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] /article/4709060.html

[2] /article/4709061.html

[3] /article/9632340.html