什么是HibernateUtil中的INSTANCE？

我是Java的Hibernate框架的新手。我有一个HibernateUtil类的代码，我不明白INSTANCE来自哪里。它持有的HibernateUtil类的情况下，如何？什么是HibernateUtil中的INSTANCE？

的代码是这样的......

import java.util.Properties; 
import org.hibernate.Session; 
import org.hibernate.SessionFactory; 
import org.hibernate.cfg.Configuration; 
import org.hibernate.service.ServiceRegistry; 
import org.hibernate.service.ServiceRegistryBuilder; 

public enum HibernateUtil { 

INSTANCE; // what about this??? 
public static SessionFactory sessionFactory = null; 

private synchronized SessionFactory initialiseSessionFactory() { 

    if (sessionFactory == null) { 
     Configuration config = new Configuration(); 

     config.addAnnotatedClass(demo1.class); 
     config.addAnnotatedClass(demo2.class); 
     config.configure(); 

     //get the properties from Hibernate configuration file 
     Properties configProperties = config.getProperties(); 
     ServiceRegistryBuilder serviceRegisteryBuilder = new ServiceRegistryBuilder(); 
     ServiceRegistry serviceRegistry = serviceRegisteryBuilder.applySettings(configProperties).buildServiceRegistry(); 
     sessionFactory = config.buildSessionFactory(serviceRegistry); 

    } 
    return sessionFactory; 
} 

public Session getSession() { 
    Session hibernateSession = null; 

    if (sessionFactory == null) { 
     hibernateSession = initialiseSessionFactory().openSession(); 

    } else { 
     hibernateSession = sessionFactory.openSession(); 
      } 
    return hibernateSession; 
} 
}

来源

2014-01-23 Ankit Lamba