猫头鹰的深夜翻译：使用JAVA CompletableFuture的20例子

和前一个例子类似，将Function替换为Consumer

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static void acceptEitherExample() {
    String original = "Message";
    StringBuilder result = new StringBuilder();
    CompletableFuture cf = CompletableFuture.completedFuture(original)
            .thenApplyAsync(s -> delayedUpperCase(s))
            .acceptEither(CompletableFuture.completedFuture(original).thenApplyAsync(s -> delayedLowerCase(s)),
                    s -> result.append(s).append("acceptEither"));
    cf.join();
    assertTrue("Result was empty", result.toString().endsWith("acceptEither"));
}

注意这里的两个Stage都是同步运行的，第一个stage将字符串转化为大写之后，第二个stage将其转化为小写。

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static void runAfterBothExample() {
    String original = "Message";
    StringBuilder result = new StringBuilder();
    CompletableFuture.completedFuture(original).thenApply(String::toUpperCase).runAfterBoth(
            CompletableFuture.completedFuture(original).thenApply(String::toLowerCase),
            () -> result.append("done"));
    assertTrue("Result was empty", result.length() > 0);
}

Biconsumer支持同时对两个Stage的结果进行操作。

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static void thenAcceptBothExample() {
    String original = "Message";
    StringBuilder result = new StringBuilder();
    CompletableFuture.completedFuture(original).thenApply(String::toUpperCase).thenAcceptBoth(
            CompletableFuture.completedFuture(original).thenApply(String::toLowerCase),
            (s1, s2) -> result.append(s1 + s2));
    assertEquals("MESSAGEmessage", result.toString());
}

如果CompletableFuture想要合并两个阶段的结果并且返回值，我们可以使用方法thenCombine。这里的计算流都是同步的，所以最后的getNow()方法会获得最终结果，即大写操作和小写操作的结果的拼接。

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static void thenCombineExample() {
    String original = "Message";
    CompletableFuture cf = CompletableFuture.completedFuture(original).thenApply(s -> delayedUpperCase(s))
            .thenCombine(CompletableFuture.completedFuture(original).thenApply(s -> delayedLowerCase(s)),
                    (s1, s2) -> s1 + s2);
    assertEquals("MESSAGEmessage", cf.getNow(null));
}

和之前的例子类似，只是这里用了不同的方法：即两个阶段的操作都是异步的。那么thenCombine也会异步执行，及时它没有Async后缀。

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static void thenCombineAsyncExample() {
    String original = "Message";
    CompletableFuture cf = CompletableFuture.completedFuture(original)
            .thenApplyAsync(s -> delayedUpperCase(s))
            .thenCombine(CompletableFuture.completedFuture(original).thenApplyAsync(s -> delayedLowerCase(s)),
                    (s1, s2) -> s1 + s2);
    assertEquals("MESSAGEmessage", cf.join());
}

我们可以使用thenCompose来完成前两个例子中的操作。

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static void thenComposeExample() {
    String original = "Message";
    CompletableFuture cf = CompletableFuture.completedFuture(original).thenApply(s -> delayedUpperCase(s))
            .thenCompose(upper -> CompletableFuture.completedFuture(original).thenApply(s -> delayedLowerCase(s))
                    .thenApply(s -> upper + s));
    assertEquals("MESSAGEmessage", cf.join());
}

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static void anyOfExample() {
    StringBuilder result = new StringBuilder();
    List messages = Arrays.asList("a", "b", "c");
    List futures = messages.stream()
            .map(msg -> CompletableFuture.completedFuture(msg).thenApply(s -> delayedUpperCase(s)))
            .collect(Collectors.toList());
    CompletableFuture.anyOf(futures.toArray(new CompletableFuture[futures.size()])).whenComplete((res, th) -> {
        if(th == null) {
            assertTrue(isUpperCase((String) res));
            result.append(res);
        }
    });
    assertTrue("Result was empty", result.length() > 0);
}

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static void allOfExample() {
    StringBuilder result = new StringBuilder();
    List messages = Arrays.asList("a", "b", "c");
    List futures = messages.stream()
            .map(msg -> CompletableFuture.completedFuture(msg).thenApply(s -> delayedUpperCase(s)))
            .collect(Collectors.toList());
    CompletableFuture.allOf(futures.toArray(new CompletableFuture[futures.size()]))
        .whenComplete((v, th) -> {
            futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null))));
            result.append("done");
        });
    assertTrue("Result was empty", result.length() > 0);
}

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static void allOfAsyncExample() {
    StringBuilder result = new StringBuilder();
    List messages = Arrays.asList("a", "b", "c");
    List futures = messages.stream()
            .map(msg -> CompletableFuture.completedFuture(msg).thenApplyAsync(s -> delayedUpperCase(s)))
            .collect(Collectors.toList());
    CompletableFuture allOf = CompletableFuture.allOf(futures.toArray(new CompletableFuture[futures.size()]))
            .whenComplete((v, th) -> {
                futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null))));
                result.append("done");
            });
    allOf.join();
    assertTrue("Result was empty", result.length() > 0);
}

下面展示了一个实践CompletableFuture的场景：

先通过调用cars()方法异步获得Car列表。它将会返回一个CompletionStage>。cars()方法应当使用一个远程的REST端点来实现。

我们将该Stage和另一个Stage组合，另一个Stage会通过调用rating(manufactureId)来异步获取每辆车的评分。

当所有的Car对象都填入评分后，我们调用allOf()来进入最终Stage，它将在这两个阶段完成后执行

在最终Stage上使用whenComplete()，打印出车辆的评分。

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cars().thenCompose(cars -> {
    List updatedCars = cars.stream()
            .map(car -> rating(car.manufacturerId).thenApply(r -> {
                car.setRating(r);
                return car;
            })).collect(Collectors.toList());
    CompletableFuture done = CompletableFuture
            .allOf(updatedCars.toArray(new CompletableFuture[updatedCars.size()]));
    return done.thenApply(v -> updatedCars.stream().map(CompletionStage::toCompletableFuture)
            .map(CompletableFuture::join).collect(Collectors.toList()));
}).whenComplete((cars, th) -> {
    if (th == null) {
        cars.forEach(System.out::println);
    } else {
        throw new RuntimeException(th);
    }
}).toCompletableFuture().join();

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Java CompletableFuture 详解
Guide To CompletableFuture

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