Java-IO之BufferedOutputStream（缓冲输出流）

BufferedOutputStream是缓冲输出流，继承于FilterOutputStream，作用是为另外一个输出流提供换从功能。

主要函数列表：

BufferedOutputStream(OutputStream out)
BufferedOutputStream(OutputStream out, int size)
synchronized void close()
synchronized void flush()
synchronized void write(byte[] buffer, int offset, int length)
synchronized void write(int oneByte)

示例代码：

public class BufferedOutputStreamTest {

    private static final int LEN = 5;
    // 对应英文字母“abcddefghijklmnopqrsttuvwxyz”
    private static 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 static void main(String[] args) {
        testBufferedOutputStream() ;
    }

    /**
     * BufferedOutputStream的API测试函数
    */
    private static void testBufferedOutputStream() {

        // 创建“文件输出流”对应的BufferedOutputStream
        // 它对应缓冲区的大小是16，即缓冲区的数据>=16时，会自动将缓冲区的内容写入到输出流。
        try {
            File file = new File("out.txt");
            OutputStream out =new BufferedOutputStream(new FileOutputStream(file), 16);
            // 将ArrayLetters数组的前10个字节写入到输出流中
            out.write(ArrayLetters, 0, 20);
            // 将“换行符\n”写入到输出流中
            out.write('\n');

            // TODO!
            out.flush();

            out.close();
            } catch (FileNotFoundException e) {
            e.printStackTrace();
            } catch (SecurityException e) {
            e.printStackTrace();
            } catch (IOException e) {
            e.printStackTrace();
            }
    }
}

运行结果是文件里有20个字符：abcdefghijklmnopqrst，由于这边设置的缓冲区大小是16，当输入的是20个字符时超过了16，不再使用缓冲区，直接将数据写入

基于JDK8的BufferedOutputStream类的源码：

public class BufferedOutputStream extends FilterOutputStream {
    /**
     * The internal buffer where data is stored.
     */
    //字符数组
    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.
     */
    //字符数组中有效的字节
    protected int count;

    /**
     * Creates a new buffered output stream to write data to the
     * specified underlying output stream.
     *
     * @param   out   the underlying output stream.
     */
    //构造函数，字节数组大小是8*1024
    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];
    }

    /** Flush the internal buffer */
    //让缓冲数据进行写
    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.
     */
    //写一个字节
    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.
     */
    //从b中off位置开始写len个字节
    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. */
               //当输入的长度大于缓冲区的长度时，直接写，不在缓冲

            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
     */
    //将缓冲数据写完
    public synchronized void flush() throws IOException {
        flushBuffer();
        out.flush();
    }
}