Java Spring:AnnotationSessionFactoryBean,Hibernate-Dialect AutoDetection

Question

我使用以下spring应用程序上下文:

<!-- Hibernate session factory -->
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="hibernateProperties">
      <props>
        <!--
             <prop key="hibernate.dialect">${hibernate.dialect}</prop>
          -->
            <prop key="hibernate.show_sql">${hibernate.show_sql}</prop>
            <prop key="hibernate.max_fetch_depth">${hibernate.max_fetch_depth}</prop>

            <prop key="hibernate.cache.use_second_level_cache">${hibernate.cache.use_second_level_cache}</prop>
            <prop key="hibernate.cache.use_query_cache">${hibernate.cache.use_query_cache}</prop>
            <prop key="hibernate.cache.provider_class">${hibernate.cache.provider}</prop>

            <prop key="hibernate.connection.pool_size">10</prop>
            <prop key="hibernate.jdbc.batch_size">1000</prop>
            <prop key="hibernate.bytecode.use_reflection_optimizer">true</prop>

        </props>
    </property>
    <property name="annotatedClasses">
        <list>
          ...
       </list>
    </property>

    <property name="schemaUpdate" value="${hibernate.schemaUpdate}"/>
</bean>

然后问题是:虽然Hibernate应该支持Dialect-Autodetection,但是这个代码不起作用: Uppon Application-Init抛出以下异常:

org.hibernate.HibernateException: The dialect was not set. Set the property hibernate.dialect.
    at org.hibernate.dialect.Dialect.instantiateDialect(Dialect.java:256)
    at org.hibernate.dialect.Dialect.getDialect(Dialect.java:234)
    at org.hibernate.dialect.Dialect.getDialect(Dialect.java:249)
    at org.springframework.orm.hibernate3.LocalSessionFactoryBean$3.doInHibernate(LocalSessionFactoryBean.java:957)
    at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:419)
    at org.springframework.orm.hibernate3.HibernateTemplate.execute(HibernateTemplate.java:339)
    at org.springframework.orm.hibernate3.LocalSessionFactoryBean.updateDatabaseSchema(LocalSessionFactoryBean.java:953)
    at org.springframework.orm.hibernate3.LocalSessionFactoryBean.afterSessionFactoryCreation(LocalSessionFactoryBean.java:843)
    at org.springframework.orm.hibernate3.AbstractSessionFactoryBean.afterPropertiesSet(AbstractSessionFactoryBean.java:213)

我可以通过取消注释hibernate.dialect属性来避免此异常.为什么我要这样做？为什么不能休眠只是像它应该的那样自动检测方言？

我想删除dialect-property来解决这个问题.

Answer 1

Hibernate本身尝试在Configuration.buildSessioNFactory()调用期间自动检测方言- 通常,但并非总是如此.

但是,Spring代码没有 - 这就是你获得异常的地方.LocalSessionFactoryBean.updateDatabaseSchema()方法使用Dialect.getDialect()方法获取Dialect需要明确设置方言的实例.

我不太清楚为什么会这样做,虽然我怀疑它是防止Hibernate错误地检测你的方言错误并且因此弄乱你的数据库的预防措施(因为更新架构可能是非常灾难性的).

你的选择是:

1. 明确指定方言
2. 设置schemaUpdate为false
3. 扩展LocalSessionFactoryBean并重写updateDatabaseSchema()方法以从会话工厂实例中获取(已经自动检测到的)方言.

你必须将它强制转换为SessionFactoryImplementor:

public void updateDatabaseSchema() throws DataAccessException {
  final Dialect dialect = ((SessionFactoryImplementor) getSessionFactory()).getDialect();
  ...
  hibernateTemplate.execute(new HibernateCallback() {
    ...
    // you already have the dialect so you don't need to get it using this:
    // Dialect dialect = Dialect.getDialect(getConfiguration().getProperties());
    ...
  );
}