Handling errors

Definition

If a step in a chain fails, the exception travels down the chain and later steps are skipped. exceptionally supplies a fallback value. handle runs whether the step succeeded or failed. join throws the failure wrapped in a CompletionException.

Example code

A fallback value and a handle step

import java.util.concurrent.CompletableFuture;

class Demo {
    public static void main(String[] args) {
        CompletableFuture<Integer> failing = CompletableFuture.supplyAsync(() -> {
            if (true) throw new IllegalStateException("service is down");
            return 1;
        });

        int fallback = failing.exceptionally(error -> -1).join();          // only runs on failure
        System.out.println("fallback: " + fallback);

        String text = failing.handle((value, error) ->                      // runs in both cases
                error == null ? "ok: " + value : "failed: " + error.getCause().getMessage()).join();
        System.out.println(text);

        try {
            failing.join();
        } catch (java.util.concurrent.CompletionException e) {
            System.out.println("join threw: " + e.getCause());
        }
    }
}

// Output:
// fallback: -1
// failed: service is down
// join threw: java.lang.IllegalStateException: service is down