使用Spring注入EntityManager（空指针异常） [英] Injecting EntityManager using Spring ( Null Pointer Exception )

问题描述

这是我的ApplicationContext.xml的代码

Here's the code insidy my ApplicationContext.xml

    <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实现方案

and here's my Dao Implementation

public class TeacherDaoImpl implements TeacherDao {

@Autowired
private EntityManager entityManager;

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

}

}

这是我的主类

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"));

}

}

请帮助，我得到一个空指针异常

Please help, i am getting a null pointer exception

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)

我已经在2天内解决了这个问题，但是我仍然找不到任何可以解决这个问题的资源。

ive been solving this problem in 2 days but still i cant find any resources that may solve this problem. i appreciate if you give me your opinion,answers or any idea that might help me solving this,

ps：我对学习春天感到陌生

ps: i am new on learning spring

推荐答案

由于您是自己实例化 TeacherDaoImpl （使用 new EntityManager 以及NPE。

Since you are instantiating TeacherDaoImpl yourself (with the new keyword) within main, Spring is not injecting the EntityManager and hence the NPE.

字段 TeacherDaoImpl.entityManager 与 @PersistenceContext 并注释 TeacherDaoImpl 带有 @Component 的类，让Spring为您实例化它。然后在您的主体中，获得该bean的所有权：

Annotate the field TeacherDaoImpl.entityManager with @PersistenceContext and annotate the TeacherDaoImpl class with @Component to have Spring instantiate it for you. Then in your main, get a hold of that bean:

TeacherDao dao = applicationContext.getBean(TeacherDao.class);
// ...

这两个指令似乎也是不必要的：

Also these two directives seem to be unnecessary:

<context:annotation-config />
<context:spring-configured />

在使用< context：component-scan时隐含前者/> 。仅当您在代码中使用 @Configurable 时，后者才有用。

The former is implied when you are using <context:component-scan />. The latter is only useful if you are using @Configurable in your code.

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