code: public void purchaseReservedInstance(String instanceId, int reservationLength, String reservationTimeUnit) {
          this.purchaseReservedInstance(new PurchaseReservedInstanceRequeset()
                  .withInstanceId(instanceId)
                  .withBilling(new Billing().withReservation(new Reservation()
                                  .withReservationLength(reservationLength)
                                  .withReservationTimeUnit(reservationTimeUnit))));
      }
docs: PurchaseReserved the instance with fixed duration.
  
  You can not purchaseReserved the instance which is resizing.
  
  This is an asynchronous interface,
  you can get the latest status by invoke {@link #getInstance(GetInstanceRequest)}
  
  @param instanceId The id of the instance.
  @param reservationLength The fixed duration to purchaseReserved,available is [1,2,3,4,5,6,7,8,9,12,24,36]
  @param reservationTimeUnit The timeUnit to renew the instance,only support "month" now.