can't read the uploaded blob in app engine

as pointed out on开发者_Python百科 the overview page of blobstore api page

An app can read a Blobstore value a portion at a time using an API call. can anybody provide a snippet doing exactly that?

You want the [fetchData method][1]. It takes a blob key and a range, and returns that range of bytes from the file.

[1]: http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/blobstore/BlobstoreService.html#fetchData(com.google.appengine.api.blobstore.BlobKey, long, long)