How to combine 3 or more CompletionStages?

Question

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)); 

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)); 

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

Answer 1

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()) ); 

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

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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())); 

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

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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))))); 

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