Java IO学习之缓冲输入流（BufferedInputStream）

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<len; i++) { 
      // 若能继续读取下一个字节，则读取下一个字节 
        if (in.available() >= 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<bufferedinputstream, byte[]> 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 
   * <code>mark</code> method before subsequent calls to the 
   * <code>reset</code> method fail. 
   * whenever the difference between <code>pos</code> 
   * and <code>markpos</code> exceeds <code>marklimit</code>, 
   * then the mark may be dropped by setting 
   * <code>markpos</code> to <code>-1</code>. 
   * 
   * @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 <code>bufferedinputstream</code> 
   * and saves its argument, the input stream 
   * <code>in</code>, for later use. an internal 
   * buffer array is created and stored in <code>buf</code>. 
   * 
   * @param  in  the underlying input stream. 
   */ 
  //带inputstream的构造函数 
  public bufferedinputstream(inputstream in) { 
      this(in, default_buffer_size); 
  } 
 
  /** 
   * creates a <code>bufferedinputstream</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 {@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 <code>read</code> 
   * method of <code>inputstream</code>. 
   * 
   * @return   the next byte of data, or <code>-1</code> 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. 
   * 
   * <p> this method implements the general contract of the corresponding 
   * <code>{@link inputstream#read(byte[], int, int) read}</code> method of 
   * the <code>{@link inputstream}</code> class. as an additional 
   * convenience, it attempts to read as many bytes as possible by repeatedly 
   * invoking the <code>read</code> method of the underlying stream. this 
   * iterated <code>read</code> continues until one of the following 
   * conditions becomes true: <ul> 
   * 
   *  <li> the specified number of bytes have been read, 
   * 
   *  <li> the <code>read</code> method of the underlying stream returns 
   *  <code>-1</code>, indicating end-of-file, or 
   * 
   *  <li> the <code>available</code> method of the underlying stream 
   *  returns zero, indicating that further input requests would block. 
   * 
   * </ul> if the first <code>read</code> on the underlying stream returns 
   * <code>-1</code> to indicate end-of-file then this method returns 
   * <code>-1</code>. otherwise this method returns the number of bytes 
   * actually read. 
   * 
   * <p> 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 <code>-1</code> 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 <code>skip</code> 
   * method of <code>inputstream</code>. 
   * 
   * @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. 
   * <p> 
   * this method returns the sum of the number of bytes remaining to be read in 
   * the buffer (<code>count - pos</code>) 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 <code>mark</code> 
   * method of <code>inputstream</code>. 
   * 
   * @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 <code>reset</code> 
   * method of <code>inputstream</code>. 
   * <p> 
   * if <code>markpos</code> is <code>-1</code> 
   * (no mark has been set or the mark has been 
   * invalidated), an <code>ioexception</code> 
   * is thrown. otherwise, <code>pos</code> is 
   * set equal to <code>markpos</code>. 
   * 
   * @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 <code>mark</code> 
   * and <code>reset</code> methods. the <code>marksupported</code> 
   * method of <code>bufferedinputstream</code> returns 
   * <code>true</code>. 
   * 
   * @return a <code>boolean</code> indicating if this stream type supports 
   *     the <code>mark</code> and <code>reset</code> 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() 
    } 
  } 
}