java - HibernateUtil中的INSTANCE是什么？

我是Java的Hibernate框架的新手。
我有一个HibernateUtil类的代码，我不知道INSTANCE的来源。
它具有HibernateUtil类的作用，如何？？？

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;
}
}

最佳答案

HibernateUtil正在实现单例模式：该类只能有一个实例，并且该实例是HibernateUtil.INSTANCE。