Java8 thenCompose 和 thenComposeAsync 之间的区别

Question

给定这段代码：

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
                )))

能否改写为：

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

这段代码是我从书上的例子里拿来的，说这两种解法不一样，但我不明白为什么。

Answer 1

让我们考虑一个如下所示的函数：

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()。

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，则为调用线程）。

thenComposeAsync 将在提供的执行程序上调用 generateRequest()，否则在默认的 ForkJoinPool 上调用。

Java8 thenCompose 和 thenComposeAsync 之间的区别

Difference between Java8 thenCompose and thenComposeAsync

java

java-8

completable-future