User problem: Fetch the order summary of a certain customer.

By default, as in the preceding examples, the asynchronous tasks are executed in threads obtained from the global ForkJoinPool.commonPool(). By simply logging  Thread.currentThread().getName(), we see something as ForkJoinPool.commonPool-worker-3.

But we can also use an explicit Executor custom thread pool. All the CompletableFuture methods that are capable of running asynchronous tasks provide a flavor that takes Executor.

Here is an example of using a single thread pool:

public static void fetchOrderSummaryExecutor() {

  ExecutorService executor = Executors.newSingleThreadExecutor();

  CompletableFuture<String> cfOrderSummary 
      = CompletableFuture.supplyAsync(() -> {

    logger.info(() -> "Fetch order summary by: "
      + Thread.currentThread().getName());
    Thread.sleep(500);

    return "Order Summary #91022";
  }, executor);

  // wait for summary to be available, this is blocking
  String summary = cfOrderSummary.get();
  logger.info(() -> "Order summary: " + summary + "
");
  executor.shutdownNow();
}