mapreduce步骤概述如下

1) InputFormat :

 - Validates the Input.

 - Splits the input files.        

 - Gives Record Reader Implementation to give input to Mapper.

2)Mapper Phase

3)Shuffle and Sort Phase

4)Reducer Phase

5) OutputFormat     

 - Validates the Output Specification.  

 - Provides Record Writer Implementation used to write the output files.

reducer不使用记录读取器。。。