本來(lái)要先看BufferedFSInputStream的，但是它實(shí)現(xiàn)了Seekable和PositionedReadable接口，就先看這兩個(gè)，再看它會(huì)比較容易理解些

 1 package org.apache.hadoop.fs; 2  3 import java.io.*; 4  5 /** Stream that permits seeking. */ 6 //提供按位置查找功能的接口 7 public interface Seekable { 8   /** 9    * Seek to the given offset from the start of the file.10    * The next read() will be from that location.  Can't11    * seek past the end of the file.12    */13   void seek(long pos) throws IOException;14   //從指定文件中的位置pos，對(duì)文件流進(jìn)行前向搜索。15   /**16    * Return the current offset from the start of the file17    */18   long getPos() throws IOException;19 //返回文件流中當(dāng)前偏移位置。 20   /**21    * Seeks a different copy of the data.  Returns true if 22    * found a new source, false otherwise.23    */24   boolean seekToNewSource(long targetPos) throws IOException;25   //從targetPos位置搜索文件數(shù)據(jù)的一個(gè)不同拷貝，搜索到則返回true，否則返回false。26 }