ExecutorService.submit(Task) vs CompletableFuture.supplyAsync(Task, Executor)

Question

To run some stuff in parallel or asynchronously I can use either an ExecutorService: <T> Future<T> submit(Runnable task, T result); or the CompletableFuture Api:static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor); (Lets assume I use in both cases the same Executor)

Besides the return type Future vs. CompletableFuture are there any remarkable differences. Or When to use what?

And what are the differences if I use the CompletableFuture API with default Executor (the method without executor)?

Graphemics

CompletableFuture is newer, and has a lot more capabilities with a superior API. I recommend using it whenever possible. Future is woefully lacking. – Isomeric

are there some facts behind? i don't want to refactor code because an api is just "newer" imho there could be executorservices that return CompetableFuture. – Graphemics

The fundamental problem with Future is that you have to waste threads waiting for them to complete. CompletableFuture flips the script and has the future invoke callbacks when it is finished. You don't need to have a thread blocked on get(). – Isomeric

It's only a problem if you don't want them to wait. For instance, with Fork/Join the left side of the computation needs to wait for the right side to complete before returning. – Nordgren

John can you probably make your last comment an answer - with little example? – Graphemics

Answer 1

Besides the return type Future vs. CompletableFuture are there any remarkable differences. Or When to use what?

It's rather simple really. You use the Future when you want the executing thread to wait for async computation response. An example of this is with a parallel merge/sort. Sort left asynchronously, sort right synchronously, wait on left to complete (future.get()), merge results.

You use a CompleteableFuture when you want some action executed, with the result after completion, asynchronously from the executed thread. For instance: I want to do some computation asynchronously and when I compute, write the results to some system. The requesting thread may not need to wait on a result then.

You can mimic the above example in a single Future executable, but the CompletableFuture offers a more fluent interface with better error handling.

It really depends on what you want to do.

And what are the differences if i use the CompletableFutureApi with default Executor (the method without executor)?

It will delegate to ForkJoin.commonPool() which is a default size to the number of CPUs on your system. If you are doing something IO intensive (reading and writing to the file system) you should define the thread pool differently.

If it's CPU intensive, using the commonPool makes most sense.

Nordgren

thanks. so if i don't need the answer (in case of runnable) or i don't need the features of CompletableFuture, there is no difference? Could you probably giv short example what the main advantage is? – Graphemics

This answer doesn't make it clear why you'd want Future, ever. The advantage of CompletableFuture (async triggering of follow-on actions) is unique to it, but there is no advantage for Future - the "wait for result" is equally applicable to CompletableFuture. – Markland

Also ForkJoin.commonPool() is shared across the whole JVM, calculating streams, and other tasks with no specific thread pool mentioned. – Photomontage

I think @Graphemics is asking difference btw ExecutorService vs. CompletableFuture instead Future vs. CompletableFuture. – Clavicytherium

One main difference is what @Chinmay mentioned below: none of them get executed unless we call .get method and during this time, while executorService.submit()/execute() executes the logic once you do it. – Clavicytherium

An example code would have made this answer much clearer IMO. – Gallstone

@BeeOnRope, one advantage/disadvantage of Future is that it tightly coupled with a computation thread and cancel() method could be used to interrupt the thread. Whereas CompletableFuture have no meaning of related thread. – Kaleb

Answer 2

CompletableFuture has rich features like chaining multiple futures, combining the futures, executing some action after future is executed (both synchronously as well as asynchronously), etc.

However, CompletableFuture is no different than Future in terms of performance. Even when combine multiple instances of CompletableFuture (using .thenCombine and .join in the end), none of them get executed unless we call .get method and during this time, the invoking thread is blocked. I feel in terms of performance, this is not better than Future.

Please let me know if I am missing some aspect of performance here.

Anorexia

Answer 3

This clarified for me the difference between future an completable future a bit more: Difference between Future and Promise

CompletableFuture is more like a promise.

Graphemics