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Domain models and metadata
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and we use entity class when the Java implementation follows the EJB 3.0 and JPA specifications. Again, you shouldn t be too concerned about these differences, because the ultimate goal is to apply the persistence aspect as transparently as possible. Almost every Java class can be a persistent class, or a POJO, or an entity class if some good practices are followed. Hibernate works best with a domain model implemented as POJOs. The few requirements that Hibernate imposes on your domain model implementation are also best practices for the POJO implementation, so most POJOs are Hibernatecompatible without any changes. Hibernate requirements are almost the same as the requirements for EJB 3.0 entity classes, so a POJO implementation can be easily marked up with annotations and made an EJB 3.0 compatible entity. A POJO declares business methods, which define behavior, and properties, which represent state. Some properties represent associations to other userdefined POJOs. A simple POJO class is shown in listing 3.1. This is an implementation of the User entity of your domain model.
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Listing 3.1 POJO implementation of the User class
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public class User implements Serializable { private String username; private Address address; public User() {}
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Declaration of Serializable
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No-argument class constructor
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public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public MonetaryAmount calcShippingCosts(Address fromLocation) { ... } Business method }
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Property accessor methods
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Implementing the domain model
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Hibernate doesn t require that persistent classes implement Serializable. However, when objects are stored in an HttpSession or passed by value using RMI, serialization is necessary. (This is likely to happen in a Hibernate application.) The class can be abstract and, if needed, extend a nonpersistent class. Unlike the JavaBeans specification, which requires no specific constructor, Hibernate (and JPA) require a constructor with no arguments for every persistent class. Hibernate calls persistent classes using the Java Reflection API on this constructor to instantiate objects. The constructor may be nonpublic, but it has to be at least package-visible if runtime-generated proxies will be used for performance optimization. Proxy generation also requires that the class isn t declared final (nor has final methods)! (We ll come back to proxies in chapter 13, section 13.1, Defining the global fetch plan. ) The properties of the POJO implement the attributes of the business entities for example, the username of User. Properties are usually implemented as private or protected instance variables, together with public property accessor methods: a method for retrieving the value of the instance variable and a method for changing its value. These methods are known as the getter and setter, respectively. The example POJO in listing 3.1 declares getter and setter methods for the username and address properties. The JavaBean specification defines the guidelines for naming these methods, and they allow generic tools like Hibernate to easily discover and manipulate the property value. A getter method name begins with get, followed by the name of the property (the first letter in uppercase); a setter method name begins with set and similarly is followed by the name of the property. Getter methods for Boolean properties may begin with is instead of get. You can choose how the state of an instance of your persistent classes should be persisted by Hibernate, either through direct access to its fields or through accessor methods. Your class design isn t disturbed by these considerations. You can make some accessor methods nonpublic or completely remove them. Some getter and setter methods do something more sophisticated than access instance variables (validation, for example), but trivial accessor methods are common. Their primary advantage is providing an additional buffer between the internal representation and the public interface of the class, allowing independent refactoring of both. The example in listing 3.1 also defines a business method that calculates the cost of shipping an item to a particular user (we left out the implementation of this method).
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