有关EntityManager为空值的问题

Question

    我在写一个session bean时出了问题，
         @PersistenceContext(unitName="abc")
private EntityManager em;
    对这个代码进行测试后发现他一直为空值。而且我也配置了persistence.xml文件。不知道如果persistence.xml文件写错了，会不会让它为空值？
     还有一个问题是，我想用下面的代码给这个变量赋值也不行。是没有执行这个代码,是不是PostConstruct注解的问题。
         @PostConstruct
        public void initContext(){
            final EntityManagerFactory emf = 
                    Persistence.createEntityManagerFactory("abc");
            em = emf.createEntityManager();
            System.out.println("-----------------------------------");
            System.out.println("Invoke initContext()");
            System.out.println("-----------------------------------");
        }另外，我的服务器是glassfish. persistence.xml如下
<?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="abc" transaction-type="JTA">
    <jta-data-source>jdbc/AAA</jta-data-source>
    <class>cn.edu.hit.blogsystem.entity.User</class>
    <exclude-unlisted-classes>true</exclude-unlisted-classes>
    <properties/>
  </persistence-unit>
</persistence>

Answer 1

EJB代码和测试代码贴出来看看。

Answer 2

接口的实现类，也就是session bean
@Stateless(name="UserManagerFacade")
public class UserManager implements UserManagerFacade
{
        @PersistenceContext(unitName="BlogSystemPU")
private EntityManager em;
        
        //这个方法我用于测呀啊PostConstruct注解的。也没有成功
        @PostConstruct
        public void initContext(){
            final EntityManagerFactory emf = 
                    Persistence.createEntityManagerFactory("BlogSystemPU");
            em = emf.createEntityManager();
            System.out.println("-----------------------------------");
            System.out.println("Invoke initContext()");
            System.out.println("-----------------------------------");
        }
        
        //这个是构造器。有被执行。
public UserManager(){
            System.out.println("-----------------------------------");
            System.out.println("INIT UserManager");
            System.out.println("-----------------------------------");
}

//一些接口的实现方法。
         public Object persistenceEntity(Object entity){
               System.out.println(em);                //输出为null
               System.out.println(entity);             //有值。
               em.persist(entity);
       return entity;
        }
}《persistence.xml    》
<?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="BlogSystemPU" transaction-type="RESOURCE_LOCAL">
    <jta-data-source>jdbc/blogsystem</jta-data-source>  
   =====该行的数据源配置不知道是不是正确的？在我的glassfish中有一个jdbc/blogsystem的数据库连接。=====
    <class>cn.edu.hit.blogsystem.entity.User</class>
    <exclude-unlisted-classes>true</exclude-unlisted-classes>
    <properties/>
  </persistence-unit>
</persistence>在我的客户端程序里代码如下：
              Properties pro = new Properties();
            pro.load(TestJPAClient.class.getResourceAsStream("jndi.properties"));
            final Context context = new InitialContext(pro);
            UserManagerFacade face = (UserManagerFacade) context.lookup("UserManagerFacade");
            System.out.println("-----------------------------------------------");
            System.out.println(face);
            System.out.println("-----------------------------------------------");
            User user = new User();
            user.setUserId(1);
            user.setUserName("zer");
            user.setPassword("123456");
            face.persistenceEntity(user); //客户端运行到此处出错。空引用错误。

Answer 3

//////////////以下是客户端的报错。 服务端的输出在楼上的UserManager类中
-----------------------------------------------
cn.edu.hit.blogsystem.facade._UserManagerFacade_Wrapper@2a666f0f
-----------------------------------------------
javax.ejb.EJBException: nested exception is: java.rmi.ServerException: RemoteExc
eption occurred in server thread; nested exception is:
        java.rmi.RemoteException: null; nested exception is:
        java.lang.NullPointerException
java.rmi.ServerException: RemoteException occurred in server thread; nested exce
ption is:
        java.rmi.RemoteException: null; nested exception is:
        java.lang.NullPointerException
        at com.sun.corba.ee.impl.javax.rmi.CORBA.Util.mapSystemException(Util.ja
va:243)
        at com.sun.corba.ee.impl.presentation.rmi.StubInvocationHandlerImpl.priv
ateInvoke(StubInvocationHandlerImpl.java:205)
        at com.sun.corba.ee.impl.presentation.rmi.StubInvocationHandlerImpl.invo
ke(StubInvocationHandlerImpl.java:152)
        at com.sun.corba.ee.impl.presentation.rmi.bcel.BCELStubBase.invoke(BCELS
tubBase.java:225)
        at cn.edu.hit.blogsystem.facade.__UserManagerFacade_Remote_DynamicStub.p
ersistenceEntity(cn/edu/hit/blogsystem/facade/__UserManagerFacade_Remote_Dynamic
Stub.java)
        at cn.edu.hit.blogsystem.facade._UserManagerFacade_Wrapper.persistenceEn
tity(cn/edu/hit/blogsystem/facade/_UserManagerFacade_Wrapper.java)
        at cn.edu.hit.blogsystem.client.TestJPAClient.main(TestJPAClient.java:32
)
Caused by: java.rmi.RemoteException: null; nested exception is:
        java.lang.NullPointerException
        at com.sun.enterprise.iiop.POAProtocolMgr.mapException(POAProtocolMgr.ja
va:251)
        at com.sun.ejb.containers.BaseContainer.postInvoke(BaseContainer.java:13
86)
        at com.sun.ejb.containers.BaseContainer.postInvoke(BaseContainer.java:13
16)
        at com.sun.ejb.containers.EJBObjectInvocationHandler.invoke(EJBObjectInv
ocationHandler.java:210)
        at com.sun.ejb.containers.EJBObjectInvocationHandlerDelegate.invoke(EJBO
bjectInvocationHandlerDelegate.java:117)
        at $Proxy63.persistenceEntity(Unknown Source)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at com.sun.corba.ee.impl.presentation.rmi.ReflectiveTie._invoke(Reflecti
veTie.java:154)
        at com.sun.corba.ee.impl.protocol.CorbaServerRequestDispatcherImpl.dispa
tchToServant(CorbaServerRequestDispatcherImpl.java:687)
        at com.sun.corba.ee.impl.protocol.CorbaServerRequestDispatcherImpl.dispa
tch(CorbaServerRequestDispatcherImpl.java:227)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.handleRequest
Request(CorbaMessageMediatorImpl.java:1846)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.handleRequest
(CorbaMessageMediatorImpl.java:1706)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.handleInput(C
orbaMessageMediatorImpl.java:1088)
        at com.sun.corba.ee.impl.protocol.giopmsgheaders.RequestMessage_1_2.call
back(RequestMessage_1_2.java:223)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.handleRequest
(CorbaMessageMediatorImpl.java:806)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.dispatch(Corb
aMessageMediatorImpl.java:563)
        at com.sun.corba.ee.impl.protocol.CorbaMessageMediatorImpl.doWork(CorbaM
essageMediatorImpl.java:2567)
        at com.sun.corba.ee.impl.orbutil.threadpool.ThreadPoolImpl$WorkerThread.
run(ThreadPoolImpl.java:555)
Caused by: java.lang.NullPointerException
        at cn.edu.hit.blogsystem.facade.impl.UserManager.persistenceEntity(UserM
anager.java:55)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at com.sun.enterprise.security.application.EJBSecurityManager.runMethod(
EJBSecurityManager.java:1067)
        at com.sun.enterprise.security.SecurityUtil.invoke(SecurityUtil.java:176
)
        at com.sun.ejb.containers.BaseContainer.invokeTargetBeanMethod(BaseConta
iner.java:2895)
        at com.sun.ejb.containers.BaseContainer.intercept(BaseContainer.java:398
6)
        at com.sun.ejb.containers.EJBObjectInvocationHandler.invoke(EJBObjectInv
ocationHandler.java:203)