Timeouts and your own executor

Definition

Always limit how long you wait: orTimeout fails the future after a time, and completeOnTimeout gives a default value instead. For blocking work, pass your own executor to the Async methods instead of using the shared common pool.

Example code

A slow call that is cut off, running on a dedicated pool

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

class Demo {
    public static void main(String[] args) {
        ExecutorService ioPool = Executors.newFixedThreadPool(4);          // a pool just for blocking calls

        CompletableFuture<String> slow = CompletableFuture.supplyAsync(() -> {
            try { Thread.sleep(2_000); } catch (InterruptedException ignored) { }
            return "late answer";
        }, ioPool);

        String result = slow
                .completeOnTimeout("default answer", 200, TimeUnit.MILLISECONDS)   // give up after 200 ms
                .join();
        System.out.println(result);

        CompletableFuture<String> strict = CompletableFuture.supplyAsync(() -> {
            try { Thread.sleep(2_000); } catch (InterruptedException ignored) { }
            return "late";
        }, ioPool).orTimeout(200, TimeUnit.MILLISECONDS);                       // fail after 200 ms
        try {
            strict.join();
        } catch (java.util.concurrent.CompletionException e) {
            System.out.println("timed out: " + e.getCause().getClass().getSimpleName());
        }
        ioPool.shutdownNow();
    }
}

// Output:
// default answer
// timed out: TimeoutException