延时队列

DelayQueue是一个支持延时获取元素的使用优先级队列实现的无界的阻塞队列。

在创建元素时可以指定多久才能从队列中获取当前元素，只有在延迟期满时才能从队列中提取元素。

类图如下：

DelayQueue的定义以及构造函数如下：

public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
    implements BlockingQueue<E> {

    private final transient ReentrantLock lock = new ReentrantLock();
    private final PriorityQueue<E> q = new PriorityQueue<E>();

    /**
     * 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;

    /**
     * Condition signalled when a newer element becomes available
     * at the head of the queue or a new thread may need to
     * become leader.
     */
    private final Condition available = lock.newCondition();

    /**
     * Creates a new {@code DelayQueue} that is initially empty.
     */
    public DelayQueue() {}

    /**
     * Creates a {@code DelayQueue} initially containing the elements of the
     * given collection of {@link Delayed} instances.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public DelayQueue(Collection<? extends E> c) {
        this.addAll(c);
    }

解读：

DelayQueue 内部使用 PriorityQueue 存放数据，使用 ReentrantLock 实现线程同步。

重点解释一下变量 leader：

其使用基于 Leader-Follower模式的变体，用于减少不必要的线程等待。

当一个线程调用队列的 take 方法变为 leader 线程后，它会调用 available. awaitNanos(delay) 等待 delay 时间；其他线程（follwer线程) 则调用 available. await()进行无限等待。

leader 线程延迟时间过期后会退出 take 方法，并通过调用 available.signal()方法唤醒一个 follwer线程，被唤醒的 follwer线程被选举为新的 leader线程。 

Note：

队列中的元素必须实现Delayed接口和Comparable接口，也就是说DelayQueue里面的元素必须有public void compareTo(To)和long getDelay(TimeUnit unit)方法存在。

原因：由于每个元素都有一个过期时间，所以要实现获取当前元素还剩下多少时间就过期了的接口；由于DelayQueue底层使用优先级队列来实现，所以要实现元素之间相互比较的接口。

Delayed接口的定义如下：

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);
}

解读：

此接口继承自Comparable接口。

Comparable接口的定义如下：

public interface Comparable<T> {
    public int compareTo(T o);
}

添加元素

offer方法的代码如下：

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return {@code true}
     * @throws NullPointerException if the specified element is null
     */
    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();
        }
    }

解读：

该方法在获取独占锁之后调用优先级队列的offer方法实现入队

    /**
     * Inserts the specified element into this priority queue.
     *
     * @return {@code true} (as specified by {@link Queue#offer})
     * @throws ClassCastException if the specified element cannot be
     *         compared with elements currently in this priority queue
     *         according to the priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null)
            throw new NullPointerException();
        modCount++;
        int i = size;
        if (i >= queue.length)
            grow(i + 1);
        siftUp(i, e);
        size = i + 1;
        return true;
    }

解读：

如果待插入元素 e 为 null，则抛出 NullPointerException 异常。

由于DelayQueue是无界队列，所以方法一直返回 true。

Note：

前述offer中 q.peek()方法返回的并不一定是当前添加的元素。

如果 q.peek()方法返回的是 e，则说明当前元素 e将是最先过期的，于是重置 leader线程为 null，进而激活 avaliable变量对应的条件队列里的一个线程，告诉它队列里面有元素了。

put方法的代码如下：

    /**
     * Inserts the specified element into this delay queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) {
        offer(e);
    }

解读：

此方法直接调用offer方法来实现。

获取元素

poll方法的代码如下：

    /**
     * Retrieves and removes the head of this queue, or returns {@code null}
     * if this queue has no elements with an expired delay.
     *
     * @return the head of this queue, or {@code null} if this
     *         queue has no elements with an expired delay
     */
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E first = q.peek();
            return (first == null || first.getDelay(NANOSECONDS) > 0)
                ? null
                : q.poll();
        } finally {
            lock.unlock();
        }
    }

解读：

如果队列里面没有过期元素，则返回null；否则返回队首元素。

take方法的代码如下：

    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * until an element with an expired delay is available on this queue.
     *
     * @return the head of this queue
     * @throws InterruptedException {@inheritDoc}
     */
    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 <= 0L)
                        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();
        }
    }

解读：

如果队列里面没有过期元素，则等待。

示例

可以将延时队列DelayQueue运用在以下场景中：

　　（1）缓存系统的设计：可以用DelayQueue保存元素的有效期，使用一个线程循环查询DelayQueue，一旦能从DelayQueue中获取元素，则表示缓存有效期到了。

　　（2）定时任务调度：使用DelayQueue保存当天将要执行的任务和执行时间，一旦从DelayQueue中获取到任务就开始执行任务，比如TimerQueue就是使用DelayQueue实现的。

具体示例如下：

（1）Student对象作为DelayQueue的元素，其必须实现Delayed接口的两个方法

package com.test;

import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;

public class Student implements Delayed {//必须实现Delayed接口
    
    private String name;
    private long submitTime;// 交卷时间
    private long workTime;// 考试时间

    public Student(String name, long submitTime) {
        this.name = name;
        this.workTime = submitTime;
        this.submitTime = TimeUnit.NANOSECONDS.convert(submitTime, TimeUnit.MILLISECONDS) + System.nanoTime();
        System.out.println(this.name + " 交卷，用时" + workTime);
    }

    public String getName() {
        return this.name + " 交卷，用时" + workTime;
    }
    
    //必须实现getDelay方法
    public long getDelay(TimeUnit unit) {
        //返回一个延迟时间
        return unit.convert(submitTime - System.nanoTime(), unit.NANOSECONDS);
    }

    //必须实现compareTo方法
    public int compareTo(Delayed o) {
        Student that = (Student) o;
        return submitTime > that.submitTime ? 1 : (submitTime < that.submitTime ? -1 : 0);
    }

}

（2）主线程程序

package com.test;

import java.util.concurrent.DelayQueue;


public class DelayQueueTest {
    public static void main(String[] args) throws Exception {
        
        // 新建一个等待队列
        final DelayQueue<Student> bq = new DelayQueue<Student>();
        for (int i = 0; i < 5; i++) {
            Student student = new Student("学生"+i,Math.round((Math.random()*10+i)));
            bq.put(student); // 将数据存到队列里！
        }
        //获取但不移除此队列的头部；如果此队列为空，则返回 null。
        System.out.println(bq.peek().getName());
    }
}

小结：