Reattaching an entity to lazy load a collection (JPA/Hibernate)

Question

Can anyone help me with a JPA issue that I thought would have been simple. I'm trying to write a general-purpose lazy load manager in my JPA persistence framework, so layers higher up the application stack can access lazy-loaded data without having to deal with the specifics.

I have a lazy load manager:

public class JpaLazyLoader extends AbstractJpaDAO<Void> implements LazyLoader
{

  public JpaLazyLoader()
  {
    super( void.class ); 
  }

  @Transactional(readOnly=true)
  public <T,E> T get( ILazyGetter<T,E> p_getter ) throws Exception {
    // reattach the object to the session

    E l_entity = getEntityManager().merge( p_getter.getEntity() );

    // return the getter data
    return p_getter.get( l_entity );
  }
}

The lazy getter is like this:

public interface ILazyGetter<T,E> extends Serializable
{
  public E getEntity();
  public T get( E p_entity ) throws Exception;
}

And the idea is that it would be used like this:

          return m_lazyLoader.get( new ILazyGetter<Collection<Child>, Parent>() {

            private static final long serialVersionUID = 1L;

            public Parent getEntity() {
              return getValue(); // get the parent object from somewhere
            }

            public Collection<Child> get( Parent p_entity ) throws Exception {
              // children are a lazy-loaded Set<Child>
              return p_entity.getChildren();
            }
          } );

The annotations in Parent are like this:

@Entity(name="parent")
public class Parent implements Serializable {

  private static final long serialVersionUID = 1L;

  ....

  @Id
  @Column(name="id")
  @GeneratedValue(strategy=GenerationType.AUTO)
  protected Long id;

  @OneToMany
  @JoinTable
  (
      name="parent_child_associations",
      joinColumns={ @JoinColumn(name="parent_id", referencedColumnName="id") },
      inverseJoinColumns={ @JoinColumn(name="child_id", referencedColumnName="id", unique=true) }
  )
  protected Set<Child> children;

}

The parent object was loaded in a different transaction and then detached. I hoped it would be a trivial thing to reattach the parent to another session (within the @transactional bit) but I can't get it to work. I've tried optimistic/pessimistic/none locking before and after the merge but none seem to work.

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: test.Parent.children, could not initialize proxy - no Session
 at     org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:566)
 at     org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
 at     org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:545)
 at     org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:124)
 at org.hibernate.collection.internal.PersistentSet.iterator(PersistentSet.java:180)

Is this just the wrong way to go about things? If so, what is the 'correct' way? If not, what am I doing wrong?

Thanks for any help

Answer 1

OK well thanks to Andre I've got it working.

I simply hadn't initialised the collection object, instead I was just calling parent.getChildren(), which only returned the proxy object rather than forcing the fetch.

The updated get method is below for anyone who has a similar need:

@Transactional(readOnly=true)
public <T,E> T get( ILazyGetter<T,E> p_getter ) throws Exception {
  // reattach the object to the session    
  E l_entity = getEntityManager().merge( p_getter.getEntity() );

  T l_lazyData = p_getter.get( l_entity );

  // Just getting the data doesn't necessarily initialize it - 
  // explicitly initialize it depending on the data type.
  intializeEntity( l_lazyData );

  // return the getter data
  return l_lazyData;
}

/**
 * Attempt to initialize the entity so it is fully lazy loaded while in the transaction
 * @param p_entity
 */
protected void intializeEntity( Object p_entity ) {
  if( p_entity != null && p_entity instanceof Collection ) {
    Collection<?> l_entity = (Collection<?>)p_entity;
    l_entity.size(); // force the collection to load
  }
}