AWS list only direct children of a subfolder.
AmazonS3 s3 = new AmazonS3Client(new AWSConfiguration());
ListObjectsRequest listObjectRequest = new ListObjectsRequest().
withBucketName(AWSConfiguration.BUCKET).
withPrefix(prefix).
withDelimiter("/");
ObjectListing objectListing = s3.listObjects(listObjectRequest).getCommonPrefixes();