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an EJB container either replaces (EJB 2.1, entity beans with CMP) or implements (EJB 3.0, Java Persistence entities) the persistence aspect. Hibernate persistent classes and the EJB 3.0 entity programming model offer transparent persistence. Hibernate and Java Persistence also provide automatic persistence. Let s explore both terms in more detail and find an accurate definition.
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Transparent and automated persistence
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We use transparent to mean a complete separation of concerns between the persistent classes of the domain model and the persistence logic, where the persistent classes are unaware of and have no dependency on the persistence mechanism. We use automatic to refer to a persistence solution that relieves you of handling low-level mechanical details, such as writing most SQL statements and working with the JDBC API. The Item class, for example, doesn t have any code-level dependency on any Hibernate API. Furthermore:
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Hibernate doesn t require that any special superclasses or interfaces be inherited or implemented by persistent classes. Nor are any special classes used to implement properties or associations. (Of course, the option to use both techniques is always there.) Transparent persistence improves code readability and maintenance, as you ll soon see. Persistent classes can be reused outside the context of persistence, in unit tests or in the user interface (UI) tier, for example. Testability is a basic requirement for applications with rich domain models. In a system with transparent persistence, objects aren t aware of the underlying data store; they need not even be aware that they are being persisted or retrieved. Persistence concerns are externalized to a generic persistence manager interface in the case of Hibernate, the Session and Query. In JPA, the EntityManager and Query (which has the same name, but a different package and slightly different API) play the same roles.
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Transparent persistence fosters a degree of portability; without special interfaces, the persistent classes are decoupled from any particular persistence solution. Our business logic is fully reusable in any other application context. You could easily change to another transparent persistence mechanism. Because JPA follows the same basic principles, there is no difference between Hibernate persistent classes and JPA entity classes.
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Implementing the domain model
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By this definition of transparent persistence, certain nonautomated persistence layers are transparent (for example, the DAO pattern) because they decouple the persistence-related code with abstract programming interfaces. Only plain Java classes without dependencies are exposed to the business logic or contain the business logic. Conversely, some automated persistence layers (including EJB 2.1 entity instances and some ORM solutions) are nontransparent because they require special interfaces or intrusive programming models. We regard transparency as required. Transparent persistence should be one of the primary goals of any ORM solution. However, no automated persistence solution is completely transparent: Every automated persistence layer, including Hibernate, imposes some requirements on the persistent classes. For example, Hibernate requires that collection-valued properties be typed to an interface such as java.util.Set or java.util.List and not to an actual implementation such as java.util.HashSet (this is a good practice anyway). Or, a JPA entity class has to have a special property, called the database identifier. You now know why the persistence mechanism should have minimal impact on how you implement a domain model, and that transparent and automated persistence are required. What kind of programming model should you use What are the exact requirements and contracts to observe Do you need a special programming model at all In theory, no; in practice, however, you should adopt a disciplined, consistent programming model that is well accepted by the Java community.
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Writing POJOs and persistent entity classes
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As a reaction against EJB 2.1 entity instances, many developers started talking about Plain Old Java Objects (POJOs),1 a back-to-basics approach that essentially revives JavaBeans, a component model for UI development, and reapplies it to the business layer. (Most developers now use the terms POJO and JavaBean almost synonymously.) The overhaul of the EJB specification brought us new lightweight entities, and it would be appropriate to call them persistence-capable JavaBeans. Java developers will soon use all three terms as synonyms for the same basic design approach. In this book, we use persistent class for any class implementation that is capable of persistent instances, we use POJO if some Java best practices are relevant,
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POJO is sometimes also written Plain Ordinary Java Objects. This term was coined in 2002 by Martin Fowler, Rebecca Parsons, and Josh Mackenzie.
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