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Java技术专题-教你如何使用【精巧好用】的DelayQueue(延时队列)

延时队列前提

  1. 定时关闭空闲连接:服务器中,有很多客户端的连接,空闲一段时间之后需要关闭之。
  2. 定时清除额外缓存:缓存中的对象,超过了空闲时间,需要从缓存中移出。
  3. 实现任务超时处理:在网络协议滑动窗口请求应答式交互时,处理超时未响应的请求。
  4. 应用在session超时管理:网络应答通讯协议的请求超时处理。

痛点方案机制

  • 一种比较暴力的办法就是,使用一个后台线程,遍历所有对象,挨个检查。这种笨笨的办法简单好用,但是对象数量过多时,可能存在性能问题,检查间隔时间不好设置,间隔时间过大,影响精确度,多小则存在效率问题。

  • 而且做不到按超时的时间顺序处理。 这场景,使用DelayQueue最适合了。

DelayQueue是java.util.concurrent中提供的一个很有意思的类。很巧妙,非常棒!但是java doc和Java SE 5.0的source中都没有Sample。我最初在阅读ScheduledThreadPoolExecutor源码时,发现DelayQueue的妙用。

本文将会对DelayQueue做一个介绍,然后列举应用场景。并且提供一个Delayed接口的实现和Sample代码。

  • DelayQueue是一个BlockingQueue,其特化的参数是Delayed。

  • Delayed扩展了Comparable接口,比较的基准为延时的时间值,Delayed接口的实现类getDelay的返回值应为固定值(final)。

  • DelayQueue内部是使用PriorityQueue实现的。

  • DelayQueue = BlockingQueue + PriorityQueue + Delayed

DelayQueue的关键元素BlockingQueue、PriorityQueue、Delayed。可以这么说,DelayQueue是一个使用优先队列(PriorityQueue)实现的BlockingQueue,优先队列的比较基准值是时间。

基本定义如下

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

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

public class DelayQueue<E extends Delayed> implements BlockingQueue<E> { 
    private final PriorityQueue<E> q = new PriorityQueue<E>();
}

DelayQueue内部的实现使用了一个优先队列。当调用DelayQueue的offer方法时,把Delayed对象加入到优先队列q中。如下:

public boolean offer(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        E first = q.peek();
        q.offer(e);
        if (first == null || e.compareTo(first) < 0)
            available.signalAll();
        return true;
    } finally {
        lock.unlock();
    }
}

DelayQueue的take方法,把优先队列q的first拿出来(peek),如果没有达到延时阀值,则进行await处理。如下:

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(TimeUnit.NANOSECONDS);
                if (delay > 0) {
                    long tl = available.awaitNanos(delay);
                } else {
                    E x = q.poll();
                    assert x != null;
                    if (q.size() != 0)
                        available.signalAll(); // wake up other takers
                    return x;
                }
            }
        }
    } finally {
        lock.unlock();
    }
}

以下是Sample,是一个缓存的简单实现。共包括三个类Pair、DelayItem、Cache。如下:

public class Pair<K, V> {
    public K first;
    public V second;
    public Pair() {}
    public Pair(K first, V second) {
        this.first = first;
        this.second = second;
    }
}
import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

public class DelayItem<T> implements Delayed {

    /** Base of nanosecond timings, to avoid wrapping */
    private static final long NANO_ORIGIN = System.nanoTime();

    /**
     * Returns nanosecond time offset by origin
     */
    final static long now() {
        return System.nanoTime() - NANO_ORIGIN;
    }
    /**
     * Sequence number to break scheduling ties, and in turn to guarantee FIFO order among tied
     * entries.
     */
    private static final AtomicLong sequencer = new AtomicLong(0);

    /** Sequence number to break ties FIFO */
    private final long sequenceNumber;

    /** The time the task is enabled to execute in nanoTime units */
    private final long time;

    private final T item;

    public DelayItem(T submit, long timeout) {
        this.time = now() + timeout;
        this.item = submit;
        this.sequenceNumber = sequencer.getAndIncrement();
    }

    public T getItem() {
        return this.item;
    }

    public long getDelay(TimeUnit unit) {
        long d = unit.convert(time - now(), TimeUnit.NANOSECONDS);
        return d;
    }

    public int compareTo(Delayed other) {
        if (other == this) // compare zero ONLY if same object
            return 0;
        if (other instanceof DelayItem) {
            DelayItem x = (DelayItem) other;
            long diff = time - x.time;
            if (diff < 0)
                return -1;
            else if (diff > 0)
                return 1;
            else if (sequenceNumber < x.sequenceNumber)
                return -1;
            else
                return 1;
        }
        long d = (getDelay(TimeUnit.NANOSECONDS) - other.getDelay(TimeUnit.NANOSECONDS));
        return (d == 0) 0 : ((d < 0) -1 : 1);
    }
}

以下是Cache的实现,包括了put和get方法,还包括了可执行的main函数

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.DelayQueue;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;
public class Cache<K, V> {
 private static final Logger LOG = Logger.getLogger(Cache.class.getName());
    private ConcurrentMap<K, V> cacheObjMap = new ConcurrentHashMap<K, V>();
    private DelayQueue<DelayItem<Pair<K, V>>> q = new DelayQueue<DelayItem<Pair<K, V>>>();
    private Thread daemonThread;
    public Cache() {
        Runnable daemonTask = new Runnable() {
            public void run() {
                daemonCheck();
            }
        };
        daemonThread = new Thread(daemonTask);
        daemonThread.setDaemon(true);
        daemonThread.setName("Cache Daemon");
        daemonThread.start();
    }

    private void daemonCheck() {
        if (LOG.isLoggable(Level.INFO))
            LOG.info("cache service started.");
        for (;;) {
            try {
                DelayItem<Pair<K, V>> delayItem = q.take();
                if (delayItem != null) {
                    // 超时对象处理
                    Pair<K, V> pair = delayItem.getItem();
                    cacheObjMap.remove(pair.first, pair.second); // compare and remove
                }
            } catch (InterruptedException e) {
                if (LOG.isLoggable(Level.SEVERE))
                    LOG.log(Level.SEVERE, e.getMessage(), e);
                break;
            }
        }
        if (LOG.isLoggable(Level.INFO))
            LOG.info("cache service stopped.");
    }

    // 添加缓存对象
    public void put(K key, V value, long time, TimeUnit unit) {
        V oldValue = cacheObjMap.put(key, value);
        if (oldValue != null)
            q.remove(key);
        long nanoTime = TimeUnit.NANOSECONDS.convert(time, unit);
        q.put(new DelayItem<Pair<K, V>>(new Pair<K, V>(key, value), nanoTime));
    }

    public V get(K key) {
        return cacheObjMap.get(key);
    }

    // 测试入口函数
    public static void main(String[] args) throws Exception {
        Cache<Integer, String> cache = new Cache<Integer, String>();
        cache.put(1, "aaaa", 3, TimeUnit.SECONDS);
        Thread.sleep(1000 * 2);
        {
            String str = cache.get(1);
            System.out.println(str);
        }
        Thread.sleep(1000 * 2);
        {
            String str = cache.get(1);
            System.out.println(str);
        }
    }
}

运行Sample,main函数执行的结果是输出两行,第一行为aaa,第二行为null。

延时队列参数配置热刷新

配置中心勿喷,场景不一样

  • 缓存延时队列的信息都存在配置文件中,比如缓存数量配置、延时超时时间,事件的超时时间等等。当需要该这些配置的值时都需要重新启动进程,改动的配置才会生效,有时候线上的应用不能容忍这种停服。

  • Apache Common Configuration给我们提供了可以检测文件修改后配置可短时间生效的功能。具体用法如下:

import org.apache.commons.configuration.ConfigurationException;
import org.apache.commons.configuration.PropertiesConfiguration;
import org.apache.commons.configuration.reloading.FileChangedReloadingStrategy;
import org.apache.log4j.Logger;

public class SystemConfig {
    private static Logger logger = Logger.getLogger(SystemConfig.class);
    private static  PropertiesConfiguration config;
    static {
        try {
            //实例化一个PropertiesConfiguration
            config = new PropertiesConfiguration("/Users/hzwangxx/
                                                 IdeaProjects/app-test/src/main/resources/conf.properties");
            //设置reload策略,这里用当文件被修改之后reload(默认5s中检测一次)
            config.setReloadingStrategy(new FileChangedReloadingStrategy());
        } catch (ConfigurationException e) {
            logger.error("init static block error. ", e);
        }
    }

    public static synchronized String getProperty(String key) {
        return (String) config.getProperty(key);
    }

    public static void main(String[] args) throws InterruptedException {
        for (;;) {
            System.out.println(SystemConfig.getProperty("key"));
            Thread.sleep(2000);
        }
    }
}

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