把long转化为byte数组

public byte[] encoder(Long obj, int maxwidth) throws Exception {
   if (obj == null) {
    return new byte[] {};
   }
   if (maxwidth > 8) {
    return new byte[] {};
   }
   long value = obj.longValue();
   byte[] b = new byte[maxwidth];
   for (int i = 0; i < maxwidth; i++) {
    b[i] = (byte) ((value >>> ((maxwidth - 1) * 8 - i * 8)) & 0xFF);
   }
   return b;

  }