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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:
If this is possible how would you prevent that?
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Spring uses the underlying database implementation for transactions, so they are as thread safe as the underlying database can be. Transaction isolation is a different issue from thread-safety.
The @Transactional annotation is the metadata that specifies the semantics of the transactions on a method. We have two ways to rollback a transaction: declarative and programmatic. In the declarative approach, we annotate the methods with the @Transactional annotation.
When using proxies, you should apply the @Transactional annotation only to methods with public visibility. If you do annotate protected, private or package-visible methods with the @Transactional annotation, no error is raised, but the annotated method does not exhibit the configured transactional settings.
The usage of the @Repository annotation or @Transactional . @Repository is not needed at all as the interface you declare will be backed by a proxy the Spring Data infrastructure creates and activates exception translation for anyway.
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.
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