Chain few CompletableFuture operations

Question

I have few separate operations that i want to chain in the same style like we have for Promises in JS.

So currently i have such code:

    CompletableFuture<Data1> data1 = ...;
    CompletableFuture<Data2> data2 = ...;

    // Fetch in parallel!
    CompletableFuture.allOf(
            data1,
            data2
    ).join(); // wait 2 threads to finish

    CompletableFuture<Data3> data3 = ... // i need data1 here
    data3.join();

Is it possible to rewrite it in one chain ? Kind of:

CompletableFuture.allOf(
        data1,
        data2
)
.then((result1, result2) -> return another CompletableFuture)
.then(result3 -> { ...do other stuff here})

Answer 1

When using allOf , there is no way around calling join on the input futures to get the values in a dependent function (though you'll know that these calls won't block).

However, when you have exactly two futures to combine, there is no need to use allOf ; you can combine them smoothly:

CompletableFuture<FinalResult> f = data1
    .thenCombine(data2, (result1, result2) -> another CompletableFuture)
    .thenCompose(Function.identity())
    .thenAccept(result3 -> …);

thenAccept will evaluate a function mapping a result value to a new result value, whereas thenCompose will evaluate a function mapping a result value to a new future. The method thenCombine is the two-future variant of thenAccept , allowing to map two values to a new value.

But there is no two-future variant of thenCombine , so the solution is to treat the future created from the two intermediate values like a value (getting a future of a future), followed by thenCompose(Function.identity()) to map the “value” to a future. Then, we can chain a function for processing the third result, once the future has been completed.

Answer 2

CompletableFuture.allOf(...) returns a CompletableFuture<Void> . It knows nothing about the types that will be returned by the givne CompletableFuture s:

the results, if any, of the given CompletableFuture s are not reflected in the returned CompletableFuture , but may be obtained by inspecting them individually.

So you will have to get the returned values of data1 and data2 if you want to use them.