https://apache.googlesource.com/hadoop-common/+/trunk/hadoop-mapreduce-project/hadoop-mapreduce-examples/src/main/java/org/apache/hadoop/examples/MultiFileWordCount.java
private void init() throws IOException { if (reader == null) { FSDataInputStream in = fs.open(file); in.seek(segmentOffset); reader = new Reader<K, V>(conf, in, segmentLength, codec); } }