hadoop InputFormat getSplits

 /** Splits files returned by {@link #listStatus(JobConf)} when
   * they‘re too big.*/ 
  public InputSplit[] getSplits(JobConf job, int numSplits)
    throws IOException {
    //计时器，
    StopWatch sw = new StopWatch().start();
    //
    FileStatus[] files = listStatus(job);
    
    // Save the number of input files for metrics/loadgen
    //设置配置中文件个数mapreduce.input.fileinputformat.numinputfiles
    job.setLong(NUM_INPUT_FILES, files.length);
    // 计算所有文件的大小总和
    long totalSize = 0;                           // compute total size
    for (FileStatus file: files) {                // check we have valid files
      if (file.isDirectory()) {
        throw new IOException("Not a file: "+ file.getPath());
      }
      totalSize += file.getLen();
    }
    // 每个split目标大小，用总的文件大小 / （max（设置的split个数，1）），
    long goalSize = totalSize / (numSplits == 0 ? 1 : numSplits);
    // 每个split大小的最小值，读取mapreduce.input.fileinputformat.split.minsize配置，如果没有配置的话那么
    // 取minSplitSize =1
    long minSize = Math.max(job.getLong(org.apache.hadoop.mapreduce.lib.input.
      FileInputFormat.SPLIT_MINSIZE, 1), minSplitSize);

    // 生成 splits
    ArrayList<FileSplit> splits = new ArrayList<FileSplit>(numSplits);
    NetworkTopology clusterMap = new NetworkTopology();
    //遍历文件列表
    for (FileStatus file: files) {
      //获取一个文件路径
      Path path = file.getPath();
      //获取文件大小
      long length = file.getLen();
      if (length != 0) {
        FileSystem fs = path.getFileSystem(job);
        BlockLocation[] blkLocations;
        //判断file是否包含file的location，也就是，是否包含BlockLocation等信息，
        if (file instanceof LocatedFileStatus) {
          blkLocations = ((LocatedFileStatus) file).getBlockLocations();
        } else {
          //去构造BlockLocation信息
          blkLocations = fs.getFileBlockLocations(file, 0, length);
        }
        //判断文件是否可以切分
        if (isSplitable(fs, path)) {
          //获取文件的BlockSize大小
          long blockSize = file.getBlockSize();
          //splitSize最终由 goalSize（设置的每个split大小的目标值），minSize（设置的每个split大小的最小值），blockSize（file的block数量）三个值所决定，逻辑关系如下：
          // Math.max(minSize, Math.min(goalSize, blockSize))
          // Math.max(minSize, Math.min((totalSize / (numSplits == 0 ? 1 : numSplits)), blockSize))
          // numSplits这个设置，只有在totalSize/numSplits < blockSize才会生效
          // minSize 只有在大于blockSize的时候才会生效
          long splitSize = computeSplitSize(goalSize, minSize, blockSize);
          //文件为读取长度
          long bytesRemaining = length;
          //如果剩余的大小/split的大小大雨1.1，那么就商城生成一个split
          while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
            String[][] splitHosts = getSplitHostsAndCachedHosts(blkLocations,
                length-bytesRemaining, splitSize, clusterMap);
            splits.add(makeSplit(path, length-bytesRemaining, splitSize,
                splitHosts[0], splitHosts[1]));
            bytesRemaining -= splitSize;
          }
          //剩余的一点点数据也要生成一个split，
          if (bytesRemaining != 0) {
            String[][] splitHosts = getSplitHostsAndCachedHosts(blkLocations, length
                - bytesRemaining, bytesRemaining, clusterMap);
            splits.add(makeSplit(path, length - bytesRemaining, bytesRemaining,
                splitHosts[0], splitHosts[1]));
          }
        } else {
          String[][] splitHosts = getSplitHostsAndCachedHosts(blkLocations,0,length,clusterMap);
          splits.add(makeSplit(path, 0, length, splitHosts[0], splitHosts[1]));
        }
      } else { 
        //Create empty hosts array for zero length files
        splits.add(makeSplit(path, 0, length, new String[0]));
      }
    }
    sw.stop();
    if (LOG.isDebugEnabled()) {
      LOG.debug("Total # of splits generated by getSplits: " + splits.size()
          + ", TimeTaken: " + sw.now(TimeUnit.MILLISECONDS));
    }
    return splits.toArray(new FileSplit[splits.size()]);
  }