（四）BufferedOutputStream源码学习

1. BufferedOutputStream源码比较简单，利用一个字节数组作为缓存区，当缓存区存满或者调用flush（）函数时，才会将数据写入到底层输出流中。
2. write(buffer)函数中做了优化，当一次性输入的字节数组超过缓存区时，直接写入底层输出流

package java.io;public class BufferedOutputStream extends FilterOutputStream {    /**     * 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;    /**     * 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 &lt;= 0.     */    public BufferedOutputStream(OutputStream out, int size) {        super(out);        if (size <= 0) {            throw new IllegalArgumentException("Buffer size <= 0");        }        buf = new byte[size];    }    /** Flush the internal buffer */    private void flushBuffer() throws IOException {        if (count > 0) {            out.write(buf, 0, count);            count = 0;        }    }    /**     * Writes the specified byte to this buffered output stream.     *     * @param      b   the byte to be written.     * @exception  IOException  if an I/O error occurs.     */    public synchronized void write(int b) throws IOException {        if (count >= buf.length) {            flushBuffer();        }        buf[count++] = (byte)b;    }    /**     * Writes <code>len</code> bytes from the specified byte array     * starting at offset <code>off</code> to this buffered output stream.     *     * <p> Ordinarily this method stores bytes from the given array into this     * stream's buffer, flushing the buffer to the underlying output stream as     * needed.  If the requested length is at least as large as this stream's     * buffer, however, then this method will flush the buffer and write the     * bytes directly to the underlying output stream.  Thus redundant     * <code>BufferedOutputStream</code>s will not copy data unnecessarily.     *     * @param      b     the data.     * @param      off   the start offset in the data.     * @param      len   the number of bytes to write.     * @exception  IOException  if an I/O error occurs.     */    public synchronized void write(byte b[], int off, int len) throws IOException {        if (len >= buf.length) {            /* If the request length exceeds the size of the output buffer,               flush the output buffer and then write the data directly.               In this way buffered streams will cascade harmlessly. */            flushBuffer();            out.write(b, off, len);            return;        }        if (len > buf.length - count) {            flushBuffer();        }        System.arraycopy(b, off, buf, count, len);        count += len;    }    /**     * Flushes this buffered output stream. This forces any buffered     * output bytes to be written out to the underlying output stream.     *     * @exception  IOException  if an I/O error occurs.     * @see        java.io.FilterOutputStream#out     */    public synchronized void flush() throws IOException {        flushBuffer();        out.flush();    }}