listObjectsでは最大1000件までしか一度に取得できないため、再帰的に取得するようにする必要があります。
try { ObjectListing listObjects = s3.listObjects(BACKEAT, key); do { List<S3ObjectSummary> objectSummaries = listObjects.getObjectSummaries(); for (S3ObjectSummary summary : objectSummaries) { System.out.printf(summary.getBucketName()); } listObjects = s3.listNextBatchOfObjects(listObjects); } while (listObjects.getMarker() != null); } catch (AmazonServiceException ace) { throw new RuntimeException("error"); } catch (AmazonClientException ace) { throw new RuntimeException("error"); }