java – PersistenceAnnotationBeanPostProcessor有什么用吗？

根据其 JavaDoc,PersistenceAnnotationBeanPostProcessor似乎负责向EntityManager注入注释@PersistenceContext.这似乎意味着没有这个bean在Spring应用程序上下文xml中声明,@PersistenceContext注释将不起作用.

然而,根据我的实验,这不是事实.

persistence.xml中

<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:\derbydb\mydb"/>
    <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" />
 -->

在UserDAOImpl

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

    @PersistenceContext
    protected EntityManager entityManager;

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

无论我评论还是取消注释persistenceAnnotation bean,结果是一样的.离开这个bean并不会伤害它,但是这个bean有什么用？

我使用的是Spring 3.0.5.

有人会提供一个场景,取出这个bean会导致失败？

另外我也不喜欢创建一个空的持久化单元来愚弄Spring.幸运的是,这个问题已经在Spring 3.1.0中解决了.