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What s what in EJB 3
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Figure 1.4 Most traditional enterprise applications have at least four layers. 1) The presentation layer is the actual user interface and can either be a browser or a desktop application. 2) The business logic layer defines the business rules. 3) The persistence layer deals with interactions with the database. 4) The database layer consists of a relational database such as Oracle that stores the persistent objects.
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EJB is obviously not a presentation layer technology. EJB is all about robust sup-
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port for implementing the business logic and persistence layers. Figure 1.5 shows how EJB supports these layers via its services. In section 1.3 we ll go into more detail on EJB services. And in section 1.2 we ll explore EJB bean types. For now, just note that the bean types called session beans and message-driven beans (MDBs) reside in and use the services in the
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Figure 1.5 The component services offered by EJB 3 at each supported application layer. Note that each service is independent of the other, so you are for the most part free to pick the features important for your application. You ll learn more about services in section 1.3.
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EJB overview
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business logic tier, and the bean types called entities reside in and use services in the persistence tier. The traditional four-tier layered architecture is not perfect. One of the most common criticisms is that it undermines the OO ideal of modeling the business domain as objects that encapsulate both data and behavior. Because the traditional architecture focuses on modeling business processes instead of the domain, the business logic tier tends to look more like a database-driven procedural application than an OO one. Since persistence-tier components are simple data holders, they look a lot like database record definitions rather than first-class citizens of the OO world. As you ll see in the next section, DDD proposes an alternative architecture that attempts to solve these perceived problems. Domain-driven design The term domain-driven design (DDD) may be relatively new but the concept is not (see Domain-Driven Design: Tackling Complexity in the Heart of Software, by Eric Evans [Addison-Wesley Professional, 2003]). DDD emphasizes that domain objects should contain business logic and should not just be a dumb replica of database records. Domain objects are known as entities in EJB 3 (see section 1.2 for a discussion on entities). With DDD, the Catalog and Customer objects in a trading application are typical examples of entities, and they may contain business logic. In his excellent book POJOs in Action (Manning, 2006), author Chris Richardson points out the problem in using domain objects just as a data holder.
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Some developers still view persistent objects simply as a means to get data in and out of the database and write procedural business logic. They develop what Fowler calls an anemic domain model .... Just as anemic blood lacks vitality, anemic object models only superficially model the problem and consist of classes that implement little or no behavior.
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Yet, even though its value is clear, until this release of EJB, it was difficult to implement DDD. Chris goes on to explain how EJB 2 encouraged procedural code:
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J2EE developers write procedural-style code [because] it is encouraged by the EJB architecture, literature, and culture, which place great emphasis on EJB components. EJB 2 components are not suitable for implementing an object model.
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Admittedly, implementing a real domain model was almost impossible with EJB 2 because beans were not POJOs and did not support many OO features. such as inheritance and polymorphism. Chris specifically targets entity beans as the problem:
What s what in EJB 3
EJB 2 entity beans, which are intended to represent business objects, have numerous limitations that make it extremely difficult to use them to implement a persistent object model.
The good news is that EJB 3 enables you to easily follow good object-oriented design or DDD. The entities defined by EJB 3 Java Persistence API (JPA) support OO features, such as inheritance or polymorphism. It s easy to implement a persistence object model with the EJB 3 JPA. More importantly, you can easily add business logic to your entities, so that implementing a rich domain model with EJB 3 is a trivial task. Note, though, that many people don t like adding complex business logic in the domain object itself and prefer creating a layer for procedural logic referred to as the service layer or application layer (see Patterns of Enterprise Application Architecture, by Martin Fowler [Addison-Wesley Professional, 2002]). The application layer is similar to the business logic layer of the traditional four-tier architecture, but is much thinner. Not surprisingly, you can use session beans to build the service layer. Whether you use the traditional four-tier architecture or a layered architecture with DDD, you can use entities to model domain objects, including modeling state and behavior. We ll discuss domain modeling with JPA entities in chapter 7. Despite its impressive services and vision, EJB 3 is not the only act in town. You can combine various technologies to more or less match EJB services and infrastructure. For example, you could use Spring with other open source technologies such as Hibernate and AspectJ to build your application, so why choose EJB 3 Glad you asked...
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