CompletableFuture already completed with an exception

问题

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.

回答1:

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);

回答2:

Java 9 provides CompletableFuture.failedFuture​(Throwable ex) that does exactly that.

来源：https://stackoverflow.com/questions/49116855/completablefuture-already-completed-with-an-exception