（四）BufferedOutputStream源碼學習

1. BufferedOutputStream源碼比較簡單，利用一個位元組數組作為緩存區，當緩存區存滿或者調用flush（）函數時，才會将資料寫入到底層輸出流中。
2. write(buffer)函數中做了優化，當一次性輸入的位元組數組超過緩存區時，直接寫入底層輸出流

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, );
    }

    /**
     * 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 <= ) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

    /** Flush the internal buffer */
    private void flushBuffer() throws IOException {
        if (count > ) {
            out.write(buf, , count);
            count = ;
        }
    }

    /**
     * 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();
    }
}