Web site design and hosting - CHAPTER 11 Implementing conversations // Notify seller and
CHAPTER 11 Implementing conversations // Notify seller and winner … return item; } … } The EJB container injects the desired components based on your declaration of fields with @EJB the interface names ItemDAO and PaymentDAO are enough information for the container to find the required components. Let s focus on the transaction and persistence-context propagation rules that apply to this component assembly. Propagation rules First, a system transaction is required and is started if a client calls ManageAuction.endAuction(). The transaction is therefore committed by the container when this method returns. This is the scope of the system transaction. Any other stateless component methods that are called and that either require or support transactions (like the DAO methods) inherit the same transaction context. If you use an EntityManager in any of these stateless components, the persistence context you re working with is automatically the same, scoped to the system transaction. Note that this isn t the case if you use JPA in a Java SE application: The EntityManager instance defines the scope of the persistence context (we elaborated on this earlier). When ItemDAO and PaymentDAO, both stateless components, are invoked inside the system transaction, both inherit the persistence context scoped to the transaction. The container injects an EntityManager instance into itemDAO and paymentDAO with the current persistence context behind the scenes. (Internally, if a client obtains a ManageAuction controller, the container grabs an idle ManageAuctionBean instance from its pool of stateless beans, injects an idle stateless ItemDAOBean and PaymentDAOBean, sets the persistence context on all the components that need it, and returns the ManageAuction bean handle to the client for invocation. This is of course somewhat simplified.) These are the formalized rules for persistence-context scoping and propagation: If a container-provided (through injection or obtained through lookup) EntityManager is invoked for the first time, a persistence context begins. By default, it s transaction-scoped and closes when the system transaction is committed or rolled back. It s automatically flushed when the transaction is committed.
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