Hibernate 5: Why am I Getting the \'org.hibernate.MappingException: Unknown Entity\' Error and How Do I Fix It?

Hibernate 5: Resolving "org.hibernate.MappingException: Unknown Entity" Error

Problem:

You've encountered the "org.hibernate.MappingException: Unknown entity" error while attempting to integrate Hibernate 5.0 with MySQL. This error occurs with Hibernate versions 5.0.0 and 5.0.1 but not with Hibernate 4.3.9.

Cause:

The issue arises due to a change in the way SessionFactory is created with Hibernate 5. Using the Configuration().buildSessionFactory(sr) method as in earlier versions of Hibernate no longer works, as it discards mapping information.

Solution:

To resolve this error, there are two approaches:

1. Simplified Configuration:

For standard configuration files like hibernate.cfg.xml and hibernate.properties, you can create SessionFactory without using ServiceRegistry:

<code class="java">SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();</code>

2. Using StandardServiceRegistryBuilder for Loading Properties:

If you have properties in files other than hibernate.properties, you can use StandardServiceRegistryBuilder to load them:

Loading Properties as a Resource:

<code class="java">ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().
    configure().loadProperties("hibernate-h2.properties").build();
SessionFactory sf = new Configuration().buildSessionFactory(serviceRegistry);</code>

Loading Properties from a File System Path:

<code class="java">File propertiesPath = new File("some_path");
ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().
    configure().loadProperties(propertiesPath).build();
SessionFactory sf = new Configuration().buildSessionFactory(serviceRegistry);</code>

Additional Notes:

• The Hibernate 5 tutorial contains an incorrect approach for creating SessionFactory. It demonstrates the method new Configuration().configure().buildSessionFactory(new StandardServiceRegistryBuilder().build()), which does not properly configure Hibernate.
• This issue affects Hibernate versions 5.0.0 and 5.0.1. For Hibernate 5.1 and later, the original configuration method Configuration().buildSessionFactory(sr) works as expected.

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