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Implementing domain objects with JPA
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@Entity public class User { ... String userId; String username; String email; ... }
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@Entity public class Seller extends User { ...
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@Entity public class Bidder extends User { ...
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Because the parent User class is declared an entity, all the inherited fields like username and email are persisted when either the Seller or Bidder entity is saved. A slightly counterintuitive nuance you should note is that this would not be the case if the User class were not an entity itself. Rather, the value of the inherited properties would be discarded when either Seller or Bidder is persisted. The preceding code snippet also demonstrates an interesting weakness the User class could be persisted on its own, which is not necessarily desirable or appropriate application behavior. One way to avoid this problem is to declare the User class abstract, since abstract entities are allowed but cannot be directly instantiated or saved. In any case, this is probably better OO design anyway. Since JPA supports entity inheritance, the relationship between entities and queries may be polymorphic. We discuss handling polymorphic queries in chapter 10. Obviously the ultimate goal of persistence is to save the properties of the entity into the database (such as the name and modification date for the Category entity in listing 7.1). However, things are not as simple as they seem, and there are a few twists regarding entity data persistence that you need to have a good grasp of.
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7.2.2 Persisting entity data An entity, because it is a persistent object, has some state that is stored into the database. In this section we discuss access types, how to define a transient field, and data types supported by JPA.
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Field- vs. property-based persistence An entity maintains its state by using either fields or properties (via setter and getter methods). Although JavaBeans object property-naming conventions have been
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widely used in the Java platform for a good number of years, some developers consider these conventions to be overkill and would rather access instance variables directly. The good news is that JPA supports this paradigm (whether it should is an open question we ll express our viewpoint in a moment). Defining O/R mapping using fields or instance variables of entity is known as field-based access, and using O/R mapping with properties is known as property-based access. If you want to use field-based access, you can declare all your POJO persisted data fields public or protected and ask the persistence provider to ignore getters/ setters altogether. You would only have to provide some indication on at least one of the instance variables that they are to be used for persistence directly. You can do this by using the @Id annotation that we ll discuss next that applies to either a property or a field. Depending on your inclination, this transparent flexibility may or may not seem a little counterintuitive. In the early releases of the EJB 3 specification, the @Entity annotation had an element named accessType for explicitly specifying the persistence data storage type as either FIELD or PROPERTY. As you ll see, O/R mapping using XML provides an element named access to specify the access type. However, many developers do not like this element and want the additional flexibility of having the JPA provider dynamically determine the access type based on entity field usage patterns. The following snippet shows what field-based persistence might look like:
@Entity public class Category { @Id public Long id; public String name; public Date modificationDate; public Category() {} }
Here, the persistence provider would infer that the id, name, and modificationDate public fields should be persisted since the @Id annotation is used on the id field. The annotations would have been applied to getters if we did not intend to use fields for persistence instead of properties.
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