Java-IO之BufferedInputStream（缓冲输入流）

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()
        }
    }
}