code: public static <T> Single<T> toSingle(ListenableFuture<T> listenableFuture) {
          return RxJava2FutureUtils.createSingle(SpringFutureUtils.createValueSource(listenableFuture));
      }
docs: Converts {@link ListenableFuture} to  {@link io.reactivex.Single}.
  The original future is canceled upon unsubscribe.