相关疑难解决方法(0)

我有一个关于CompletableFuture方法的问题:

public <U> CompletableFuture<U> thenApply(Function<? super T, ? extends U> fn)

事情是JavaDoc说的就是这样:

返回一个新的CompletionStage,当该阶段正常完成时,将使用此阶段的结果作为所提供函数的参数执行.有关特殊完成的规则​​,请参阅CompletionStage文档.

线程怎么样？这将在哪个线程中执行？如果未来由线程池完成怎么办？

Given this piece of code:

public List<String> findPrices(String product){
    List<CompletableFuture<String>> priceFutures =
    shops.stream()
         .map(shop -> CompletableFuture.supplyAsync(
                () -> shop.getPrice(product), executor))
         .map(future -> future.thenApply(Quote::parse))
         .map(future -> future.thenCompose(quote ->
                CompletableFuture.supplyAsync(
                        () -> Discount.applyDiscount(quote), executor
                )))
         .collect(toList());

    return priceFutures.stream()
                       .map(CompletableFuture::join)
                       .collect(toList());
}

This part of it:

.map(future -> future.thenCompose(quote ->
                CompletableFuture.supplyAsync(
                        () -> Discount.applyDiscount(quote), executor
                )))

Could it be rewrite as:

.map(future -> 
    future.thenComposeAsync(quote -> Discount.applyDiscount(quote), executor))

I took this code from an example of a book and says the two solutions are …

此代码引自Java 8 in Action,也出现在本书11.4.3中.

public Stream<CompletableFuture<String>> findPricesStream(String product) {
    return shops.stream()
            .map(shop -> CompletableFuture.supplyAsync(() -> shop.getPrice(product), executor))
            .map(future -> future.thenApply(Quote::parse))
            .map(future -> future.thenCompose(quote -> CompletableFuture.supplyAsync(() -> Discount.applyDiscount(quote), executor)));
}

沿着代码,编写器附上如下图,表示applyDiscount()同一个线程中的工作getPrice(),我强烈怀疑:这里有两个不同的Async后缀,这意味着第二个调用应该在另一个线程中.

我使用以下代码在本地测试它:

private static void testBasic() {
    out.println("*****************************************");
    out.println("********** TESTING thenCompose **********");
    CompletableFuture[] futures = IntStream.rangeClosed(0, LEN).boxed()
            .map(i -> CompletableFuture.supplyAsync(() -> runStage1(i), EXECUTOR_SERVICE))
            .map(future -> future.thenCompose(i -> CompletableFuture.supplyAsync(() -> runStage2(i), EXECUTOR_SERVICE)))
            .toArray(size -> new CompletableFuture[size]);
    CompletableFuture.allOf(futures).join();
}

输出进一步证明了我的想法,这是正确的吗？

***************************************** …

我正在学习 CompletableFuture API，有一个例子：

CompletableFuture.completedFuture(url)
                 .thenComposeAsync(this::readPage, executor)
                 .thenApply(this::getImageURLs)
                 .thenApply(this::saveFoundImages)
                 .....

我有一个问题：如果我将thenComposeAsync(...)方法作为第一个调用，链中的其他方法会在executor我通过参数传递的方法中执行，还是应该使用async调用其他方法以在特定执行程序中异步执行？