How to install the Appengine SDK to the Maven local repository?
Note that this needs to be done only once.
  $ mvn gae:unpack
  Note that you don't need to manually download any SDK.

How to generate the WAR?
  $ mvn package
  $ mvn war:war
  Note that this must be done before running the webapp locally and before
  deploying it to Appengine.

How to run the webapp locally?
  $ mvn gae:run

How to deploy the webapp to Appengine?
  $ mvn gae:update

If you have made any changes to the code or pom.xml file of demo here,
you may need to run Maven commands like ```mvn clean install -U```
(especially one hierarchy above i.e at java/ folder) to resolve any
dependency issues w.r.t new developement version.