[已解决] HibernateException：当未设置“hibernate.dialect”时，对 DialectResolutionInfo 的访问不能为空

我按照Hibernate 官方文档创建了一个版本为4.3.5.Final的基本 Hibernate 应用程序。当我执行该应用程序时，hibernate.HibernateException: Access to DialectResolutionInfo cannot be null when 'hibernate.dialect' not set尽管所有配置对我来说似乎都很好，但我还是得到了。

HibernateException：当未设置“hibernate.dialect”时，对 DialectResolutionInfo 的访问不能为空

我花了很长时间才解决这个问题，结果发现问题出在SessionFactory实例的创建方式上。我创建 SessionFactory 实例的实用程序类如下所示。

package com.journaldev.hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class SessionFactoryUtil {

	private static SessionFactory sessionFactory;

	private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
       	SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml")
        				.buildSessionFactory(new StandardServiceRegistryBuilder().build());

            return sessionFactory;
        }
        catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

	public static SessionFactory getSessionFactory() {
		if(sessionFactory == null) sessionFactory = buildSessionFactory();
        return sessionFactory;
    }
}

我得到了以下堆栈跟踪：

May 07, 2014 7:11:59 PM org.hibernate.cfg.Configuration getConfigurationInputStream
INFO: HHH000040: Configuration resource: hibernate.cfg.xml
May 07, 2014 7:12:00 PM org.hibernate.cfg.Configuration addResource
INFO: HHH000221: Reading mappings from resource: employee.hbm.xml
May 07, 2014 7:12:00 PM org.hibernate.cfg.Configuration doConfigure
INFO: HHH000041: Configured SessionFactory: null
May 07, 2014 7:12:00 PM org.hibernate.engine.jdbc.connections.internal.ConnectionProviderInitiator initiateService
WARN: HHH000181: No appropriate connection provider encountered, assuming application will be supplying connections
Initial SessionFactory creation failed.org.hibernate.HibernateException: Access to DialectResolutionInfo cannot be null when 'hibernate.dialect' not set
Exception in thread "main" java.lang.NullPointerException
	at com.journaldev.hibernate.main.HibernateMain.main(HibernateMain.java:38)

从日志中可以清楚地看出，Configuration 类能够找到Hibernate配置文件并且已经hibernate.dialect定义，因此异常没有任何意义。

修复 HibernateException：当未设置“hibernate.dialect”时，对 DialectResolutionInfo 的访问不能为空

经过一番研究和调试，我终于解决了这个问题。官方文档中缺少的部分是我们需要将配置属性应用于StandardServiceRegistryBuilder。当我将实用程序类更改为以下内容时，一切都运行良好。

package com.journaldev.hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class SessionFactoryUtil {

	private static SessionFactory sessionFactory;

	private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
        	Configuration configuration = new Configuration();
        	configuration.configure("hibernate.cfg.xml");
        	System.out.println("Hibernate Configuration loaded");

        	//apply configuration property settings to StandardServiceRegistryBuilder
        	ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build();
        	System.out.println("Hibernate serviceRegistry created");

        	SessionFactory sessionFactory = configuration
        						.buildSessionFactory(serviceRegistry);

            return sessionFactory;
        }
        catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

	public static SessionFactory getSessionFactory() {
		if(sessionFactory == null) sessionFactory = buildSessionFactory();
        return sessionFactory;
    }
}

注意StandardServiceRegistryBuilder().applySettings传递配置属性的调用。Hibernate 仍在不断发展，每个版本都有很多变化，我觉得文档部分已经落后了。我希望这能帮助人们解决问题，而不是浪费大量时间进行调试。