BufferedInputStream(InputStream in) 
BufferedInputStream(InputStream in, int size) 
synchronized int available() 
void close() 
synchronized void mark(int readlimit) 
boolean markSupported() 
synchronized int read() 
synchronized int read(byte[] buffer, int offset, int byteCount) 
synchronized void reset() 
synchronized long skip(long byteCount) 

public class BufferedInputStreamTest { 
 
  private static final int LEN = 5; 
 
  public static void main(String[] args) { 
    testBufferedInputStream() ; 
  } 
  private static void testBufferedInputStream() { 
 
    // 创建BufferedInputStream字节流，内容是ArrayLetters数组 
    try { 
      File file = new File("file.txt"); 
      InputStream in =new BufferedInputStream(new FileInputStream(file), 512); 
 
      // 从字节流中读取5个字节。“abcde”，a对应0x61，b对应0x62，依次类推... 
      for (int i=0; i= 0) { 
        // 读取“字节流的下一个字节” 
        int tmp = in.read(); 
        System.out.printf("%d : 0x%s\n", i, Integer.toHexString(tmp)); 
        } 
      } 
 
      // 若“该字节流”不支持标记功能，则直接退出 
      if (!in.markSupported()) { 
        System.out.println("make not supported!"); 
        return ; 
      } 
 
      // 标记“当前索引位置”，即标记第6个位置的元素--“f” 
      // 1024对应marklimit 
      in.mark(1024); 
 
      // 跳过22个字节。 
      in.skip(22); 
 
      // 读取5个字节 
      byte[] buf = new byte[LEN]; 
      in.read(buf, 0, LEN); 
      // 将buf转换为String字符串。 
      String str1 = new String(buf); 
      System.out.printf("str1=%s\n", str1); 
 
      // 重置“输入流的索引”为mark()所标记的位置，即重置到“f”处。 
      in.reset(); 
      // 从“重置后的字节流”中读取5个字节到buf中。即读取“fghij” 
      in.read(buf, 0, LEN); 
      // 将buf转换为String字符串。 
      String str2 = new String(buf); 
      System.out.printf("str2=%s\n", str2); 
 
      in.close(); 
    } catch (FileNotFoundException e) { 
      e.printStackTrace(); 
    } catch (SecurityException e) { 
      e.printStackTrace(); 
    } catch (IOException e) { 
      e.printStackTrace(); 
    } 
  } 
} 

0 : 0x61
1 : 0x62
2 : 0x63
3 : 0x64
4 : 0x65
str1=12345
str2=fghij

public class BufferedInputStream extends FilterInputStream { 
 
  private static int DEFAULT_BUFFER_SIZE = 8192;//默认缓冲区大小为8X1024 
  private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8; 
  protected volatile byte buf[]; //缓冲数组 
 
  /** 
   * Atomic updater to provide compareAndSet for buf. This is 
   * necessary because closes can be asynchronous. We use nullness 
   * of buf[] as primary indicator that this stream is closed. (The 
   * "in" field is also nulled out on close.) 
   */ 
  private static final AtomicReferenceFieldUpdater bufUpdater = 
      AtomicReferenceFieldUpdater.newUpdater(BufferedInputStream.class, byte[].class, "buf"); 
  //值在0到buf.len之间 
  protected int count; 
  //在buffer的当前位置，下一个字符被读取 
  protected int pos; 
  //值为mark函数最近被调用的时候，值为-1到pos 
  protected int markpos = -1; 
 
  /** 
   * The maximum read ahead allowed after a call to the 
   * mark method before subsequent calls to the 
   * reset method fail. 
   * Whenever the difference between pos 
   * and markpos exceeds marklimit, 
   * then the mark may be dropped by setting 
   * markpos to -1. 
   * 
   * @see   java.io.BufferedInputStream#mark(int) 
   * @see   java.io.BufferedInputStream#reset() 
   */ 
 
  protected int marklimit; 
 
  /** 
   * Check to make sure that underlying input stream has not been 
   * nulled out due to close; if not return it; 
   */ 
  // 
  private InputStream getInIfOpen() throws IOException { 
    InputStream input = in; 
    if (input == null) 
      throw new IOException("Stream closed"); 
    return input; 
  } 
 
  /** 
   * Check to make sure that buffer has not been nulled out due to 
   * close; if not return it; 
   */ 
  private byte[] getBufIfOpen() throws IOException { 
    byte[] buffer = buf; 
    if (buffer == null) 
      throw new IOException("Stream closed"); 
    return buffer; 
  } 
 
  /** 
   * Creates a BufferedInputStream 
   * and saves its argument, the input stream 
   * in, for later use. An internal 
   * buffer array is created and stored in buf. 
   * 
   * @param  in  the underlying input stream. 
   */ 
  //带InputStream的构造函数 
  public BufferedInputStream(InputStream in) { 
      this(in, DEFAULT_BUFFER_SIZE); 
  } 
 
  /** 
   * Creates a BufferedInputStream 
   * with the specified buffer size, 
   * and saves its argument, the input stream 
   * in, for later use. An internal 
   * buffer array of length size 
   * is created and stored in buf. 
   * 
   * @param  in   the underlying input stream. 
   * @param  size  the buffer size. 
   * @exception IllegalArgumentException if {@code size <= 0}. 
   */ 
  //带InputStream和大小的构造函数 
  public BufferedInputStream(InputStream in, int size) { 
    super(in); 
    if (size <= 0) { 
        throw new IllegalArgumentException("Buffer size <= 0"); 
    } 
    buf = new byte[size]; 
  } 
 
  /** 
   * Fills the buffer with more data, taking into account 
   * shuffling and other tricks for dealing with marks. 
   * Assumes that it is being called by a synchronized method. 
   * This method also assumes that all data has already been read in, 
   * hence pos > count. 
   */ 
  // 
  private void fill() throws IOException { 
    byte[] buffer = getBufIfOpen(); 
    if (markpos < 0) 
      pos = 0;      /* no mark: throw away the buffer */ 
    else if (pos >= buffer.length) /* no room left in buffer */ 
      if (markpos > 0) { /* can throw away early part of the buffer */ 
        int sz = pos - markpos; 
        System.arraycopy(buffer, markpos, buffer, 0, sz); 
        pos = sz; 
        markpos = 0; 
      } else if (buffer.length >= marklimit) { 
         markpos = -1;  /* buffer got too big, invalidate mark */ 
         pos = 0;    /* drop buffer contents */ 
      } else if (buffer.length >= MAX_BUFFER_SIZE) { 
        throw new OutOfMemoryError("Required array size too large"); 
      } else {      /* grow buffer */ 
        int nsz = (pos <= MAX_BUFFER_SIZE - pos) ? 
        pos * 2 : MAX_BUFFER_SIZE; 
        if (nsz > marklimit) 
          nsz = marklimit; 
        byte nbuf[] = new byte[nsz]; 
        System.arraycopy(buffer, 0, nbuf, 0, pos); 
        if (!bufUpdater.compareAndSet(this, buffer, nbuf)) { 
          // Can't replace buf if there was an async close. 
          // Note: This would need to be changed if fill() 
          // is ever made accessible to multiple threads. 
          // But for now, the only way CAS can fail is via close. 
          // assert buf == null; 
          throw new IOException("Stream closed"); 
        } 
        buffer = nbuf; 
      } 
    count = pos; 
    int n = getInIfOpen().read(buffer, pos, buffer.length - pos); 
    if (n > 0) 
      count = n + pos; 
    } 
 
  /** 
   * See 
   * the general contract of the read 
   * method of InputStream. 
   * 
   * @return   the next byte of data, or -1 if the end of the 
   *       stream is reached. 
   * @exception IOException if this input stream has been closed by 
   *             invoking its {@link #close()} method, 
   *             or an I/O error occurs. 
   * @see    java.io.FilterInputStream#in 
   */ 
  //读下一个字节，没有数据返回-1 
  public synchronized int read() throws IOException { 
    if (pos >= count) { 
      fill(); 
      if (pos >= count) 
        return -1; 
    } 
    return getBufIfOpen()[pos++] & 0xff; 
  } 
 
  /** 
   * Read characters into a portion of an array, reading from the underlying 
   * stream at most once if necessary. 
   */ 
  private int read1(byte[] b, int off, int len) throws IOException { 
    int avail = count - pos; 
    if (avail <= 0) { 
      /* If the requested length is at least as large as the buffer, and 
        if there is no mark/reset activity, do not bother to copy the 
        bytes into the local buffer. In this way buffered streams will 
        cascade harmlessly. */ 
      if (len >= getBufIfOpen().length && markpos < 0) { 
        return getInIfOpen().read(b, off, len); 
      } 
      fill(); 
      avail = count - pos; 
      if (avail <= 0) return -1; 
    } 
    int cnt = (avail < len) ? avail : len; 
    System.arraycopy(getBufIfOpen(), pos, b, off, cnt); 
    pos += cnt; 
    return cnt; 
  } 
 
  /** 
   * Reads bytes from this byte-input stream into the specified byte array, 
   * starting at the given offset. 
   * 
   * 
 This method implements the general contract of the corresponding 
   * {@link InputStream#read(byte[], int, int) read} method of 
   * the {@link InputStream} class. As an additional 
   * convenience, it attempts to read as many bytes as possible by repeatedly 
   * invoking the read method of the underlying stream. This 
   * iterated read continues until one of the following 
   * conditions becomes true: 
 
   * 
   *  
 The specified number of bytes have been read, 
   * 
   *  
 The read method of the underlying stream returns 
   *  -1, indicating end-of-file, or 
   * 
   *  
 The available method of the underlying stream 
   *  returns zero, indicating that further input requests would block. 
   * 
   * 
 If the first read on the underlying stream returns 
   * -1 to indicate end-of-file then this method returns 
   * -1. Otherwise this method returns the number of bytes 
   * actually read. 
   * 
   * 
 Subclasses of this class are encouraged, but not required, to 
   * attempt to read as many bytes as possible in the same fashion. 
   * 
   * @param   b   destination buffer. 
   * @param   off  offset at which to start storing bytes. 
   * @param   len  maximum number of bytes to read. 
   * @return   the number of bytes read, or -1 if the end of 
   *       the stream has been reached. 
   * @exception IOException if this input stream has been closed by 
   *             invoking its {@link #close()} method, 
   *             or an I/O error occurs. 
   */ 
  // 
  public synchronized int read(byte b[], int off, int len)throws IOException 
  { 
    getBufIfOpen(); // Check for closed stream 
    if ((off | len | (off + len) | (b.length - (off + len))) < 0) { 
      throw new IndexOutOfBoundsException(); 
    } else if (len == 0) { 
      return 0; 
    } 
 
    int n = 0; 
    for (;;) { 
      int nread = read1(b, off + n, len - n); 
      if (nread <= 0) 
        return (n == 0) ? nread : n; 
      n += nread; 
      if (n >= len) 
        return n; 
      // if not closed but no bytes available, return 
      InputStream input = in; 
      if (input != null && input.available() <= 0) 
        return n; 
    } 
  } 
 
  /** 
   * See the general contract of the skip 
   * method of InputStream. 
   * 
   * @exception IOException if the stream does not support seek, 
   *             or if this input stream has been closed by 
   *             invoking its {@link #close()} method, or an 
   *             I/O error occurs. 
   */ 
  //跳过n长的数据 
  public synchronized long skip(long n) throws IOException { 
    getBufIfOpen(); // Check for closed stream 
    if (n <= 0) { 
      return 0; 
    } 
    long avail = count - pos; 
 
    if (avail <= 0) { 
      // If no mark position set then don't keep in buffer 
      if (markpos <0) 
        return getInIfOpen().skip(n); 
 
      // Fill in buffer to save bytes for reset 
      fill(); 
      avail = count - pos; 
      if (avail <= 0) 
        return 0; 
    } 
 
    long skipped = (avail < n) ? avail : n; 
    pos += skipped; 
    return skipped; 
  } 
 
  /** 
   * Returns an estimate of the number of bytes that can be read (or 
   * skipped over) from this input stream without blocking by the next 
   * invocation of a method for this input stream. The next invocation might be 
   * the same thread or another thread. A single read or skip of this 
   * many bytes will not block, but may read or skip fewer bytes. 
   * 
 
   * This method returns the sum of the number of bytes remaining to be read in 
   * the buffer (count - pos) and the result of calling the 
   * {@link java.io.FilterInputStream#in in}.available(). 
   * 
   * @return   an estimate of the number of bytes that can be read (or skipped 
   *       over) from this input stream without blocking. 
   * @exception IOException if this input stream has been closed by 
   *             invoking its {@link #close()} method, 
   *             or an I/O error occurs. 
   */ 
  //返回还有多少数据可以读 
  public synchronized int available() throws IOException { 
    int n = count - pos; 
    int avail = getInIfOpen().available(); 
    return n > (Integer.MAX_VALUE - avail)? Integer.MAX_VALUE: n + avail; 
  } 
 
  /** 
   * See the general contract of the mark 
   * method of InputStream. 
   * 
   * @param  readlimit  the maximum limit of bytes that can be read before 
   *           the mark position becomes invalid. 
   * @see   java.io.BufferedInputStream#reset() 
   */ 
  public synchronized void mark(int readlimit) { 
    marklimit = readlimit; 
    markpos = pos; 
  } 
 
  /** 
   * See the general contract of the reset 
   * method of InputStream. 
   * 
 
   * If markpos is -1 
   * (no mark has been set or the mark has been 
   * invalidated), an IOException 
   * is thrown. Otherwise, pos is 
   * set equal to markpos. 
   * 
   * @exception IOException if this stream has not been marked or, 
   *         if the mark has been invalidated, or the stream 
   *         has been closed by invoking its {@link #close()} 
   *         method, or an I/O error occurs. 
   * @see    java.io.BufferedInputStream#mark(int) 
   */ 
  public synchronized void reset() throws IOException { 
    getBufIfOpen(); // Cause exception if closed 
    if (markpos < 0) 
      throw new IOException("Resetting to invalid mark"); 
    pos = markpos; 
  } 
 
  /** 
   * Tests if this input stream supports the mark 
   * and reset methods. The markSupported 
   * method of BufferedInputStream returns 
   * true. 
   * 
   * @return a boolean indicating if this stream type supports 
   *     the mark and reset methods. 
   * @see   java.io.InputStream#mark(int) 
   * @see   java.io.InputStream#reset() 
   */ 
  //是否支持标记 
  public boolean markSupported() { 
    return true; 
  } 
 
  /** 
   * Closes this input stream and releases any system resources 
   * associated with the stream. 
   * Once the stream has been closed, further read(), available(), reset(), 
   * or skip() invocations will throw an IOException. 
   * Closing a previously closed stream has no effect. 
   * 
   * @exception IOException if an I/O error occurs. 
   */ 
  //关闭资源  
  public void close() throws IOException { 
    byte[] buffer; 
    while ( (buffer = buf) != null) { 
      if (bufUpdater.compareAndSet(this, buffer, null)) { 
        InputStream input = in; 
        in = null; 
        if (input != null) 
          input.close(); 
        return; 
      } 
      // Else retry in case a new buf was CASed in fill() 
    } 
  } 
}