GridFSDBFile cannot be cast to org.springframework.web.multipart.MultipartFile - spring

Question

I'm coding a spring mvc webapp that uses images of type MultipartFile which i convert to byte[] and then to Inputstream and store it in MongoDB using GridFsTemplate.
Now the problem is I want to display the stored images in a webpage but whenever I try to, the database returns the image file as GridFSDBFiles and so tosses the following exception:
java.lang.ClassCastException: com.mongodb.gridfs.GridFSDBFile cannot be cast to org.springframework.web.multipart.MultipartFile
This is my DAO for storing images:
public void saveScan(Scan scan) throws IOException {
String owner = String.valueOf(scan.getPatientId());
String fileName = String.valueOf(scan.getPatientId() + "" + scan.getScanType());
Date date = new Date();
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("MM/dd/YYYY HH:mm a");
String uploadTime = simpleDateFormat.format(date);
System.out.println("the scan type is " + scan.getScanType());
DBObject metaData = new BasicDBObject();
metaData.put("owner", owner);
metaData.put("fileName", fileName);
metaData.put("uploadTime", uploadTime);
byte[] scanBytes = scan.getScan().getBytes();
InputStream inputStream = new ByteArrayInputStream(scanBytes);
scanDaoImpl.SaveScan(inputStream, fileName, "image/jpeg", metaData);
}
And this is for retrieving the images:
public MultipartFile findOneScan(BigInteger patientId) {
MultipartFile multipartFile = (MultipartFile) gridFsTemplate
.findOne(new Query(Criteria.where("metadata.owner").is(patientId)));
return multipartFile;
And this is my controller for getting images
#ResponseBody
#RequestMapping(value = "/patients/{id}/scan", produces = MediaType.IMAGE_JPEG_VALUE)
public ResponseEntity<byte[]> scanImage(#PathVariable("id") BigInteger id) throws IOException {
logger.debug("scanImage() is finding Image to display");
byte[] bs = patientScanServiceImpl.findOne(id).getBytes();
HttpHeaders httpHeaders = new HttpHeaders();
httpHeaders.setContentType(MediaType.IMAGE_JPEG);
httpHeaders.setCacheControl(CacheControl.noCache().getHeaderValue());
return new ResponseEntity<byte[]>(bs, httpHeaders, HttpStatus.OK);
}
This is my thymeleaf image tag:
<span>
<img th:src="#{/patients/{patientid}/scan(patientid=${patient.id})}" width="250" height="250"/>
</span>

Answer 1

Now I have more insight in this, and I have finally found a solution. You cannot directly cast a gridFSDBFile straight to byte[ ]; it must first be converted to OutputStream and then byte[ ] if it must be displayed. So I allowed my DAO method to return a GridFSDBFile but in the service layer I converted the GridFSDBFile to ByteArrayOutputStream and then to byte[ ].
Now my DAO method for retrieving images is
public GridFSDBFile findOneScan(BigInteger patientId, String scanType) {
String fileName = String.valueOf(patientId + "" + scanType);
GridFSDBFile gridFSDBFile = gridFsTemplate.findOne(new Query(Criteria.where("metadata.fileName").is(fileName)));
return gridFSDBFile;
And my service layer which feeds the controller is
public byte[] findOne(BigInteger patientId, String scanType) throws IOException {
GridFSDBFile gridFSDBFile = scanRepository.findOneScan(patientId, scanType);
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
gridFSDBFile.writeTo(outputStream);
byte[] bs = outputStream.toByteArray();
return bs;
}
And the whole thing works fine.