@PersistenceUnit注释不会创建EntityManageFactory emf = null

Question

我尝试使用Sun Java PetStore演示.
在CatalogFacade类中,有以下注释:

@PersistenceUnit(unitName="myPetStorePU")
private EntityManagerFactory emf;

在CatalogFacade Sun的所有方法中都有:

    EntityManager em = emf.createEntityManager();

但是在尝试createEntityManager时,我得到emf的空指针异常.但是......如果我在该行上方添加以下行

    EntityManagerFactory emf = javax.persistence.Persistence.createEntityManagerFactory("myPetStorePU");
    EntityManager em = emf.createEntityManager();

然后emf成功创建,持久性单元myPetStorePU也成功连接到数据库.所以它看起来像persistence.xml语法和它的位置是正确的.我想理解为什么注释不起作用,因为我认为只有使用注释而不是在每个方法中添加createEntityManagerFactory行是有原因的.

我的src/META-INF/persistence.xml文件如下所示:

<persistence-unit name="myPetStorePU">
    <description>Petstore Persistence Unit</description>
    <provider>oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider</provider>

  <class>com.sun.javaee.blueprints.petstore.model.Tag</class>
  <class>com.sun.javaee.blueprints.petstore.model.SellerContactInfo</class>
  <class>com.sun.javaee.blueprints.petstore.model.Product</class>
  <class>com.sun.javaee.blueprints.petstore.model.Item</class>
  <class>com.sun.javaee.blueprints.petstore.model.Category</class>
  <class>com.sun.javaee.blueprints.petstore.model.Address</class>
  <class>com.sun.javaee.blueprints.petstore.model.ZipLocation</class>
    <properties>
        <property name="toplink.jdbc.driver" value="oracle.jdbc.driver.OracleDriver"/>
        <property name="toplink.jdbc.url" value="jdbc:oracle:thin:@#############"/>
        <property name="toplink.jdbc.user" value="####"/>
        <property name="toplink.jdbc.password" value="#####"/>
        <property name="toplink.logging.level" value="INFO"/>
    </properties>

</persistence-unit>

编辑:CatalogFacade位于petstore.model包中并实现ServletContextListener

<listener>
    <listener-class>com.sun.javaee.blueprints.petstore.model.CatalogFacade</listener-class>
</listener>

在index.jsp中Sun有以下内容:

<%
CatalogFacade cf = (CatalogFacade)config.getServletContext().getAttribute("CatalogFacade");
List<Tag> tags=cf.getTagsInChunk(0, 12);
%>


public List<Tag> getTagsInChunk(int start, int chunkSize) {
//The next line is required since the @PersistenceUnit annotation at the top of this class does not work
    EntityManagerFactory emf = javax.persistence.Persistence.createEntityManagerFactory("myPetStorePU");
    EntityManager em = emf.createEntityManager();
    System.out.println("Entity manager " + emf);
    Query query = em.createQuery("SELECT t FROM Tag t ORDER BY t.refCount DESC, t.tag");
    List<Tag> tags = query.setFirstResult(start).setMaxResults(chunkSize).getResultList();
    em.close();
    return tags;
}

Answer 1

如果带注释的对象不是由容器(spring/CDI/EJB容器)管理的,则不会向其注入任何内容.

因此,根据您的环境,获取该对象的上下文实例.

如果您没有使用上述任何技术(spring/CDI/EJB) - 那么就不能使用@PersistenceUnit和@PersistenceContext.使用手动方式获取装置.