Java IO：BufferedOutputStream使用详解及源码分析

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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
}