GAE BlobStore : After Uploading a file successfully, google appengine throws null exception when calling user.getEmail()

Question

I am using Google blobstore service to upload a file. I did the following steps

• Login in to the application using the UserService
• I used the blobstoreservice.createUploadUrl(successpath)
• After the file is upload, the callback url 'sucesspath' is called.
• In this 'successpath' servlet, I am calling the UserService.getUserService.
• The above call return the User object as null

Why is the User object null, after the callback from the blobstoreservice.

Thanks in advance!