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14.4.4 Client applications
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In EJB 3, we use the EntityManager API to perform entity operations such as persisting, finding, and removing entities. In EJB 2 the home interface acted as a factory interface to provide methods to create, remove, and find entity bean instances. Clients used these methods to persist, remove, and query bean instances. In the new EJB 3 world, client applications should be modified to use the EntityManager API. Let s say we have an EJB 2 newUser method in the BazaarAdminBean that is a session fa ade used to create an instance of a User entity bean as follows:
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Migrating CMP 2 entity beans to the EJB 3 JPA
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public void newUser(UserDTO user) throws CreateException, FinderException, NamingException { User userLocal = getUserLocalHome().create( user.getUserId(), user.getFirstName(), user.getLastName(), user.getUserType()); ... }
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Our example code uses a DTO named UserDTO. After migration, the client code (where User is an entity) will look much simpler:
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public void newUser(User user) { getEntityManager().persist(user); }
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Similarly, you can migrate other operations, such as removal, or queries to use the EntityManager API. You have to remove the client code to handle exceptions such as CreateException and FinderException that were previously required to be handled in CMP entity beans but are no longer imposed. Translating entity home to a session fa ade It s fairly effortless to migrate the home interface to a session fa ade by moving all factory methods such as create, remove, and find to this fa ade. We first create a UserLocalHome interface:
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public interface UserLocalHome { User create(String userId, String firstName, String lastName, String userType) throws CreateException; User findByPrimaryKey(String primaryKey) throws FinderException; Collection findAll() throws FinderException; Collection findByFirstName(String name) throws FinderException; }
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This interface is exactly the same as before except it does not extend EJBLocalHome. The session bean implements the UserLocalHome interface and implements all factory methods using the EntityManager API as follows:
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@Stateless public class UserLocalHomeBean implements UserLocalHome { @PersistenceContext EntityManager em;
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Migrating to EJB 3
User create(String userId, String firstName, String lastName, String userType) throws CreateException { User user = new User(userId, firstName, lastName, userType); try { em.persist(user); } catch (Exception e) { throw new CreateException(e.getMessage()); } returnuser; } User findByPrimaryKey(String primaryKey) throws FinderException { try { return (User)em.find(User.class, primaryKey); } catch (Exception e) { throw new FinderException(e.getMessage()); } } }
This code throws the EJB 2 specific exceptions that may be expected by clients. You can use this session bean fa ade to mimic the home interface, thereby reducing the number of changes required on the client side. This concludes our discussion on migrating applications using CMP entity beans to the EJB 3 JPA. As we told you at the start of this section, migrating your CMP beans to JPA is the most involved of the tasks you re likely to undertake when upgrading to EJB 3. Revising your domain model, using DTOs and session facades, and the required API changes will all help you achieve your migration goal. Your EJB 2 applications may already use DTOs and session facades, in which case much of this is familiar to you. Either way, we ve provided a roadmap that will lead you down the migration trail. At this point, we know exactly what you re thinking OK, what you re probably thinking. What about all that JDBC code You guys got any tips for converting it to JPA It just so happens we do, and if you mosey on over to the next section you can see what these tips are.
Migrating JDBC DAOs to use the EJB 3 JPA
14.5 Migrating JDBC DAOs to use the EJB 3 JPA
Almost every Java developer in the universe has used JDBC to persist data to a database. If your code also uses the DAO pattern, then migrating to the EJB 3 JPA may be a relatively painless process. Migration from JDBC requires three major tasks:
Identify your entities. Change the DAO implementation classes to use the EntityManager API to perform entity operations instead of using JDBC. Migrate your SQL queries to use JPQL and the EntityManager API.
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