ClassNotFoundException is thrown when calling a serviceTask in a workflow if the ProcessEngine is running in OSGI container.
The exception comes from ExpressionFactory class while it tries to create a new instance of the default "org.activiti.engine.impl.juel.ExpressionFactoryImpl".
Reproducing the issue: see ActivitiInAction_BookOsgiApp.patch. Install the necessary components in Karaf and try to run the osgiProcess workflow.
Possible solution: see ExpressionFactoryReflectUtil.patch