public List<InputSplit> getSplits(JobContext job) throws IOException {
　　 //获取块的最少大小
    long minSize = Math.max(getFormatMinSplitSize(), getMinSplitSize(job));
    //获取块的最大的size,默认是long.maxvalue
    long maxSize = getMaxSplitSize(job);

   
    List<InputSplit> splits = new ArrayList<InputSplit>();
    //获取输入文件的相关文件信息
    List<FileStatus> files = listStatus(job);
    for (FileStatus file: files) {
      Path path = file.getPath();
      long length = file.getLen();
      if (length != 0) {
        BlockLocation[] blkLocations;
        if (file instanceof LocatedFileStatus) {
          blkLocations = ((LocatedFileStatus) file).getBlockLocations();
        } else {
          FileSystem fs = path.getFileSystem(job.getConfiguration());
          blkLocations = fs.getFileBlockLocations(file, 0, length);
        }
        if (isSplitable(job, path)) {
          long blockSize = file.getBlockSize();
          long splitSize = computeSplitSize(blockSize, minSize, maxSize);

          long bytesRemaining = length;
　　　　　　//循环切分文件makeSplit（length-bytesRemaining//起始偏移量，splitSize//切片大小）， 
          /** 
           *如文件大小为500M，切分大小为128M,那么循环makeSplit 
           *1.makeSplit(128M,128M) 
           *2.makeSplit(256M,128M) 
           *3.makeSplit(384M,128M) 
           *循环切成3个InputSplit */
          while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
            int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);
            splits.add(makeSplit(path, length-bytesRemaining, splitSize,
                        blkLocations[blkIndex].getHosts(),
                        blkLocations[blkIndex].getCachedHosts()));
            bytesRemaining -= splitSize;
          }

          if (bytesRemaining != 0) {
            int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);
            splits.add(makeSplit(path, length-bytesRemaining, bytesRemaining,
                       blkLocations[blkIndex].getHosts(),
                       blkLocations[blkIndex].getCachedHosts()));
          }
        } else { // not splitable
          splits.add(makeSplit(path, 0, length, blkLocations[0].getHosts(),
                      blkLocations[0].getCachedHosts()));
        }
      } else { 
        //Create empty hosts array for zero length files
        splits.add(makeSplit(path, 0, length, new String[0]));
      }
    }
    // Save the number of input files for metrics/loadgen
    job.getConfiguration().setLong(NUM_INPUT_FILES, files.size());
    sw.stop();
    if (LOG.isDebugEnabled()) {
      LOG.debug("Total # of splits generated by getSplits: " + splits.size()
          + ", TimeTaken: " + sw.elapsedMillis());
    }
    return splits;
  }
               

public RecordReader<LongWritable, Text>     createRecordReader(InputSplit split,                       TaskAttemptContext context) {    String delimiter = context.getConfiguration().get(        "textinputformat.record.delimiter");    byte[] recordDelimiterBytes = null;    if (null != delimiter)      recordDelimiterBytes = delimiter.getBytes(Charsets.UTF_8);    return new LineRecordReader(recordDelimiterBytes);}public class LineRecordReader extends RecordReader<LongWritable, Text> {  //判断Inputsplit是否有下一行内容，并且读取这一行内容  public boolean nextKeyValue() throws IOException {    if (key == null) {      key = new LongWritable();    }    key.set(pos);    if (value =http://www.mamicode.com/= null) {"Skipped line of size " + newSize + " at pos " +                (pos - newSize));    }    if (newSize == 0) {      key = null;      value = http://www.mamicode.com/null;>

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {  。。。。  public abstract class Context    implements MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {  }  public void run(Context context) throws IOException, InterruptedException {    setup(context);    try {      //循序读取Inputsplit每一行的内容，每读一行调用一次map函数      while (context.nextKeyValue()) {        map(context.getCurrentKey(), context.getCurrentValue(), context);      }    } finally {      当完成读取一个Inputsplit时，就调用这个cleanup函数      cleanup(context);    }  }  。。。。}public class MapContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT>     extends TaskInputOutputContextImpl<KEYIN,VALUEIN,KEYOUT,VALUEOUT>     implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {    private RecordReader<KEYIN,VALUEIN> reader;    public KEYIN getCurrentKey() throws IOException, InterruptedException {      return reader.getCurrentKey();    }  public VALUEIN getCurrentValue() throws IOException, InterruptedException {    return reader.getCurrentValue();  }  public boolean nextKeyValue() throws IOException, InterruptedException {    return reader.nextKeyValue();  }}context.nextKeyValue() 就是 reader.nextKeyValue();