Java8 thenCompose和thenComposeAsync之间的区别

问题描述：

给出这段代码:

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());
}

这部分:

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

可以改写为:

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

我从一本书的示例中获取了这段代码，并说这两种解决方案是不同的，但是我不明白为什么.

I took this code from an example of a book and says the two solutions are different, but I do not understand why.

答

让我们考虑一个看起来像这样的函数:

Let's consider a function that looks like this:

public CompletableFuture<String> requestData(String parameters) {
    Request request = generateRequest(parameters);
    return CompletableFuture.supplyAsync(() -> sendRequest(request));
}

区别在于调用哪个线程generateRequest().

The difference will be with respect to which thread generateRequest() gets called on.

thenCompose将在与上游任务相同的线程上调用generateRequest()(如果上游任务已经完成，则调用该线程).

thenCompose will call generateRequest() on the same thread as the upstream task (or the calling thread if the upstream task has already completed).

thenComposeAsync将在提供的执行程序(如果提供)上调用generateRequest()，否则将在默认的ForkJoinPool上调用.

thenComposeAsync will call generateRequest() on the provided executor if provided, or on the default ForkJoinPool otherwise.

Java8 thenCompose和thenComposeAsync之间的区别

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