CompletableFuture already completed with an exception

Question

CompletableFuture.completedFuture() returns a CompletedFuture that is already completed with the given value.

How do we construct a CompletableFuture that is already completed exceptionally?

Meaning, instead of returning a value I want the future to throw an exception.

In which context do you need to do that? I think in a lot of contexts, just throwing the exception would do the job (e.g. with Spring `@Async` or with `thenCompose()`) — Didier L, Mar 06 '18 at 14:57
@DidierL I've got a method that executes quick tasks synchronously, but returns a `CompletionStage`. Why? Because the caller wants to validate input parameters synchronously before chaining asynchronous `CompletionStage`s after it, and it needs any exceptions thrown by precondition validation to get handled by `exceptionally()`. — Gili, Mar 06 '18 at 19:46

score 31 · Accepted Answer · edited Aug 21 '19 at 23:20

31

Unlike Java 9 and later, Java 8 does not provide a static factory method for this scenario. The default constructor can be used instead:

CompletableFuture<T> future = new CompletableFuture<>();
future.completeExceptionally(exception);

edited Aug 21 '19 at 23:20

M. Justin

answered Mar 05 '18 at 18:15

Gili

score 15 · Answer 2 · answered Mar 27 '18 at 12:42

15

answered Mar 27 '18 at 12:42

Didier L

2 Answers2