EJB 3 Transaction attribute for read only method

Question

I have a method that returns lot of data, should I use @TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED) for this method. The method perform a JPA query an loads the full content of a table (about 1000 rows).

Answer 1

The client to this method - is that already in a transaction? When you use NotSupported the caller transaction will be suspended. If not I would say, just put Never as the transaction type. Never is better since callers know they are not supposed to call this method from inside a transaction. A more straight forward contract.

We always use Never for methods that do more processing so that developers are aware right off the bat not to call if they are involved in a transaction already. Hope it helps.

Answer 2

I would care to disagree as it seldom happens that user is not in a transaction in almost all the systems. The best approach is to use NOT SUPPORTED so that the transaction is suspended if the caller is in any transaction already. NEVER is troublesome unless you have a series of calls which are all in NO TRANSACTION scope. In short, NOT SUPPORTED is the type one should use.

Answer 3

As far as I know (at least this is the case with Hibernate), you cannot use JPA outside of a transaction as the entity manager's lifecycle is linked to the transaction's lifecycle. So the actual method that does the query must be transactional.

However, you can set it to TransactionAttributeType.REQUIRES_NEW; this would suspend any existing transaction, start a new one, and stop it when the method returns. That means all your entities would be detached by the time they reach the caller, which is what it sounds like you're trying to achieve.

In more complex systems, it pays to completely separate your data layer from your business layer and create a new set of object. Your method will then call the JPA query, then use the entities returned to populate objects from your business layer, and return those. That way the caller can never get their hands on the actual JPA entities and you are free to do in your data layer what you want, since now it's just an implementation detail. (Heck, you could change the database call to a remote API call and your caller wouldn't have to know.)