通俗易懂的JUC源码剖析-DelayQueue

前言

DelayQueue是BlockingQueue接口的实现类，它是带有延时功能的无界阻塞队列，意思就是每个元素都有过期时间，当从队列获取元素时，只有过期元素才会出队列。队列的头部元素是最快要过期的元素。

实现原理

先来看看它的类结构：

public class DelayQueue extends AbstractQueue
    implements BlockingQueue {
}

可以看到，存放的元素必须实现了Delayed接口，它的定义如下：

public interface Delayed extends Comparable {
  long getDelay(TimeUnit unit);
}

再来看关键属性：

private final transient ReentrantLock lock = new ReentrantLock();
private final PriorityQueue q = new PriorityQueue();
private Thread leader = null;
private final Condition available = lock.newCondition();

再来看重要方法：
offer()

public boolean offer(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
         q.offer(e);
         if (q.peek() == e) {
             leader = null;
             available.signal();
         }
         return true;
    } finally {
        lock.unlock();
    }
}

poll()

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()

public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        for (;;) {
            E first = q.peek();
            if (first == null)
                available.await();
            else {
                long delay = first.getDelay(NANOSECONDS);
                if (delay <= 0)
                    return q.poll();
                first = null; // don't retain ref while waiting
                if (leader != null)
                    available.await();
                else {
                    Thread thisThread = Thread.currentThread();
                    leader = thisThread;
                    try {
                        available.awaitNanos(delay);
                    } finally {
                        if (leader == thisThread)
                            leader = null;
                    }
                }
            }
        }
    } finally {
        if (leader == null && q.peek() != null)
            available.signal();
        lock.unlock();
  }
}

明天再分析吧，先睡了，晚安全世界！