Java IO：ByteArrayInputStream使用详解及源码分析

1 使用方法

　　ByteArrayInputStream 包含一个内部缓冲区，该缓冲区包含从流中读取的字节。内部计数器跟踪 read 方法要提供的下一个字节。ByteArrayOutputStream实现了一个输出流，其中的数据被写入一个 byte 数组。缓冲区会随着数据的不断写入而自动增长。可使用 toByteArray()和 toString()获取数据。

1.1 方法介绍

　　ByteArrayInputStream提供的API如下：

// 构造函数    ByteArrayInputStream(byte[] buf)    ByteArrayInputStream(byte[] buf, int offset, int length)    synchronized int         available() //能否读取字节流的下一字节    void                     close() //关闭字节流    synchronized void        mark(int readlimit) //保存当前位置    boolean                  markSupported() //是否支持mark    synchronized int         read() //读取下一字节    synchronized int         read(byte[] buffer, int offset, int length) //将字节流写入buffer数组    synchronized void        reset() //重置索引到mark位置    synchronized long        skip(long byteCount) //跳过n个字节}

1.2 使用示例

public class TestByteArray {    // 对应英文字母“abcddefghijklmnopqrsttuvwxyz”    private final byte[] ArrayLetters = {            0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F,            0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7A    };    public void testByteArrayInputStream() {        //创建字节流,以ArrayLetters初始化        ByteArrayInputStream inputStream = new ByteArrayInputStream(ArrayLetters);        //读取5个字节        int i = 0;        System.out.print("前5个字节为: ");        while (i++ < 5) {            //是否可读            if (inputStream.available() >= 0) {                int buf = inputStream.read();                System.out.printf("0x%s ", Integer.toHexString(buf));            }        }        System.out.println();        //是否支持标记        if (!inputStream.markSupported()) {            System.out.println("该字节流不支持标记");        } else {            System.out.println("该字节流支持标记");        }        //标记, 已经读取5个字节,标记处为0x66        System.out.println("标记该字节流为位置为0x66(f)");        inputStream.mark(0);        //跳过2个字节        inputStream.skip(2);        //读取5个字节到buffer        byte [] buffer = new byte[5];        inputStream.read(buffer, 0, 5);        System.out.println("buffer: " + new String(buffer));        //重置        inputStream.reset();        inputStream.read(buffer, 0, 5);        System.out.println("重置后读取5个字符为: " + new String(buffer));    }}

　　运行结果如下：

前5个字节为: 0x61 0x62 0x63 0x64 0x65该字节流支持标记标记该字节流为位置为0x66(f)buffer: hijkl重置后读取5个字符为: fghij

2 源码分析

2.1构造函数

　　ByteArrayInputStream有两个构造函数，区别是初始化内容选择。

/** * Creates a <code>ByteArrayInputStream</code> * so that it  uses <code>buf</code> as its * buffer array. * The buffer array is not copied. * The initial value of <code>pos</code> * is <code>0</code> and the initial value * of  <code>count</code> is the length of * <code>buf</code>. * * @param   buf   the input buffer. */public ByteArrayInputStream(byte buf[]) {    this.buf = buf; //缓冲数组    this.pos = 0; //当前位置    this.count = buf.length; //输入流字节数}/** * Creates <code>ByteArrayInputStream</code> * that uses <code>buf</code> as its * buffer array. The initial value of <code>pos</code> * is <code>offset</code> and the initial value * of <code>count</code> is the minimum of <code>offset+length</code> * and <code>buf.length</code>. * The buffer array is not copied. The buffer's mark is * set to the specified offset. * * @param   buf      the input buffer. * @param   offset   the offset in the buffer of the first byte to read. * @param   length   the maximum number of bytes to read from the buffer. */public ByteArrayInputStream(byte buf[], int offset, int length) {    this.buf = buf; //缓冲数组    this.pos = offset; //当前位置为传入buf的offset    this.count = Math.min(offset + length, buf.length); //输入流字节数    this.mark = offset; //标记}

2.2 read方法

　　read方法有两个，不带参数的read()每次读取字节流中一个字节，带参数的read(byte b[], int off, int len)将字节流从当前位置开始，写入len个字节到b中，写入开始位置为off。

/** * 读取字节流当前字节 * @return 一个字节 */public synchronized int read() {    return (pos < count) ? (buf[pos++] & 0xff) : -1; //&0xff为限制返回值为一个字节,即8位}/** * 将字节流当前位置开始的len个字节写入到 b从off开始的len个位置 * @param b * @param off * @param len * @return */public synchronized int read(byte b[], int off, int len) {    if (b == null) {        throw new NullPointerException();    } else if (off < 0 || len < 0 || len > b.length - off) {        throw new IndexOutOfBoundsException();    }    if (pos >= count) { //超出字节流范围        return -1;    }    int avail = count - pos; //可读取的字节数量    if (len > avail) {        len = avail;    }    if (len <= 0) {        return 0;    }    System.arraycopy(buf, pos, b, off, len); //将buf从pos位置开始的字节复制到b从off开始的位置,共复制len长    pos += len;    return len;}

2.4 skip方法

/** * Skips <code>n</code> bytes of input from this input stream. Fewer * bytes might be skipped if the end of the input stream is reached. * The actual number <code>k</code> * of bytes to be skipped is equal to the smaller * of <code>n</code> and  <code>count-pos</code>. * The value <code>k</code> is added into <code>pos</code> * and <code>k</code> is returned. * * @param   n   the number of bytes to be skipped. * @return  the actual number of bytes skipped. */public synchronized long skip(long n) {    long k = count - pos; //剩余字节数    if (n < k) {        k = n < 0 ? 0 : n;    }    pos += k;    return k;}

2.5 mark和reset方法

/** * Set the current marked position in the stream. * ByteArrayInputStream objects are marked at position zero by * default when constructed.  They may be marked at another * position within the buffer by this method. * <p> * If no mark has been set, then the value of the mark is the * offset passed to the constructor (or 0 if the offset was not * supplied). * * <p> Note: The <code>readAheadLimit</code> for this class *  has no meaning. * * @since   JDK1.1 */public void mark(int readAheadLimit) {    mark = pos;}/** * Resets the buffer to the marked position.  The marked position * is 0 unless another position was marked or an offset was specified * in the constructor. */public synchronized void reset() {    pos = mark;}

参考：

[1] http://www.cnblogs.com/skywang12345/p/io_02.html
[2] http://www.cnblogs.com/skywang12345/p/io_03.html
[3] http://blog.csdn.net/rcoder/article/details/6118313