Check out the LibPQFactory[1]. I haven't used it myself (only looked over the code when writing another SSL factory) but it's supposed to handle using client certificates for auth. It's meant to mimic the behavior of the libpq library and generally accepts the same defaults for flags/cert locations.
[1]: https://github.com/pgjdbc/pgjdbc/blob/master/org/postgresql/ssl/jdbc4/LibPQFactory.java
Regards,
Sehrope Sarkuni
Founder & CEO | JackDB, Inc. | http://www.jackdb.com/