Question

我是JPA的佼佼者，当我在servlet中使用@PersistenceUnit时，我遇到了问题。 EntityManagerFactory没有被注入，我不知道为什么。

public class MyServlet extends HttpServlet
{   

  @PersistenceUnit
  private EntityManagerFactory emf = null; 

  public MyServlet ()
  {
    super();        
  }

  @Override
  public void init(ServletConfig config) throws ServletException
  {
    super.init(config);
  }


  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Writer w = response.getWriter();
    w.append("HELLO HELLO");
    w.close();
  }

}

我已经配置了persistence.xml文件（在META-INF目录中），如下所示：

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" 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">
<persistence-unit name="MiddlewareExpedElectrModel" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.ejb.HibernatePersistence</provider>     
    <non-jta-data-source>java:/OracleDS</non-jta-data-source>
    <class>com.ieci.mugeju.middleware.model.entities.FechaUltimaSolicitudProcesada</class>
    <properties>         
     <property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.JBossTransactionManagerLookup"/>
     <property name="hibernate.show_sql" value="true"/>
     <property name="hibernate.dialect" value="org.hibernate.dialect.Oracle10gDialect"/>         
  </properties>     
</persistence-unit>

为什么不注入EntityManagerFactory？

我正在使用Jboss 4.2.3，我还使用oracle-ds.xml文件配置了数据源。

更新

如果我尝试以编程方式获取EntityManagerFactory，则效果很好：

 EntityManagerFactory emf = Persistence.createEntityManagerFactory("MiddlewareExpedElectrModel");

但是我希望通过注入获得一个EntityManagerFactory实例。谢谢！

Answer 1

你能试试吗

public class MyServlet extends HttpServlet {
//this is thread-safe
@PersistenceUnit(unitName="MiddlewareExpedElectrModel")
private EntityManagerFactory emf;

据我所知，EntityManager接口中的方法不是线程安全的，并且可能不会在多个并发请求之间共享。因此，不要将EntityManager注入servlet实例变量。

这不是线程安全的，并且避免使用

@PersistenceContext(unitName="MiddlewareExpedElectrModel")
private EntityManager em;

您仍然可以在servlet类类型级别注入EntityManager，并在请求处理期间需要时查找它。

@PersistenceContext(unitName="MiddlewareExpedElectrModel", name="persistence/em")
public class MyServlet extends HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
try {
  InitialContext ic = new InitialContext();
  EntityManager em =
   (EntityManager) ic.lookup("java:comp/env/persistence/em");
  } catch (NamingException ex) {
    ...
  }

Answer 2

我在glassfish服务器上的应用程序中遇到了同样的问题。直到我从web.xml中删除servlet声明并且使用批注声明servlet时，entitymanager工厂始终为空。

@WebServlet("/GetRevisioniServlet")  // <-- this annotation solved all my problems
public class GetRevisioniServlet extends HttpServlet {

@PersistenceUnit(unitName = "clerpPU")
private EntityManagerFactory entityManagerFactory;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

        EntityManager em = null;
        em = entityManagerFactory.createEntityManager(); 
        .....
    }
 }

没有注入EntityManagerFactory - JPA

2 个答案: