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The EntityManager Interface in Java Create Code 3/9 in Java The EntityManager Interface




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The EntityManager Interface generate, create ansi/aim code 39 none for java projects Barcode FAQs The EntityManag Code39 for Java er API creates and removes persistent entity instances, finds entities by the entity s primary key, and allows queries to be run on entities.. 19 Introduction to the Java Persistence API Managing Entities Container-Managed Entity Managers With a containe barcode 3 of 9 for Java r-managed entity manager, an EntityManager instance s persistence context is automatically propagated by the container to all application components that use the EntityManager instance within a single Java Transaction API (JTA) transaction. JTA transactions usually involve calls across application components. To complete a JTA transaction, these components usually need access to a single persistence context.

This occurs when an EntityManager is injected into the application components by means of the javax.persistence.PersistenceContext annotation.

The persistence context is automatically propagated with the current JTA transaction, and EntityManager references that are mapped to the same persistence unit provide access to the persistence context within that transaction. By automatically propagating the persistence context, application components don t need to pass references to EntityManager instances to each other in order to make changes within a single transaction. The Java EE container manages the lifecycle of container-managed entity managers.

To obtain an EntityManager instance, inject the entity manager into the application component:. @PersistenceCon text EntityManager em;. Application-Managed Entity Managers With an applica tion-managed entity manager, on the other hand, the persistence context is not propagated to application components, and the lifecycle of EntityManager instances is managed by the application. Application-managed entity managers are used when applications need to access a persistence context that is not propagated with the JTA transaction across EntityManager instances in a particular persistence unit. In this case, each EntityManager creates a new, isolated persistence context.

The EntityManager and its associated persistence context are created and destroyed explicitly by the application. They are also used when directly injecting EntityManager instances can t be done because EntityManager instances are not thread-safe. EntityManagerFactory instances are thread-safe.

Applications create EntityManager instances in this case by using the createEntityManager method of javax.persistence.EntityManagerFactory.

To obtain an EntityManager instance, you first must obtain an EntityManagerFactory instance by injecting it into the application component by means of the javax.persistence.PersistenceUnit annotation:.

The Java EE 6 Tutorial: Basic Concepts Managing Entities @PersistenceUni Code39 for Java t EntityManagerFactory emf;. Then obtain an EntityManager from the EntityManagerFactory instance:. EntityManager e barcode 3 of 9 for Java m = emf.createEntityManager();. Application-man aged entity managers don t automatically propagate the JTA transaction context. Such applications need to manually gain access to the JTA transaction manager and add transaction demarcation information when performing entity operations. The javax.

transaction.UserTransaction interface defines methods to begin, commit, and roll back transactions. Inject an instance of UserTransaction by creating an instance variable annotated with @Resource:.

@Resource UserT Java 3 of 9 ransaction utx;. To begin a tran saction, call the UserTransaction.begin method. When all the entity operations are complete, call the UserTransaction.

commit method to commit the transaction. The UserTransaction.rollback method is used to roll back the current transaction.

The following example shows how to manage transactions in an application that uses an application-managed entity manager:. @PersistenceCon Code 39 Extended for Java text EntityManagerFactory emf; EntityManager em; @Resource UserTransaction utx; ...

em = emf.createEntityManager(); try { utx.begin(); em.

persist(SomeEntity); em.merge(AnotherEntity); em.remove(ThirdEntity); utx.

commit(); } catch (Exception e) { utx.rollback(); }. Finding Entities Using the EntityManager The EntityManag jar 3 of 9 barcode er.find method is used to look up entities in the data store by the entity s primary key:. @PersistenceCon text EntityManager em; public void enterOrder(int custID, Order newOrder) { Customer cust = em.find(Customer.class, custID); cust.

getOrders().add(newOrder);. 19 Introduction to the Java Persistence API Managing Entities newOrder.setCustomer(cust); }. Managing an Entity Instance s Lifecycle You manage enti spring framework USS Code 39 ty instances by invoking operations on the entity by means of an EntityManager instance. Entity instances are in one of four states: new, managed, detached, or removed..

New entity inst ances have no persistent identity and are not yet associated with a persistence context. Managed entity instances have a persistent identity and are associated with a persistence context. Detached entity instances have a persistent identity and are not currently associated with a persistence context.

Removed entity instances have a persistent identity, are associated with a persistent context, and are scheduled for removal from the data store..
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