使用Spring注入EntityManager(Null指针异常)[重复]

这是我的ApplicationContext.xml中的代码

    <context:spring-configured />
<context:annotation-config />
<context:component-scan base-package="com.apsas.jpa" />
<tx:annotation-driven />

<bean id="entityManagerFactory"
    class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
    <property name="persistenceUnitName" value="testjpa" />
</bean>

<bean id="entityManager"
    class="org.springframework.orm.jpa.support.SharedEntityManagerBean">
    <property name="entityManagerFactory" ref="entityManagerFactory" />
</bean>
    class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor" />

这是我的Dao Implementation

public class TeacherDaoImpl implements TeacherDao {

@Autowired
private EntityManager entityManager;

@Transactional
public Teacher addTeacher(Teacher teacher) {
    entityManager.persist(teacher);
    return teacher;

}

}

这是我的主课

public class TestApp {

public static void main(String[] args) {

    ApplicationContext applicationContext = new ClassPathXmlApplicationContext(
            "config/ApplicationContext.xml");       

    TeacherDao teacherDao = new TeacherDaoImpl();       
    Teacher teacher1 =  teacherDao.addTeacher(new Teacher("First Teacher"));

}

}

请帮忙,我得到一个空指针异常

Exception in thread "main" java.lang.NullPointerException
at com.apsas.jpa.dao.impl.TeacherDaoImpl.addTeacher(TeacherDaoImpl.java:22)
at com.apsas.jpa.main.TestApp.main(TestApp.java:26)

我在两天内解决了这个问题,但仍然找不到任何可以解决这个问题的资源。如果你能给我你的意见、答案或任何可能帮助我解决这个问题的想法,我将不胜感激,

ps:我是春季学习新手