This will capture a pointer to the classlaoder in which the SC was created. So addJars can be called from anywhere and it will always augment this class loader.

I think this means that the class will be visible to (a) the thread that created the sc and (b) any threads created by that thread. Though it would be good to verify that the context class loader is passed on to child threads or they delegate to that of the parent.

This does mean that a thread entirely outside of the SparkContext-creating thread and its children won't have the class loaded. I think that's actually desirable given that you may have a case where mutliple SparkContext's are created in the same JVM.