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Manipulating entities with EntityManager
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Context ctx = new InitialContext(); EntityManager em =(EntityManager) ctx.lookup( "java:comp/env/pu/actionBazaar"); ut.begin(); em.persist(item); ut.commit(); ... } }
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The other alternative is to use an application-managed EntityManager with a JTA transaction. It is worth noting that EntityManagerFactory is thread-safe.
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Now that you are familiar with working with EntityManager inside the container, let s move on to the application-managed EntityManager.
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9.2.2 Application-managed EntityManager An application-managed EntityManager wants nothing to do with a Java EE container. This means that you must write code to control every aspect of the EntityManager s lifecycle. By and large, application-managed EntityManagers are most appropriate for environments where a container is not available, such as Java SE or a lightweight web container like Tomcat. However, a justification to use application-managed EntityManagers inside a Java EE container is to maintain fine-grained control over the EntityManager lifecycle or transaction management. For this reason, as well as to maintain flexibility, the EJB 3 API provides a few conveniences for using application-managed EntityManagers inside a container. This happens to suit us well too, because it provides an effective approach to exploring application-managed EntityManagers (see listing 9.2) by reusing the code in listing 9.1.
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Listing 9.2 ItemManagerBean using an application-managed EntityManager
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@Stateless public class ItemManagerBean implements ItemManager { @PersistenceUnit private EntityManagerFactory entityManagerFactory; private EntityManager entityManager; public ItemManagerBean() {} @PostConstruct
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Injects EntityManagerFactory instance
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Creating EntityManager instances
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public void initialize() { entityManager = entityManagerFactory.createEntityManager(); } Creates ... EntityManager public Item updateItem(Item item) { entityManager.joinTransaction(); Explicitly joins JTA transaction entityManager.merge(item); return item; } ... @PreDestroy public void cleanup() { if (entityManager.isOpen()) { Closes EntityManager entityManager.close(); } } ...
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It should be fairly obvious that we are more or less explicitly doing what the container did for us behind the scenes in listing 9.1. First we inject an instance of EntityManagerFactory b. We create an EntityManager using the injected entity manager factory after the bean is constructed C and close it E before the bean is destroyed, mirroring what the container does automatically. The same is true when we explicitly join a container-managed JTA transaction D before performing an EntityManager operation. EntityManagerFactory As you can see in listing 9.2, we get an instance of an application-managed EntityManager using the EntityManagerFactory interface. If you have used JDBC, this is essentially the same idea as creating a Connection from a DriverManager factory. In a Java EE environment, you have the luxury of using the @PersistenceUnit annotation to inject an instance of an EntityManagerFactory, just as we do in listing 9.2. This useful annotation is defined as follows:
@Target({TYPE, METHOD, FIELD}) @Retention(RUNTIME) public @interface PersistenceUnit { String name() default ""; String unitName() default ""; }
The name and unitName elements serve exactly the same purpose as they do for the @PersistenceContext annotation. While the name element can be used to point to
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Figure 9.5 These relationships exist between various important classes in the javax.persistence package for using JPA outside the Java EE container.
the JNDI name of the EntityManagerFactory, the unitName element is used to specify the name of the underlying persistence unit. Figure 9.5 shows the relationships between important interfaces made available by JPA outside the container. The EntityManagerFactory s createEntityManager method creates an application-managed EntityManager. This is probably the most commonly used method in the interface, in addition to the close method. We don t explicitly close the factory in listing 9.2 since the container takes care of cleaning up all resources it injects (unlike the EntityManager, which is created programmatically and is explicitly closed). Table 9.2 lists all the methods in the EntityManagerFactory interface. As you can see, most of them are fairly self-explanatory.
Table 9.2 The EntityManager factory is used to create an instance of an application-managed EntityManager. Method EntityManager createEntityManager() EntityManager createEntityManager(Map map) Purpose Creates an application-managed EntityManager.
Creates an application-managed EntityManager with a specified Map. The Map contains vendor-specific properties to create the manager. Closes the EntityManagerFactory. Checks whether the EntityManagerFactory is open.
void close() boolean isOpen()
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