Spring equivalent of #Stateless EJB with multiple #PersistenceContext - spring

Question

I've been banging my head on the wall for the past couple of days trying to get this to work but I've not been able to.
I wrote a JPA EntityManagerProvider recently using EJB:
EntityManagerProviderBean.java
#Stateless
public class EntityManagerProviderBean {
#PersistenceContext(unitName="PU1")
private EntityManager entityManager1;
#PersistenceContext(unitName="PU2")
private EntityManager entityManager2;
public EntityManager getEntityManager() {
return (...) ? entityManager1: entityManager2;
}
}
And then of course I can inject the EJB wherever needed like this:
UserFacade.java
#Stateless
public class UserFacade {
#EJB
private EntityManagerProviderBean emProvider;
private EntityManager em = emProvider.getEntityManager();
...
}
Now I'm trying to do something similar using Spring, using annotations, and without doing anything in XML. I can't seem to figure out a way to inject the EntityManager. Anything that I do leads to a NullPointerException. For example, I tried to inject the EntityManager manually without relying on my EntityManagerProviderBean, like this:
UserFacadeSpring.java
public class UserFacadeSpring {
#PersistenceContext(unitName="PU1")
private EntityManager em;
...
}
But this gives me a NullPointerException. So the EntityManager is not being injected at all and I'm not sure what's wrong.
So two questions basically:
How can I inject the EntityManager using Spring?
How can I use my existing EntityManagerProviderBean EJB in Spring? What modifications do I need to make?
Any help in this matter will be greatly appreciated. As you can tell I'm a complete noob to Spring. I tried to read the guide but everything's flying over my head at the moment. I actually did try to do something half-baked but it didn't work either (I either get NullPointerException or BeanNotFoundException, I must have used every combination of #Component, #Bean, #Autowired annotations I think!):
EntityManagerProviderSpring.java
#Component
public class EntityManagerProviderSpring {
#PersistenceContext(unitName="PU1")
private EntityManager entityManager1;
#PersistenceContext(unitName="PU2")
private EntityManager entityManager2;
#Bean
public EntityManager getEntityManager() {
return (...) ? entityManager1: entityManager2;
}
}
Main.java
public class Main {
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(EntityManagerProviderSpring.class);
EntityManagerProviderSpring emProvider = context.getBean(EntityManagerProviderSpring.class);
EntityManager em = emProvider.getEntityManager();
...
}
Thanks!
EDIT: I'm using Hibernate with JPA and the following is my persistence.xml file:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="PU1" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>...</class>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/db1?zeroDateTimeBehavior=convertToNull"/>
<property name="javax.persistence.jdbc.user" value="pu1_username"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="javax.persistence.jdbc.password" value="pu1_pwd"/>
<property name="hibernate.cache.provider_class" value="org.hibernate.cache.NoCacheProvider"/>
</properties>
</persistence-unit>
<persistence-unit name="PU2" transaction-type="RESOURCE_LOCAL">
...
</persistence-unit>
</persistence>

Answer 1

To have the exact same functionality on dependency injection level, it is impossible to map stateless ejb bean to a spring bean.
EJB #Singleton -> Spring #Component (or #Service or #Controller or #Repository)
EJB #Stateful -> Spring #Component#Scope("prototype") (or #Service#Scope("prototype") or #Controller#Scope("prototype") or #Repository#Scope("prototype"))
EJB #Stateless -> DOES NOT EXIST IN SPRING.
Stateless beans are very handy for this multiple persistence contexts... However, since Spring doesn't offer stateless bean. You have to use custom Spring setup to apply multiple persistence context. It won't work with only persistence.xml like EJB, unfortunately :(.
So to answer your questions:
Read Spring doc... Plenty of examples everywhere. Shortly, In Spring 4 Java Config you can use #EnableTransactionManagement, a transaction manager and an entity manager factory per persistence context.
http://docs.spring.io/spring/docs/current/spring-framework-reference/html/ejb.html. This is how you do it. Not a nice way since using jndi, but it is a solution...