Why Does Hibernate 5.0 Throw 'Unknown Entity' MappingException, and How Can I Fix It?

Hibernate MappingException: Unknown Entity in Hibernate 5

When integrating Hibernate 5.0 with MySQL, you may encounter the error "org.hibernate.MappingException: Unknown entity." This issue is prevalent in versions 5.0.0 and 5.0.1, but not in Hibernate 4.3.9.

Diagnosis

The issue stems from a mismatch in the Hibernate 5 configuration code. The following code fragment from your "HibernateMain.java" class is problematic:

Configuration configuration = new Configuration();
configuration.configure();

ServiceRegistry sr = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build();

SessionFactory sf = configuration.buildSessionFactory(sr);

In Hibernate 5, using configuration.buildSessionFactory(sr) leads to a loss of mapping information acquired during configuration.configure().

Solution

To resolve this issue, employ the following approach without ServiceRegistry:

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

Additional Loading Options for Properties

• Loading Properties as a Resource:

ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().
    configure().loadProperties("hibernate-h2.properties").build();
SessionFactory sf = new Configuration().buildSessionFactory(serviceRegistry);  

• Loading Properties from a File System Path:

File propertiesPath = new File("some_path");
ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().
    configure().loadProperties(propertiesPath).build();
SessionFactory sf = new Configuration().buildSessionFactory(serviceRegistry);

Conclusion

Avoid using the incorrect configuration method presented in the Hibernate 5 tutorial 1.1.6. Utilize the aforementioned solutions to rectify the "Unknown entity" issue and establish a successful Hibernate 5 integration with MySQL.

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