使用最小堆实现，最近要到达时间的节点放在堆顶，每个节点都会附带到期时间，依次作为堆调整的依据。看poll()的源码就明白延迟队列的秘密了
public RunnableScheduledFuture<?> poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
RunnableScheduledFuture<?> first = queue[0];
if (first == null || first.getDelay(NANOSECONDS) > 0)
return null;
else
return finishPoll(first);
} finally {
lock.unlock();
}
}