Java IO流 BufferedInputStream、BufferedOutputStream的基本使用

　　　　　　　　BufferedInputStream、BufferedOutputStream的基本使用

　　BufferedInputStream是FilterInputStream流的子类，FilterInputStream又是InputStream流的子类。一个BufferedInputStream为另一个输入流添加了功能，即缓冲输入和支持mark和reset方法的功能。当BufferedInputStream创建时，就会创建一个内部缓冲区数组。当从流中读取或跳过字节时，内部缓冲区将根据需要从所包含的输入流中重新填充，一次有多个字节。mark操作会记住输入流中的一点，并且reset

操作会导致从最近的mark操作之后读取的所有字节在从包含的输入流中取出新的字节之前重新读取。

　　以下例子比较了FileInputStream和BufferedInputStream读取文件的效率，发现BufferedInputStream的效率还是要高很多，虽然两者都是一个字节一个字节的进行读取，但BufferedInputStream因为有一个内部缓冲数组。所以当它第一次从磁盘读取时，它就会突然从磁盘中读取很多字节进入缓冲区，然后再从缓冲区里面进行一个字节一个字节读取。经测试，它的读取效率比FileInputStream读取效率更高、读取数据所用的时间更少。FileInputStream的使用可参考https://www.cnblogs.com/jhtian/p/14110083.html

测试中的文件如下：

/*
 * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.io;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

/**
 * A <code>BufferedInputStream</code> adds
 * functionality to another input stream-namely,
 * the ability to buffer the input and to
 * support the <code>mark</code> and <code>reset</code>
 * methods. When  the <code>BufferedInputStream</code>
 * is created, an internal buffer array is
 * created. As bytes  from the stream are read
 * or skipped, the internal buffer is refilled
 * as necessary  from the contained input stream,
 * many bytes at a time. The <code>mark</code>
 * operation  remembers a point in the input
 * stream and the <code>reset</code> operation
 * causes all the  bytes read since the most
 * recent <code>mark</code> operation to be
 * reread before new bytes are  taken from
 * the contained input stream.
 *
 * @author  Arthur van Hoff
 * @since   JDK1.0
 */
public
class BufferedInputStream extends FilterInputStream {

    private static int DEFAULT_BUFFER_SIZE = 8192;

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;

    /**
     * The internal buffer array where the data is stored. When necessary,
     * it may be replaced by another array of
     * a different size.
     */
    protected volatile byte buf[];

    /**
     * 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");

    /**
     * The index one greater than the index of the last valid byte in
     * the buffer.
     * This value is always
     * in the range <code>0</code> through <code>buf.length</code>;
     * elements <code>buf[0]</code>  through <code>buf[count-1]
     * </code>contain buffered input data obtained
     * from the underlying  input stream.
     */
    protected int count;

    /**
     * The current position in the buffer. This is the index of the next
     * character to be read from the <code>buf</code> array.
     * <p>
     * This value is always in the range <code>0</code>
     * through <code>count</code>. If it is less
     * than <code>count</code>, then  <code>buf[pos]</code>
     * is the next byte to be supplied as input;
     * if it is equal to <code>count</code>, then
     * the  next <code>read</code> or <code>skip</code>
     * operation will require more bytes to be
     * read from the contained  input stream.
     *
     * @see     java.io.BufferedInputStream#buf
     */
    protected int pos;

    /**
     * The value of the <code>pos</code> field at the time the last
     * <code>mark</code> method was called.
     * <p>
     * This value is always
     * in the range <code>-1</code> through <code>pos</code>.
     * If there is no marked position in  the input
     * stream, this field is <code>-1</code>. If
     * there is a marked position in the input
     * stream,  then <code>buf[markpos]</code>
     * is the first byte to be supplied as input
     * after a <code>reset</code> operation. If
     * <code>markpos</code> is not <code>-1</code>,
     * then all bytes from positions <code>buf[markpos]</code>
     * through  <code>buf[pos-1]</code> must remain
     * in the buffer array (though they may be
     * moved to  another place in the buffer array,
     * with suitable adjustments to the values
     * of <code>count</code>,  <code>pos</code>,
     * and <code>markpos</code>); they may not
     * be discarded unless and until the difference
     * between <code>pos</code> and <code>markpos</code>
     * exceeds <code>marklimit</code>.
     *
     * @see     java.io.BufferedInputStream#mark(int)
     * @see     java.io.BufferedInputStream#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.
     */
    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}.
     */
    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
     */
    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.
     */
    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&nbsp;- 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.
     */
    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()
     */
    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.
     */
    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()
        }
    }
}

测试代码：

package com.tianjh.io.inputstream;

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.IOException;

/**
 * Created on 2020/12/10
 * A BufferedInputStream为另一个输入流添加了功能，即缓冲输入和支持mark和reset方法的功能
 * BufferedInputStream创建时，就会创建一个内部缓冲区数组。当从流中读取或跳过字节时，内部缓
 * 冲区将根据需要从所包含的输入流中重新填充，一次有多个字节。mark操作会记住输入流中的一点，并
 * 且reset操作会导致从最近的mark操作之后读取的所有字节在从包含的输入流中取出新的字节之前重新
 * 读取
 **/
public class BufferedInputStreamDemo {
    public static void main(String[] args) {
        // 1.FileInputStream的read方法读取
        try {
            // 使用字符串类型的文件名创建一个文件输入流对象来读取文件
            FileInputStream fileInputStream = new FileInputStream("D:/BufferedInputStream.java");
            // 获得遍历之前的时间
            long startTime = System.currentTimeMillis();
            int size;
            // read方法返回-1,表示文件结尾
            while ((size = fileInputStream.read()) != -1) {}
            // 关闭此输入流并释放与流相关联的任何系统资源
            fileInputStream.close();
            // 获得遍历所需要的时间
            long endTime = System.currentTimeMillis() - startTime;
            System.out.println("FileInputStream的read遍历文件所用时间为: "+endTime);
            // expected output: FileInputStream的read遍历文件所用时间为: 63
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 2.BufferedInputStream的read方法读取
        try {
            // 使用字符串类型的文件名创建一个文件输入流对象来读取文件
            FileInputStream fileInputStream = new FileInputStream("D:/BufferedInputStream.java");
            BufferedInputStream bufferedInputStream = new BufferedInputStream(fileInputStream);
            // 获得遍历之前的时间
            long startTime = System.currentTimeMillis();
            int size;
            // read方法返回-1,表示文件结尾 BufferedInputStream的read方法也是一个字节一个字节读取，但是它因为有一个
            // 内部缓冲数组。所以当它第一次从磁盘读取时，它就会突然从磁盘中读取很多字节进入缓冲区，然后再从缓冲区里面进行
            // 一个字节一个字节读取。所以它的读取效率比FileInputStream读取效率更高
            while ((size = bufferedInputStream.read()) != -1) {}
            // 关闭此输入流并释放与流相关联的任何系统资源
            bufferedInputStream.close();
            fileInputStream.close();
            // 获得遍历所需要的时间
            long endTime = System.currentTimeMillis() - startTime;
            System.out.println("BufferedInputStream的read遍历文件所用时间为: "+endTime);
            // expected output: BufferedInputStream的read遍历文件所用时间为: 1
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

上述列子输出结果为：

　　FileInputStream的read遍历文件所用时间为: 63
　　BufferedInputStream的read遍历文件所用时间为: 1

 

BufferedOutputStream它内部也有一个buffer（缓存），它的原理和BufferedInputStream流一样，当写数据时，可以进行批量的写数据。以下是做一个文件的拷贝Demo，实现先从指定文件 D:/BufferedInputStream.java 读取数据然后再把读取到的数据写入到指定文件 D:/BufferedOutputStream.java 中。

package com.tianjh.io.outputstream;

import java.io.*;

/**
 * Created on 2020/12/10
 * BufferedOutputStream它内部有一个buffer（缓存），当写数据时，可以批量的写
 * 它的工作原理和BufferedInputStream是一样的
 * 实现先从指定文件 D:/BufferedInputStream.java 读取数据然后再把读取到的数据
 * 写入到指定文件  D:/BufferedOutputStream.java 中
 **/
public class BufferedOutputStreamDemo {
    public static void main(String[] args) {
        // 1.BufferedInputStream的read方法读取字节
        // 2.BufferedOutputStream的write方法写入字节
        try {
            String sourceFile = "D:/BufferedInputStream.java";
            String targetFile = "D:/BufferedOutputStream.java";
            // 使用字符串类型的文件名创建一个文件输入流对象来读取文件
            FileInputStream fis = new FileInputStream(sourceFile);
            BufferedInputStream bis = new BufferedInputStream(fis);
            FileOutputStream fos = new FileOutputStream(targetFile);
            BufferedOutputStream bos = new BufferedOutputStream(fos);

            int readCount;
            byte[] readByte = new byte[256];
            while ((readCount = bis.read(readByte, 0, readByte.length)) != -1) {
                bos.write(readByte, 0, readCount);
            }
            bis.close();
            bos.close();
            System.out.println("拷贝完成！");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

上述列子输出结果为：

　　拷贝完成！

 拷贝成功之后的文件BufferedOutputStream和BufferedInputStream文件内容完全是一样的！

/*
 * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.io;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

/**
 * A <code>BufferedInputStream</code> adds
 * functionality to another input stream-namely,
 * the ability to buffer the input and to
 * support the <code>mark</code> and <code>reset</code>
 * methods. When  the <code>BufferedInputStream</code>
 * is created, an internal buffer array is
 * created. As bytes  from the stream are read
 * or skipped, the internal buffer is refilled
 * as necessary  from the contained input stream,
 * many bytes at a time. The <code>mark</code>
 * operation  remembers a point in the input
 * stream and the <code>reset</code> operation
 * causes all the  bytes read since the most
 * recent <code>mark</code> operation to be
 * reread before new bytes are  taken from
 * the contained input stream.
 *
 * @author  Arthur van Hoff
 * @since   JDK1.0
 */
public
class BufferedInputStream extends FilterInputStream {

    private static int DEFAULT_BUFFER_SIZE = 8192;

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;

    /**
     * The internal buffer array where the data is stored. When necessary,
     * it may be replaced by another array of
     * a different size.
     */
    protected volatile byte buf[];

    /**
     * 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");

    /**
     * The index one greater than the index of the last valid byte in
     * the buffer.
     * This value is always
     * in the range <code>0</code> through <code>buf.length</code>;
     * elements <code>buf[0]</code>  through <code>buf[count-1]
     * </code>contain buffered input data obtained
     * from the underlying  input stream.
     */
    protected int count;

    /**
     * The current position in the buffer. This is the index of the next
     * character to be read from the <code>buf</code> array.
     * <p>
     * This value is always in the range <code>0</code>
     * through <code>count</code>. If it is less
     * than <code>count</code>, then  <code>buf[pos]</code>
     * is the next byte to be supplied as input;
     * if it is equal to <code>count</code>, then
     * the  next <code>read</code> or <code>skip</code>
     * operation will require more bytes to be
     * read from the contained  input stream.
     *
     * @see     java.io.BufferedInputStream#buf
     */
    protected int pos;

    /**
     * The value of the <code>pos</code> field at the time the last
     * <code>mark</code> method was called.
     * <p>
     * This value is always
     * in the range <code>-1</code> through <code>pos</code>.
     * If there is no marked position in  the input
     * stream, this field is <code>-1</code>. If
     * there is a marked position in the input
     * stream,  then <code>buf[markpos]</code>
     * is the first byte to be supplied as input
     * after a <code>reset</code> operation. If
     * <code>markpos</code> is not <code>-1</code>,
     * then all bytes from positions <code>buf[markpos]</code>
     * through  <code>buf[pos-1]</code> must remain
     * in the buffer array (though they may be
     * moved to  another place in the buffer array,
     * with suitable adjustments to the values
     * of <code>count</code>,  <code>pos</code>,
     * and <code>markpos</code>); they may not
     * be discarded unless and until the difference
     * between <code>pos</code> and <code>markpos</code>
     * exceeds <code>marklimit</code>.
     *
     * @see     java.io.BufferedInputStream#mark(int)
     * @see     java.io.BufferedInputStream#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.
     */
    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}.
     */
    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
     */
    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.
     */
    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&nbsp;- 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.
     */
    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()
     */
    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.
     */
    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()
        }
    }
}