Java IO学习之缓冲输入流（BufferedInputStream）

Java IO  BufferedInputStream

概要：

BufferedInputStream是缓冲输入流，继承于FilterInputStream,作用是为另一个输入流添加一些功能，本质上是通过一个内部缓冲数组实现的。例如，在新建某输入流对应的BufferedInputStream后，当通过read()读取数据时，BufferedInputStream会将输入流的数据分批的填入到缓冲区中，每当缓冲区的数据读完之后，输入流会再次填充数据缓冲区，直到读完数据。

BufferedInputStream主要的函数列表：

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)

示例代码：

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();
    }
  }
}

运行结果：

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

基于JDK8的BufferInputStream代码：

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()
    }
  }
}

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