[Solved] 当未设置 ‘hibernate.dialect’ 时，无法将 DialectResolutionInfo 访问为 null 的 HibernateException

我按照Hibernate官方文档创建了一个使用4.3.5.Final版本的基本Hibernate应用程序。当我执行该应用程序时，尽管所有配置看起来都很好，但我却收到了hibernate.HibernateException: 当’hibernate.dialect’未设置时，无法访问DialectResolutionInfo的错误信息。

当未设置“hibernate.dialect”时，无法将访问DialectResolutionInfo设置为null的Hibernate异常。

解决这个问题花了我很长时间，结果发现是SessionFactory实例创建方式的问题。我用下面的方法创建SessionFactory实例的辅助类。

package com.Olivia.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;
    }
}

而且我得到了以下的堆栈跟踪：

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.Olivia.hibernate.main.HibernateMain.main(HibernateMain.java:38)

从日志中可以明确看出，Configuration类能够找到hibernate配置文件，并且已经定义了hibernate.dialect，所以异常没有任何意义。

当未设置‘hibernate.dialect’时，修复HibernateException：访问DialectResolutionInfo不能为空。

经过一些研究和调试，我成功修复了它。在官方文件中缺失的部分是我们需要将配置属性应用于StandardServiceRegistryBuilder。当我把实用类更改如下，一切都顺利运行。

package com.Olivia.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的调用，其中传递了配置属性。Hibernate仍在不断发展，每个版本都带来了许多变化，我觉得文档部分滞后了。希望这能帮助某人解决问题，而不是浪费很多时间进行调试。