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2.3 Entity beans are a horse of a different color
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The night after we hear the big cat, we re on the trail again. Everyone knows that cats hunt nocturnally. His shadow stalks us on every trail. Though we never encounter him, the big cat still creates mischief, distracting us from the trail long enough to cause us to crash or lose our way. Three years go by, and although no one has yet seen the big cat, he continues to plague many riders in the Texas hill country. Like night rides in the Texas hill country, it s not always clear where the danger lies. The trail, the night, and the fear of both intangible and concrete danger all contribute to the overall risk. Likewise, the EJB landscape hides peril. It s important to discriminate between real and imagined danger. Let s cast some more light on the trail itself: the EJB framework. In the earlier sections, we ve seen that the EJB architecture forces the developer to conform to certain design restrictions to realize the benefits provided by an application server. These are relatively small problems, like tree roots on a mountain bike trail. In this section, we ll encounter a more serious obstacle entity beans. Many people who approached the EJB specification a couple years ago didn t quite understand what session beans did. Entity beans, on the other hand, seemed simple. Entity beans provide a mechanism for dealing with data stored in a database as Java objects, so the entity beans can be used to implement a persistent domain object model. If you implement persistent classes such as Employee, Invoice, and PurchaseOrder as entity beans, everything will be great. Right Wrong. Entity beans are not merely classes whose instances may be stored to a database; they are full-fledged EJB components, capable of being deployed to an EJB container. Entity beans differ from session beans in that they support a more durable persistent state than that supported by session beans. (Keep in mind that stateful session beans can lose state in the event of a crash.) However, the persistent state supported by entity beans does not really map to the needs of a persistent domain object model. The additional services contained within the entity bean specification may be useful in isolation, but they present complexities and constraints that get in the way when you need to use entity beans to implement a persistent domain object model. In this section, we ll talk about the origins of entity beans and what they were designed to accomplish. We ll look at how the industry has used them in the past, and we ll consider the changes made in EJB2 to accommodate the industry s desires. We ll determine when it s appropriate to use entity beans and when to avoid them. By the end of this chapter, you will understand why using entity beans
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The bitter cost
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seems a bit clumsy, and you ll see that the entity bean and EJB specifications require this awkwardness. We will not discuss how to use entity beans or how not to use them, for that matter. That topic will be covered in greater depth in chapter 7. Here we ll simply focus on the cost of the service.
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2.3.1 The black sheep of the bean family
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Entity beans are one of three types of beans defined in the EJB specification. Like session beans and message-driven beans, you can access entity beans from remote JVMs. Entity bean methods can also participate in global transactions governed by a transaction manager. Further, you can control access to entity bean methods via the security services provided by an EJB container. In addition to these common services available to all Enterprise JavaBeans, entity beans provide a mechanism for persisting data into a database. Because of this persistence mechanism, J2EE users primarily use entity beans to implement persistent domain object models. To do so, you define an entity bean for each class that must be persisted into a database. This approach has problems because it means that the domain classes must adhere to all EJB specification rules. Often all we want is a way to make application-defined objects persistent. As table 2.4 illustrates, we don t need all the services built into EJB. As you can see, we need only the persistence services and not the declarative transaction, security, or distributed object services
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Table 2.4 Many EJB services typically go to waste. The problem is that the EJB container provides coarse-grained services, but persistence needs to be a fine-grained service. Persistence, security, and distributed declarative transactions do not belong at the fine-grained level. Service Declarative, distributed transactions Security and authentication Distributed object access failover scalability Persistence Coarse Coarse Coarse Granularity
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