Multithreading: CompletableFuture.exceptionally supplies a fallback

exceptionally only runs when the upstream future completed with an exception, producing a substitute value so the chain can still complete normally. If the future had succeeded, exceptionally would simply be skipped and the original value kept.

Code

CompletableFuture<Integer> risky = CompletableFuture.supplyAsync(() -> {
    throw new ArithmeticException("bad math");
});
CompletableFuture<Integer> recovered = risky.exceptionally(ex -> -1);
System.out.println("Recovered value: " + recovered.get());

Output

Recovered value: -1