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You may want to work around this by having your servlets implement SingleThreadModel. Although doing so may seem harmless, keep in mind that this is a deprecated feature (since servlet 2.4) and we don t recommend using it because of the many limitations it imposes on applications. This topic is beyond the scope of this book (see www.esus.com/ javaindex/j2ee/servlets/servletdiffthread.html).
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The right way to use a container-managed EntityManager is by avoiding dependency injection and instead performing a JNDI lookup. Bear in mind that the JNDI lookup mechanism can be used in any nonmanaged or managed class. To use a container-managed EntityManager, you must first establish the references to the persistence unit using either the @PersistenceContext annotation at the class level or the persistence-context-ref element in web.xml. If you re using a managed class such as a servlet or JSF managed bean, then you can establish the reference as follows:
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@PersistenceContext(name = "actionBazaar/EntityManager", unitName = "actionBazaar") public class ActionBazaarBidControllerServlet extends HttpServlet { private EntityManager entityManager; ... }
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If you are using a web framework like Struts where the action classes are not managed, then you can t use annotations to establish references to the persistence context. In this case you will have to use the persistence-context-ref element in the web.xml as follows:
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<persistence-context-ref> <persistence-context-ref-name> actionBazaar/EntityManager </persistence-context-ref-name> <persistence-unit-name>actionBazaar</persistence-unit-name> </persistence-context-ref>
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Regardless of which previous approach you use, your next step is to grab an instance of an EntityManager by using JNDI:
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Context context = new InitialContext(); EntityManager entityManager =
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Effectively integrating EJB 3 across your application tiers
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(EntityManager)context.lookup( "java:comp/env/actionBazaar/EntityManager");
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One thing you need to remember when you don t use session beans to access the entities is that you have to programmatically manage your transactions since CMT is not available in the web container. You must use the UserTransaction API in the following way:
@Resource private UserTransaction ut; ... ut.begin(); ... entityManager.persist(item); ... ut.commit();
You ll recall that in chapter 6 we discussed the disadvantages of programmatic transaction; it s also evident from the previous code that you have to write errorprone code to manage transactions. If you plan to deploy your web module to a Java EE container, an EJB container is included as well. We recommend that you take full advantage of your container by using a session bean to fa ade your entity operations. With this approach, you don t have to worry about programmatic transactions, and you receive extra benefits, such as the ability to inject EntityManager and the power of extended persistence context. Next we ll discuss how you can use an application-managed EntityManager with the JTA transaction manager.
12.3.2 Using an application-managed EntityManager with JTA transactions
As you know, we don t typically recommend using an application-managed EntityManager, but in some cases it s your best choice. For instance, you may not want to use JNDI to grab a container-managed EntityManager instance because you are a fan of dependency injection. In that case, there is another option. You can consider using the @PersistenceUnit annotation to inject an instance of an EntityManagerFactory, and then create an application-managed instance of EntityManager as follows:
public class ActionBazaarBidControllerServlet extends HttpServlet { @PersistenceUnit(unitName = "actionBazaar") private EntityManagerFactory emf; ... }
Using JPA from the web tier
Just remember that you have to manage the lifecycle of an application-managed EntityManager (see listing 12.4); the container is not going to do it for you in this situation.
Listing 12.4 Using an application-managed entity manager with a JTA transaction
try { Begins ... transaction ut.begin(); em = emf.createEntityManager(); em.persist(item); ... ut.commit(); ... } catch (Exception e) { try { ut.rollback(); } catch (Exception e) { } finally { Closes EntityManager em.close(); } }
Creates EntityManager
In listing 12.4 we are doing a lot of housekeeping to manage the lifecycle of the EntityManager and to manually manage transactions. Unless you have a specific reason to use an application-managed EntityManager, we recommend using the Session Fa ade design pattern (discussed in 12.2.3) with CMT, and a containermanaged EntityManager. The EJB 3 incarnation of session beans are lightweight POJOs, which make life easier by not requiring the developer to manage the lifecycle of entity managers or transactions.
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