how to keep jpa session in the thread - spring

Question

Now I use JPA in my web application. And I have a class like this:
class A {
...
#OneToMany
private List<B> list;
...
}
When it is in a HTTP request, I can use a.getList() successful. But in a schedule thread, it throws the exception:
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: A.list, could not initialize proxy - no Session
Can I keep the session in the schedule thread just like the http request thread?

Answer 1

Actually, when spring handle the http request, it start the transaction by the interceptor org.springframework.orm.hibernate3.support.OpenSessionInViewInterceptor or the filter org.springframework.orm.hibernate3.support.OpenSessionInViewFilter. So if we want to keep the session, we can start and commit the transaction by our self.
Here is the code:
public class BackendThread {
#Autowired
private PlatformTransactionManager platformTransactionManager;
#Override
public void onMessage(Message message, byte[] pattern) {
new TransactionTemplate(platformTransactionManager).execute(new TransactionCallback<Void>() {
#Override
public Void doInTransaction(TransactionStatus transactionStatus) {
...
your code here
...
return null;
}
});
}
}

Answer 2

TransactionTemplate is helpful if you use JPA. I didn't use it and created a session/transactions on my own in background thread, so that each task had its own environment:
Inject entity manager or get it from Spring context or pass it as reference:
#PersistenceContext
private EntityManager entityManager;
Then create a new entity manager, to avoid using a shared one:
EntityManager em = entityManager.getEntityManagerFactory().createEntityManager();
Now you can start transaction and use Spring DAO, Repository, JPA, etc
private void save(EntityManager em) {
try
{
em.getTransaction().begin();
<your database changes>
em.getTransaction().commit();
}
catch(Throwable th) {
em.getTransaction().rollback();
throw th;
}
}