using @Transactional for thread safety

Question

I have a question about how @Transactional annotation alone manages code and transactions execution. Given a correctly setup Spring application and the following code:

@Transactional
public void withdraw(int amount) {
    if(isEnoughFunds(amount)) {
        decreaseFunds(amount);
    }
}

Is it possible for the following scenario to occur:

• funds == 100; amount == 100
• thread A enters withdraw / transaction A starts
• thread A executes isEnoughFunds which evaluates to true
• thread B enters withdraw / transaction B starts
• thread B executes isEnoughFunds which evaluates to true
• thread A executes decreaseFunds / thread A locks db record
• thread B waits for thread A to commit transaction and release write lock
• thread A exits withdraw / transaction A commits
• thread B executes decreaseFunds / thread B locks db record
• thread B exits withdraw / transaction B commits
• funds == -100

If this is possible how would you prevent that?

Answer 1

Yes, it is possible, depending on the isolation level. To prevent it, you can explicitly obtain a read lock from the database before calling ifEnoughFunds(). The lock will be released at the end of the transaction. In this scenario, thread B will always wait for thread's A transaction to commit before checking.