GAE：如何通过未使用Java部署在GAE上的外部Web服务来访问存储在GAE Blobstore中的数据/文件？

Question

Within GAE, I have an application that stores files in the GAE blobstore service. After the data is stored, it should also be send to an external Web Service not deployed on GAE. I use Java.

My first idea was to use JAX-RPC (the Java API for XML-based RPC) and SAAJ (SOAP with Attachments API for Java) as described in http://www.ibm.com/developerworks/xml/library/x-tippass/ but I was not able to deploy the Axis2 framework on GAE because I got socket exceptions (not allowed in GAE). The idea was to push the data to the external web service.

In my second try, I wanted to pull the data from GAE, which means to access the file directly in the Blobstore by the external Web Service. In the Admin console of GAE, it is possible to download the blobs. Therefore, I thought, I can do the same with my external Web Service by passing the key of the blob to the external Web Service to use java.net.URL to fetch my file, similarly described describe here: http://code.google.com/intl/de-DE/appengine/docs/java/urlfetch/overview.html . This link looks like this:

String blobFileUrl = "https://appengine.google.com/blobstore/download?app_id=s~gae_app&key=" + gaeBlobKey;

However, it did not work because the webservice/www-user needs to be authenticated and authorized to access the file. As far as I know, Google does not provide a Java API to login in GAE from an external web service.

Is it possible to access blobs/data of GAE with an external Web Service, which is not deployed on GAE, using Java? If so, how is it possible?

Answer 1

You can create a servlet in your GAE App that simply serves a blob given a blob key.

The url would be something like this:

http://appid.appspot.com/blobserve?blobKey=3asdfg324gdasdf

and your servlet would look like this:

public void doGet(HttpServletRequest req, HttpServletResponse res)
throws IOException {
    BlobKey blobKey = new BlobKey(req.getParameter("blobKey"));
    blobstoreService.serve(blobKey, res);
}