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有一家商店提供的价格是以欧元#xff08;EUR#xff09;计价的#xff0c;但是你希望以美元的方式提供给你的客户。你可以用异步的方式向商店查询指定商品的价格#xff0c;同时从远程的汇率服务那里查到欧元和美元之间的汇率。当二者都结束时#xff0c;再将这两个…需求
有一家商店提供的价格是以欧元EUR计价的但是你希望以美元的方式提供给你的客户。你可以用异步的方式向商店查询指定商品的价格同时从远程的汇率服务那里查到欧元和美元之间的汇率。当二者都结束时再将这两个结果结合起来用返回的商品价格乘以当时的汇率得到以美元计价的商品价格。
实现源码
public class ExchangeService {public enum Money {USD(1.0), EUR(1.35387), GBP(1.69715), CAD(.92106), MXN(.07683);private final double rate;Money(double rate) {this.rate rate;}}public static double getRate(Money source, Money destination) {return getRateWithDelay(source, destination);}private static double getRateWithDelay(Money source, Money destination) {Util.delay();return destination.rate / source.rate;}
}public class BestPriceFinder {private final ListShop shops Arrays.asList(new Shop(BestPrice),new Shop(LetsSaveBig),new Shop(MyFavoriteShop),new Shop(BuyItAll) new Shop(ShopEasy));private final Executor executor Executors.newFixedThreadPool(shops.size(), ExecuterThreadFactoryBuilder.build());
}实现方案
你对一个CompletableFuture对象调用了thenCompose方法并向其传递了第二个CompletableFuture而第二个CompletableFuture又需要使用第一个CompletableFuture的执行结果作为输入。但是另一种比较常见的情况是你需要将两个完全不相干的CompletableFuture对象的结果整合起来而且你也不希望等到第一个任务完全结束才开始第二项任务。你应该使用thenCombine方法。thenCombine方法也提供有一个Async的版本。这里如果使用thenCombineAsync会导致BiFunction中定义的合并操作被提交到线程池中由另一个任务以异步的方式执行。
方案1
public ListString findPricesInUSD(String product) {ListCompletableFutureDouble priceFutures new ArrayList();for (Shop shop : shops) {CompletableFutureDouble futurePriceInUSD CompletableFuture.supplyAsync(() - shop.getPrice(product), executor).thenCombine(CompletableFuture.supplyAsync(() - ExchangeService.getRate(Money.EUR, Money.USD), executor), (price, rate) - price * rate);priceFutures.add(futurePriceInUSD);}ListString prices priceFutures.stream().map(CompletableFuture::join).map(price - price is price).collect(Collectors.toList());return prices;
}方案2
public ListString findPricesInUSD2(String product) {ListCompletableFutureString priceFutures new ArrayList();for (Shop shop : shops) {CompletableFutureString futurePriceInUSD CompletableFuture.supplyAsync(() - shop.getPrice(product), executor).thenCombine(CompletableFuture.supplyAsync(() - ExchangeService.getRate(Money.EUR, Money.USD), executor), (price, rate) - price * rate).thenApply(price - shop.getName() price is price);priceFutures.add(futurePriceInUSD);}ListString prices priceFutures.stream().map(CompletableFuture::join).collect(Collectors.toList());return prices;
}方案3
public ListString findPricesInUSD3(String product) {ListCompletableFutureString priceFutures shops.stream().map(shop - CompletableFuture.supplyAsync(() - shop.getPrice(product)).thenCombine(CompletableFuture.supplyAsync(() - ExchangeService.getRate(Money.EUR, Money.USD)), (price, rate) - price * rate).thenApply(price - shop.getName() price is price)).collect(Collectors.toList());ListString prices priceFutures.stream().map(CompletableFuture::join).collect(Collectors.toList());return prices;
}方案4使用Java 7中提供的特性完成上述功能
// 为了更直观地感受一下使用CompletableFuture在代码可读性上带来的巨大提升尝试仅使用Java 7中提供的特性实现以下如上实现
public ListString findPricesInUSDJava7(String product) {ExecutorService executor Executors.newCachedThreadPool();ListFutureDouble priceFutures new ArrayList();for (Shop shop : shops) {final FutureDouble futureRate executor.submit(new CallableDouble() {public Double call() {return ExchangeService.getRate(Money.EUR, Money.USD);}});FutureDouble futurePriceInUSD executor.submit(new CallableDouble() {public Double call() {try {double priceInEUR shop.getPrice(product);return priceInEUR * futureRate.get();} catch (InterruptedException | ExecutionException e) {throw new RuntimeException(e.getMessage(), e);}}});priceFutures.add(futurePriceInUSD);}ListString prices new ArrayList();for (FutureDouble priceFuture : priceFutures) {try {prices.add( price is priceFuture.get());}catch (ExecutionException | InterruptedException e) {e.printStackTrace();}}return prices;
}我们能看到创建流水线对同步和异步操作进行混合操作有多么简单随着处理任务和需要合并结果数目的增加这种声明式程序设计的优势也愈发明显。
测试
public static void main(String[] args) {StopWatch stopWatch new StopWatch(性能比较);execute(combined USD findPricesInUSDJava7, () - bestPriceFinder.findPricesInUSDJava7(myPhone27S), stopWatch);execute(combined USD CompletableFuture, () - bestPriceFinder.findPricesInUSD(myPhone27S), stopWatch);execute(combined USD CompletableFuture v2, () - bestPriceFinder.findPricesInUSD2(myPhone27S), stopWatch);execute(combined USD CompletableFuture v3, () - bestPriceFinder.findPricesInUSD3(myPhone27S), stopWatch);StopWatchUtils.logStopWatch(stopWatch);
}
private static void execute(String msg, SupplierListString s, StopWatch stopWatch) {stopWatch.start(msg);System.out.println(s.get());stopWatch.stop();System.out.println();
}[ price is 91.040141702717, price is 125.1710573102377, price is 158.16078708139523, price is 136.45612204497712, price is 130.0591978746988][ price is 145.62246618545896, price is 123.78887748261509, price is 142.17561724598045, price is 147.48700495707948, price is 148.2901027867818][BestPrice price is 126.3823279607243, LetsSaveBig price is 124.52723804111048, MyFavoriteShop price is 129.1051274535831, BuyItAll price is 114.36072566615553, ShopEasy price is 121.11783256695436][BestPrice price is 168.06251816668828, LetsSaveBig price is 148.38498827435606, MyFavoriteShop price is 119.0272869408407, BuyItAll price is 115.15446874021768, ShopEasy price is 153.35355439427738]性能比较 total cost time 6045 ms
combined USD findPricesInUSDJava7 : 1008 ms, 16.67%
combined USD CompletableFuture : 2009 ms, 33.23%
combined USD CompletableFuture v2 : 2015 ms, 33.33%
combined USD CompletableFuture v3 : 1012 ms, 16.74%
