PersistenceAnnotationBeanPostProcessor 有什么用处吗?

2022-01-18 00:00:00 spring entitymanager jakarta-ee java spring-data

根据它的JavaDoc，PersistenceAnnotationBeanPostProcessor 似乎负责用注解@PersistenceContext 注入EntityManager.它似乎暗示没有在 Spring 应用程序上下文 xml 中声明此 bean，@PersistenceContext 注释将不起作用.

According to its JavaDoc, PersistenceAnnotationBeanPostProcessor seems to be responsible for injecting the EntityManager with the annotation @PersistenceContext. It appears to imply without this bean declared in the Spring application context xml, the @PersistenceContext annotation won't work.

但是，根据我的实验，事实并非如此.

However, based on my experiments, this is not the truth. 

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence 
        http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
    version="1.0">
    <persistence-unit name="default" transaction-type="RESOURCE_LOCAL" />
</persistence>

Spring 应用程序上下文 XML

<context:component-scan base-package="com.test.dao" />

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="persistenceUnitName" value="default"/>
    <property name="jpaVendorAdapter">
        <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
            <property name="showSql" value="true"/>
            <property name="generateDdl" value="true"/>
            <property name="databasePlatform" value="org.hibernate.dialect.DerbyDialect"/>
        </bean>
    </property>
</bean>

<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
    <property name="driverClassName" value="org.apache.derby.jdbc.ClientDriver"/>
    <property name="url" value="jdbc:derby://localhost:1527/c:derbydbmydb"/>
    <property name="username" value="APP"/>
    <property name="password" value="APP"/>
</bean>

<tx:annotation-driven/>

<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
    <property name="entityManagerFactory" ref="entityManagerFactory" />
</bean>

<!-- 
    <bean id="persistenceAnnotation" class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor" />
 -->

UserDaoImpl

@Repository("userDao")
public class UserDaoImpl implements UserDao {

    @PersistenceContext
    protected EntityManager entityManager;

    @Transactional
    public void save(User user) {
            entityManager.persist(user);
    }
}

无论我注释或取消注释persistenceAnnotation bean，结果都是一样的.把豆子留在身边也无妨，但是这豆子有什么用呢?

Whether I comment or uncomment the persistenceAnnotation bean, the result is the same. It doesn't hurt to leave the bean around, but what's the use of this bean?

我使用的是 Spring 3.0.5.

I am using Spring 3.0.5.

有人能提供一个取出这个 bean 会导致失败的场景吗?

Could someone provide a scenario where taking out this bean will result in failure?

我也不喜欢仅仅为了愚弄 Spring 而创建一个空的持久化单元.幸运的是，这个问题已经在 Spring 3.1.0 中得到解决.

Also I am not fond of creating an empty persistence unit just to fool Spring. Luckily this problem has been addressed in Spring 3.1.0.

推荐答案

PersistenceAnnotationBeanPostProcessor 由 元素透明激活.准确地说，是 <context:annotation-config/> 元素激活了 bean，但是这个元素又被 <context:component-scan/><透明地激活了/代码>.

The PersistenceAnnotationBeanPostProcessor transparently activated by the <context:component-scan /> element. To be precise it's the <context:annotation-config /> element that activates the bean but this element in turn gets transparently activated by <context:component-scan />.

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