利用java内存映射文件机制实现CRC循环冗余校验

import java.nio.MappedByteBuffer;   
import java.nio.channels.FileChannel;   
import java.util.zip.CRC32;   
public static void main(String[] args){   
            try {                               //对
import java.nio.MappedByteBuffer;  
import java.nio.channels.FileChannel;  
import java.util.zip.CRC32;  
public static void main(String[] args){  
            try {                               //对文件进行crc校验  
            long begin = System.currentTimeMillis();  
            FileInputStream in = new FileInputStream("code.py");//指定目标文件  
            FileChannel channel = in.getChannel(); //从文件中获取一个通道  
            CRC32 crc = new CRC32();  
            int length = (int)channel.size();  
            MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, length); //用只读模式从该通道获取字节缓冲，实现文件到内存的映射  
            for(int i = 0;i<length;i++)