public static <T> T getWithDcl(Supplier<T> supplier, Supplier<T> factory, Object mutex) {
    T object = supplier.get();
    if (object == null) {
      synchronized (mutex) {
        object = supplier.get();
        if (object == null) {
          object = factory.get();
        }
      }
    }

    return object;
  }

  // Usage
  Locks.getWithDcl(() -> pool.poll(), () -> factory.apply(this), pool);