auto commit is off, I checked the hibernate logging.

code is like this:

try{
tx.begin();
em.persist(entity);
tx.commit();
}catch(Exception ex)
{
tx.rollabck();
}

after em.persist() is executed, the data is in my database! tx.commit() seems to be useless here.

anyone can answer this question?

all managed objects need to have assigned a primary key before it can be added to the persistence context.
I guess you're using a type of primary key which is generated by the database (like mysql autoincrements?) so Hibernate needs to send the INSERT statement to the database in order to have a PK assigned. This doesn't change semantics as of course using the transaction is still needed, as you are still able to rollback the database transaction until the call to commit(); it might be interesting to consider this for performance tuning, but your code should behave the same way.

Thank you so much!
Yes, the primary key is generated by MySQL. Did not realize that could cause me so much trouble especially in terms of performance. I guess that also applies to calling em.merge(): in order to Save or Update an entity what I ended up doing is to find the object by using its unique key and based on whether it can be found or not, call em.persist() or change the loaded entity.
Is there a good practice to solve this kind of problems or should I dump mysql auto-increments ?

that shouldn't be a huge hit in terms of performance :/
Even if you get the next value of a sequence, you'll have to ask the DB for a value. If your really need to avoid the DB rountrip, use uuid generated values (Hibernate provides several generators), or when possible assign a key yourself.

I got another problem because of that behavior. When I use em.merge() to save a record. Hibernate always send an insert statement to database first which fails every time because of duplicate unique key.
So in order to save a record, instead of one line em.merge(entity), I have to do something like this:

public void save(MobileAppOwner entity) {
try {
MobileAppOwner existingEntity = this.find(entity.getMobileApp(), entity.getPerson());

existingEntity.setMobileApp(entity.getMobileApp());
existingEntity.setPerson(entity.getPerson());
existingEntity.setStatus(entity.getStatus());
existingEntity.setLastUpdatedTime(entity.getLastUpdatedTime());

em.merge(entity);
} catch (javax.persistence.NoResultException e) {
em.persist(entity);
}

}

What is the best practice to solve this problem? This has been bothering me for months, any help will be greatly appreciated.

Sorry I don't understand; why are you using merge to save a record? it should be used only to update an existing record. To save a new entity use persist.

Hi Sanne :
Thanks for your reply!
This is because in my case, the data I received from the caller does not contain primary key, which is generated by mysql database(MyISAM btw). It dose include unique constrain tough. So in my code I have to take care of either inserting or updating a record.
So say I am using merge only to update a record, in my situation, i.e, primary key generated by MySQL database, what would you recommend to handle the update? Is it possible to just em.merge(entity) to do that? Thanks.

yet another question here:
Since I am using MyISAM store engine, I found transaction rollback method does not really work here. I figured this is because myisam does not support transaction.
I guess I have to simulate rollback by code like this:

//somewhere in code entity1 inserted into db

try {
em.getTransaction().begin();
em.persist(entity2);

em.getTransaction().commit();
} catch (RuntimeException ex) {
logger.error(ex,ex);

// em.getTransaction().rollback();
// do our own rollback
em.delete(entity1);
}

what is your suggestion?

I'd really suggest you to configure MySQL to use InnoDB, so you have foreign keys and transactions. Why should you be using MyISAM?