1，MD5簡介

1）Message Digist Algorithm 5(消息摘要算法); SHA(Secure hash algorithm)安全hash算法。
2）Java已經實現(xiàn)了MD5、SHA1算法。java.security.MessageDigest冈敛，String和文件的MD5以及SHA1結果。the array of bytes for the resulting hash value.
3）md5是一個固定長度128比特(bit)的串1和0的組合鸣皂，md5的32位表示使用16進制表示二進制抓谴，md5的16位表示32位表示去掉前8位和后8位。

2寞缝，MD5加密字符串

private static final char[] HEX = {'0', '1', '2', '3', '4', '5', '6', '7',
            '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

    public static String md5String(String input) throws Exception {
        MessageDigest md = MessageDigest.getInstance("md5");
        byte[] inputBytes = input.getBytes();
        byte[] resultByte = md.digest(inputBytes);

        int length = resultByte.length;
        char[] result = new char[length * 2];

        int index = 0;
        for(int i = 0; i < length; i++){
            int val = resultByte[i];//轉換成int類型
            if(val < 0){
                val = val + 256;//負數(shù)使用補碼表示齐邦，eg: -1 + 256，等效于只取負數(shù)補碼的低8位(原byte類型)第租。
            }

            result[index++] = HEX[val/16];//使用除k取整法轉成16
            result[index++] = HEX[val % 16];
        }

        return result.toString();
    }

3措拇，MD5加密文件

public static String md5File(String path) throws Exception{
        MessageDigest md = MessageDigest.getInstance("md5");
        File file = new File(path);
        InputStream inputStream = new FileInputStream(file);

        DigestInputStream digestInputStream = new DigestInputStream(inputStream, md);
        byte[] buffer =new byte[256 * 1024];
        while (digestInputStream.read(buffer) > 0){
            md = digestInputStream.getMessageDigest();
        }

        byte[] resultByte = md.digest();
        int length = resultByte.length;
        char[] result = new char[length * 2];

        int index = 0;
        for(int i = 0; i < length; i++){
            result[index++] = HEX[resultByte[i] >>> 4 & 0xf];//使用位運算來進行16進制轉換
            result[index++] = HEX[resultByte[i] & 0xf];
        }

        return result.toString();
    }