hadoop SequenceFileRecordReader源代码

package org.apache.hadoop.mapred;

import java.io.IOException;


import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.*;
import org.apache.hadoop.util.ReflectionUtils;

/** An {@link RecordReader} for {@link SequenceFile}s. */
@InterfaceAudience.Public
@InterfaceStability.Stable
/**
*hadoop中一个读取文件中某个片段（由其内部的start、length决定片段）的类，继承自RecordReader，可以作为InputFomat中 public *RecordReader<K, V> getRecordReader(InputSplit split,JobConf job, Reporter reporter)方法的返回的结果，处理FileSplit 。
*  K V对应map中K V，其每次取得K V，要作为map的输入来做的
**/
public class SequenceFileRecordReader<K, V> implements RecordReader<K, V> {
  
  private SequenceFile.Reader in;   //读取的Reader
  private long start;               //文件在Reader的开始位置
  private long end;                 //结束位置，一个hdfs文件在Reader中是连续的？
  private boolean more = true;
  protected Configuration conf;

  public SequenceFileRecordReader(Configuration conf, FileSplit split)
    throws IOException {
    Path path = split.getPath();
    FileSystem fs = path.getFileSystem(conf);
    this.in = new SequenceFile.Reader(fs, path, conf);
    this.end = split.getStart() + split.getLength();
    this.conf = conf;

    if (split.getStart() > in.getPosition())
      in.sync(split.getStart());                  // sync to start

    this.start = in.getPosition();    // 一个hdfs的开始位置，但不是在Reader中的开始位置，是以取Reader的开始位置  ？？
    more = start < end;
  }


  /** The class of key that must be passed to {@link
   * #next(Object, Object)}.. */
  public Class getKeyClass() { return in.getKeyClass(); }

  /** The class of value that must be passed to {@link
   * #next(Object, Object)}.. */
  public Class getValueClass() { return in.getValueClass(); }
  /**
  *每次调用next(K key, V value)方法需要传入相应的K 和 V 对象，是以以下像个方法构造K 、V的实例，
  *以便调用next(K key, V value)，传给map方法
  *
  */
  @SuppressWarnings("unchecked")
  public K createKey() {
    return (K) ReflectionUtils.newInstance(getKeyClass(), conf);
  }
  
  @SuppressWarnings("unchecked")
  public V createValue() {
    return (V) ReflectionUtils.newInstance(getValueClass(), conf);
  }
   /**
   *取值的函数
   */
  public synchronized boolean next(K key, V value) throws IOException {
    if (!more) return false;
    long pos = in.getPosition();
    boolean remaining = (in.next(key) != null);
    if (remaining) {
      getCurrentValue(value);
    }
    if (pos >= end && in.syncSeen()) {
      more = false;
    } else {
      more = remaining;
    }
    return more;
  }
  /**
   *用于跳过某个key 
   */
  protected synchronized boolean next(K key)
    throws IOException {
    if (!more) return false;
    long pos = in.getPosition();
    boolean remaining = (in.next(key) != null);
    if (pos >= end && in.syncSeen()) {
      more = false;
    } else {
      more = remaining;
    }
    return more;
  }
  
  protected synchronized void getCurrentValue(V value)
    throws IOException {
    in.getCurrentValue(value);
  }
  
  /**
   * Return the progress within the input split
   * @return 0.0 to 1.0 of the input byte range
   */
  public float getProgress() throws IOException {
    if (end == start) {
      return 0.0f;
    } else {
      return Math.min(1.0f, (in.getPosition() - start) / (float)(end - start));
    }
  }
  
  public synchronized long getPos() throws IOException {
    return in.getPosition();
  }
  
  protected synchronized void seek(long pos) throws IOException {
    in.seek(pos);
  }
  public synchronized void close() throws IOException { in.close(); }
  
}

  hadoop中一个读取文件中某个片段（由其内部的start、length决定片段）的类，继承自RecordReader，可以作为InputFomat中 public RecordReader<K, V> getRecordReader(InputSplit split,JobConf job, Reporter reporter)方法的返回的结果，处理FileSplit 。

可以读取FileSplit中的key、Value对，以便map可以使用 。