Editing Hibernate (section)

===Configuration and Startup===
If there is a file named import.sql e.g. in the root directory of an [[JEE#beans_jar_file|EJB jar file]] and hibernate.hbm2ddl.auto in the [[JavaJEEJPA#persistence.xml|persistence.xml]] the file is imported at startup of a JBoss application server.

====SessionFactory====
A '''session factory''' represents a particular logical data-store configuration (s.[[Hibernate#hibernate.cfg.xml|hibernate.cfg.xml]]). The [[JavaJEEJPA|JPA]] aquivalent of the session factory is the entity manager factory, the aquivalent of the session factory configuration is the [[JavaJEEJPA#Persistance_Unit|persistence unit]]. In most hibernate applications a SessionFactory should be instantiated once during application initialization. There are advanced options to store the factory like JNDI. A simple and pragmatic approach is to use a static global variable and static initialization (s. example uweheuer application server.HibernateUtil).

====Session====
A '''session''' is a particular unit of work, that means a set of data access operations. The aquivalent in JPA is the [[JavaJEEJPA#Entity_Manager|entity manager]].

====Startup====
A typical example of a Hibernate startup procedure:

 SessionFactory sf = new Configuration().configure().buildSessionFactory();

or

 uweheuerSessionFactory = new AnnotationConfiguration().configure("uweheuer.hibernate.cfg.xml").buildSessionFactory();

When <code>new configuration()</code> is called Hibernate searches for a file named <code>hibernate.properties</code> in the root of the classpath. If found they are loaded into the configuration object. When <code>configure()</code> is called, Hibernate searches for a file named <code>hibernate.cfg.xml</code>. Bei Web Application ist die Ablage unter \WEB-INF\classes. Its possible to have more than one for multiple databases, one for each database (e.g. project 'uweheuer', files uweheuer.hibernate.cfg.xml and uweswiki.hibernate.cfg.xml). 

====hibernate.cfg.xml====

 <hibernate-configuration>
  <session-factory>
   <property name="hbm2ddl.auto">[update|create]</property>

Update scheint die beste Einstellung, Änderungen an der Klasse bewirken beim Initialisieren der Anwendung (Reload for a web application) eine Änderung in der zugehörigen Tabelle. Daten gehen nicht verloren. Allerdings werden z.B. neue java long members in der DB mit NULL initialisiert. Dies führt in der Regel zu einer Exception beim Laden.