src/main/java/io/github/biezhi/wechat/utils/MD5Checksum.java
public static String getMD5Checksum(String filename) {
       try {
           byte[] b = createChecksum(filename);
           String result = "";

           for (int i=0; i < b.length; i++) {
               result += Integer.toString( ( b[i] & 0xff ) + 0x100, 16).substring( 1 );
           }
           return result;
       } catch (Exception e){
           return "";
       }
   }
public static String getMD5Checksum(String filename) {
       try {
           byte[] b = createChecksum(filename);
           String result = "";

           for (int i=0; i < b.length; i++) {
               result += Integer.toString( ( b[i] & 0xff ) + 0x100, 16).substring( 1 );
           }
           return result;
       } catch (Exception e){
           return "";
       }
   }
a byte array to a HEX string