Creación y finalización de CompletableFutures

¿Qué es CompletableFuture?

supplyAsync: inicio de un cálculo asíncrono

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    // runs on ForkJoinPool.commonPool()
    return fetchDataFromNetwork();
});
System.out.println("Non-blocking — this runs immediately");

runAsync: ejecutar y olvidar

CompletableFuture<Void> task = CompletableFuture.runAsync(() ->
    sendEmail("user@example.com", "Welcome!"));
task.thenRun(() -> System.out.println("Email sent"));

Executor personalizado

ExecutorService pool = Executors.newFixedThreadPool(10);
CompletableFuture<User> future = CompletableFuture.supplyAsync(
    () -> userService.findById(42L), pool
);

completedFuture: valor ya completado

CompletableFuture<String> cached = CompletableFuture.completedFuture("cached-value");
cached.thenAccept(System.out::println); // runs immediately

Completar manualmente un future

CompletableFuture<String> promise = new CompletableFuture<>();
// In another thread or callback:
promise.complete("done!");       // signals success
// promise.completeExceptionally(new RuntimeException("fail")); // signals failure
String result = promise.get();   // blocks until completed

get() y join()

try {
    String r = future.get(5, TimeUnit.SECONDS); // checked exceptions
} catch (TimeoutException | InterruptedException | ExecutionException e) { ... }
// Or:
String r = future.join(); // throws CompletionException (unchecked)

getNow() para comprobaciones sin bloqueo

String result = future.getNow("loading...");
System.out.println(result); // "loading..." if not yet done

isDone, isCancelled, isCompletedExceptionally

if (future.isDone())                    System.out.println("Completed");
if (future.isCancelled())               System.out.println("Cancelled");
if (future.isCompletedExceptionally()) System.out.println("Failed");

cancel() para gestionar tiempos de espera

CompletableFuture<String> future = fetchAsync();
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
scheduler.schedule(() -> future.cancel(true), 3, TimeUnit.SECONDS);

completeOnTimeout (Java 9 o posterior)

CompletableFuture<String> result = fetchAsync()
    .completeOnTimeout("default", 3, TimeUnit.SECONDS);
System.out.println(result.join()); // "default" if fetch took > 3s

Comprobación rápida

Resumen