CHAPTER 2 Starting (Web hosting companies) a project annotations on the
Monday, October 22nd, 2007CHAPTER 2 Starting a project annotations on the private fields of the class, starting with @Id and @Generated- Value for the database identifier mapping. The JPA persistence provider detects that the @Id annotation is on a field and assumes that it should access properties on an object directly through fields at runtime. If you placed the @Id annotation on the getId() method, you d enable access to properties through getter and setter methods by default. Hence, all other annotations are also placed on either fields or getter methods, following the selected strategy. Note that the @Table, @Column, and @JoinColumn annotations aren t necessary. All properties of an entity are automatically considered persistent, with default strategies and table/column names. You add them here for clarity and to get the same results as with the XML mapping file. Compare the two mapping metadata strategies now, and you ll see that annotations are much more convenient and reduce the lines of metadata significantly. Annotations are also type-safe, they support autocompletion in your IDE as you type (like any other Java interfaces), and they make refactoring of classes and properties easier. If you re worried that the import of the JPA interfaces will bind your code to this package, you should know that it s only required on your classpath when the annotations are used by Hibernate at runtime. You can load and execute this class without the JPA interfaces on your classpath as long as you don t want to load and store instances with Hibernate. A second concern that developers new to annotations sometimes have relates to the inclusion of configuration metadata in Java source code. By definition, configuration metadata is metadata that can change for each deployment of the application, such as table names. JPA has a simple solution: You can override or replace all annotated metadata with XML metadata files. Later in the book, we ll show you how this is done. Let s assume that this is all you want from JPA annotations instead of XML. You don t want to use the JPA programming interfaces or query language; you ll use Hibernate Session and HQL. The only other change you need to make to your project, besides deleting the now obsolete XML mapping file, is a change in the Hibernate configuration, in hibernate.cfg.xml:
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