I have been involved in this for a few weeks - since ThoughtWorks is a Google Enterprise Partner - and it’s been a very interesting time. This post and a few others will take a closer look at what I’ve been experimenting with.
First of all, GAE/J is not based on Dalvik, as far as I can tell. It is a full Java implementation, so you compile your applications locally, using any standard JDK and then upload them. Google recommends Java 6 for this, but Java 5 works too.
The actual interface to GAE/J uses the standard Java Servlet API, so if you have something that works with it, chances are you won’t have to do many changes to your application.
Google also gives access to several different APIs, including the User service, Memcache service, Mail service, URL fetching service, Image service and DataStore service. These all give access to different pieces of the Google machinery. For me, the most interesting parts were the User service, that makes it possible to use the regular Google authentication infrastructure, and the DataStore service that makes it a snap to use Googles data storage infrastructure. For regular Java applications, you can use either JDO or JPA to interact with the DataStore, but Google also gives access to the low level APIs too.
As part of the GAE/J release, you get access to a local development server. It tries to mimic the full environment as closely as possibly. For the specific type of application Google expects most people to write, it works very well - but if you go outside of this beaten path, many things get a bit shaky. I ended up not using it very much.
So, GAE/J is a very cool platform to target cloud applications to. Obviously Python is still a valid choice too, but the combination of apps built in Python and applications running on GAE/J seems like a very powerful choice.
ThoughtWorks has recently been spending much time in this area and we have gotten some good experience with it. We look forward to be able to work with applications for Google App Engine, written in Java, or any of the other languages supported. (If you follow todays blog posts, you will see that I’m not the only ThoughtWorker who has explored alternative languages on this platform).
My esteemed colleagues have also written up their experiences with the Java pieces of Google App Engine. You can read it here: http://paulhammant.com/blog/google-app-engine-for-java-with-rich-ruby-clients.html, http://elhumidor.blogspot.com/ and http://blog.sriramnarayan.com/.