There are some people reporting issues when they try to install OpenNMS with OpenJDK JRE instead of the JDK. See here:
Especially when a Server with graphical environment installs an OpenJDK runtime environment. With installing OpenNMS OpenJDK development kit will not be installed when a JRE is present. The documentation should have a hint about checking the OpenJDK environment to prevent users running in this kind of issues.