@JsonIgnore
  public String getScheduleExpression() {
    StringBuilder expression = new StringBuilder();
    expression.append("0"); // seconds
    expression.append(" ");
    expression.append(minutes);
    expression.append(" ");
    expression.append(hours);
    expression.append(" ");
    expression.append(daysOfMonth);
    expression.append(" ");
    expression.append(month);
    expression.append(" ");
    expression.append(dayOfWeek);
    if (year != null && !year.isEmpty()) {
      expression.append(" ");
      expression.append(year);
    }
    return expression.toString();
  }
Return a cron expression from the schedule fields.
Example: "0 0 12 * * ?"
@return
ambari-server/src/main/java/org/apache/ambari/server/state/scheduler/Schedule.java