Conversations with EJB 3.0 EntityManager em; public Item
Conversations with EJB 3.0 EntityManager em; public Item getAuction(Long itemId) { return em.find(Item.class, itemId); } public boolean sellerHasEnoughMoney(User seller) { boolean sellerCanAffordIt = (Boolean) em.createQuery(”select…”).getSingleResult(); return sellerCanAffordIt; } @Remove public void endAuction(Item item, User buyer) { // Set winning bid // Charge seller // Notify seller and winner item.setBuyer(…); em.flush(); } } This bean implements the three methods of the ManageAuction interface (we don t have to show you this interface). First, it s a stateful EJB; the container creates and reserves an instance for a particular client. When a client obtains a handle to this EJB for the first time, a new instance is created and a new extended persistence context is injected by the container. The persistence context is now bound to the lifecycle of the EJB instance and is closed when the method marked as @Remove returns. Notice how you can read the methods of the EJB like a story of your conversation, one step after another. You can annotate several methods with @Remove; for example, you can add a cancel() method to undo all conversation steps. This is a strong and convenient programming model for conversations, all built-in for free with EJB 3.0. Next is the problem of automatic flushing. All methods of the ManageAuction- Bean require a transaction; you declare this on the class level. The sellerHasEnoughMoney() method, step two in the conversation, flushes the persistence context before executing the query and again when the transaction of that method returns. To prevent that, you declare that the injected persistence context should be in FlushMode.MANUAL, a Hibernate extension. It s now your responsibility to flush the persistence context whenever you want to write the queued SQL DML to the database you do this only once at the end of the conversation. Your transaction assembly is now decoupled from the flush behavior of the persistence engine.
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