FileInputStream和BufferedInputStream

 

万恶的javaeye，编辑器能不能优化下，每次修改样式都变形了。

 

FileInputStream 属于数据源

BufferedInputStream 属于FileInputStream的一个装饰

 

 

BufferedInputStream 有个内部缓冲区当read时会先把缓冲区填满，然后下次读取是直接从缓冲区读取。当读取的位置大于缓冲区时会再一次加载缓冲区。

 

read()和read(byte[] buf, int off, int len)处理方式一样，区别在于后者一次返回多个数据，但是同样都是先放入缓冲区，然后再读取。

private void fill() throws IOException {

       byte abyte0[] = getBufIfOpen();

       if (markpos < 0)

           pos = 0;

       else if (pos >= abyte0.length)

           if (markpos > 0) {

              int i = pos - markpos;

              System.arraycopy(abyte0, markpos, abyte0, 0, i);

              pos = i;

              markpos = 0;

           } else if (abyte0.length >= marklimit) {

              markpos = -1;

              pos = 0;

           } else {

              int j = pos * 2;

              if (j > marklimit)

                  j = marklimit;

              byte abyte1[] = new byte[j];

              System.arraycopy(abyte0, 0, abyte1, 0, pos);

              abyte0 = abyte1;

           }

       count = pos;

       int k = getInIfOpen().read(abyte0, pos, abyte0.length - pos);

       if (k > 0)

           count = k + pos;

    }

 

    public synchronized int read() throws IOException {

       if (pos >= count) {

           fill();

           if (pos >= count)

              return -1;

       }

       return getBufIfOpen()[pos++] & 255;

    }

 

至于性能问题，我们都知道文件的物理读取性能肯定要大于内存读取，FileInputStream.read()相当于一次物理读取，而BufferedInputStream .read()大部分相当于一次内存读取。

 

 

 

同理FileOutputStream和BufferedOutputStream原理也是一样，也是有个缓冲区，每次write相当于写入缓冲区，当缓冲区慢了后在物理写入文件。

private void flushBuffer()

        throws IOException

    {

        if(count > 0)

        {

            out.write(buf, 0, count);

            count = 0;

        }

    }

 

    public synchronized void write(int i)

        throws IOException

    {

        if(count >= buf.length)

            flushBuffer();

        buf[count++] = (byte)i;

    }

 

 

 

测试

 

public static void bufferTest() throws IOException{

       BufferedInputStream is = new BufferedInputStream(new FileInputStream("d:/A.exe"));

       BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("d:/B.exe"));

       byte[] buffer = new byte[100];

      

       System.out.println("buffer start "+new Date());

       int size = 0 ;

       while((size=is.read(buffer, 0, buffer.length))!=-1){

           out.write(buffer);

       }

       is.close();

       out.close();

       System.out.println("buffer end "+new Date());

    }

   

    public static void ioTest()throws IOException{

       FileInputStream is = new FileInputStream("d:/A.exe");

       FileOutputStream out = new FileOutputStream("d:/C.exe");

       byte[] buffer = new byte[100];

      

       System.out.println("io start "+new Date());

       int size = 0 ;

       while((size=is.read(buffer, 0, buffer.length))!=-1){

           out.write(buffer);

       }

       is.close();

       out.close();

       System.out.println("io end "+new Date());

    }

 

文件大概有1G,用buffer大概用了20秒，用原始方式等了1分钟还没完，直接强制终止。