Java Executor でブロックせずにタスク完了通知を取得する方法?

class CallbackTask implements Runnable {

  private final Runnable task;
  private final Callback callback;

  CallbackTask(Runnable task, Callback callback) {
    this.task = task;
    this.callback = callback;
  }

  public void run() {
    task.run();
    callback.complete();
  }

}

import java.util.concurrent.CompletableFuture;

public class GetTaskNotificationWithoutBlocking {

  public static void main(String... argv) throws Exception {
    ExampleService svc = new ExampleService();
    GetTaskNotificationWithoutBlocking listener = new GetTaskNotificationWithoutBlocking();
    CompletableFuture<String> f = CompletableFuture.supplyAsync(svc::work);
    f.thenAccept(listener::notify);
  }

}

class ExampleService {

  String work() {
    // Code to perform long-running task
    return "Result from long-running task";
  }

}

class GetTaskNotificationWithoutBlocking {

  void notify(String msg) {
    // Code to handle the message from the completed task
  }

}