should package core ,math and collections to one Jar package for org.apache.mahout.cf.taste.hadoop.item.RecommenderJob.
because RecommenderJob used classes (for example org.apache.mahout.math.VectorWritable )of the math module of mahout project,
but math and core module is the separated jar package.
so when work on the hadoop env ,the class of math module can not load to classloader in the datanode.
it will cause class not found exception.
the work around is package all mahout classes to one package manually.