How can I get hold of realm object (JDBCRealm) within Java app?

Question

I have configured JDBCRealm Tomcat with the following:

I have a few questions:
1) How can I get hold of this realm object within my Java app?
2) Even if I get the instance of the JDBCRealm - how do I get the list of all roles available?

Comments

What's the purpose of doing this?

If I wanted to get a list of my users or roles, I would use JDBC and pull them from the database. Then I don't have to tie my application to Tomcat's internal classes.