MapReduce Input Split And Custom Input Format

透過實作InputFormat interface來實現客制的input split

Interface InputFormat<K,V>

1. 對job的input進行確認驗證(i.e.configuration的確認)與生效。
   Validate the input-specification of the job.
2. 將輸入的檔案（在HDFS中為blocks）轉換成為Input Split(logical chunks of type InputSplit)，而每一個Input Split將會被分別指派給Mapper進行處理。
   Split-up the input file(s) into logical InputSplits, each of which is then assigned to an individual Mapper.
3. 實作RecordReader，用以產生InputSplit的key/value pairs(實際描述Mapper每次要處理的單元)，送進Mapper處理。
   Provide the RecordReader implementation to be used to glean input records from the logical InputSplit for processing by the Mapper.