/** 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()]);

  }