This is the default mapper that is chosen when no mapper is specified in the MapReduce driver class.
It implements identity function, which directly writes all its key-value pairs into output.
It is defined in old MapReduce API (MR1) in: org.apache.Hadoop.mapred.lib.package
This class is used to run multiple mappers in a single map task.
The output of the first mapper becomes the input to the second mapper, second to third and so on.
It is defined in: org.apache.Hadoop.mapreduce.lib.chain.ChainMapperpackage