@SuppressWarnings("unchecked")
  public static <K2, V2> Class<Mapper<ImmutableBytesWritable, Result, K2, V2>>
    getMapperClass(JobContext job) {
    return (Class<Mapper<ImmutableBytesWritable, Result, K2, V2>>) job.getConfiguration()
      .getClass(MAPPER_CLASS, Mapper.class);
  }
Get the application's mapper class.
@param <K2> the map's output key type
@param <V2> the map's output value type
@param job  the job
@return the mapper class to run
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/MultithreadedTableMapper.java