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()        }    }}