Eclipselink JPA MySQL Prevent Transactions Concurrency

Question

I have a situation, where data has concurrency in transaction.

Example: User 1 create Transaction 1 with Item 1 qty 3 and Item 2 qty 5. User 2 create Transaction 2 with Item 1 qty 6 and Item 2 qty 7.

Every Item has each their own balance (like in warehouse, item in and out).

How can I prevent concurrency that item balance from item 1 and item 2 count correctly, if transaction 1 and transaction 2 run in the same time from diffrence users?

Item 1 should has balance 9 (3 + 6), not 3 or 6, because of overwrite. Item 2 should has balance 12 (5 + 7).

I had experience in ColdFusion. When I used function cftransaction, every sql query will lock data and table (not all table, only in cftransaction) until commit or rollback called (correct me if I misunderstand).

What until now I apply:

In persistence.xml

<property name="eclipselink.session.customizer"
            value="com.sft.FlexiSessionCustomizer" />

In java file:

public class FlexiSessionCustomizer implements SessionCustomizer {

    @Override
    public void customize(Session session) throws Exception {
        DatabaseLogin databaseLogin = (DatabaseLogin) session
              .getDatasourceLogin();
        databaseLogin
              .setTransactionIsolation(DatabaseLogin.TRANSACTION_SERIALIZABLE);
    }

}

Insert method (it's pure insert to database):

public boolean insertDataBean(Object data) {

    if (data == null) {
        logger.error("Data is not valid.");
        return false;
    }

    boolean result = true;

    EntityManager em = this.createEntityManager();

    data.setCreatedDate(new Timestamp((new Date()).getTime()));

    EntityTransaction entr = em.getTransaction();

    try {
        entr.begin();
        em.persist(data);
        entr.commit();

    } catch (Exception e) {
        logger.error("ERROR INSERT : " + data.getClass().getName() + " | "
              + e.getMessage());
        result = false;
    }

    em.close();

    return result;
}

Call the insert method:

insertDataBean(transaction1); // from user 1

insertDataBean(transaction2); // from user 2

Is that enough to prevent concurrency?

What I think, maybe, I should change the insert code like:

public boolean insertDataBean(Object data) {

        if (data == null) {
            logger.error("Data is not valid.");
            return false;
        }

        boolean result = true;

        EntityManager em = this.createEntityManager();

        data.setCreatedDate(new Timestamp((new Date()).getTime()));

        EntityTransaction entr = em.getTransaction();

        try {
            entr.begin();

            // all code, like read item balance and calculation item balance? 
            // should query use the same entity manager (em) or new entity manager will be the same?

            em.persist(data);

            // Other code transaction

            entr.commit();

        } catch (Exception e) {
            logger.error("ERROR INSERT : " + data.getClass().getName() + " | "
                  + e.getMessage());
            result = false;
        }

        em.close();

        return result;
    }

Do I still need to appy Pessimistic-locking?

Maybe someone can help me, to prevent the concurrency in transactions?

Thank you in advance.