java延迟队列DelayQueue使用及原理

概述

　　java延迟队列提供了在指定时间才能获取队列元素的功能，队列头元素是最接近过期的元素。没有过期元素的话，使用poll()方法会返回null值，超时判定是通过getDelay(TimeUnit.NANOSECONDS)方法的返回值小于等于0来判断。延时队列不能存放空元素。

延时队列实现了Iterator接口，但iterator()遍历顺序不保证是元素的实际存放顺序。

队列元素

　　DelayQueue<E extends Delayed>的队列元素需要实现Delayed接口，该接口类定义如下:

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);
}

　　由Delayed定义可以得知，队列元素需要实现getDelay(TimeUnit unit)方法和compareTo(Delayed o)方法, getDelay定义了剩余到期时间，compareTo方法定义了元素排序规则，注意，元素的排序规则影响了元素的获取顺序，将在后面说明。

内部存储结构　　

DelayedQuene的元素存储交由优先级队列存放。

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>();//元素存放

DelayedQuene的优先级队列使用的排序方式是队列元素的compareTo方法，优先级队列存放顺序是从小到大的，所以队列元素的compareTo方法影响了队列的出队顺序。

若compareTo方法定义不当，会造成延时高的元素在队头，延时低的元素无法出队。

获取队列元素

　　非阻塞获取

    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();
        }
    }
------------------------------------------------------------------------------------------------------------------------
　　PriorityQueue队列peek()方法。

public E peek() {
    return (size == 0) ? null : (E) queue[0];
}

　　由代码我们可以看出，获取元素时，总是判断PriorityQueue队列的队首元素是否到期，若未到期，返回null，所以compareTo()的方法实现不当的话，会造成队首元素未到期，当队列中有到期元素却获取不到的情况。因此，队列元素的compareTo方法实现需要注意。

　　阻塞方式获取

public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E first = q.peek();
                if (first == null) //没有元素，让出线程，等待java.lang.Thread.State#WAITING
                    available.await();
                else {
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0) // 已到期，元素出队
                        return q.poll();
                    first = null; // don't retain ref while waiting
                    if (leader != null) 
                        available.await();// 其它线程在leader线程TIMED_WAITING期间，会进入等待状态，这样可以只有一个线程去等待到时唤醒，避免大量唤醒操作

else { Thread thisThread = Thread.currentThread(); leader = thisThread; try { available.awaitNanos(delay);// 等待剩余时间后，再尝试获取元素，他在等待期间，由于leader是当前线程，所以其它线程会等待。 } finally { if (leader == thisThread) leader = null; } } } } } finally { if (leader == null && q.peek() != null) available.signal(); lock.unlock(); } }

示例：

package org.dromara.hmily.demo.springcloud.account.service;
 
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.concurrent.DelayQueue;
import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;
 
/**
 * @description: 延时队列测试
 * @author: hh
 */
public class DelayedQueneTest {
 
    public static void main(String[] args) throws InterruptedException {
        Item item1 = new Item("item1", 5, TimeUnit.SECONDS);
        Item item2 = new Item("item2",10, TimeUnit.SECONDS);
        Item item3 = new Item("item3",15, TimeUnit.SECONDS);
        DelayQueue<Item> queue = new DelayQueue<>();
        queue.put(item1);
        queue.put(item2);
        queue.put(item3);
        System.out.println("begin time:" + LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
        for (int i = 0; i < 3; i++) {
            Item take = queue.take();
            System.out.format("name:{%s}, time:{%s}\n",take.name, LocalDateTime.now().format(DateTimeFormatter.ISO_DATE_TIME));
        }
    }
 
}
 
class Item implements Delayed{
    /* 触发时间*/
    private long time;
    String name;
 
    public Item(String name, long time, TimeUnit unit) {
        this.name = name;
        this.time = System.currentTimeMillis() + (time > 0? unit.toMillis(time): 0);
    }
 
    @Override
    public long getDelay(TimeUnit unit) {
        return time - System.currentTimeMillis();
    }
 
    @Override
    public int compareTo(Delayed o) {
        Item item = (Item) o;
        long diff = this.time - item.time;
        if (diff <= 0) {// 改成>=会造成问题
            return -1;
        }else {
            return 1;
        }
    }
 
    @Override
    public String toString() {
        return "Item{" +
                "time=" + time +
                ", name='" + name + '\'' +
                '}';
    }
}<br><br>

　　运行结果：每5秒取出一个

begin time:2019-05-31T11:58:24.445
name:{item1}, time:{2019-05-31T11:58:29.262}
name:{item2}, time:{2019-05-31T11:58:34.262}
name:{item3}, time:{2019-05-31T11:58:39.262}

　　修改compareTo方法 diff >= 0 后的运行结果：在15秒之后几乎同时取出，

begin time:2019-05-31T12:02:50.157
name:{item3}, time:{2019-05-31T12:03:04.959}
name:{item2}, time:{2019-05-31T12:03:04.999}
name:{item1}, time:{2019-05-31T12:03:05}