IBM supplies JDBC drivers for the iSeries and DB2 is the built-in database. There's no real difference between using JDBC to interface with an iSeries machine and using JDBC to interface with any other database/OS.
For details, check out the docs at ibm.com.
Bjoke: A "Bully Joke". A Statement or action made with malicious intent - unless challenged. At which point it magically transforms into "I was just funnin'" or "What's the matter, can't take a joke?"