Editing Hibernate (section)

==Introduction==
Hibernate is an open source object/relational mapping tool for Java. Hibernate lets you develop persistent classes following common Java idiom - including association, inheritance, polymorphism, composition and the Java collections framework.<br>
[[Image:HibernateArchitecture.jpg]]<br>
Hibernate makes use of persistent objects commonly called as POJO (POJO = "Plain Old Java Object".) along with XML mapping documents for persisting objects to the database layer. The term POJO refers to a normal Java objects that does not serve any other special role or implement any special interfaces of any of the Java frameworks (EJB, JDBC, DAO, JDO, etc...). Hibernate uses runtime reflection to determine the persistent properties of a class. The objects to be persisted are defined in a mapping document, which serves to describe the persistent fields and associations, as well as any subclasses or proxies of the persistent object. The mapping documents are compiled at application startup time and provide the framework with necessary information for a class. Additionally, they are used in support operations, such as generating the database schema or creating stub Java source files.
In order to provide a clean POJO programming model, Hibernate hides itself inside of your POJO by using either its own implementation of the JDK collections or by using a CGLIB proxy to surround an object reference, depending on the type of association it is managing. Since object graphs can be quite large, and in some cases infinite, it is mandatory to draw the line somewhere when loading an object and claim these associations as lazy. This deferral means that at some later point, it may be necessary to fetch the associated objects from the database when this line is crossed.

===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.