How do I download a image file from google webapp engine?

AppEngineFile AFE = new AppEngineFile(FILESYSTEM + alist.get(0).getPath());
BlobKey bk = FileServiceFactory.getFileService().getBlobKey(AFE);


if("image/jpeg".equals(alist.get(0).getContentType()) ){
    resp.setContentType("image/jpeg");
} 

resp.setHeader("Content-Disposition", "attachment; filename=\"" + alist.get(0).getTitle() + "\"");

FileService fileService = FileServiceFactory.getFileService();
AppEngineFile file = null;

try {
    file = fileService.getBlobFile(bk);
} catch (FileNotFoundException e) {
    e.printStackTrace();
}

FileReadChannel ch = null;

try {
    ch = fileService.openReadChannel(file, false);
} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (LockException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

byte[] barray = new byte[MAXSIZE];

ByteArrayOutputStream baos = new ByteArrayOutputStream();

try {
    ByteBuffer bb = ByteBuffer.wrap(barray);
    int nRead;
    while ((nRead=ch.read(bb)) != -1) {
        for (int i=0; i < nRead; i++) {
            try {
                baos.write(barray[i]);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        bb.clear();
    }
} catch (IOException e) {
    e.printStackTrace();
}

resp.setCon开发者_开发百科tentLength(baos.size());

// resp.getOutputStream().write(baos.toByteArray()); // <= if I use it, this message, "Error 6 (net::ERR_FILE_NOT_FOUND): The file or directory could not be found.", is showed.

// out.print(baos); // <= if I use this, I can download a file but it is byte code.            

baos.flush();
baos.close();

How do I fix whole this code for downloading an image file? because If I use Number 1, it make a error, which is "Error 6 (net::ERR_FILE_NOT_FOUND): The file or directory could not be found.", is showed." or if i use number 2, it looks fine but the type of stored file is byte code. This means is not image file.

Who can give me any idea or example?

I ran into this problem a while back. You cannot read files uploaded with the application. They are considered static application blobs and do not exist in any way accessible to your code.

Take a look here and notice that the only available option right now is BLOBSTORE. If you want something to be readable by application code, it must be stored in the Blobstore or as a BlobProperty on an object in the Datastore (which is very inefficient, use the Blobstore if you can).