Beginning Java EE 6 with GlassFish 3, Second Edition

CHAPTER 2 ■ JAVA PERSISTENCEknown as named queries, are defined using either annotations or XML metadata. The previous JPQLstatement can, for example, be defined as a named query on the Book entity.Listing 2-2 shows a Book entity defining the findBookByTitle named query using the @NamedQueryannotation.Listing 2-2. A findBookByTitle Named Query@Entity@NamedQuery(name = "findBookByTitle", query = "SELECT b FROM Book b WHERE b.title ='H2G2'")public class Book {@Id @GeneratedValueprivate Long id;@Column(nullable = false)private String title;private Float price;@Column(length = 2000)private String description;private String isbn;private Integer nbOfPage;private Boolean illustrations;}// Constructors, getters, settersAs you will see in Chapter 4, the EntityManager.createNamedQuery() method is used to execute thequery and return a list of Book entities that match the search criteria.Callbacks and ListenersEntities are just Plain Old Java Objects (POJOs). When they are managed by the entity manager, theyhave a persistence identity, and their state is synchronized with the database. When they are notmanaged (i.e., they are detached from the entity manager), they can be used like any other Java class.This means that entities have a life cycle, as shown in Figure 2-3. When you create an instance of theBook entity with the new operator, the object exists in memory, and JPA knows nothing about it (it caneven end up being garbage collected). When it becomes managed by the entity manager, its state ismapped and synchronized with the BOOK table. Calling the EntityManager.remove() method deletes thedata from the database, but the Java object continues living in memory until it gets garbage collected.49