Why I have to declare each and every class in my hibernate.cfg.xml when using annotations?

Question

Why it isn't enough to set the @Entity annotation?

Am I missing the point here e.g. performance?

Answer 1

The annotation is not enough because hibernate does not know where your annotated classes live without some sort of explicit declaration. It could, in theory, scan every single class in the classpath and look for the annotation but this would be very very expensive for larger projects.

You can use spring which has a helper that can allow you to specify the package(s) that your hibernate objects are in and it will just scan these packages for @Entity. If you have all your objects in a small number of fixed packages this works well.

E.g.

  <bean id="referenceSessionFactory" class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="packagesToScan">
      <array>
        <value>com.xxx.hibernate.objects</value>
      </array>
    </property>
  </bean>

The above is the Spring declaration. If you aren't familiar with the above syntax you can just construct it programmatically.

AnnotationSessionFactoryBean sfb = new AnnotationSessionFactoryBean();
sfb.setDataSource( ds );
sfb.setHibernateProperties( hibProps);
sfb.setPackagesToScan( ... );
sfb.initialise();
SessionFactory sf = sfb.getObject();

It supports a bunch of config options so you can use raw properties or pass in a pre-config'd datasource.

Answer 2

You don't if you set hibernate.archive.autodetection property to true. There is a performance issue here as Hibernate will search the jar files for the JPA annotation. Note that, it will also initializes those classes.