I\'m trying to deploy one of my java apps in GAE and I have used MongoDB. Unfortunately GAE doesn\'t allow me to create databases, however it suggests using Google Datastore
If you really want to use MongoDB anyway, you'll have to find a third-party host that provides a REST interface through which you can make your transactions. I've personally tried and recommend MongoLab on App Engine. Then, in your code, use an HttpUrlConnection object to send/receive requests if you're doing it with Java.