GeneratePresignedUrlRequest generatePresignedUrlRequest =
new GeneratePresignedUrlRequest(bucket, key);
generatePresignedUrlRequest.setMethod(HttpMethod.GET); // Default.
generatePresignedUrlRequest.setExpiration(expiration);
URL s = s3Client.generatePresignedUrl(generatePresignedUrlRequest);
JsonObject response = new JsonObject();
response.addProperty("thumbnailUrl", s.toString());
resp.getWriter().write(response.toString());