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Migrating CMP 2 entity beans to the EJB 3 JPA
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@Stateless(name = "PlaceBid") @RemoteHome(PlaceBidHome.class) public class PlaceBidBean implements PlaceBid { }
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If you want to maintain backward compatibility with local EJB 2 clients, then you can use the @LocalHome annotation. You must remember that you can t use the @javax.ejb.LocalHome and @javax.ejb.RemoteHome annotations in the home interfaces but only in the bean classes. This concludes our discussion on migrating session beans. There s not much to it, is there Let s now move to a discussion on migrating MDBs before we jump into more complex migration tasks involving CMP entity beans.
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14.3 Migrating message-driven beans
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MDBs have been simple Java classes since their introduction in EJB 2, and do
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not require remote or home interfaces, or unnecessary lifecycle methods. Not many changes to MDBs transpired in EJB 3, so migration of MDBs to EJB 3 is an easy task. Unlike EJB 2, EJB 3 doesn t require that your MDB class implement the javax.ejb.MessgeDrivenBean interface, and the deployment descriptor is now optional. You may prefer to use the @MessageDriven and @ActivationConfigProperty annotations instead of the complementary deployment descriptors to activate configuration properties. You can migrate your lifecycle methods to custom methods and annotate them with the @PostConstruct and @PreDestroy annotations. Optionally you can change this such that resources and services use dependency injection instead of JNDI lookup. These migration steps are similar to what we discussed for session beans in section 14.2.2. This is all that s involved to update your MDBs for EJB 3! Piece of cake! Simple as pie! OK, now we re getting hungry The story for entity beans is not so bright; this is one of the areas that was completely overhauled for EJB 3. Are you ready to dive into the EJB 3 migration waters for EJB 2 entity beans You go right ahead to the next section; we ll meet you there.
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14.4 Migrating CMP 2 entity beans to the EJB 3 JPA
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Migrating CMP entity beans to the EJB 3 JPA is the most complex and involved migration task we ll discuss. It requires careful planning and a thorough understanding of your application.
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Migrating to EJB 3
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You now know that there are significant differences between CMP entity beans and EJB 3 entities. CMP entity beans are coarse-grained data objects, whereas EJB 3 entities are lightweight domain objects that represent finegrained data. EJB 3 entities are regular Java classes and lack some features that CMP 2 entity beans provide, such as remoteness, declarative security, and transactions. We agree with the point of view that imposing those features was a bad idea in the first place. However, you ll have to implement workarounds if you used those features. If you avoided them by using best practices and design patterns, then you are probably in good shape and your overall migration process will be smoother than it would be otherwise. EJB 2 provided the ability to access an entity bean remotely by using remote interfaces, but design experts recommended against this. As you ll recall from chapter 12, we recommend you use a session fa ade as a shield to front entity beans, and use DTOs to transfer data between clients and the server. If you ve followed these guidelines, it will simplify your migration efforts. Another benefit of using the DAO design pattern is that it may enable you to migrate the persistence tier easily without much impact on the business logic tier.
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Signpost up ahead: dead end for BMP entity beans
EJB 2 supported entity beans with bean-managed persistence (BMP), where the code was developed using an API such as JDBC for persisting bean instances. BMP was quite popular initially because CMP implementations from most application servers were not stable. It was also useful when applications required nonrelational data. However, there are some inherent issues with BMP, such as N+1 problems. Unfortunately, there is no clear migration path for BMP entity beans to EJB 3. This translates into what seems like a dead end for BMP entity beans. As EJB 3 requires support for EJB 2, BMP entity beans will be supported in EJB 3 containers. If you re using BMP entity beans with a relational database, we recommend you consider migrating these to use the EJB 3 JPA. Otherwise, you may have a huge support issue lurking in your code base!
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