* add docs

* fix javadoc warning

* add maven plugin to check code format

* apply AOSP code format

* add rat plugin for license check

* add missing license header

* run rat in validate phase

* upgrade and make logmessage as a single package

* omit OB unrelated stuffs

* omit duplicant set

* add comment and cleanup redundant class

* update java doc

* add javadoc to listeners

* cleanup exception class