Hmac-md5加密

	    String key = "ThisIsAKeyYouMakeIt";   //自己写一个key字符串（解码的时候也要用)
        byte[] callURLByte = null;
        try {
            callURLByte = callURL.getBytes("UTF8");
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        Mac mac;
        SecretKey secretKey = new SecretKeySpec(key.getBytes(), "HmacMD5"); //传入的key需要编程byte数组
        mac = Mac.getInstance(secretKey.getAlgorithm());
        mac.init(secretKey);
        byte[] result = mac.doFinal(callURLByte);     //得到的结果同样是byte数组
        StringBuilder sign = new StringBuilder();
        
		// Integer.toHexString是得到16进制数

        for (byte b : result) {
            if (Integer.toHexString(0xFF & b).length() == 1) {
                sign.append("0" + Integer.toHexString(0xFF & b));  //为MD5加密是32位的16进制，和0xFF做与运算，是为了让高24为补0
            } else {
                sign.append(Integer.toHexString(0xFF & b));
            }
        }