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; } @Override public 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 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。
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]], ...