java.util.Date expiration = new java.util.Date();
long msec = expiration.getTime();
msec += 1000 * 60 * 60; // 1 hour.
expiration.setTime(msec);
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());