如何组合3个或更多完成阶段？ [英] How to combine 3 or more CompletionStages?

问题描述

如果有2个完成阶段，我可以与 thenCombine 方法结合使用：

If have 2 CompletionStages I can combine them with thenCombine method:

CompletionStage<A> aCompletionStage = getA();
CompletionStage<B> bCompletionStage = getB();
CompletionStage<Combined> combinedCompletionStage =
    aCompletionStage.thenCombine(bCompletionStage, (aData, bData) -> combine(aData, bData));

如果我有3个或更多完成阶段，我可以创建一个 thenCombine 方法，但我必须使用临时对象来传递结果。例如，这里有一个解决方案使用对和 Triple 从 org.apache.commons .lang3.tuple 包：

If I have 3 or more CompletionStages I can make a chain of thenCombine methods, but I have to use temporary objects to pass results. For example, here is a solution using Pair and Triple from the org.apache.commons.lang3.tuple package:

CompletionStage<A> aCompletionStage = getA();
CompletionStage<B> bCompletionStage = getB();
CompletionStage<C> cCompletionStage = getC();
CompletionStage<D> dCompletionStage = getD();

CompletionStage<Combined> combinedDataCompletionStage =
        aCompletionStage.thenCombine(bCompletionStage, (Pair::of))
                .thenCombine(cCompletionStage, (ab, c) ->
                        Triple.of(ab.getLeft(), ab.getRight(), c))
                .thenCombine(dCompletionStage, (abc, d) ->
                        combine(abc.getLeft(), abc.getMiddle(), abc.getRight(), d));

有更好的方法来组合多个完成阶段的结果吗？

Is there a better way to combine results from multiple CompletionStages?

使用 CompletableFuture 的方法来组合多个阶段， 。如果您的 CompletionStage 不是 CompletableFuture ，您仍然可以使用 .toCompletableFuture ）：

The only way to combine multiple stages that scales well with a growing number of stages, is to use CompletableFuture. If your CompletionStages aren’t CompletableFutures you may still convert them using .toCompletableFuture():

CompletableFuture<A> aCompletionStage = getA().toCompletableFuture();
CompletableFuture<B> bCompletionStage = getB().toCompletableFuture();
CompletableFuture<C> cCompletionStage = getC().toCompletableFuture();
CompletableFuture<D> dCompletionStage = getD().toCompletableFuture();

CompletionStage<Combined> combinedDataCompletionStage = CompletableFuture.allOf(
    aCompletionStage, bCompletionStage, cCompletionStage, dCompletionStage)
    .thenApply(ignoredVoid -> combine(
        aCompletionStage.join(), bCompletionStage.join(),
        cCompletionStage.join(), dCompletionStage.join()) );

这包含比通过 thenCombine

This contains more boilerplate than combining two stages via thenCombine but the boilerplate doesn’t grow when adding more stages to it.

请注意，即使您的原始 thenCombine 方法，你不需要 Triple ，是足够的：

Note that even with your original thenCombine approach, you don’t need a Triple, a Pair is sufficient:

CompletionStage<Combined> combinedDataCompletionStage =
    aCompletionStage.thenCombine(bCompletionStage, (Pair::of)).thenCombine(
        cCompletionStage.thenCombine(dCompletionStage, Pair::of),
        (ab, cd) -> combine(ab.getLeft(), ab.getRight(), cd.getLeft(), cd.getRight()));

但是，如果您想合并更多阶段，它的扩展性不好。

Still, it doesn’t scale well if you want to combine more stages.

中间的解决方案（关于复杂性）可能是：

An in-between solution (regarding complexity) might be:

CompletionStage<Combined> combinedDataCompletionStage = aCompletionStage.thenCompose(
    a -> bCompletionStage.thenCompose(b -> cCompletionStage.thenCompose(
        c -> dCompletionStage.thenApply(d -> combine(a, b, c, d)))));

这个结构比较简单，但是在更多的阶段不能很好地扩展。

That’s simpler in its structure but still doesn’t scale well with more more stages.

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