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Hibernate/Spring: failed to lazily initialize - no session or session was closed

Answer 1

+2 A:

i think you should not use the hibernate session transactionnal methods, but let spring do that.

add this to your spring conf:

<bean id="txManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
    <property name="sessionFactory" ref="mySessionFactory" />
</bean>

<bean id="transactionTemplate" class="org.springframework.transaction.support.TransactionTemplate">
    <property name="transactionManager" ref="txManager"/>
</bean>

and then i would modify your test method to use the spring transaction template :

public static void main(String[] args) {
    // init here (getting dao and transaction template)

    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // do your hibernate stuff in here : call save, list method, etc
        }
    }
}

as a side note, @OneToMany assocations are lazy by default, so you don't need to annotate it lazy. (@*ToMany are LAZY by default, @*ToOne are EAGER by default)

EDIT: here is now what is happening from hibernate point of view:

open session (with transaction start)
save a user and keep it in the session (see the session cache as an entity hashmap where the key is the entity id)
save an event and keep it in the session
save another event and keep it in the session
... same with all the save operations ...
then load all users (the "from Users" query)
at that point hibernate see that it has already the object in its session, so discard the one it got from the request and return the one from the session.
your user in the session does not have its event collection initialized, so you get null.
...

Here is some points to enhance your code :

in your model, when collection ordering is not needed, use Set, not List for your collections (private Set events, not private List events)
in your model, type your collections, otherwise hibernate won't which entity to fetch (private Set<Event> events)
when you set one side of a bidirectionnal relation, and you wish to use the mappedBy side of the relation in the same transaction, set both sides. Hibernate will not do it for you before the next tx (when the session is a fresh view from the db state).

So to address the point above, either do the save in one transaction, and the loading in another one :

public static void main(String[] args) {
    // init here (getting dao and transaction template)
    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // save here
        }
    }

    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // list here
        }
    }
}

or set both side:

...
event1.setUser(user);
...
event2.setUser(user);
...
user.setEvents(Arrays.asList(event1,event2));
...

(Also do not forget to address the code enhancement points above, Set not List, collection typing)

Thierry 2010-06-14 22:29:58

I tried your solution, and I got rid of the Lazy problem - unfortunately, not it does not load any Events at all: user.getEvents() Produces a null pointer (altough I can see in the db and from the first iteration that the user does have events associated). Is it possible that hibernateTemplate.find doe snot resolve the dependencies?

Niko 2010-06-15 06:56:05

I uploaded my test-project with your suggestion already in place. maybe you can find out whats going on.

Niko 2010-06-15 07:10:03

updated the answer.

Thierry 2010-06-15 12:38:22

Answer 2

A:

The issue is that your dao is using one hibernate session but the lazy load of the user.getName (I assume that is where it throws) is happening outside that session -- either not in a session at all or in another session. Typically we open up a hibernate session before we make DAO calls and don't close it until we are done with all lazy loads. Web requests are usually wrapped in a big session so these problems do not happen.

Typically we have wrapped our dao and lazy calls in a SessionWrapper. Something like the following:

public class SessionWrapper {
    private SessionFactory sessionFactory;
    public void setSessionFactory(SessionFactory sessionFactory) {
        this.hibernateTemplate = new HibernateTemplate(sessionFactory);
    }
    public <T> T runLogic(Callable<T> logic) throws Exception {
        Session session = null;
        // if the session factory is already registered, don't do it again
        if (TransactionSynchronizationManager.getResource(sessionFactory) == null) {
            session = SessionFactoryUtils.getSession(sessionFactory, true);
            TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));
        }

        try {
            return logic.call();
        } finally {
            // if we didn't create the session don't unregister/release it
            if (session != null) {
                TransactionSynchronizationManager.unbindResource(sessionFactory);
                SessionFactoryUtils.releaseSession(session, sessionFactory);
            }
        }
    }
}

Obviously the SessionFactory the same SessionFactory that was injected into your dao.

In your case, you should wrap the entire listUserWithEvent body in this logic. Something like:

public List listUserWithEvent() {
    return sessionWrapper.runLogic(new Callable<List>() {
        public List call() {
            List users = hibernateTemplate.find("from User");
            for (User user : users) {
                System.out.println("LIST : " + user.getName() + ":");
                user.getEvents().size();
            }
        }
    });
}

You will need to inject the SessionWrapper instance into your daos.

Gray 2010-06-14 22:57:45

Unfortunately I don't really get this to work. Do you mean that you DAOImpl extend the SessionWrapper? And who (in my example) would call runLogic and what would be the Callable? Maybe its cleaerer when you extend the example and adapt it to the sample code I provided.Also, my question is specific to non-web, because I want to use the same model/daos I use in web also for stand-alone applications (Our data crawlers).

Niko 2010-06-15 07:00:20

Sorry it was confusing. I've added a section with a runLogic example. You shouldn't need to change your dao at all. It should be injected with the same SessionFactory injected into the SessionWrapper. The SessionWrapper then gets injected into the daos.

Gray 2010-06-15 12:35:39

That's what the spring transaction template is doing. (it's just missing Typing :/)

Thierry 2010-06-15 12:42:06

The spring transaction template also opens a database transaction right? We didn't want to have to do that with our session wrapper.

Gray 2010-06-15 13:09:49

What do you mean open a database tx ? It opens a connection to the database, exactly like getting a session from the sessionFactory. Enabling a transaction is just the fact of setting the autoCommit flag from on to off on the connection.

Thierry 2010-06-16 21:10:05

Depends on how it is implemented. In hibernate I'm pretty sure you are starting a SavePoint as well which causes the database to have to save state so you can rollback. Right?

Gray 2010-06-17 14:07:42

Answer 3

+1 A:

I got here looking for a hint regarding a similar problem. I tried the solution mentioned by Thierry and it didnt work. After that I tried these lines and it worked:

SessionFactory sessionFactory = (SessionFactory) context.getBean("sessionFactory");
Session session = SessionFactoryUtils.getSession(sessionFactory, true);
TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));

Indeed what I'm doing is a batch process that must leverage Spring existings managers/services. After loading the context and doing some invocations I founded the famous issue "failed to lazily initialize a collection". Those 3 lines solved it for me.

Fran Jiménez 2010-08-05 16:42:17

this really did the trick - thanks a lot

Niko 2010-08-11 09:39:36

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Hibernate/Spring: failed to lazily initialize - no session or session was closed

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