func4 allows you to use db4o for authentication through the JAAS spec for Java applications. It has been used for webapps in Tomcat 5 and Jetty 6. Currently it should be considered in beta.

Note: The password field used by func4 is a String and is therefore considered insecure. It means that if someone could get access to the JVM memory passwords could be stolen since they will remain in memory till garbage collection cleans them up. Since I run this on server based webapps if they get access to the JVM memory I have a lot bigger problems.

http://code.google.com/p/func4/