优势：
IO读写性能提高（OS负责），也可引做共享内存，减少IO操作，提升并发性；
应用crash，保证这部分内容还能写的进去文件。在我们调用channel.write(bytebuffer)之后，具体何时写入磁盘、
bytebuffer中内容暂存于哪里（os cache）等相关一系列问题，就交由OS本身负责了。
public static void main(String[] args) throws IOException
{
FileInputStream fileInputStream = new FileInputStream("f:"+ File.separator +"a.txt");
FileOutputStream fileOutputStream = new FileOutputStream("f:" + File.separator + "b.txt");
FileChannel inChannel = fileInputStream.getChannel();
FileChannel outChannel= fileOutputStream.getChannel();
ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
long start = System.currentTimeMillis();
while(true)
{
int eof = inChannel.read(byteBuffer);
if(eof == -1 ) break;
byteBuffer.flip();
outChannel.write(byteBuffer);
byteBuffer.clear();
}
System.out.println("spending : " + (System.currentTimeMillis()-start));
inChannel.close();
outChannel.close();
}
}