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Implementing domain models
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Listing 7.1 Category domain object in Java
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package ejb3inaction.actionbazaar.model; import java.sql.Date; public class Category { protected Long id;
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Plain Java object Instance variable uniquely identifying object
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protected String name; protected Date modificationDate;
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Object attribute instance variable
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protected Set<Item> items; protected Category parentCategory; protected Set<Category> subCategories; public Category() {} public Long getId() { return this.id; } public void setId(Long id) { this.id = id; } public String getName() { return this.name; } public void setName(String name) { this.name = name; } public Date getModificationDate() { return this.modificationDate; }
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Instance variables for relationships
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Getters and setters for each instance variable
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public void setModificationDate(Date modificationDate) { this.modificationDate = modificationDate; } public Set<Item> getItems() { return this.items; } public void setItems(Set<Item> items) { this.items = items; } public Set<Category> getSubCategories() { return this.subCategories; }
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Implementing domain objects with JPA
public void setSubCategories(Set<Category> subCategories) { this.subCategories = subCategories; } public Category getParentCategory() { return this.parentCategory; }
Getters and setters for each instance variable
public void setParentCategory(Category parentCategory) { this.parentCategory = parentCategory; } }
The Category POJO b has a number of protected instance fields D E, each with corresponding setters and getters that conform to JavaBeans naming conventions F. In case you are unfamiliar with them, JavaBeans rules state that all instance variables should be nonpublic and made accessible via methods that follow the getXX and setXX pattern used in listing 7.1, where XX is the name of the property (instance variable). Other than name and modificationDate, all the other properties have a specific role in domain modeling and persistence. The id field is used to store a unique number that identifies the category C. The items property stores all the items stored under a category and represents a many-to-many relationship between items and categories. The parentCategory property represents a self-referential many-to-one relationship between parent and child categories. The subCategories property maintains a one-to-many relationship between a category and its subcategories. The Category class as it stands in listing 7.1 is a perfectly acceptable Java implementation of a domain object. The problem is that the EJB 3 persistence provider has no way of distinguishing the fact that the Category class is a domain object instead of just another random Java object used for business logic, presentation, or some other purpose. Moreover, note that the properties representing relationships do not make direction or multiplicity clear. Lastly, the persistence provider also needs to be told about the special purpose of the id property. We ll start solving some of these problems by using JPA annotations next, starting with identifying the Category class as a domain object.
7.2 Implementing domain objects with JPA
In the previous few sections you learned about domain modeling concepts and identified part of the ActionBazaar domain model. Also, we briefly introduced
Implementing domain models
some commonly used metadata annotations supported by JPA. In this section, you ll see some of the JPA annotations in action as we implement part of the domain model using the EJB 3 JPA. We ll start with the @Entity: annotation that converts a POJO to an entity. Then you ll learn about field- and property-based persistence and entity identity. Finally, we ll discuss embedded objects.
7.2.1 The @Entity annotation The @Entity annotation marks a POJO as a domain object that can be uniquely identified. You may think of the annotation as the persistence counterpart of the @Stateless, @Stateful, and @MessageDriven annotations. Mark the Category class as an entity as follows:
@Entity public class Category { ... public Category() { ... } public Category(String name) { ... } ... }
As the code snippet demonstrates, all nonabstract entities must have either a public or a protected no-argument constructor. The constructor is used to create a new entity instance by using the new operator as follows:
Category category = new Category();
One of the coolest features of JPA is that since entities are POJOs, they support a full range of OO features like inheritance and polymorphism, with a few persistencerelated nuances thrown in. You can have an entity extend either another entity or even a nonentity class. For example, it would be good design to extend both the Seller and Bidder domain object classes from a common User class, as shown in figure 7.4. As the code snippet that follows shows, this class could store information common to all users like the user ID, username, and email address:
Figure 7.4 Inheritance support with entities. Bidder and Seller entities extend the User entity class.
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