Spring Jpa并发事务和陈旧数据

Question

I'm using spring jpa in my project.

I've a service with a synchonized method:

@Service("venditaCustom")
@Transactional
public class VenditaCustomRepositoryImpl implements VenditaCustomRepository {

@Override   
public synchronized <S extends Vendita> S salva(S vendita, Long idPrenotazione)      throws Exception {
    if (idPrenotazione == null || idPrenotazione < 0) {
        return salva(vendita);
    } else {
        return salvaDaPrenotazione(vendita, idPrenotazione);
    }
}

This method send request to the method salva or salvaDaPrenotazione.

public <S extends Vendita> S salva(S vendita) throws Exception {
....do many operation on object Vendita.....
int numeroFiscale = getNumeroBiglietto();   
...
manager.persist(vendita);
manager.flush();

the method getNumeroBiglietto() do a query to obtain the last sequence number:

private int getNumeroBiglietto() {      
    String sQuery = "SELECT MAX(numero) FROM Biglietto WHERE anno = :anno ";
    Query q = manager.createQuery(sQuery);
    q.setParameter("anno", new GregorianCalendar().get(Calendar.YEAR));
    Integer maxNum = 0;
    try {
        maxNum = (Integer) q.getSingleResult();         
        if (maxNum == null)
            maxNum = 0;
            } catch (NoResultException e) {

        maxNum = 0;
    } catch (Exception e) {
        log.error("", e);
        maxNum = 0;
    }
    if (maxNum == null || maxNum == 0) {
        maxNum = PRIMO_NUMERO_FISCALE_BIGLIETTI;
    } else {
        maxNum++;
    }
    log.trace("maxNum " + maxNum);
    return maxNum;
}

The method salvaDaPrenotazione is quite similar to salva():

    public <S extends Vendita> S salvaDaPrenotazione(S vendita, Long idPrenotazione)     throws Exception {
 ...do many action on object Vendita
int numeroFiscale = getNumeroBiglietto();
.....
manager.persist(vendita);
manager.flush();

The problem: I do this action in sequence:

• call indirectly method salvaDaPrenotazion() with big data so the method take long time to complete (about 7s)
• before first method finish I call indirectly method salva()

You can see that: - when you enter in method salvaDaPrenotazione() you get the correct MAX(numero) (is the last value of sequence saved on db) - at the end of the method salvaDaPrenotazion() after persist and flush doing a call to getNumeroBiglietto() I've the correct updated value from db - when you enter in the method salva() and i get the value MAX(numero) from db I've a wrong value! I've a stale value, the same I got at the start of method salvaDaPrenotazione() as if this transaction don't see the commit of the other transaction. So the transaction fails with a

org.hibernate.exception.ConstraintViolationException

Note that:

• there is a unique enter point that is

  public synchronized S salva(S vendita, Long idPrenotazione) throws Exception {

and this method is syncronized.

• the transaction isolation is DEFAULT in spring jpa and READ_COMMITTED in my.cnf (I'm using Mysql)
• If you retry to call indirectly the method salva() after first fail it works!

I don't understand where can be the problem. I don't think is related to cache problem, maybe to isolation problem.

Answer 1

Are you trying to use this for sequencing? It won't work as every process calling that query will all get the same value, unless you guarantee they are only executed sequentially via locking. Pessimistic locking would need to be done on the query so that no other process can issue the same query until the first process is done its update.
Rather than reinventing the wheel though, you should just use a database or JPA sequencing strategy, see http://en.wikibooks.org/wiki/Java_Persistence/Identity_and_Sequencing