Java IO：ByteArrayOutputStream使用详解及源码分析

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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

　　ByteArrayOutputStream提供的API如下：

// 构造函数ByteArrayOutputStream()ByteArrayOutputStream(int size)void    close() //关闭字节流synchronized void    reset() //重置计数器int     size() //获取当前计数synchronized byte[]  toByteArray() //将字节流转换为字节数组String  toString(int hibyte) //将字节流转换为StringString  toString(String charsetName)String  toString()synchronized void    write(byte[] buffer, int offset, int len) //写入字节数组buffer到字节流, offset是buffer的起始位置synchronized void    write(int oneByte) //写入一个字节到字节流synchronized void    writeTo(OutputStream out) //写输出流到其他输出流out
}

1.2 使用示例

public void testByteArrayOutputStream() {byte [] letter = {'h', 'i', 'j', 'k'};//新建字节流ByteArrayOutputStream outputStream = new ByteArrayOutputStream();//写入abcdefgint i = 'a'; //awhile (i < 'h') {outputStream.write(i);i++;}System.out.println("当前字节流中的内容有: " + outputStream.toString());//写入多个outputStream.write(letter, 1, 3);System.out.println("写入letter数组中的第2,3,4个字母字节流中的内容有: " + outputStream.toString());System.out.println("当前output字节流中的字节数为: " + outputStream.size());byte [] byteArr = outputStream.toByteArray();i = 0;System.out.print("byte数组内容为: ");while (i < byteArr.length) {System.out.print(byteArr[i++] + " ");}System.out.println();OutputStream cloneOut = new ByteArrayOutputStream();try {outputStream.writeTo(cloneOut);System.out.println("cloneOut的内容为: " + cloneOut.toString());} catch (IOException e) {e.printStackTrace();}}

　　运行结果如下：

当前字节流中的内容有: abcdefg
写入letter数组中的第2,3,4个字母字节流中的内容有: abcdefgijk
当前output字节流中的字节数为: 10
byte数组内容为: 97 98 99 100 101 102 103 105 106 107
cloneOut的内容为: abcdefgijk

2 源码分析

2.1构造函数

　　ByteArrayOutputStream有两个构造函数,区别是初始大小不同。

/*** Creates a new byte array output stream. The buffer capacity is* initially 32 bytes, though its size increases if necessary.*/
public ByteArrayOutputStream() {this(32);
}/*** Creates a new byte array output stream, with a buffer capacity of* the specified size, in bytes.** @param   size   the initial size.* @exception  IllegalArgumentException if size is negative.*/
public ByteArrayOutputStream(int size) {if (size < 0) {throw new IllegalArgumentException("Negative initial size: "+ size);}buf = new byte[size];
}

2.2 write方法

/*** Writes the specified byte to this byte array output stream.** @param   b   the byte to be written.*/
public synchronized void write(int b) {ensureCapacity(count + 1); //增加容量, 容量不够则加倍buf[count] = (byte) b; //写入字节count += 1;
}/*** Writes <code>len</code> bytes from the specified byte array* starting at offset <code>off</code> to this byte array output stream.** @param   b     the data.* @param   off   the start offset in the data.* @param   len   the number of bytes to write.*/
public synchronized void write(byte b[], int off, int len) {if ((off < 0) || (off > b.length) || (len < 0) ||((off + len) - b.length > 0)) {throw new IndexOutOfBoundsException();}ensureCapacity(count + len); //增加容量,容量不够则加倍System.arraycopy(b, off, buf, count, len); //写入字节数组count += len;
}

2.3 writeTo方法

/*** Writes the complete contents of this byte array output stream to* the specified output stream argument, as if by calling the output* stream's write method using <code>out.write(buf, 0, count)</code>.** @param      out   the output stream to which to write the data.* @exception  IOException  if an I/O error occurs.*/
public synchronized void writeTo(OutputStream out) throws IOException {out.write(buf, 0, count); //将 当前OutputStream的buf中内容写到out中
}

2.4 toString , toByteArray方法

/*** Creates a newly allocated byte array. Its size is the current* size of this output stream and the valid contents of the buffer* have been copied into it.** @return  the current contents of this output stream, as a byte array.* @see     java.io.ByteArrayOutputStream#size()*/
public synchronized byte toByteArray()[] {return Arrays.copyOf(buf, count); //返回信得数组
}/*** Converts the buffer's contents into a string decoding bytes using the* platform's default character set. The length of the new <tt>String</tt>* is a function of the character set, and hence may not be equal to the* size of the buffer.** <p> This method always replaces malformed-input and unmappable-character* sequences with the default replacement string for the platform's* default character set. The {@linkplain java.nio.charset.CharsetDecoder}* class should be used when more control over the decoding process is* required.** @return String decoded from the buffer's contents.* @since  JDK1.1*/
public synchronized String toString() {return new String(buf, 0, count); //返回String对象
}

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

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