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Asked: 2026-06-13T21:54:45+00:00

I’m a newbie in Finagle. I’m now reading someone’s code and found that a

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I’m a newbie in Finagle. I’m now reading someone’s code and found that a Future object is reused in different join operations. My question is will that cause the Future object to be executed multiple times (in each join), or will it only execute once and store the result for later joins?

Example:

Future<A> a= b
            .join(c)
            .flatMap(new SomeFunctionReturningA());

Future<Tuple2<A, B>> future1 = a.join(b);
Future<D> future2 = future1.flatMap(new SomeFunctionReturningD()); 
future2.get();

So will b be executed twice, or just once?

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1 Answer

  1. Editorial Team

    A Future is just a container for a value, and the value will only be set one time!
    The Future[T] have different states:

    • Empty
    • Set with a value of type T
    • Set with an Exception

    When you use map/flatMap with a function f on a Future A, you will just create a new Future B that will be the result of the previous one transformed by the function f.

    Note that:

    • If the Future A is not yet ‘filled’ you will have a not yet filled B
    • The thread that set the value a in A will also execute f(a) and set the value in B
    • If A is already ‘filled’ then the caller of map/flatMap will also execute f(a), but it will NOT recompute the value of A.
    • You can also use onSuccess/onFailure that will just register some code to be executed when the future gets its value.
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