From OpenNMS

Supported OpenJDK Versions

OpenNMS allows startup with some minimally-tested versions of OpenJDK. As of OpenNMS 1.6.6 and 1.7.4, you will be able to run with build 14 and higher of both OpenJDK 1.6.0 (IcedTea6) and OpenJDK 1.7.0.

If, when running runjava, you get the error:

 <jdk> is not a Sun JVM and only Sun JVMs are supported.
 You can use the '-f' option to ignore this check, but you will be on your own for support.

You are running an unsupported version of OpenJDK. If you wish to attempt to run it anyways, see the below instructions.

Forcing OpenNMS to Use an Unsupported JDK

This is work in progress, do not expect any support from the opennms team on this You may hit terrible bugs that eat children

Installation is modified as follows :

./runjava -f -S /usr/lib/jvm/java-1.6.0-openjdk/bin/java

It will abundantly complain, but does the work anyway.

Modify the "install" script by adding "-f" in the command line

./install -dis

edit the "opennms" startup script to add

RUNJAVA_OPTIONS=" -f "

Work isn't finished, because jetty will also complain :

OpenNMS recommends the official Sun JVM.  You can edit /opt/opennms/jetty-webapps/opennms/WEB-INF/web.xml and change the value for the 'dontBlameOpenNMS'
context parameter from 'false' to 'true', but don't blame OpenNMS for any errors that occur without switching back to a supported JVM and setting the
property back to 'false', first.

Do as the guy says, and you should be able to start OpenNMS !

Reporting a Working JDK

If you JDK was reported as unsupported but, after following these instructions, works anyways, please open a bug and let us know.