Planned lessonMID-006-L03

Errors in async code

On This Lesson 4 topics

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

propagate and handle failures.

An exception completes the stage exceptionally; dependent stages are skipped and carry it forward.
exceptionally(fn): recover with a fallback value.
handle((v, ex) -> ...): see both result and error; whenComplete observes without changing.
join() throws CompletionException; get() throws ExecutionException; the cause is wrapped.
An unobserved failed future loses the error silently; always attach handling or read it.

Draft outline details

java

CompletableFuture<List<Review>> r = reviewsF.exceptionally(ex -> List.of()); // degrade gracefully

What does a downstream thenApply do when its source failed?

Show answer

It is skipped and the failure propagates.

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