Prevent Dozer from triggering Hibernate lazy loading

Question

I am using Spring transactions so the transaction is still active when POJO to DTO conversion occurs.

I would like to prevent Dozer from triggering lazy loading, so that hidden sql queries never occur : all fetching has to be done explicitly via HQL (to get the best control on performances).

1. Is it a good practice (I can't find it documented anywhere) ?

2. How to do it safely ?

I tried this before DTO conversion :

PlatformTransactionManager tm = (PlatformTransactionManager) SingletonFactoryProvider.getSingletonFactory().getSingleton("transactionManager");
tm.commit(tm.getTransaction(new DefaultTransactionDefinition()));

I don't know what happens to the transaction, but the Hibernate session doesn't get closed, and the lazy loading still occurs.

I tried this :

SessionFactory sf = (SessionFactory) SingletonFactoryProvider.getSingletonFactory().getSingleton("sessionFactory");
sf.getCurrentSession().clear();
sf.getCurrentSession().close();

And it prevents lazy loading, but is it a good practice to manipulate session directly in the application layer (which is called "facade" in my project) ? Which negative side effects should I fear ? (I've already seen that tests involving POJO -> DTO conversions could no more be launched through AbstractTransactionnalDatasource Spring test classes, because this classes try to trigger a rollback on a transaction which is no more linked to an active session).

I've also tried to set propagation to NOT_SUPPORTED or REQUIRES_NEW, but it reuse the current Hibernate session, and doesn't prevent lazy loading.

Answer 1

The only generic solution I have found for managing this (after looking into Custom Converters, Event Listeners & Proxy Resolvers) is by implementing a Custom Field Mapper. I found this functionality tucked away in the Dozer API (I don't believe it is documented in the User Guide).

A simple example is as follows;

public class MyCustomFieldMapper implements CustomFieldMapper 
{
    public boolean mapField(Object source, Object destination, Object sourceFieldValue, ClassMap classMap, FieldMap fieldMapping) 
    {       
        // Check if field is a Hibernate collection proxy
        if (!(sourceFieldValue instanceof AbstractPersistentCollection)) {
            // Allow dozer to map as normal
            return false;
        }

        // Check if field is already initialized
        if (((AbstractPersistentCollection) sourceFieldValue).wasInitialized()) {
            // Allow dozer to map as normal
            return false;
        }

        // Set destination to null, and tell dozer that the field is mapped
        destination = null;
        return true;
    }   
}

This will return any non-initialized PersistentSet objects as null. I do this so that when they are passed to the client I can differentiate between a NULL (non-loaded) collection and an empty collection. This allows me to define generic behaviour in the client to either use the pre-loaded set, or make another service call to retrieve the set (if required). Additionally, if you decide to eagerly load any collections within the service layer then they will be mapped as usual.

I inject the custom field mapper using spring:

<bean id="dozerMapper" class="org.dozer.DozerBeanMapper" lazy-init="false">
    <property name="mappingFiles">
        ...
    </property>
    <property name="customFieldMapper" ref="dozerCustomFieldMapper" />
</bean>
<bean id="dozerCustomFieldMapper" class="my.project.MyCustomFieldMapper" />

I hope this helps anyone searching for a solution for this, as I failed to find any examples when searching the Internet.

Answer 2

A variation on the popular version above, makes sure to catch both PersistentBags, PersistentSets, you name it...

public class LazyLoadSensitiveMapper implements CustomFieldMapper {

public boolean mapField(Object source, Object destination, Object sourceFieldValue, ClassMap classMap, FieldMap fieldMapping) {
    //if field is initialized, Dozer will continue mapping

    // Check if field is derived from Persistent Collection
    if (!(sourceFieldValue instanceof AbstractPersistentCollection)) {
        // Allow dozer to map as normal
        return false;
    }

    // Check if field is already initialized
    if (((AbstractPersistentCollection) sourceFieldValue).wasInitialized()) {
        // Allow dozer to map as normal
        return false;
    }

    return true;
}

}

Answer 3

I didn't get the above to work (probably different versions). However this works fine

public class HibernateInitializedFieldMapper implements CustomFieldMapper {
    public boolean mapField(Object source, Object destination, Object sourceFieldValue, ClassMap classMap, FieldMap fieldMapping) {
        //if field is initialized, Dozer will continue mapping
        return !Hibernate.isInitialized(sourceFieldValue));
    }
}

Answer 4

Have you considered disabling lazy loading altogether?

It doesn't really seem to jive with the patterns you state you would like to use:

I would like to prevent Dozer from triggering lazy loading, so that hidden sql queries never occur : all fetching has to be done explicitly via HQL (to get the best control on performances).

This suggests you would never want to use lazy loading.

Dozer and the Hibernate-backed beans you pass to it are blissfully ignorant of each other; all Dozer knows is that it is accessing properties in the bean, and the Hibernate-backed bean is responding to calls to get() a lazy-loaded collection just as it would if you were accessing those properties yourself.

Any tricks to make Dozer aware of the Hibernate proxies in your beans or vice versa would, IMO, break down the layers of your app.

If you don't want any "hidden SQL queries" fired at unexpected times, simply disable lazy-loading.