-1 I like the patch, but there are just a couple of things that need fixing:
1) The mapping should be between # and ArrayList<ACL> rather than # and ACL. I think it further simplifies things and will make serialization/deserialization go even faster. convertACLs should return a long for example.
2) When you serialize the # to ACL map and the ACL to # map, you only need to serialize once. I would serialize the # to ACL map. When you deserialize you can rebuild the ACL to # map at the same time since they have the same information.