Every custom format reading keys of type K and values of type V must subclass InputFormat<K, V>. Usually it is easier to subclass FileInputFormat<K, V> – the file listing and splitting is then solved by the FileInputFormat itself.

We start by creating FileAsPathInputFormat, which reads any file, splits it and for each split return exactly one input pair (file_path, start-length) with types (Text, Text), where file_path is path to the file and start-length is a string containing two dash-separated numbers: start offset of the split and length of the split.

public static class FileAsPathInputFormat extends FileInputFormat<Text, Text> {
  public static class FileAsPathRecordReader extends RecordReader<Text, Text> {
    private Path file;
    long start, length;
    private Text key, value;
 
    public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException {
      FileSplit split = (FileSplit) genericSplit;
      file = split.getPath();
      start = split.getStart();
      length = split.getLength();
      key = null;   
      value = null; 
    }               
    public boolean nextKeyValue() throws IOException {
      if (key != null) return false;
 
      key = new Text(file.toString());
      value = new Text(String.format("%d-%d", start, length));
 
      return true;  
    }               
 
    public Text getCurrentKey() { return key; }
    public Text getCurrentValue() { return value; }
    public float getProgress() { return (key == null) ? 0 : 1; }
    public synchronized void close() throws IOException {}
  }
 
  public RecordReader<Text, Text> createRecordReader(InputSplit split, TaskAttemptContext context) {
    return new FileAsPathRecordReader();
  }   
 
  protected boolean isSplittable(JobContext context, Path filename) {
    CompressionCodec codec = new CompressionCodecFactory(context.getConfiguration()).getCodec(filename);
    return codec == null;
  }
}

We start by creating WholeFileInputFormat, which reads any file and return exactly one input pair (input_path, file_content) with types (Text, BytesWritable). The format does not allow file splitting – each file will be processed by exactly one mapper.

The main functionality lays in WholeFileRecordReader, a subclass of RecordReader<Text, BytesWritable.

public class WholeFileInputFormat extends FileInputFormat<Text, BytesWritable> {
  // Helper class, which does the actual work -- reads the (path, content) input pair.
  public static class WholeFileRecordReader extends RecordReader<Text, BytesWritable> {
    private Path file;
    int length;
    private boolean value_read;
    private Text key;
    private BytesWritable value;
    DataInputStream in;
 
    public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException {
      FileSplit split = (FileSplit) genericSplit;
      file = split.getPath();
      length = (int) split.getLength();
      key = null;
      value = null;
      value_read = false;
 
      FileSystem fs = file.getFileSystem(context.getConfiguration());
      in = fs.open(split.getPath());
 
      CompressionCodecFactory compressionCodecs = new CompressionCodecFactory(context.getConfiguration());
      CompressionCodec codec = compressionCodecs.getCodec(file);
      if (codec != null)
        in = new DataInputStream(codec.createInputStream(in));
    }
 
    public boolean nextKeyValue() throws IOException {
      if (value_read) return false;
 
      byte[] data = new byte[length];
      in.readFully(data);
 
      key = new Text(file.toString());
      value = new BytesWritable(data);
      value_read = true;
 
      return true;
    }
 
    public Text getCurrentKey() { return key; }
    public BytesWritable getCurrentValue() { return value; }
    public float getProgress() { return value_read ? 0 : 1; }
    public synchronized void close() throws IOException { if (in != null) { in.close(); in = null; } }
  }
 
  // Use the helper class as a RecordReader in out file format.
  public RecordReader<Text, BytesWritable> createRecordReader(InputSplit split, TaskAttemptContext context) {
    return new WholeFileRecordReader();
  }
 
  // Do not allow splitting.
  protected boolean isSplittable(JobContext context, Path filename) {
    return false;
  }
}