The .jpi cache files are from a java plug-in, they are a sticky cache; i.e., the disk cache created and controlled by java plug-in which the browser cannot overwrite.
The jar file will contain the class files and auxiliary resources associated with applets and applications.
I wouldn’t think they are taking up that much space that you would need to delete them for disk space. I would say that it is safe to delete these files as long as they are not part of a java program that you have.

