Hibernate, Java : no session or session was closed

Question

I already saw this kind of problem on Stackoverflow but nothing could help me to resolve my problem.

I am novice in Hibernate and have a project to be made in Java and MySQL, and thus use hibernate. I managed to reach my data, to modify them, to deletethem, but I block on a method because I have one exception which arrives.. And seen that I understand still not all the threads I manage not to remove this bug:

Here is my error :

org.hibernate.LazyInitializationException Grave: failed to lazily initialize a collection of role: DAO.User.files, no session or session was closed org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: DAO.User.files, no session or session was closed at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:358) at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:350) at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:343) at org.hibernate.collection.PersistentSet.add(PersistentSet.java:189) at DAO.UserDAO.removeGroupAddUserFiles(UserDAO.java:252) at javaapplication5.JavaApplication5.main(JavaApplication5.java:37)

And Here are my Java classes :

public static void main(String[] args) {
        GroupDAO grDAO = new GroupDAO();
        List[] lists = grDAO.removeGroup(grDAO.getGroupById(1));

        FileDAO fiDAO = new FileDAO();
        fiDAO.removeGroupAddFiles(lists);

        UserDAO usDAO = new UserDAO();
        usDAO.removeGroupAddUserFiles(lists);
    }

GroupDAO.java :

public List[] removeGroup(Group group) {
        Set<File> setFiles = group.getFiles();
        Set<User> setUsers = group.getUsers_1();
        List[] lists = new List[2];

        Iterator<File> itFiles = setFiles.iterator();
        Iterator<User> itUsers = setUsers.iterator();
        List<File> listFiles = new ArrayList<File>();
        List<User> listUsers = new ArrayList<User>();
        while (itFiles.hasNext()) {
            listFiles.add(itFiles.next());
        }
        while (itUsers.hasNext()) {
            listUsers.add(itUsers.next());
        }

        lists[0] = listUsers;
        lists[1] = listFiles;

        org.hibernate.Transaction tx = session.beginTransaction();
        session.delete(group);
        tx.commit();

        return lists;
    }

FileDAO.java :

public List[] removeGroupAddFiles(List[] lists) {
        System.out.println("5 : " + session.isOpen());
        System.out.println("6 : " + session.isOpen());
        Iterator<File> itFile = lists[1].iterator();
        System.out.println("7 : " + session.isOpen());

        org.hibernate.Transaction tx = session.beginTransaction();
        while (itFile.hasNext()) {
            System.out.println("8 : " + session.isOpen());
            Iterator<User> itUser = lists[0].iterator();
            System.out.println("9 : " + session.isOpen());
            File f = itFile.next();
            while (itUser.hasNext()) {
                System.out.println("10 : " + session.isOpen());
                System.out.println("11 : " + session.isOpen());
                User u = itUser.next();
                System.out.println("12 : " + session.isOpen());
                File fCopie = new File(u, f.getName() + "_" + u.getFirstName() + "_" + u.getLastName(), f.getExtension(), f.getSize(), f.getCreatedAt(), f.getUpdateAt());
                System.out.println("13 : " + session.isOpen());
                session.save(fCopie);
                System.out.println("14 : " + session.isOpen());
            }
        }
        tx.commit();
        return lists;
    }

UserDAO.java :

public List[] removeGroupAddUserFiles(List[] lists) {
        System.out.println("15 : " + session.isOpen());
        Iterator<User> itUser = lists[0].iterator();
        System.out.println("16 : " + session.isOpen());

        org.hibernate.Transaction tx = session.beginTransaction();

        while (itUser.hasNext()) {
            System.out.println("17 : " + session.isOpen());
            Iterator<File> itFile = lists[1].iterator();
            System.out.println("18 : " + session.isOpen());
            User u = itUser.next();
            while (itFile.hasNext()) {
                System.out.println("19 : " + session.isOpen());
                File f = itFile.next();
                System.out.println("20 : " + session.isOpen());
                try {
                    u.getFiles().add(f);
                } catch (LazyInitializationException e) {
                    e.printStackTrace();
                }
                System.out.println("21 : " + session.isOpen());
            }
            try {
                session.update(u);
            } catch (ConstraintViolationException e) {
                e.printStackTrace();
            }
            System.out.println("22 : " + session.isOpen());
        }
        tx.commit();
        return lists;
    }

My code is redundant, I know him(it), it was exactly to try to avoid the problem of session that closed.

The session closes in the line:

U.getFiles () .add (f);

And HibernateUtil.java :

public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
// load from different directory
            SessionFactory sessionFactory = new Configuration().configure("/hibernate.cfg.xml").buildSessionFactory();

            return sessionFactory;

        } catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

    public static void shutdown() {
// Close caches and connection pools
        getSessionFactory().close();
    }
}

I put a display of session.isOpen() everywhere in my code as well as a try wrestling with the error. The program thus continues, and shows me systematically true, whether it is before or after the exception!

Answer 1

General problem is: you open transaction, create Hibernate object with FetchType LAZY. Hibernate creates proxy for collection, which will load objects on first usage of collections. You close transaction, and try to access that collection. Hibernate session is expired, because transaction is closed, so you got error.

You should redesign your code so to never return object with unitialized proxies out of transaction block. You should either load collections or evict object from session or do not map the collection authomatically (remove them from POJO).