It is important to understand how the BlobService workflow works. Following is the typical workflow of a blobservice.
- The client first creates an upload URL where the image would be uploaded. Since it is hard (not possible) to do a decent authentication scheme around blobservice, I decided to use a appengine endpoint to create the upload URL for me. The code for appengine endpoint implementation is very simple.
- Now that we have the URL, the first thing that we need to do is to convert the image into a byte array that we can upload. I have created following utility function to do just that. Basically I don't want to store full sized images, I have created this function that takes a Bitmap and scales the image and converts it to a byte array. The images is scaled with actual aspect ratio with the width fixed at maximum PRODUCT_IMAGE_WIDTH.
- Now that we have the image as a byte array, we upload the image from the client using this URL. Since google has removed the old HTTPClient, I had to figure out how to use OkHTTP which seems to be the new HTTP client that needs to be used. It was really not very hard to do this, it is just that documentation around OKHttp is very sparse and it takes some trial and error to make this work. We set a response header so that the client can receive the cloud key that is generated by the blob service. We need to know this key to retrieve the image later.
- Let's look at the implementation of blobservice. We basically need to create a servlet to handle the upload request. The doPost of the servlet should handle the upload. WIth some trial and error, I figured out that the blobservice API looks for a multipart payload with image byte array and header with the name "file". You can also provide the bytearray type, otherwise it decodes it with the filename extension that you provide as part of the value.
- Retrieving the image is also very simple. You need another servlet to do that. In my case I have a servlet with a GET method which returns the image.