How to lazily load references when the object is detached from session ?

[Kumar Raja]

Hi All,

I have read that in order to lazily load objects and its references, the object and its references should always be attached to a session. This makes sense as, the proxies are associated with the session and out of session proxies does not have any value and can not be used. However, in most of the cases, especially in a multitier environment, a client's request may pass through many layers and at some where below the stack, the objects are retrieved under a session and returned back to calling object. In the calling object, there may/may not be having any references to session (may be that is completely unaware of a thing called Hibernate). In such cases, how can the objects and its references be still loaded lazily. I may better explain my question using an example.

Assume, I have two entities Person and Event.
Every Person can attain one or more events and I'm having a one-many unidirectional association between Person and Event.

Person.java

     
public class Person {
	private Long id=null;
	private int age=0;
	private String fname=null;
	private String lname=null;
	private String ssn=null;
	private Set<Event> events=new HashSet<Event>();

Person.hbm.xml

    
<hibernate-mapping package="com.hibertut.beans">
	<class name="Person" table="PERSON" lazy="true">
		<id name="id" column="PERSON_ID" type="long">
			<generator class="native"></generator>
		</id>
                                  ..............................................................
                                  .............................................................
		<set name="events" table="PERSON_EVENT" lazy="true">
			<key column="PERSON_ID"/>
			<many-to-many column="EVENT_ID" class="Event" />
		</set>
	</class>
</hibernate-mapping>

Event.java

     
public class Event {
                private Long id=null;
	private String title=null;
	private Date date=null;

Event.hbm.xml

     
<hibernate-mapping package="com.hibertut.beans">
	<class name="Event" table="EVENT" lazy="true">
		<id name="id" column="EVENT_ID" type="long">
			<generator class="native"></generator>
		</id>
                      .................................................
                      ................................................	
</class>
</hibernate-mapping>

After defining these files, I'm using a client application to persist Events and Persons and establishing associations between these two. I'm successful till here.

Now, I'm attempting to retrieve objects and my method is as follows

    
public Object retrieve(long id,String objType){
		Object object=null;
		session=sessionFactory.openSession();
		if(session!=null){
			if (objType.equalsIgnoreCase("Event")){
				object=(Event)session.load(Event.class,new Long(id));
			}
			else if (objType.equalsIgnoreCase("Person")){
				object=(Person)session.load(Person.class,new Long(id));
			}
		}
		session.close();
		return object;
	}

As you can see from this method this is returning only the proxies as I'm using load method instead of get. But when I try to invoke the method

 
        e=(Event)hiber.retrieve(1L,"Event");      
        System.out.println(e.title);

on the reference I get for these objects, I get exception [b]1203 [main] ERROR org.hibernate.LazyInitializationException - could not initialize proxy - no Session. [b]. I can understand the reason for this exception, as at this point, object being referred by "e" is not associated with any session.

I can quickly overcome this issue, if I use "get" instead of "load", though I'm more interested in using load.

But what if, Event has references to some other entities, say "Address". How can I call
e.getAddress() like method, when I'm completely out of session control, except that I mention the reference retrieval as eager loading.

Is there any work around, where I can access objects and their references lazily, but yet outside a hibernate session.

Thanks

[David Newton]

No, you need to be attached to a Hibernate session--this seems self-evident to me.

Options include using an OSiV-style filter or loading more data earlier in the process.