I'm new to app engine, so bear it with me.
I've a App-Engine web app project that does multiple things and populates Database using JDO persistence. App takes long time to execute, so I prefer running it offline (as time limit is much higher). Now where is this local JDO data persisted? I can access it at http://localhost:8888/_ah/admin How can I export this database to App Engine?
There is a bulk uploader for the appengine (written in python) that can be used to upload data stored in datastore for your app.
Unfortunately there is no equivalent tool (provided by google) in java.
On your second question, the local dev server stores data in
<app-dir>/WEB-INF/appengine-generated/local_db.bin
This forum suggests that you will have to write a script and use datanucleus's rest api to do that for your java appengine app.