Can anyone provide a tutorial on using mapDB?

I know very little about java and even less about mapDB.

I am trying to modify an Apache-Tomcat application. All I need
to do is store a variable away that was created with a javascript function.
And then subsequently, a client would need to access it.

I was going to us php/mysql but the Apache-Tomcat that the app is using does not support php.

Any help would be very much appreciated.