使用DelayQueue、ConcurrentHashMap、FutureTask實(shí)現(xiàn)的緩存工具類(lèi)。

DelayQueue 簡(jiǎn)介

DelayQueue是一個(gè)支持延時(shí)獲取元素的無(wú)界阻塞隊(duì)列。DelayQueue內(nèi)部隊(duì)列使用PriorityQueue來(lái)實(shí)現(xiàn)。隊(duì)列中的元素必須實(shí)現(xiàn)Delayed接口，在創(chuàng)建元素時(shí)可以指定多久才能從隊(duì)列中獲取當(dāng)前元素。只有在延遲期滿(mǎn)時(shí)才能從隊(duì)列中提取元素。

DelayQueue非常有用，可以將DelayQueue運(yùn)用在以下應(yīng)用場(chǎng)景。

緩存系統(tǒng)的設(shè)計(jì)：可以用DelayQueue保存緩存元素的有效期，使用一個(gè)線(xiàn)程循環(huán)查詢(xún)
DelayQueue，一旦能從DelayQueue中獲取元素時(shí)，表示緩存有效期到了。

定時(shí)任務(wù)調(diào)度：使用DelayQueue保存當(dāng)天將會(huì)執(zhí)行的任務(wù)和執(zhí)行時(shí)間，一旦從
DelayQueue中獲取到任務(wù)就開(kāi)始執(zhí)行，比如TimerQueue就是使用DelayQueue實(shí)現(xiàn)的。

ConcurrentHashMap和FutureTask，詳見(jiàn)以下：

http://www.jianshu.com/p/d10256f0ebea

FutureTask 源碼分析

緩存工具類(lèi)實(shí)現(xiàn)

支持緩存多長(zhǎng)時(shí)間，單位毫秒。

支持多線(xiàn)程并發(fā)。
比如：有一個(gè)比較耗時(shí)的操作，此時(shí)緩沖中沒(méi)有此緩存值，一個(gè)線(xiàn)程開(kāi)始計(jì)算這個(gè)耗時(shí)操作，而再次進(jìn)來(lái)線(xiàn)程就不需要再次進(jìn)行計(jì)算，只需要等上一個(gè)線(xiàn)程計(jì)算完成后（使用FutureTask）返回該值即可。

import java.util.concurrent.Callable; import java.util.concurrent.CancellationException; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import java.util.concurrent.DelayQueue; import java.util.concurrent.Delayed; import java.util.concurrent.ExecutionException; import java.util.concurrent.Future; import java.util.concurrent.FutureTask; import java.util.concurrent.TimeUnit;public class CacheBean<V> {// 緩存計(jì)算的結(jié)果private final static ConcurrentMap<String, Future<Object>> cache = new ConcurrentHashMap<>();// 延遲隊(duì)列來(lái)判斷那些緩存過(guò)期private final static DelayQueue<DelayedItem<String>> delayQueue = new DelayQueue<>();// 緩存時(shí)間private final int ms;static {// 定時(shí)清理過(guò)期緩存Thread t = new Thread() {@Overridepublic void run() {dameonCheckOverdueKey();}};t.setDaemon(true);t.start();}private final Computable<V> c;/*** @param c Computable*/public CacheBean(Computable<V> c) {this(c, 60 * 1000);}/*** @param c Computable* @param ms 緩存多少毫秒*/public CacheBean(Computable<V> c, int ms) {this.c = c;this.ms = ms;}public V compute(final String key) throws InterruptedException {while (true) {//根據(jù)key從緩存中獲取值Future<V> f = (Future<V>) cache.get(key);if (f == null) {Callable<V> eval = new Callable<V>() {public V call() {return (V) c.compute(key);}};FutureTask<V> ft = new FutureTask<>(eval);//如果緩存中存在此可以，則返回已存在的valuef = (Future<V>) cache.putIfAbsent(key, (Future<Object>) ft);if (f == null) {//向delayQueue中添加key，并設(shè)置該key的存活時(shí)間delayQueue.put(new DelayedItem<>(key, ms));f = ft;ft.run();}}try {return f.get();} catch (CancellationException e) {cache.remove(key, f);} catch (ExecutionException e) {e.printStackTrace();}}}/*** 檢查過(guò)期的key，從cache中刪除*/private static void dameonCheckOverdueKey() {DelayedItem<String> delayedItem;while (true) {try {delayedItem = delayQueue.take();if (delayedItem != null) {cache.remove(delayedItem.getT());System.out.println(System.nanoTime() + " remove " + delayedItem.getT() + " from cache");}} catch (InterruptedException e) {e.printStackTrace();}}}}class DelayedItem<T> implements Delayed {private T t;private long liveTime;private long removeTime;public DelayedItem(T t, long liveTime) {this.setT(t);this.liveTime = liveTime;this.removeTime = TimeUnit.MILLISECONDS.convert(liveTime, TimeUnit.MILLISECONDS) + System.currentTimeMillis();}@Overridepublic int compareTo(Delayed o) {if (o == null)return 1;if (o == this)return 0;if (o instanceof DelayedItem) {DelayedItem<T> tmpDelayedItem = (DelayedItem<T>) o;if (liveTime > tmpDelayedItem.liveTime) {return 1;} else if (liveTime == tmpDelayedItem.liveTime) {return 0;} else {return -1;}}long diff = getDelay(TimeUnit.MILLISECONDS) - o.getDelay(TimeUnit.MILLISECONDS);return diff > 0 ? 1 : diff == 0 ? 0 : -1;}@Overridepublic long getDelay(TimeUnit unit) {return unit.convert(removeTime - System.currentTimeMillis(), unit);}public T getT() {return t;}public void setT(T t) {this.t = t;}@Overridepublic int hashCode() {return t.hashCode();}@Overridepublic boolean equals(Object object) {if (object instanceof DelayedItem) {return object.hashCode() == hashCode() ? true : false;}return false;}}

Computable 接口

public interface Computable<V> {V compute(String k);}

測(cè)試類(lèi)

public class FutureTaskDemo {public static void main(String[] args) throws InterruptedException {// 子線(xiàn)程Thread t = new Thread(() -> {CacheBean<String> cb = new CacheBean<>(k -> {try {System.out.println("模擬計(jì)算數(shù)據(jù)，計(jì)算時(shí)長(zhǎng)2秒。key=" + k);TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {e.printStackTrace();}return "你好：" + k;}, 5000);try {while (true) {System.out.println("thead2:" + cb.compute("b"));TimeUnit.SECONDS.sleep(1);}} catch (InterruptedException e) {e.printStackTrace();}});t.start();// 主線(xiàn)程while (true) {CacheBean<String> cb = new CacheBean<>(k -> {try {System.out.println("模擬計(jì)算數(shù)據(jù)，計(jì)算時(shí)長(zhǎng)2秒。key=" + k);TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {e.printStackTrace();}return "你好：" + k;}, 5000);System.out.println("thead1:" + cb.compute("b"));TimeUnit.SECONDS.sleep(1);}} }

執(zhí)行結(jié)果：

兩個(gè)線(xiàn)程同時(shí)訪(fǎng)問(wèn)同一個(gè)key的緩存。從執(zhí)行結(jié)果發(fā)現(xiàn)，每次緩存失效后，同一個(gè)key只執(zhí)行一次計(jì)算，而不是多個(gè)線(xiàn)程并發(fā)執(zhí)行同一個(gè)計(jì)算然后緩存。

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