
 * interim.  Whenever the head of the queue is replaced with
 * an element with an earlier expiration time, the leader
 * field is invalidated by being reset to null, and some
 * waiting thread, but not necessarily the current leader, is
 * signalled.  So waiting threads must be prepared to acquire
 * and lose leadership while waiting.
 */
private Thread leader = null;

/**
 * Condition signalled when a newer element becomes available
 * at the head of the queue or a new thread may need to
 * become leader.
 */
// 条件，代表如果有数据则通知Follower线程，唤醒线程处理队列内容。
private final Condition available = lock.newCondition();

 public boolean offer(E e) {
     // 获取到锁
     final ReentrantLock lock = this.lock;
     lock.lock();
     try {
         // 将元素存储到PriorityQueue优先队列中
         q.offer(e);
         // 如果第一个元素是当前元素，说明之前队列中为空，则先将Leader设置为空，通知等待线程可以争抢Leader了。
         if (q.peek() == e) {
            leader = null;
            available.signal();
        }
        // 返回成功
        return true;
    } finally {
        lock.unlock();
    }
}

public void put(E e) {
    offer(e);
}

 public E take() throws InterruptedException {
     final ReentrantLock lock = this.lock;
     // 获取可中断的锁。
     lock.lockInterruptibly();
     try {
         // 循环获取数据。
         for (;;) {
             // 获取最早过期的元素，但是不弹出对象。
             E first = q.peek();
            // 如果最早过期的元素为空，说明队列为空，则线程直接进入无限期等待，并且让出锁。
            if (first == null)
                // 当前线程无限期等待，直到被唤醒，并且让出锁对象。
                available.await();
            else {
                // 获取最早过期的元素剩余过期时间。
                long delay = first.getDelay(NANOSECONDS);
                // 如果剩余过期时间小于0，则说明已经过期，反之还没有过期。
                if (delay <= )
                    // 如果已经过期直接获取最早过期的元素，并返回。
                    return q.poll();
                // 如果剩余过期日期大于0，则会进入到这里。
                // 将刚才获取的最早过期的元素设置为空。
                first = null; // don't retain ref while waiting
                // 如果有线程争抢的Leader线程，则进行无限期等待。
                if (leader != null)
                    // 无限期等待并让出锁。
                    available.await();
                else {
                    // 获取当前线程。
                    Thread thisThread = Thread.currentThread();
                    // 设置当前线程变为Leader线程。
                    leader = thisThread;
                    try {
                        // 等待剩余等待时间。
                        available.awaitNanos(delay);
                    } finally {
                        // 将Leader设置为null。
                        if (leader == thisThread)
                            leader = null;
                    }
                }
            }
        }
    } finally {
        // 如果队列不为空，并且没有Leader则通知等待线程可以成为Leader。
        if (leader == null && q.peek() != null)
            // 通知等待线程。
            available.signal();
        lock.unlock();
    }
}

 public E poll() {
     final ReentrantLock lock = this.lock;
     lock.lock();
     try {
         E first = q.peek();
         // 如果队列为空或者队列最早过期的元素没有过期，则返回null。
         if (first == null || first.getDelay(NANOSECONDS) > 0)
             return null;
         else
            // 出队列操作。
            return q.poll();
    } finally {
        lock.unlock();
    }
}