package org.apache.hadoop.fs;

 import java.io.BufferedInputStream;

 import java.io.IOException;

 /**

  * A class optimizes reading from FSInputStream by bufferring

  */

 //通过缓存优化FSInputStream读取

 public class BufferedFSInputStream extends BufferedInputStream

 implements Seekable, PositionedReadable {

 //两个接口在前面刚看过了，功能为...

   /**

    * Creates a <code>BufferedFSInputStream</code>

    * with the specified buffer size,

    * and saves its  argument, the input stream

    * <code>in</code>, for later use.  An internal

    * buffer array of length  <code>size</code>

    * is created and stored in <code>buf</code>.

    *

    * @param   in     the underlying input stream.

    * @param   size   the buffer size.

    * @exception IllegalArgumentException if size <= 0.

    */

   public BufferedFSInputStream(FSInputStream in, int size) {

     super(in, size);

   }

 //通过跟踪父类代码知道对接了输入流“管道”，初始化了一个大小为size的buffer

   public long getPos() throws IOException {

     return ((FSInputStream)in).getPos()-(count-pos);

   }

 //返回现在的偏移量

   public long skip(long n) throws IOException {

     if (n <= 0) {

       return 0;

     }

     seek(getPos()+n);

     return n;

   }

 //跳过n长度后得到现偏移量

   public void seek(long pos) throws IOException {

     if( pos<0 ) {

       return;

     }

     // optimize: check if the pos is in the buffer

     long end = ((FSInputStream)in).getPos();

     long start = end - count;

     if( pos>=start && pos<end) {

       this.pos = (int)(pos-start);

       return;

     }

     // invalidate buffer

     this.pos = 0;

     this.count = 0;

     ((FSInputStream)in).seek(pos);

   }

 //实现了Seekable的seek方法

   public boolean seekToNewSource(long targetPos) throws IOException {

     pos = 0;

     count = 0;

     return ((FSInputStream)in).seekToNewSource(targetPos);

   }

 //.....

   public int read(long position, byte[] buffer, int offset, int length) throws IOException {

     return ((FSInputStream)in).read(position, buffer, offset, length) ;

   }

   public void readFully(long position, byte[] buffer, int offset, int length) throws IOException {

     ((FSInputStream)in).readFully(position, buffer, offset, length);

   }

   public void readFully(long position, byte[] buffer) throws IOException {

     ((FSInputStream)in).readFully(position, buffer);

   }

 }