/**   
         * Convert hex string to byte[]   
         * @param hexString the hex string   
         * @return byte[]   
         */   
        public static byte[] hexStringToBytes(String hexString) {    
            if (hexString == null || hexString.equals("")) {    
                return null;    
            }    
            hexString = hexString.toUpperCase();    
            int length = hexString.length() / 2;    
            char[] hexChars = hexString.toCharArray();    
            byte[] d = new byte[length];    
            for (int i = 0; i < length; i++) {    
                int pos = i * 2;    
                d[i] = (byte) (charToByte(hexChars[pos]) << 4 | charToByte(hexChars[pos + 1]));    
            }    
            return d;    
        } 
        
        /**   
         * Convert char to byte   
         * @param c char   
         * @return byte   
         */   
         private static byte charToByte(char c) {    
            return (byte) "0123456789ABCDEF".indexOf(c);    
        }