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7.1 Domain modeling and the JPA
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Often the first step to developing an enterprise application is creating the domain model that is, listing the entities in the domain and defining the relationships between them. In this section we ll first present a primer on domain modeling. Then we ll explore the ActionBazaar problem domain and identify actors in a domain model, such as objects, relationships, and cardinality. We ll provide a brief overview of how domain modeling is supported with the EJB 3 Java Persistence API and then build a simple domain object as a Java class.
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Domain modeling and the JPA
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7.1.1 Introducing domain models
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Although you may have been warned that domain modeling is complex, the idea behind it is pretty simple. In effect, a domain model is a conceptual image of the problem your system is trying to solve. Literally, it is made up of the objects in the system universe and the relationships or associations between them. As you can guess, an object in a domain model need not be a physical object but just a concept used by your system. A relationship, on the other hand, is an imaginary link between objects that need to know about one another. The critical thing to remember is that the domain model describes the objects and how the objects might relate to one another, but not how a system acts on the objects. We like to think of a domain model as a set of interlocking toy blocks. Each uniquely shaped block in the set is an object. The shape of each block determines how they fit with one another. Each such fit is a relationship. In the end, though, you put together the blocks into whatever configuration sparks your imagination. The master plan for putting together the final results forms the business rules of the application. The business rules are implemented by the session beans and MDBs we discussed in previous chapters, while the persistence API implements the domain model that the business rules act on. We won t talk about domain modeling much further than what is needed for explaining the concepts we just introduced. However, we encourage you to explore the topic further by checking out the excellent books written on the subject of domain modeling, most notably Patterns of Enterprise Applications Architecture by Martin Fowler (Addison-Wesley, 2002). UML class diagrams are the most popular method of creating the initial domain model. However, we are going to avoid using formal class diagrams throughout this chapter and in the rest of the book. Instead, we ll use the simplest diagrams possible, which might have a shallow resemblance to UML.
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7.1.2 The ActionBazaar problem domain
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Modeling the entire ActionBazaar domain will introduce complexity that we don t need in order to explain JPA. To avoid this unnecessary complexity, we are going to develop the core functionality of the ActionBazaar application that is directly related to buying and selling items on bid online.
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Admittedly, this is a slightly unoriginal example. We considered using an example tangential to the central theme of ActionBazaar but decided against it and remained true to the ActionBazaar core concept.
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Implementing domain models
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As figure 7.1 shows, at the heart of it ActionBazaar centers on the following activities:
Sellers post an item on ActionBazaar. Items are organized into searchable and navigable categories. Bidders place bids on items. The highest bidder wins.
If you are familiar with use cases and the list looks a lot like use cases, they really are.
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In our artificially simplistic scenario, we can pick out the domain objects by scanning the list of activities and looking for nouns: seller, item, category, bidder, bid, and order. Our goal is to identify the domain objects or entities that we want to persist in the database. In the real world, finding domain objects usually involves hours of work and many iterations spent analyzing the business problem. We ll make our initial diagram by randomly throwing together our objects into figure 7.2.
Figure 7.1 The core functionality of ActionBazaar. Sellers post items into searchable and navigable categories. Bidders bid on items and the highest bid wins.
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