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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