CompletableFuture已完成，但有异常

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.

Answer 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);

Answer 2

Java 9提供了CompletableFuture.failedFuture​(Throwable ex)来做到这一点。