I haven't worked extensively with the API, but I believe that it represents filesystem instances accessible to the JVM, not abstract technical filesystems. So

I haven't worked extensively with the API, but I believe that it represents filesystem instances accessible to the JVM, not abstract technical filesystems. So in the case of a Unix filesystem tree with various mount points, the FileSystem object would throw the exception iff the path you're trying to construct is illegal for the actual filesystem configuration in the system.