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
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 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);
}
}
}