BufferedOutputStream 是一个带缓冲区到输出流，通过设置这种输出流，应用程序就可以将各个字节写入底层输出流中，而不必针对每次字节写入调用底层系统。
俩个成员变量，一个是存储数据的内部缓冲区，一个是缓冲区中的有效字节数。
/** * The internal buffer where data is stored. */ protected byte buf[]; /** * The number of valid bytes in the buffer. This value is always * in the range <tt>0</tt> through <tt>buf.length</tt>; elements * <tt>buf[0]</tt> through <tt>buf[count-1]</tt> contain valid * byte data. */ protected int count;
构造参数可以使用默认大小，也可以指定大小。
/**
* Creates a new buffered output stream to write data to the
* specified underlying output stream.
*
* @param out the underlying output stream.
*/
public BufferedOutputStream(OutputStream out) {
this(out, 8192);
}
/**
* Creates a new buffered output stream to write data to the
* specified underlying output stream with the specified buffer
* size.
*
* @param out the underlying output stream.
* @param size the buffer size.
* @exception IllegalArgumentException if size <= 0.
*/
public BufferedOutputStream(OutputStream out, int size) {
super(out);
if (size <= 0) {
throw new IllegalArgumentException("Buffer size <= 0");
}
buf = new byte[size];
}
刷新缓冲区数据到底层输出流。
/** Flush the internal buffer */
private void flushBuffer() throws IOException {
if (count > 0) {
out.write(buf, 0, count);
count = 0;
}
}
输出一个字节。
public synchronized void write(int b) throws IOException {
//判断缓冲区字节数如果大于缓冲区到长度就刷新缓冲区
if (count >= buf.length) {
flushBuffer();
}
buf[count++] = (byte)b;
}
输出多个字节
public synchronized void write(byte b[], int off, int len) throws IOException {
//如果请求的长度大于缓冲区的长度，那么刷新缓冲区，并且直接输出到底层流
if (len >= buf.length) {
flushBuffer();
out.write(b, off, len);
return;
}
//如果缓冲区剩余空间不够,那么刷新缓冲区
if (len > buf.length - count) {
flushBuffer();
}
//将输出字节缓存到缓冲区当中
System.arraycopy(b, off, buf, count, len);
count += len;
}
刷新输出流，将缓冲区到字节输出到底层流当中
public synchronized void flush() throws IOException { flushBuffer(); out.flush(); }
java.io.BufferedOutputStream 源码分析,布布扣,bubuko.com
java.io.BufferedOutputStream 源码分析
原文：http://www.cnblogs.com/daxin/p/3772278.html