阻塞队列之七：DelayQueue延时队列

一、DelayQueue简介

　　是一个无界的BlockingQueue，用于放置实现了Delayed接口的对象，其中的对象只能在其到期时才能从队列中取走。这种队列是有序的（PriorityQueue实际存放Delayed接口对象），即队头对象的延迟到期时间最短（队列顶端总是最小的元素）。注意：不能将null元素放置到这种队列中。

　　DelayQueue在poll/take的时候，队列中元素会判定这个elment有没有达到超时时间，如果没有达到，poll返回null，而take进入等待状态。但是，除了这两个方法，队列中的元素会被当做正常的元素来对待。例如，size方法返回所有元素的数量，而不管它们有没有达到超时时间。而协调的Condition available只对take和poll是有意义的。

二、DelayQueue源码分析

2.1、DelayQueue的lock

DelayQueue使用一个可重入锁和这个锁生成的一个条件对象进行并发控制。

    private final transient ReentrantLock lock = new ReentrantLock();    //内部用于存储对象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 = null;/*** 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();

2.2、成员变量

要先了解下DelayQueue中用到的几个关键对象：

2.2.1、Delayed，一种混合风格的接口，用来标记那些应该在给定延迟时间之后执行的对象。

此接口的实现必须定义一个 compareTo()方法，该方法提供与此接口的 getDelay()方法一致的排序。

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

DelayQueue是一个BlockingQueue，其泛型类的参数是Delayed接口对象。

Delayed接口：

public interface Delayed extends Comparable<Delayed> {long getDelay(TimeUnit unit);   //返回与此对象相关的剩余延迟时间，以给定的时间单位表示。

Comparable接口：

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

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）。

2.2.2、PriorityQueue，优先级队列存放有序对象

优先队列的比较基准值是时间。详解见《阻塞队列之八：PriorityBlockingQueue优先队列》

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。

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

总结：DelayQueue内部是使用PriorityQueue实现的，DelayQueue = BlockingQueue + PriorityQueue + Delayed。

2.3、构造函数

    public DelayQueue() {}    public DelayQueue(Collection<? extends E> c) {this.addAll(c);}

    public boolean offer(E e, long timeout, TimeUnit unit) {return offer(e);}

超时的参数被忽略，因为是无界的。不会阻塞或超时。

2.4、入队

    public boolean add(E e) {return offer(e);}public void put(E e) {offer(e);}public boolean offer(E e) {final ReentrantLock lock = this.lock;lock.lock();try {q.offer(e);if (q.peek() == e) {//添加元素后peek还是e，重置leader，通知条件队列 leader = null;available.signal();}return true;} finally {lock.unlock();}}

2.5、出队

public E poll() {  final ReentrantLock lock = this.lock;  lock.lock();  try {  E first = q.peek();  if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0) //队列为空或者延迟时间未过期  return null;  else  return q.poll();  } finally {  lock.unlock();  }
}  /** * take元素，元素未过期需要阻塞 */
public E take() throws InterruptedException {  final ReentrantLock lock = this.lock;  lock.lockInterruptibly();  try {  for (;;) {  E first = q.peek();  if (first == null)  available.await(); //队列空，加入条件队列  else {  long delay = first.getDelay(TimeUnit.NANOSECONDS); //获取剩余延迟时间  if (delay <= 0) //小于0，那就poll元素  return q.poll();  else if (leader != null) //有延迟，检查leader，不为空说明有其他线程在等待，那就加入条件队列
                    available.await();  else {   Thread thisThread = Thread.currentThread();  leader = thisThread; //设置当前为leader等待  try {  available.awaitNanos(delay); //条件队列等待指定时间  } finally {  if (leader == thisThread) //检查是否被其他线程改变，没有就重置，再次循环  leader = null;  }  }  }  }  } finally {  if (leader == null && q.peek() != null) //leader为空并且队列不空，说明没有其他线程在等待，那就通知条件队列
            available.signal();  lock.unlock();  }
}  /** * 响应超时的poll */
public E poll(long timeout, TimeUnit unit) throws InterruptedException {  long nanos = unit.toNanos(timeout);  final ReentrantLock lock = this.lock;  lock.lockInterruptibly();  try {  for (;;) {  E first = q.peek();  if (first == null) {  if (nanos <= 0)  return null;  else  nanos = available.awaitNanos(nanos);  } else {  long delay = first.getDelay(TimeUnit.NANOSECONDS);  if (delay <= 0)  return q.poll();  if (nanos <= 0)  return null;  if (nanos < delay || leader != null)  nanos = available.awaitNanos(nanos);  else {  Thread thisThread = Thread.currentThread();  leader = thisThread;  try {  long timeLeft = available.awaitNanos(delay);  nanos -= delay - timeLeft;  } finally {  if (leader == thisThread)  leader = null;  }  }  }  }  } finally {  if (leader == null && q.peek() != null)  available.signal();  lock.unlock();  }
}  /** * 获取queue[0]，peek是不移除的 */
public E peek() {  final ReentrantLock lock = this.lock;  lock.lock();  try {  return q.peek();  } finally {  lock.unlock();  }
}

三、JDK或开源框架中使用

ScheduledThreadPoolExecutor中使用了DelayedWorkQueue。

应用场景

下面的应用场景是来源于网上，虽然借用DelayedQueue可以快速找到要“失效”的对象，但DelayedQueue内部的PriorityQueue的（插入、删除时的排序）也耗费资源。

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。
d)session超时管理，网络应答通讯协议的请求超时处理。

四、示例

1、缓存示例

当向缓存中添加key-value对时，如果这个key在缓存中存在并且还没有过期，需要用这个key对应的新过期时间
为了能够让DelayQueue将其已保存的key删除，需要重写实现Delayed接口添加到DelayQueue的DelayedItem的hashCode函数和equals函数
当缓存关闭，监控程序也应关闭，因而监控线程应当用守护线程

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：

package com.dxz.concurrent.delayqueue;public class Pair<K, V> {public K key;public V value;public Pair() {}public Pair(K first, V second) {this.key = first;this.value = second;}@Overridepublic String toString() {return "Pair [key=" + key + ", value=" + value + "]";}}

以下是Delayed的实现

package com.dxz.concurrent.delayqueue;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 objectreturn 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;elsereturn 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。

package com.dxz.concurrent.delayqueue;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.key, pair.value); // 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) {boolean result = q.remove(new DelayItem<Pair<K, V>>(new Pair<K, V>(key, oldValue), 0L));System.out.println("remove:="+result);}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);}public DelayQueue<DelayItem<Pair<K, V>>> getQ() {return q;}public void setQ(DelayQueue<DelayItem<Pair<K, V>>> q) {this.q = q;}// 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", 60, TimeUnit.SECONDS);cache.put(1, "aaaa", 10, TimeUnit.SECONDS);//cache.put(1, "ccc", 60, TimeUnit.SECONDS);cache.put(2, "bbbb", 30, TimeUnit.SECONDS);cache.put(3, "cccc", 66, TimeUnit.SECONDS);cache.put(4, "dddd", 54, TimeUnit.SECONDS);cache.put(5, "eeee", 35, TimeUnit.SECONDS);cache.put(6, "ffff", 38, TimeUnit.SECONDS);cache.put(1, "aaaa", 70, TimeUnit.SECONDS);for(;;) {Thread.sleep(1000 * 2);{for(Object obj : cache.getQ().toArray()) {System.out.print(((DelayItem)obj).toString());System.out.println(",");}System.out.println();}}}
}

结果片段1：（重复key的Delayed对象将从DelayedQueue中移除）

remove:=true
remove:=true
七月 04, 2017 11:28:36 上午 com.dxz.concurrent.delayqueue.Cache daemonCheck
信息: cache service started.
DelayItem [sequenceNumber=3, time=30000790187, item=Pair [key=2, value=bbbb]],
DelayItem [sequenceNumber=6, time=35000842411, item=Pair [key=5, value=eeee]],
DelayItem [sequenceNumber=7, time=38000847189, item=Pair [key=6, value=ffff]],
DelayItem [sequenceNumber=5, time=54000835925, item=Pair [key=4, value=dddd]],
DelayItem [sequenceNumber=4, time=66000803499, item=Pair [key=3, value=cccc]],
DelayItem [sequenceNumber=9, time=70000900437, item=Pair [key=1, value=aaaa]],

结果片段2：(队头对象将最先过时，可以被take()出来，这段代码在daemonCheck()方法中，即对超时对象的处理，如这里是清理session集合对象)

...
DelayItem [sequenceNumber=3, time=30000665600, item=Pair [key=2, value=bbbb]],
DelayItem [sequenceNumber=6, time=35000689152, item=Pair [key=5, value=eeee]],
DelayItem [sequenceNumber=7, time=38000694272, item=Pair [key=6, value=ffff]],
DelayItem [sequenceNumber=5, time=54000685398, item=Pair [key=4, value=dddd]],
DelayItem [sequenceNumber=4, time=66000679595, item=Pair [key=3, value=cccc]],
DelayItem [sequenceNumber=9, time=70000728406, item=Pair [key=1, value=aaaa]],DelayItem [sequenceNumber=6, time=35000689152, item=Pair [key=5, value=eeee]],
DelayItem [sequenceNumber=5, time=54000685398, item=Pair [key=4, value=dddd]],
DelayItem [sequenceNumber=7, time=38000694272, item=Pair [key=6, value=ffff]],
DelayItem [sequenceNumber=9, time=70000728406, item=Pair [key=1, value=aaaa]],
DelayItem [sequenceNumber=4, time=66000679595, item=Pair [key=3, value=cccc]],
...