MapContext

/**
 * The context that is given to the {@link Mapper}.
 * Mapper的上下文
 *
 * @param <KEYIN> the key input type to the Mapper
 * @param <VALUEIN> the value input type to the Mapper
 * @param <KEYOUT> the key output type from the Mapper
 * @param <VALUEOUT> the value output type from the Mapper
 */
@InterfaceAudience.Public
@InterfaceStability.Evolving
public interface MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> 
  extends TaskInputOutputContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {

  /**
   * Get the input split for this map.
   */
  public InputSplit getInputSplit();
  
}

MapContextImpl

MapContextImpl是MapContext接口的实现类。

/**
 * The context that is given to the {@link Mapper}.
 * @param <KEYIN> the key input type to the Mapper
 * @param <VALUEIN> the value input type to the Mapper
 * @param <KEYOUT> the key output type from the Mapper
 * @param <VALUEOUT> the value output type from the Mapper
 */
@InterfaceAudience.Private
@InterfaceStability.Unstable
public class MapContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT> 
    extends TaskInputOutputContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT> 
    implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {
  private RecordReader<KEYIN,VALUEIN> reader;
  private InputSplit split;

  /**
   * @param conf Hadoop Configuration
   * @param taskid
   * @param reader RecordReader
   * @param writer RecordWriter
   * @param committer
   * @param reporter
   * @param split InputSplit MapReduce数据输入分片，与FileInputFormat相关
   */
  public MapContextImpl(Configuration conf, TaskAttemptID taskid,
                        RecordReader<KEYIN,VALUEIN> reader,
                        RecordWriter<KEYOUT,VALUEOUT> writer,
                        OutputCommitter committer,
                        StatusReporter reporter,
                        InputSplit split) {
    super(conf, taskid, writer, committer, reporter);
    this.reader = reader;
    this.split = split;
  }

  /**
   * Get the input split for this map.
   */
  public InputSplit getInputSplit() {
    return split;
  }

  @Override
  public KEYIN getCurrentKey() throws IOException, InterruptedException {
    return reader.getCurrentKey();
  }

  @Override
  public VALUEIN getCurrentValue() throws IOException, InterruptedException {
    return reader.getCurrentValue();
  }

  @Override
  public boolean nextKeyValue() throws IOException, InterruptedException {
    return reader.nextKeyValue();
  }

}