计算文件MD5值 （hashcode）

 /**
     * 计算文件MD5
     * @return 文件MD5
     */
    public static String getMd5ByFile(String path) throws FileNotFoundException {  
        File file = new File(path);
        String value = null;  
        FileInputStream in = new FileInputStream(file);  
        try {  
            MappedByteBuffer byteBuffer = 
                    in.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, file.length());  
            MessageDigest md5 = MessageDigest.getInstance("MD5");  
            md5.update(byteBuffer);  
            BigInteger bi = new BigInteger(1, md5.digest());  
            value = bi.toString(16);  
        } catch (Exception e) {  
            return null;
        } finally {  
            if (null != in) {  
                try {  
                    in.close();  
                } catch (IOException e) {  
                    return null;
                }  
            }  
        }  
        return value;  
    }