关于Spring与hibernate整合的困惑

在整合Spring和Hibernate的过程中遇到一点问题。不太明白Spring是怎样将SessionFactory实例化的，我的applicationContext.xml如下配置

<!-- 自动扫描与装配bean -->
<context:component-scan base-package="com.bjwl"></context:component-scan>
<!-- 一、配置SessionFactory，在这里写所有的Hibernate配置
注意：hibernate配置前最好都加上"hibernate."前缀
-->
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<property name="dataSource" ref="dataSource"></property>
<property name="hibernateProperties">
<props>
<prop key="hibernate.show_sql">true</prop>
<prop key="hibernate.hbm2ddl.auto">update</prop>
</props>
</property>
<property name="mappingLocations">
<list>
<value>classpath:com/bjwl/domain/*.hbm.xml</value>
</list>
</property>
</bean>
<bean id="hibernateTemplate" class="org.springframework.orm.hibernate3.HibernateTemplate">
<property name="sessionFactory" ref="sessionFactory"></property>
</bean>

<!-- 二、配置声明式事务管理，采用基于注解的方式 -->
<bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
<property name="sessionFactory" ref="sessionFactory"></property>
</bean>
<tx:annotation-driven transaction-manager="transactionManager"/>

public class HibernateTest {
//ApplicationContext ac =  new ClassPathXmlApplicationContext("applicationContext.xml");
@Resource
private SessionFactory sessionFactory;

public void test1(){
//SessionFactory sessionFactory = (SessionFactory) ac.getBean("sessionFactory");(可以获取到sessionFactory)
System.out.println(sessionFactory);//(输出为null,也就是sessionFactory没有注入)

}
}

但是我改变了测试方法如下：

@Service
public class UserService {

@Resource
private SessionFactory sessionFactory;

@Transactional
public void saveTwoUsers() {
Session session = sessionFactory.getCurrentSession();

session.save(new Person());
// int a = 1 / 0; // 这行会抛异常
session.save(new Person());
}
}

@Test
public void testTransaction() throws Exception {
UserService userService = (UserService) ac.getBean("userService");
userService.saveTwoUsers();
}

这样又能够执行成功。sessionFactory注入成功。不清楚为什么会有这两种结果。希望能有高手能帮我解惑。