JDBC - Best path to get from here to there?

The accounting package at http://www.compiere.org is looking to make a
port to PostgreSQL. Right now, the code is tied to Oracle, and Mr. Janke
wants to see the new code have a more 'universal' link.

Is there any kind of 'shim' in Java/JDBC land that can provide some form
of 'universal' interface?

(At the moment, my java knowledge is 'lacking', ok, non-existant. And I'd
rather fight a limited number of battles - learning JAVA, getting this
code working on FreeBSD, PostgreSQL, and a workable UNIVERSAL wedge. )

Any suggestions on researching this would be more than helpful.

Linux is for people who hate Microsoft.
FreeBSD is for people who love UNIX.

Windows: "Where do want to go today?"
Linux: "Where do you want to go tomorrow?"
BSD: "Hey, are you guys coming or what?!?"