Java IO：BufferedOutputStream使用详解及源码分析

使用方法

　　BufferedOutputStream继承于FilterOutputStream，提供缓冲输出流功能。缓冲输出流相对于普通输出流的优势是，它提供了一个缓冲数组，只有缓冲数组满了或者手动flush时才会向磁盘写数据，避免频繁IO。核心思想是，提供一个缓冲数组，写入时首先操作缓冲数组。

方法介绍

　　BufferedOutputStream提供的API如下：

//构造函数
BufferedOutputStream(OutputStream out) //默认缓冲数组大小为8192
BufferedOutputStream(OutputStream out, int size)

synchronized void     close() //关闭
synchronized void     flush() //刷盘
synchronized void     write(byte[] b, int off, int len) //向输出流写数据
synchronized void     write(int b)

使用示例

public void testBufferedOutput() {
    try {
        final byte [] letters = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n'};
        /*
         *创建文件输出流out,缓冲区大小为8
         */
        OutputStream out = new BufferedOutputStream(new FileOutputStream(new File("buff.txt")), 8);
        /*将letters前6个字符写入到输出流*/
        out.write(letters, 0 ,6);
        /*此时不会写入任何数据到磁盘文件*/
        readFile();
        /*继续写入4个字符*/
        for (int i = 0; i < 4; i++) {
            out.write('g' + i);
        }
        /*此时只会写入8个字符到磁盘文件*/
        readFile();
        /*此时会把所有内容写入磁盘文件*/
        out.flush();
        readFile();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

private void readFile() {
    try {
        InputStream in = new FileInputStream("buff.txt");
        byte [] bytes = new byte[20];
        in.read(bytes, 0, bytes.length);
        System.out.println("文件中的内容为: "  + new String(bytes));
    } catch (Exception e) {
        e.printStackTrace();
    }
}

　　运行结果如下：

文件中的内容为:
文件中的内容为: abcdefgh
文件中的内容为: abcdefghij

源码分析

构造方法

　　BufferedOutputStream的构造方法有两个，区别是字节缓冲数组大小。

/**
 * 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 &lt;= 0.
 */
public BufferedOutputStream(OutputStream out, int size) {
    super(out);
    if (size <= 0) {
        throw new IllegalArgumentException("Buffer size <= 0");
    }
    buf = new byte[size];
}

write方法

　　write方法有两个重载方法，分别是协议一个字节的write(int b)和写入一个字节数组的write(byte b[], int off, int len)。下面分析第二个方法的源码。

/**
 * 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) { //如果写入长度比buf长度长,直接写入文件，不走缓冲区
        /* 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(); //将原有缓冲区内容刷盘
        out.write(b, off, len); //直接写入文件
        return;
    }
    if (len > buf.length - count) { //可用空间不足,先刷盘
        flushBuffer();
    }
    System.arraycopy(b, off, buf, count, len); //复制写入
    count += len;
}
/** Flush the internal buffer */
private void flushBuffer() throws IOException {
    if (count > 0) {
        out.write(buf, 0, count);
        count = 0;
    }
}

flush方法

/**
 * 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(); //刷盘
    out.flush(); //未做任何实现
}

参考：

[1] http://www.cnblogs.com/skywang12345/p/io_13.html
[2] http://czj4451.iteye.com/blog/1545159