Edson Carlos Ericksson Richter has written an authentication module
that uses JDBC to locate user and group information, much as the Tomcat implementation. Glassfish will have an official JDBC module in the future, but as Mr. Richter says, why wait when this one's available?
Glassfish is currently delivered with four realms: a file realm, a certificate realm, an LDAP
realm, and a Solaris realm (even in Windows). While these are useful, and its arguable that LDAP is used for most "big" enterprises, many applications don't need it; what's more, most developers probably prefer to use a database for this (as database resources are more likely to be present than LDAP instances.) It's surprising that JDBC hasn't been a higher priority for the Glassfish team, but as Mr. Richter shows, it's not impossible for users to step up to the plate.
The download is available from the blog entry, as well as detailed steps of how to install the JDBC realm.