Conversations with EJB (Web site design) 3.0 If a container-provided
Conversations with EJB 3.0 If a container-provided (through injection or obtained through lookup) EntityManager is invoked for the first time, a persistence context begins. If no system transaction is active at that time, the persistence context is short and serves only the single method call. Any SQL triggered by any such method call executes on a database connection in autocommit mode. All entity instances that are (possibly) retrieved in that EntityManager call become detached immediately. If a stateless component (such as ItemDAO) is invoked, and the caller has an active transaction and the transaction is propagated into the called component (because ItemDAO methods require or support transactions), any persistence context bound to the JTA transaction is propagated with the transaction. If a stateless component (such as ItemDAO) is invoked, and the caller doesn t have an active transaction (for example, ManageAuction.endAuction() doesn t start a transaction), or the transaction isn t propagated into the called component (because ItemDAO methods don t require or support a transaction), a new persistence context is created when the EntityManager is called inside the stateless component. In other words, no propagation of a persistence context occurs if no transaction is propagated. These rules look complex if you read only the formal definition; however, in practice they translate into a natural behavior. The persistence context is automatically scoped and bound to the JTA system transaction, if there is one you only have to learn the rules for transaction propagation to know how the persistence context is propagated. If there is no JTA system transaction, the persistence context serves a single EntityManager call. You used TransactionAttributeType.REQUIRED in almost all the examples so far. This is the most common attribute applied in transaction assemblies; after all, EJB is a programming model for transactional processing. Only once did we show TransactionAttributeType.NOT_SUPPORTED, when we discussed nontransactional data access with a Hibernate Session in chapter 10 section 10.3.3, Optional transactions with JTA . Also remember that you need nontransactional data access in JPA, to disable automatic flushing of the persistence context in a long conversation the problem of the missing FlushMode.MANUAL again. We now take a closer look at the transaction attribute types and how you can implement a conversation with EJBs and manual flushing of an extended persistence context.
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