When using Drools inside an Equinox OSGi container (Oxygen, org.eclipse.osgi-3.12.0.v20170512-1932.jar) the logic inside org.drools.relective.classloader.ProjectClassLoader#isOsgiClassLoader does not work properly, i.e. the OSGi runtime is not detected. This leads to "The Eclipse JDT Core jar is not in the classpath" because the shaded and packed compiler classes cannot be found.
Cause of the problem seems to be the fact that the EquinoxClassLoader does not implement BundleReference directly but so does its superclass. I tried and patched ProjectClassLoader with the following snippet and this seems to work: