Why is Hibernate deleting my collection entries when all I do is a list?

Question

I seem to have a rather strange problem. I display users and their roles in a JSP. For some reason, the roles are displayed only for the first time the page is loaded. I refresh the page and the roles are all deleted from the database!

My setup is standard Spring MVC, JPA + Hibernate (over Spring Data) application. (Spring 3.2.x, Hibernate 4.1.8)

I have two entities - User and Role as shown below (assume the setters and getters)

@Entity
public class User {
    @Id
    @GeneratedValue
    private int id;

    private String name;

    @ManyToMany
    @JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    private Set<Role> roles = new HashSet<>();
}

@Entity
public class Role {
    @Id
    private String id;

    private String name;
}

I have a Spring Data repository which doesn't define any extra methods.

public interface UserRepository extends CrudRepository<User, Integer> { 
}

I have a service and a controller whose relevant methods are as follows.

// service
@Transactional(readOnly = true)
public Iterable<User> findAll() {
    return userRepository.findAll();
}

// controller
@RequestMapping
public String showUsers(ModelMap model) {
    model.put("users", userService.findAll());

    return "admin/users";
}

In my JSP, I am trying to display all the users and any associated roles.

<c:forEach var="user" items="${users}">
    <tr>
        <td>${user.name}</td>
        <td>
           <c:forEach var="role" items="${user.roles}">
                ${role.name}
            </c:forEach>
        </td>
    </tr>
</c:forEach>

Like I said earlier, the records in my user_role table get deleted after this page has been rendered.

Upon enabling DEBUG for org.hibernate and enabling query logging, here is what I found in the logs:

22:36:25 DEBUG Collection dereferenced: [com.adarshr.domain.User.roles#1] [Collections.java:76]
22:36:25 DEBUG Flushed: 0 insertions, 0 updates, 0 deletions to 2 objects [AbstractFlushingEventListener.java:117]
22:36:25 DEBUG Flushed: 0 (re)creations, 0 updates, 1 removals to 1 collections [AbstractFlushingEventListener.java:124]
22:36:25 DEBUG Deleting collection: [com.adarshr.domain.User.roles#1] [AbstractCollectionPersister.java:1124]
22:36:25 DEBUG 
    delete 
    from
        user_role 
    where
        user_id=? [SqlStatementLogger.java:104]
22:36:25 DEBUG Done deleting collection [AbstractCollectionPersister.java:1182]

Quite clearly something fishy is going on here. Why is my collection being dereferenced in the first place?

Here is my JPA entity manager factory definition.

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
    <property name="dataSource" ref="mainDataSource" />
    <property name="packagesToScan" value="com.adarshr.domain" />
    <property name="persistenceProviderClass" value="org.hibernate.ejb.HibernatePersistence" />
    <property name="jpaVendorAdapter">
        <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter" />
    </property>
    <property name="jpaProperties">
        <value>
            hibernate.dialect=org.hibernate.dialect.MySQL5InnoDBDialect
            hibernate.format_sql=${hibernate.format.sql}
            hibernate.ejb.naming_strategy=org.hibernate.cfg.ImprovedNamingStrategy
            hibernate.show_sql=${hibernate.show.sql}
            hibernate.enable_lazy_load_no_trans=true
        </value>
    </property>
</bean>

Am I missing something obvious here?

Answer 1

As the code you posted seems correct, I am guessing here.

The first thing I would suggest is to remove this initialization (it seems the only place where roles are removed) which while is a good idea by itself in general (see comments below) I think may interfere with hibernate.enable_lazy_load_no_trans=true which has been known to leak connections in the past:

 private Set<Role> roles = new HashSet<>();

A second try would be to check if and what changes if you also annotate the inverse side of your ManyToMany relation with mappedBy.

A third attempt would be if eagerly loading the collection fixes the problem (aggravating enable_lazy_load_no_trans position even more), using FetchType, Hibernate.initialize() of whatever you want.

Last one is getting rid of enable_lazy_load_no_trans and use OpenSessionInView

EDIT: ahh, last one, you may* have it this bug (hibernate 4.1.8 and 4.1.9 affected): https://hibernate.onjira.com/browse/HHH-7971

So having a shot with 4.1.7 could give better results (or maybe worse).

* where may is to be intended as: "your case is so similar you are invited to send your code as a test case in the bug report".