Java-技术专区-精巧好用的DelayQueue

a) 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。
b) 缓存。缓存中的对象，超过了空闲时间，需要从缓存中移出。
c) 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。
　　一种笨笨的办法就是，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中都没有提供Sample。我最初在阅读ScheduledThreadPoolExecutor源码时，发现DelayQueue的妙用。随后在实际工作中，应用在session超时管理，网络应答通讯协议的请求超时处理。
　　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。（不了解BlockingQueue的同学，先去了解BlockingQueue再看本文）
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。
 
他们的基本定义如下
 
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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：
 
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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：
 
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<br>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：
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public class Pair<K, V> {<br>
    public K first;
 
    public V second;
 
    public Pair() {}
 
    public Pair(K first, V second) {
        this.first = first;
        this.second = second;
    }
}
--------------
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import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
 
public class DelayItem<T> implements Delayed {<br>
    /** 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。
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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> {
    <br>　　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));
    }
<br>
    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。
posted @   洛神灬殇  阅读(253)  评论(0)  编辑  收藏  举报
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