I was running some MR jobs tests and ended up with PrefixTreeCodec class not found in the YarnChildren processes.
2016-01-21 06:24:26,844 WARN [main] mapreduce.TableMapReduceUtil(785): The hbase-prefix-tree module jar containing PrefixTreeCodec is not present.
This is related to
HBASE-7434 and HBASE-7936 which address compile time concerns. This fix makes it so that jar inclusion is done at run time, and continues if it is not present (for mr unit tests that don't depend on it)