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Domain models and metadata
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In this book, we won t have much to say about business rules or about the behavior of our domain model. This isn t because we consider it unimportant; rather, this concern is mostly orthogonal to the problem of persistence. It s the state of our entities that is persistent, so we concentrate our discussion on how to best represent state in our domain model, not on how to represent behavior. For example, in this book, we aren t interested in how tax for sold items is calculated or how the system may approve a new user account. We re more interested in how the relationship between users and the items they sell is represented and made persistent. We ll revisit this issue in later chapters, whenever we have a closer look at layered application design and the separation of logic and data access.
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ORM without a domain model We stress that object persistence with full ORM is most suitable for applications based on a rich domain model. If
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your application doesn t implement complex business rules or complex interactions between entities (or if you have few entities), you may not need a domain model. Many simple and some not-so-simple problems are perfectly suited to table-oriented solutions, where the application is designed around the database data model instead of around an objectoriented domain model, often with logic executed in the database (stored procedures). However, the more complex and expressive your domain model, the more you ll benefit from using Hibernate; it shines when dealing with the full complexity of object/relational persistence.
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Now that you have a (rudimentary) application design with a domain model, the next step is to implement it in Java. Let s look at some of the things you need to consider.
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Implementing the domain model
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Several issues typically must be addressed when you implement a domain model in Java. For instance, how do you separate the business concerns from the crosscutting concerns (such as transactions and even persistence) Do you need automated or transparent persistence Do you have to use a specific programming model to achieve this In this section, we examine these types of issues and how to address them in a typical Hibernate application. Let s start with an issue that any implementation must deal with: the separation of concerns. The domain model implementation is usually a central, organizing component; it s reused heavily whenever you implement new application functionality. For this reason, you should be prepared to go to some lengths to ensure
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Implementing the domain model
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that concerns other than business aspects don t leak into the domain model implementation.
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Addressing leakage of concerns
The domain model implementation is such an important piece of code that it shouldn t depend on orthogonal Java APIs. For example, code in the domain model shouldn t perform JNDI lookups or call the database via the JDBC API. This allows you to reuse the domain model implementation virtually anywhere. Most importantly, it makes it easy to unit test the domain model without the need for a particular runtime environment or container (or the need for mocking any service dependencies). This separation emphasizes the distinction between logical unit testing and integration unit testing. We say that the domain model should be concerned only with modeling the business domain. However, there are other concerns, such as persistence, transaction management, and authorization. You shouldn t put code that addresses these crosscutting concerns in the classes that implement the domain model. When these concerns start to appear in the domain model classes, this is an example of leakage of concerns. The EJB standard solves the problem of leaky concerns. If you implement your domain classes using the entity programming model, the container takes care of some concerns for you (or at least lets you externalize those concerns into metadata, as annotations or XML descriptors). The EJB container prevents leakage of certain crosscutting concerns using interception. An EJB is a managed component, executed inside the EJB container; the container intercepts calls to your beans and executes its own functionality. This approach allows the container to implement the predefined crosscutting concerns security, concurrency, persistence, transactions, and remoteness in a generic way. Unfortunately, the EJB 2.1 specification imposes many rules and restrictions on how you must implement a domain model. This, in itself, is a kind of leakage of concerns in this case, the concerns of the container implementer have leaked! This was addressed in the EJB 3.0 specification, which is nonintrusive and much closer to the traditional JavaBean programming model. Hibernate isn t an application server, and it doesn t try to implement all the crosscutting concerns of the full EJB specification. Hibernate is a solution for just one of these concerns: persistence. If you require declarative security and transaction management, you should access entity instances via a session bean, taking advantage of the EJB container s implementation of these concerns. Hibernate in
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