writing custom functions that use external java classes on Hive

Question

I've been thinking of how to do it in Hive.

For e.g. i've a specific field in a log file that I want to extract (this is already possible in Hive) and then I want to map this field's value to something else. This mapping is determiened by own customic business logic that is coded up in a Java Class. How can I use this Java class in Hive?