org.apache.hadoop.fs-BufferedFSInputStream

 1 package org.apache.hadoop.fs;
 2 
 3 import java.io.BufferedInputStream;
 4 import java.io.IOException;
 5 
 6 
 7 /**
 8  * A class optimizes reading from FSInputStream by bufferring
 9  */
10 //通过缓存优化FSInputStream读取
11 
12 public class BufferedFSInputStream extends BufferedInputStream
13 implements Seekable, PositionedReadable {
14 //两个接口在前面刚看过了，功能为...
15   /**
16    * Creates a <code>BufferedFSInputStream</code>
17    * with the specified buffer size,
18    * and saves its  argument, the input stream
19    * <code>in</code>, for later use.  An internal
20    * buffer array of length  <code>size</code>
21    * is created and stored in <code>buf</code>.
22    *
23    * @param   in     the underlying input stream.
24    * @param   size   the buffer size.
25    * @exception IllegalArgumentException if size <= 0.
26    */
27   public BufferedFSInputStream(FSInputStream in, int size) {
28     super(in, size);
29   }
30 //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
31   public long getPos() throws IOException {
32     return ((FSInputStream)in).getPos()-(count-pos);
33   }
34 //返回现在的偏移量
35   public long skip(long n) throws IOException {
36     if (n <= 0) {
37       return 0;
38     }
39 
40     seek(getPos()+n);
41     return n;
42   }
43 //跳过n长度后得到现偏移量
44   public void seek(long pos) throws IOException {
45     if( pos<0 ) {
46       return;
47     }
48     // optimize: check if the pos is in the buffer
49     long end = ((FSInputStream)in).getPos();
50     long start = end - count;
51     if( pos>=start && pos<end) {
52       this.pos = (int)(pos-start);
53       return;
54     }
55 
56     // invalidate buffer
57     this.pos = 0;
58     this.count = 0;
59 
60     ((FSInputStream)in).seek(pos);
61   }
62 //实现了Seekable的seek方法
63   public boolean seekToNewSource(long targetPos) throws IOException {
64     pos = 0;
65     count = 0;
66     return ((FSInputStream)in).seekToNewSource(targetPos);
67   }
68 //.....
69   public int read(long position, byte[] buffer, int offset, int length) throws IOException {
70     return ((FSInputStream)in).read(position, buffer, offset, length) ;
71   }
72 
73   public void readFully(long position, byte[] buffer, int offset, int length) throws IOException {
74     ((FSInputStream)in).readFully(position, buffer, offset, length);
75   }
76 
77   public void readFully(long position, byte[] buffer) throws IOException {
78     ((FSInputStream)in).readFully(position, buffer);
79   }
80 }