android BufferedOutputStream的使用

今天，简单讲讲android里的BufferedOutputStream的使用。

  BufferedInputStream是带缓冲区的输入流，默认缓冲区大小是8M，能够减少访问磁盘的次数，提高文件读取性能；BufferedOutputStream是带缓冲区的输出流，能够提高文件的写入效率。BufferedInputStream与BufferedOutputStream分别是FilterInputStream类和FilterOutputStream类的子类，实现了装饰设计模式。

构造方法

//创建一个新的缓冲输出流，以将数据写入指定的底层输出流。
public BufferedOutputStream(OutputStream out);

//创建一个新的缓冲输出流，以将具有指定缓冲区大小的数据写入指定的底层输出流。
public BufferedOutputStream(OutputStream out,int size);

常用的方法

//向输出流中输出一个字节
public void write(int b);

//将指定 byte 数组中从偏移量 off 开始的 len 个字节写入此缓冲的输出流。
public void write(byte[] b,int off,int len);

//刷新此缓冲的输出流。这迫使所有缓冲的输出字节被写出到底层输出流中。
public void flush();

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();
}
}

说明：
BufferedOutputStream的源码非常简单，这里就BufferedOutputStream的思想进行简单说明：BufferedOutputStream通过字节数组来缓冲数据，当缓冲区满或者用户调用flush()函数时，它就会将缓冲区的数据写入到输出流中。

从源码可以看出，BufferedOutputStream的默认构造函数，缓冲区字节数组大小是8*1024，即8M.

这里简单讲讲public void
write(byte[] b,int off,int len);这个函数，这个函数可以将b数组的从off开始的len个字节写入到文件，所以当写入的数据大小在变化时，可以新建一个比较大的数组，然后通过这个函数不停写入数据，避免不停的创建不同大小的数组。

android BufferedOutputStream的使用就讲完了。

就这么简单。