Ruby-Java Bridge (RJB) allows use of Java libraries from within a c-ruby environment, see this slideshow. RJB provides an alternative when a full port to JRuby is impractical.
First install Java and build-essential
apt-get update apt-get install openjdk-6-jdk apt-get install build-essential
Now install rjb gem
export JAVA_HOME=/usr/lib/jvm/java-6-openjdk gem install rjb
The bad news is that RJB currently crashes when used with Passenger so the best alternative is to revert to Mongrel.