Java unique code generation

private static String beforeBh;
 private static String beforeNum;
public static String codeGenerate() {
   Date now = new Date();
   String behind = now.getTime()+"";
   if(beforeBh==null) {
    beforeBh = behind;
   } else if(beforeBh.equals(behind)) {
    behind = behind +beforeNum;
    beforeNum++;
   } else {
    beforeBh = behind;
    beforeNum = 0;
   }
   return behind;