我们谈一下实际的场景吧。我们在开发中,有如下场景
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
public
interface
Comparable
<
T
>
{
public int compareTo(T o);
}
public int compareTo(T o);
}
public
interface
Delayed
extends
Comparable
<
Delayed
>
{
long getDelay(TimeUnit unit);
}
long getDelay(TimeUnit unit);
}
public
class
DelayQueue
<
E
extends
Delayed
>
implements
BlockingQueue
<
E
>
{
private final PriorityQueue < E > q = new PriorityQueue < 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();
}
}
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();
}
}
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;
}
}
public K first;
public V second;
public Pair() {}
public Pair(K first, V second) {
this .first = first;
this .second = second;
}
}
--------------
以下是Delayed的实现
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 );
}
}
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);
}
}
}
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。