Potential concurrency issues when checking and updating records in a table

this is the case

Members must redeem tokens to access (unlock) a given item. The relevant database tables are:

Table 1

Table MEMBER_BALANCE: MEMBER_ID, TOKEN_BALANCE

Table 2

Table UNLOCKED_ITEM: MEMBER_ID, DATE_UNLOCKED, ITEM_ID

The check or constraint I need to perform is

1. TOKEN_BALANCE must be > 0 when the user tries to unlock the item, and
2. The user has not unlocked the same item before.

My hunch is to write a simple method in MemberService.java:

@Transactional
public void unlockItem(Member member, Item item){
    memberBalanceDAO.decrementBalance(member);
    itemDAO.unlockItem(member, item);
}

I handle the second requirement by adding constraints uniqueon MEMBER_ID/ ITEM_IDpairs on the table UNLOCKED_ITEM.

I think, the only thing I need to take care of is that the user is trying to unlock many items at the same time TOKEN_BALANCEand the requirements are not met. For example, TOKEN_BALANCE1, but the user clicks to virtually unlock both items at the same time.

Here is my MemberBalanceDAO.decrementBalancemethod:

@Transactional
public void decrementBalance(Member member) {
    MemberBalance memberBalance = this.findMemberBalance(member);
    if (memberBalance.getTokens() >= 1) {
        memberBalance.setTokens(memberBalance.getTokens() - 1);
        this.save(memberBalance);
    } else {
        throw new SomeCustomRTException("No balance");
    }
}

I don't think this will save me from the TOKEN_BALANCE=1 use case. I'm worried about having multiple unlock requests at the same time. If the balance is 1, I can get two calls at the same time, decrementBalance()both submit the balance as 0, and then both are called successfully itemDAO.unlockItem(...), right?

How should I implement this? Should I set transaction for service level method to isolation = Isolation.SERIALIZABLE? Or is there a cleaner/better way to solve this problem?