The OSGi plugin does not work with GlassFish where Felix is already included. When including in the Struts web application a bundle containing an activator class the following exception occurs:
java.lang.ClassCastException: myosgi.Activator cannot be cast to org.osgi.framework.BundleActivator
while when including a bundle with a class which implements BundleContextAware the following exception occurs:
Exception starting filter struts2 java.lang.LinkageError: loader constraint violation: loader (instance of org/apache/felix/framework/searchpolicy/ContentClassLoader) previously initiated loading for a different type with name "org/osgi/framework/BundleContext"
It also seems that the plugin uses an old version of Felix which could be (at least partly) responsible for the errors since GlassFish uses a newer one.
More details on the errors also available in this stackoverflow post