public void map(ImmutableBytesWritable key, Result value, Context context)
    throws IOException, InterruptedException {
    context.write(key, value);
  }
Pass the key, value to reduce.
@param key     The current key.
@param value   The current value.
@param context The current context.
@throws IOException          When writing the record fails.
@throws InterruptedException When the job is aborted.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/IdentityTableMapper.java