DelayQueue 使用和延时功能源码分析

DelayQueue 延迟队列使用和延时功能源码分析，先看DelayQueue 的使用

目录

1、基本使用

2、延时功能源码分析

3、总结

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1、基本使用

想要实现延时功能，需要实现 Delayed 接口，重写 getDelay 方法，在 getDelay 方法里返回延时时间

笔者定义一个 Order 类

在构造函数中传入延时的时间

package com.wsjzzcbq.java.queue;

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

/**
 * Order
 *
 * @author wsjz
 * @date 2023/09/22
 */
public class Order implements Delayed {

    /**
     * 延时时长
     */
    private long time;
    /**
     * 延时开始时间
     */
    private long start = System.currentTimeMillis();

    public Order(long time) {
        this.time = time;
    }

    public Order(long time, long start) {
        this.time = time;
        this.start = start;
    }

    @Override
    public long getDelay(TimeUnit unit) {
        return unit.convert((start + time) - System.currentTimeMillis(), TimeUnit.MILLISECONDS);
    }

    @Override
    public int compareTo(Delayed o) {
        Order order = (Order)o;
        return (int) (this.getDelay(TimeUnit.MILLISECONDS) - order.getDelay(TimeUnit.MILLISECONDS));
    }
}

延时队列使用

（1）、以现在时间为开始时间，延时获取

package com.wsjzzcbq.java.queue;

import java.time.LocalDateTime;
import java.util.concurrent.DelayQueue;

/**
 * DelayQueueLearn
 *
 * @author wsjz
 * @date 2023/09/22
 */
public class DelayQueueLearn {

    public static void main(String[] args) throws InterruptedException {
        DelayQueue delayQueue = new DelayQueue<>();
        Order order = new Order(5*1000);

        System.out.println(LocalDateTime.now());
        delayQueue.add(order);
        Order order1 = delayQueue.take();
        System.out.println(LocalDateTime.now());
        System.out.println(order1);
    }
}

延时 5 秒钟才能获取

测试运行

添加到队列后5秒钟，获取数据

（2）、以指定时间为开始时间，延时获取

以当前时间加 5 秒为开始时间，延时 5 秒钟获取

package com.wsjzzcbq.java.queue;

import java.time.LocalDateTime;
import java.time.ZoneOffset;
import java.util.concurrent.DelayQueue;

/**
 * DelayQueueLearn
 *
 * @author wsjz
 * @date 2023/09/22
 */
public class DelayQueueLearn {

    public static void main(String[] args) throws InterruptedException {
        DelayQueue delayQueue = new DelayQueue<>();
        
        //当前时间加5秒为开始时间
        LocalDateTime localDateTime = LocalDateTime.now().plusSeconds(5);
        long start = localDateTime.toInstant(ZoneOffset.of("+8")).toEpochMilli();
        System.out.println(start);
        Order order = new Order(5*1000, start);

        System.out.println(LocalDateTime.now());
        delayQueue.add(order);
        Order order1 = delayQueue.take();
        System.out.println(LocalDateTime.now());
        System.out.println(order1);
    }
}

测试运行

一共延时 10 秒钟

2、延时功能源码分析

DelayQueue 是基于 PriorityQueue（优先队列）实现的，PriorityQueue 默认是最小堆结构

我们先看 add 添加方法

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

offer 方法

    /**
     * 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();
        }
    }

先获取锁，然后调用 PriorityQueue 的 offer 方法，如果此时 PriorityQueue 的头部元素是新添加的元素，则 leader = null，并唤醒等待线程；否则直接返回 true

因为这里的 PriorityQueue 是最小堆结构，所以它能保证延时时间最小的元素最先出队（添加进去的元素 Order 对象实现了 compareTo 方法）

PriorityQueue 的 offer 方法

    /**
     * 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);
        size = i + 1;
        if (i == 0)
            queue[0] = e;
        else
            siftUp(i, e);
        return true;
    }

如果超出容量的话，调用 grow 方法扩容

如果是首次添加的话放在数组索引是0的首位

如果队列中有元素的话，调用 siftUp 方法添加

grow 方法

PriorityQueue 基于数组实现

    /**
     * Increases the capacity of the array.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        int oldCapacity = queue.length;
        // Double size if small; else grow by 50%
        int newCapacity = oldCapacity + ((oldCapacity < 64) ?
                                         (oldCapacity + 2) :
                                         (oldCapacity >> 1));
        // overflow-conscious code
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        queue = Arrays.copyOf(queue, newCapacity);
    }

siftUp 方法

    /**
     * Inserts item x at position k, maintaining heap invariant by
     * promoting x up the tree until it is greater than or equal to
     * its parent, or is the root.
     *
     * To simplify and speed up coercions and comparisons. the
     * Comparable and Comparator versions are separated into different
     * methods that are otherwise identical. (Similarly for siftDown.)
     *
     * @param k the position to fill
     * @param x the item to insert
     */
    private void siftUp(int k, E x) {
        if (comparator != null)
            siftUpUsingComparator(k, x);
        else
            siftUpComparable(k, x);
    }

默认 comparator 是 null，调用 siftUpComparable 方法

    private void siftUpComparable(int k, E x) {
        Comparable key = (Comparable) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (key.compareTo((E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = key;
    }

siftUpComparable 方法会进行比较，保证延时时间最小的元素在最上面

然后我们直接看 take 方法

    /**
     * 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 (;;) {
                //队列头部元素
                E first = q.peek();
                if (first == null)
                    //如果头部元素是null 则让当前线程等待
                    available.await();
                else {
                    //头部元素不为空，获取延时时间
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0)
                        //延时时间小于等于0，出队返回
                        return q.poll();
                    first = null; // don't retain ref while waiting
                    if (leader != null)
                        available.await();
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            //让线程等待延时时间
                            available.awaitNanos(delay);
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && q.peek() != null)
                available.signal();
            lock.unlock();
        }
    }

相关说明在代码注释中，先看队列头部元素是不是null，如果是说明当前队列为空，让线程等待；如果不为空，看头部元素延时时间，如果延时时间小于等于0，则出队返回，leader 默认是null，因此线程等待延时时间的时长，等待时间到达后，重新开始循环，此时延时时间小于等于0，出队返回，达到延时效果

关于leader 的分析，leader 这里使用了 Leader-Follower 模式的变体

    /**
     * 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;

假设没有 leader，现在有2个线程，线程A 和线程B，线程A 和线程B 都会执行 available.awaitNanos(delay) 进行等待，等待时间结束后，线程A 和线程B中只有一个能拿到元素返回，另外一个将重新等待，对于没拿到元素的线程来说一开始等待，之后等待结束被唤醒，最后再次等待，是一种资源浪费，不如一开始就让它一直等待（如果它不是leader的话）

leader 更详细的分析：https://stackoverflow.com/questions/48493830/what-exactly-is-the-leader-used-for-in-delayqueue

3、总结

DelayQueue 内部基于优先队列 PriorityQueue（最小堆结构）实现延时时间小的元素总是先出队。延时功能是通过循环加线程等待的方式实现的，先判断 PriorityQueue 中延时时间最小的元素的延时时间是否小于等于0，如果是则直接出队返回；否则让线程等待延时的时长，等待结束后，开始新一轮循环，这时延时时间肯定是小于等于0的，出队返回，达到延时的效果

至此完