Identity Mapper is the default Mapper class provided by Hadoop. when no other Mapper class is defined, Identify will be executed. It only writes the input data into output and do not perform and computations and calculations on the input data.
The class name is org.apache.hadoop.mapred.lib.IdentityMapper.
Chain Mapper is the implementation of simple Mapper class through chain operations across a set of Mapper classes, within a single map task. In this, the output from the first mapper becomes the input for second mapper and second mapper's output the input for third mapper and so on until the last mapper.
The class name is org.apache.hadoop.mapreduce.lib.ChainMapper.