EJB Views and the UML in Java

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EJB Views and the UML
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Structurally, EJBs consist of a main Java class, often called the implementation class or the bean class, and two interfaces: Home and Remote In the case of entity beans (discussed in
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there is also a primary key class The relationships between these items, as well as
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the particular J2EE base objects these items extend and implement, give the EJB its particular functionality and usefulness as a J2EE component
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All EJBs make use of a deployment descriptor to hold additional information pertaining to the component This includes information like transaction settings on business methods, settings for the bean type, security settings, and more
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EJB component types are deployed within an EJB container with the help of a deployment utility An EJB server hosts the EJB container An EJB server is often re ferred to as an enterprise application server, or most commonly as an application server The particular capabilities and level of compliance of the EJB server determine which version of the EJB specification you need to be working with
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Representing an Enterprise JavaBean in UML
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Given UML's breadth, it should not come as a surprise that there are seemingly multiple potential ways to represent an EJB in the UML Obvious candidates appear to be the two UML constructs: class and component
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A UML class is deficient in that unlike an EJB, a class is typically fine-grained In addition, a class does not offer a very self-contained representation of an EJB by itself and requires additional modeling support (such as packaging) to achieve that
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You could possibly me rge the different elements that make up an EJB and represent them as a single UML class, for example, in a fashion similar to that shown inFigure 12-1 In such a case, the class would need to have several different compartments, each containing design information pertaining to a different aspect of the complete EJB
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Figure 12-1 Example of a class-based EJB representation
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This approach might first appear to present a more compact EJB model, but the large number of necessary compartments can make working with this representation harder to understand and more prone to error It also exposes far more information to an EJB client than is necessary or desirable
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Similarly, although the UML has the notion of component, it is not quite the same as a n EJB component The UML component is more closely associated with implementation, for example, representing a physical artifact such as a source code file Furthermore, UML components are not typically modeled during analysis and design[1]
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Note that UML 14 will change the notion of a UML component and bring it closer to the
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J2EE definition of a component Another approach [2] is to use s omething that combines the capabilities of a class (more precisely, a classifier in general) as well as a package, namely a subsystem As mentioned in
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4 ,
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a subsystem is essentially a group of UML elements that represent a behavioral unit
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in a model It can have interfaces as well as operations Subsystems, unlike components, are typically significant from an analysis and design perspective and do not suffer from the fine-grained nature of classes
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Proposed by the UML EJB mapping team in response to JSR 26, Sun Community Process
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(available at javasuncom) For a good discussion of the rationale for this approach, see Modeling Components and Frameworks with UML by Cris Kobryn, Communications of the ACM, October 2000 Given the sound rationale for using a subsystem for representing an EJB, the advantages it offers, and the fact that this approach is likely to become the de facto standard, we have chosen to use the subsystem construct to represent EJBs in this book
It is best to look at EJBs from different perspectives relevant to specific roles (eg, user, developer, etc) Let's take a look at these different views of an EJB We'll discuss session beans and technology details thereafter to elaborate further on these views
Client View
The client view of an EJB includes what the client can access, which consists of the Home and Remote interfaces In a UML class diagram, the client view of an EJB is represented by a UML subsystem for reasons discussed earlier
An advantage of using the subsystem approach is the ability to expose those aspects that are of particular relevance or hide irrelevant details Figure
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